respiratory disease

respiratory disease

Introduction
 any of the diseases and disorders that affect human respiration (respiration, human).

 Diseases of the respiratory system may affect any of the structures and organs that have to do with breathing, including the nasal cavities, the pharynx (or throat), the larynx, the trachea (or windpipe), the bronchi and bronchioles, the tissues of the lungs (lung), and the respiratory muscles of the chest cage.

 The respiratory tract is the site of an exceptionally large range of disorders for three main reasons: 1) it is exposed to the environment and therefore may be affected by inhaled organisms, dusts, or gases; 2) it possesses a large network of capillaries through which the entire output of the heart has to pass, which means that diseases that affect the small blood vessels (blood vessel) are likely to affect the lung; and 3) it may be the site of “sensitivity” or allergic phenomena that may profoundly affect function.

      This article discusses the signs and symptoms of respiratory disease, the natural defenses of the human respiratory system, the methods of detecting respiratory disease, and the different diseases of the respiratory system. For more information about the anatomy of the human respiratory system and the process of respiration, see human respiratory system (respiration, human).

Signs and symptoms
      The symptoms of lung disease are relatively few. cough is a particularly important sign of all diseases that affect any part of the bronchial tree. A cough productive of sputum is the most important manifestation of inflammatory or malignant diseases of the major airways, of which bronchitis is a common example. In severe bronchitis the mucous glands lining the bronchi enlarge greatly, and, commonly, 30 to 60 ml of sputum are produced in a 24-hour period, particularly in the first two hours after awakening in the morning. An irritative cough without sputum may be caused by extension of malignant disease to the bronchial tree from nearby organs. The presence of blood in the sputum (hemoptysis) is an important sign that should never be disregarded. Although it may result simply from an exacerbation of an existing infection, it may also indicate the presence of inflammation, capillary damage, or a tumour. Hemoptysis is also a classic sign of tuberculosis of the lungs.

      The second most important symptom of lung disease is dyspnea, or shortness of breath. This sensation, of complex origin, may arise acutely, as when a foreign body is inhaled into the trachea, or with the onset of a severe attack of asthma. More often, it is insidious in onset and slowly progressive. What is noted is a slowly progressive difficulty in completing some task, such as walking up a flight of stairs, playing golf, or walking uphill. The shortness of breath may vary in severity, but in diseases such as emphysema (see below Pulmonary emphysema (respiratory disease)), in which there is irreversible lung damage, it is constantly present. It may become so severe as to immobilize the victim, and tasks such as dressing cannot be performed without difficulty. Severe fibrosis of the lung, resulting from occupational lung disease or arising from no identifiable antecedent condition, may also cause severe and unremitting dyspnea. Dyspnea is also an early symptom of congestion of the lung as a result of impaired function of the left ventricle of the heart. When this occurs, if the right ventricle that pumps blood through the lungs is functioning normally, the lung capillaries become engorged, and fluid may accumulate in small alveoli and airways. It is commonly dyspnea that first causes a patient to seek medical advice, but absence of the symptom does not mean that serious lung disease is not present, since, for example, a small lung cancer that is not obstructing an airway does not produce shortness of breath.

      Chest pain may be an early symptom of lung disease, but it is most often associated with an attack of pneumonia, in which case it is due to an inflammation of the pleura that follows the onset of the pneumonic process. Pain associated with inflammation of the pleura is characteristically felt when a deep breath is taken. The pain disappears when fluid accumulates in the pleural space, a condition known as a pleural effusion. Acute pleurisy with pain may signal a blockage in a pulmonary vessel, which leads to acute congestion of the affected part. For example, pulmonary embolism, the occlusion of a pulmonary artery by a fat deposit or by a blood clot that has dislodged from a site elsewhere in the body, can cause pleurisy. Sudden blockage of a blood vessel injures the lung tissue to which the vessel normally delivers blood. In addition, severe chest pain may be caused by the spread of malignant disease to involve the pleura, or by a tumour that arises from the pleura itself, such as a mesothelioma. Severe, intractable pain caused by such conditions may require surgery to cut the nerves that supply the affected segment. Fortunately, pain of this severity is rare.

      To these major symptoms of lung disease—coughing, dyspnea, and chest pain—may be added several others. A wheeziness in the chest may be heard. This is caused by narrowing of the airways, such as occurs in asthma. Some diseases of the lung are associated with the swelling of the fingertips (and, rarely, of the toes) called “clubbing.” Clubbing may be a feature of bronchiectasis (chronic inflammation and dilation of the major airways), diffuse fibrosis of the lung from any cause, and lung cancer. In the case of lung cancer, this unusual sign may disappear after surgical removal of the tumour. In some lung diseases, the first symptom may be a swelling of the lymph nodes (lymph node) that drain the affected area, particularly the small nodes above the collarbone in the neck; enlargement of the lymph nodes in these regions should always lead to a suspicion of intrathoracic disease. Not infrequently, the presenting symptom of a lung cancer is caused by spread of the tumour to other organs. Thus, a hip fracture from bone metastases, cerebral signs from intracranial metastases, or jaundice from liver involvement may all be the first evidence of a primary lung cancer, as may sensory changes in the legs, since a peripheral neuropathy may also be the presenting evidence of these tumours.

      The generally debilitating effect of many lung diseases is well recognized. A person with active lung tuberculosis or with lung cancer, for example, may be conscious of only a general feeling of malaise, unusual fatigue, or seemingly minor symptoms as the first indication of disease. Loss of appetite and loss of weight, a disinclination for physical activity, general psychological depression, and some symptoms apparently unrelated to the lung, such as mild indigestion or headaches (headache), may be diverse indicators of lung disease. Not infrequently, the patient may feel as one does when convalescent after an attack of influenza. Because the symptoms of lung disease, especially in the early stage, are variable and nonspecific, physical and radiographic examination of the chest are an essential part of the evaluation of persons with these complaints.

Defenses of the respiratory system
      Exposed as it is to the outside environment, the respiratory tract possesses a complicated but comprehensive series of defenses against inhaled material. As air passes through the nose, large particles of debris are filtered out by cilia (cilium) and by mucus that is secreted from the mucous membrane lining the nasal cavity. The air then travels through the pharynx, which is the last portion of the upper airway, through the larynx, which is the beginning portion of the lower airways, and into the trachea. Further filtration of the air occurs as it passes over cilia and sticky layers of mucus in the trachea. In addition, lymphatic vessels in the wall of the trachea transport cells of the immune system, such as lymphocytes (lymphocyte) and macrophages, that act to trap and destroy foreign particles. Bands of muscle that surround the cartilage of the trachea play an important role in narrowing the airway during coughing, thus providing a forceful defense mechanism by which sputum and other substances can be quickly expelled from the respiratory tract.

      In the bronchial tree, cilia (cilium) beat in unison in one direction, moving substances up and out of the airways. Covering the cilia in the bronchioles and small bronchi is a thin layer of fluid, which increases in thickness and becomes layered with mucus as the small bronchi converge into the large bronchi. When the cilia beat, foreign particles are transported along in the fluid and mucus layers. This system, known as the mucociliary escalator, carries debris as far as the pharynx, where the fluid and mucus is then swallowed and the debris eliminated by the digestive system.

      Macrophages form the first line of defense in the smaller branches of the airways. These cells, located within the alveoli of the lungs, ingest and destroy bacteria and viruses (virus) and remove small particles. They also secrete chemicals that attract other immune cells such as white blood cells (leukocyte) to the site, and hence they can initiate an inflammatory response in the lung. Particles picked up by macrophages are carried into the lymphatic system of the lung and stored in adjacent lymph nodes (lymph node) in the lung and mediastinum (the region between the lungs). Soluble particles are removed into the bloodstream and are eventually excreted by the kidneys (kidney).

Methods of investigation
      Physical examination of the chest remains important, as it may reveal the presence of an area of inflammation, a pleural effusion, or an airway obstruction. Methods of examination include physical inspection and palpation for masses, tender areas, and abnormal breathing patterns; percussion to gauge the resonance of the underlying lung; and auscultation (listening) with a stethoscope to determine pitch and loudness of breath sounds. The sounds detected with a stethoscope may reveal abnormalities of the airways, the lung tissue, or the pleural space. Examination of the sputum for bacteria allows the identification of many infectious organisms and the institution of specific treatment; sputum examination for malignant cells is occasionally helpful.

      The conventional radiological examination of the chest has been greatly enhanced by the technique of computerized tomography (CT). This technique produces a complete picture of the lungs by using X-rays (X-ray) to create two dimensional images that are integrated into one image by a computer. While the resolution of computerized tomography is much better than most other visualization techniques, lung ventilation and perfusion scanning can also be helpful in detecting abnormalities of the lungs. In these techniques, a radioactive tracer molecule is either inhaled, in the case of ventilation scanning, or injected, in the case of perfusion scanning. The ventilation scan allows visualization of gas exchange in the bronchi and trachea, and the perfusion scan allows visualization of the blood vessels in the lungs. The combined results from ventilation and perfusion scanning are important for the detection of focal occlusion of pulmonary blood vessels by pulmonary emboli.

      Although magnetic resonance imaging (MRI) plays a limited role in examination of the lung, because the technique is not well suited to imaging air-filled spaces, MRI is useful for imaging the heart and blood vessels within the thorax. positron emission tomography (PET scanning) is used to distinguish malignant lung tissue from scar tissue on tissues such as the lymph nodes (lymph node). Flexible fibre-optic (fibre optics) bronchoscopes that can be inserted into the upper airway through the mouth are used to examine the larynx, trachea, and major bronchi. By feeding a surgical instrument through a special channel of the bronchoscope, physicians can collect fluid and small tissue samples from the airways. Tissue samples are examined for histological changes that indicate certain diseases and are cultured to determine whether harmful bacteria are present.

      A number of tests are available to determine the functional status of the lung and the effects of disease on pulmonary function. Spirometry, the measurement of the rate and quantity of air exhaled forcibly from a full respiration, allows measurement of the ventilation capacity of the lungs and quantification of the degree of airflow obstruction. Ventilatory capability can be measured with a peak flow meter, which is often used in field studies. More complex laboratory equipment is necessary to measure the volumes of gas in the lung; the distribution of ventilation within the lung; airflow resistance; the stiffness of the lung, or the pressure required to inflate it; and the rate of gas transfer across the lung, which is commonly measured by recording the rate of absorption of carbon monoxide into the blood ( hemoglobin has a high affinity for carbon monoxide). Arterial blood gases and pH values indicate the adequacy of oxygenation and ventilation and are routinely measured in patients in intensive care units. Tests of exercise capability, in which workload, total ventilation, and gas exchange are compared before, during, and after exercise, are useful in assessing functional impairment and disability.

Lung transplantation (transplant)
      Early attempts at transplanting a single lung in patients with severe bilateral lung disease were not successful, but from the late 1970s bilateral lung transplantation had some striking results. Persons severely disabled by cystic fibrosis, emphysema, sarcoidosis, pulmonary fibrosis, or severe primary pulmonary hypertension can achieve nearly normal lung function several months after the procedure. Because transplantation offers the only hope for persons with severe lung disease, who may be relatively young, the techniques are being pursued aggressively in specialized centres. Availability of donor lungs is sharply limited by the number of suitable donors; for example, many people who die of severe head injuries, which presumably would leave the lungs intact, often have also suffered lung injury or lung infection. With proper selection of donor organs and proper transplantation technique, survival at one year has been reported at 90 percent.

      Many recipients of single or double lung transplantation develop bronchiolitis obliterans beginning several months or years after surgery. This complication is thought to represent gradual immunologic rejection of the transplanted tissue despite the use of immunosuppressant drugs. Brochiolitis obliterans and the constant risk of serious infection brought about by the use of immunosuppressant drugs limit survival to approximately 40 to 60 percent five years after surgery.

Morphological classification of respiratory disease
  The main divisions of the respiratory system (respiration, human) serve as a basis for the morphological description of respiratory system diseases. The upper airway consists of the nose, nasopharynx, and larynx. Below these structures lies the trachea. Thereafter the airway divides into two major airways, right and left, and then into progressively smaller tubes until finally the terminal bronchioles, which are about one millimetre in diameter, are reached. On average, 16 generations of division occur between the trachea and the terminal bronchioles. Although there is only one airway at the beginning—the trachea—there are about 650,000 terminal bronchioles. The cross-sectional area of the bronchial tree increases with increasing subdivision. The end of each terminal bronchiole opens into an acinus, so called because the structure resembles a cluster of grapes, and from this point onward the gas-exchanging portion of the lung is reached. The alveoli, or air sacs, which are divided into groups or lobules by fibrous partitions, or septa, are small hexagonal structures forming a blind end to the acinus. The wall of the acinus consists of blood capillaries (capillary), and the remaining structures are extremely thin, only providing supporting tissue for the rich capillary bed that constitutes the parenchyma, or the essential tissue of the lung itself. The parenchyma is the gas-exchanging tissue of the lung and has a surface area roughly comparable to that of a tennis court. Blood is distributed to the lung through the branching pulmonary artery, which subdivides with the bronchial tree and accompanies the smaller bronchioles into the region of the acinus to supply the capillaries of the alveolar wall. Oxygenated blood from the acini is collected into pulmonary veins (vein), which run at some little distance from the bronchioles. An interstitial space exists around the alveoli and around the bronchioles and blood vessels, and this connects the lymph nodes (lymph node) (the small masses of lymphatic tissue that occur along the course of the lymph vessels) situated in the midline of the thoracic cavity and extending in a chain up into the neck and down into the abdomen.

      Each lung is covered by a protective membrane, or pleura, which consists of a visceral layer that lines the outer surface of the lung and a parietal layer that lines the inside of the chest wall. The space between these two layers, called the intrapleural space, normally contains no air and only a few millilitres of fluid for lubrication purposes, as during breathing one layer must slide on another. The pleurae may become involved by inflammation or neoplastic disease, in which case an effusion of fluid may occur between the two layers.

      From this general description, diseases of the respiratory system may be grouped into the following categories.

Diseases of the upper airway
 The nasal sinuses (sinus) are frequently the site of both acute and chronic infections. In common with the palate and the nasopharynx, they are also the site of malignant neoplastic changes. Cancer of the larynx (laryngeal cancer) is much more common in smokers (smoking) than in nonsmokers.

      The occurrence of upper airway obstruction (particularly common in people who snore) has been documented in sleep laboratory studies. Sleep apnea, which sometimes involves upper airway obstruction, is characterized by cessation of breathing for up to a minute and by a marked fall in blood oxygen levels, thus arousing an affected individual from sleep. Sleep apnea affects approximately 4 percent of adults. It is not confined to the very obese, although it forms part of the syndrome of severe obesity in which sleep disturbance is common, and it is associated with the daytime somnolence known as the pickwickian syndrome, after Charles Dickens (Dickens, Charles)'s description of the fat boy in The Pickwick Papers. Sleep apnea is sometimes caused by relaxation of muscles around the pharynx and obstruction of the airway by the palate and tongue. It is related to narrow anatomical dimensions in this area but is also more likely to occur if alcohol (alcoholic beverage) is ingested shortly before sleep. Sleep apnea may cause a rise in systemic blood pressure, and pickwickian syndrome may affect one's performance at work and ability to do other tasks carried out during the day. In severe cases, sleep apnea leads to right ventricular heart failure.

Diseases of the major bronchi
      The major bronchi can become the seat of chronic inflammation, as in chronic bronchitis or bronchiectasis. The latter disease may be caused by the familial disease cystic fibrosis (see below Bronchiectasis (respiratory disease)). The major bronchi may also be the site of development of malignant disease.

Diseases of the smaller bronchi and bronchioles
      It is in the smaller bronchi that major obstruction commonly occurs in asthma: these bronchi contain smooth muscle in their walls, and the muscle may contract, causing airway obstruction. The small radicles of the bronchial tree, the bronchioles, are commonly involved in infective processes such as viral infections; they are also the primary site of deposition of inhaled dust and particles. Because of the large cross-sectional area of this part of the airway, considerable disease may be present in the bronchioles without affecting the expiratory flow rate. The bronchioles are occasionally the site of a primary noninfective bronchiolitis in persons with rheumatoid arthritis or other systemic inflammatory diseases.

Diseases of the alveolar ducts and alveoli (pulmonary alveolus)
      These structures are the site of primary involvement in many infections, including pneumonia, and it is on the parenchyma of the lung that the main effects of blockage of a pulmonary artery ( pulmonary embolism) occur. The capillary bed surrounding the alveoli is subject to damage, and fluid may leak through the alveolar capillaries to accumulate in the lungs (pulmonary edema (lung congestion)). The capillary bed is also extensively damaged in the condition known generally as acute respiratory distress syndrome (respiratory distress syndrome of newborns); the exact mechanism of the damage is not yet fully understood. The alveolar walls themselves may undergo diffuse interstitial thickening, a characteristic of diseases grouped under the heading of “diffuse interstitial fibrosis”; interstitial thickening may also occur as a manifestation of collagen diseases such as scleroderma. One of the common forms of emphysema, in which alveolar destruction occurs, entails early loss of tissue at the point where the bronchiole ends in the acinus, resulting in a punched-out lesion in the centriacinar region. This form of emphysema is the one that most commonly develops after years of cigarette smoking.

      The lung parenchyma is the site of the discrete aggregations of cells (cell), usually giant cells, that form the granulomas characteristic of the generalized disease known as sarcoidosis, and it is in the lung parenchyma that nodules caused by the inhalation of silica particles are found.

Diseases of the pleura
      The pleura may be involved in inflammatory or neoplastic processes, either of which may lead to fluid accumulation ( pleural effusion) between the two layers. The pleural membranes of the lungs may become perforated and spontaneously rupture, usually over a small collection of congenital blebs, or cysts (cyst) at the apex of the lung. This causes spontaneous pneumothorax, a partial or occasionally complete collapse of the lung. In the majority of cases, a pneumothorax resolves slowly of its own accord, although pleural suction may be needed to expedite recovery. If repetitive attacks occur, the blebs may be removed surgically, and the pleural membrane of the affected lung may be sealed to the pleural membrane of the inner wall of the thorax to prevent a recurrence.

      The most common disease of the pleura is caused by inflammation and is referred to as pleurisy. A pleurisy with an effusion may be the presenting symptom of pulmonary tuberculosis, and pleurisy may accompany any kind of pneumonia. When a pleural effusion in a person with bacterial pneumonia becomes infected, pus accumulates in the pleural cavity ( empyema). This complication—dreaded before the widespread availability of antibiotics (antibiotic) in the mid-20th century and after the outbreaks of antibiotic-resistant microorganisms in the late 20th and early 21st centuries—requires drainage of the pleural space. In severe instances of empyema, video-assisted thoracic surgery is performed to evacuate viscous or semisolid infected material from the space.

      Mesothelioma, a cancer of the pleura, may occur many years after inhalation of asbestos fibres (see below Asbestosis and mesothelioma (respiratory disease)). The cancerous cells of the pleura can eventually metastasize and invade nearby and distant tissues, including tissues of the neck and head.

Diseases of the mediastinum and diaphragm
      The mediastinum comprises the fibrous membrane in the centre of the thoracic cavity, together with the many important structures situated within it. Enlargement of lymph glands in this region is common, particularly in the presence of lung tumours (tumour) or as part of a generalized enlargement of lymphatic tissue in disease. Primary tumours of mediastinal structures may arise from the thymus gland or the lower part of the thyroid gland; noninvasive cysts of different kinds are also found in the mediastinum.

      The diaphragm may be incompletely formed, leading to herniation of abdominal viscera through it. In adult life an important disease involving the diaphragm is bilateral diaphragmatic paralysis. This leads to a severe reduction in vital capacity, especially when the subject is recumbent. In many cases the cause of the paralysis cannot be determined. Paralysis of the diaphragm on one side is more common and better tolerated than bilateral paralysis, although some shortness of breath on exertion is often present. The function of the diaphragm may be compromised when the lung is highly overinflated, as occurs in emphysema; diaphragmatic fatigue may limit the exercise capability of affected persons.

      Although these divisions provide a general outline of the ways in which diseases may affect the lung, they are by no means rigid. It is common for more than one part of the system to be involved in any particular disease process, and disease in one region frequently leads to involvement of other parts.

Major diseases of the respiratory system

Viral infections of the respiratory system
      A wide variety of viruses (virus) are responsible for acute respiratory disease. The common cold—frequently of viral origin—can cause inflammation of the trachea and laryngitis, and such inflammation may extend to involve the lower bronchial tree. After such episodes the ciliary lining of the bronchial tree may be damaged, but the repair process is usually rapid.

      Infections with rhinoviruses (rhinovirus) and adenoviruses (adenovirus) are especially important in children (childhood disease and disorder), in whom they cause a febrile ( fever-associated) illness, occasionally with severe bronchiolar involvement. Although recovery is usually rapid, in some infections with respiratory syncytial virus an extensive bronchiolitis develops that may be severe enough to threaten life. In epidemics (epidemic) of these diseases, occasional cases occur in which the course is complicated by inflammation of the pericardium—the membrane enclosing the heart—or by a pleural effusion.

       influenza and parainfluenza viruses are capable of causing severe illness. The influenza virus attacks many systems of the body simultaneously, but the primary site of viral replication appears to be the alveolar cells of the lung. There the virus multiplies many times over within a 24-hour period, and the pulmonary involvement may begin in the parenchyma and cause considerable consolidation and inflammation of lung tissue. Severe tracheitis, bronchitis, and bronchiolitis often occur at the same time. Another form of the disorder is that described as viral pneumonia, in which a distinguishing feature is the presence of patchy areas of atelectasis, or partial collapse of lung tissue, without extensive involvement of the bronchial tree. All these conditions are more dangerous in small children and in the elderly, and the lung that is the seat of a severe attack of influenza may quickly become secondarily infected.

      It was secondary bacterial infection that accounted for the high mortality in the influenza epidemic of 1918–19, one of the worst human catastrophes on record. Today this epidemic is more precisely called a pandemic because it affected populations around the world. It has been estimated that more than 20,000,000 people worldwide died during the outbreak; of the 20,000,000 people who suffered from the illness in the United States, approximately 850,000 died. It was a characteristic of this pandemic that young people were severely affected. The high mortality resulted from the lack of antibiotics (antibiotic) for treating the secondary bacterial infection; widespread malnutrition probably also contributed to the death rate.

      There are three immunologically distinct types of influenza virus, designated A, B, and C; parainfluenza viruses are designated by the letter D. Types A, B, and D cause epidemic disease. Within type A there are known to be at least four distinct strains. The “Asian” strain of type A was responsible for the 1957 influenza epidemic. Epidemic influenza tends to occur in two- or three-year cycles; careful study has allowed predictions to be made of their future occurrence. Although infected individuals develop lasting immunity to a particular strain following an attack of influenza, the immunity is highly specific as to type, and no protection is afforded against even closely related strains. Artificial immunization with high- potency vaccines (vaccine) is of value in protecting against previous strains, and the vaccines have been shown to ameliorate the infection in the general population. Their use is particularly indicated in elderly people whose cardiac or lung function is already compromised.

       psittacosis and ornithosis, primarily infections of birds (bird) and particularly common among parakeets (parakeet) and parrots (parrot), are transmitted to human beings by inhalation of dust particles from the droppings of infected birds. The onset of psittacosis may be quite severe, with headache, insomnia, and even delirium. Gastrointestinal symptoms such as vomiting and pain are frequent, and a cough productive of clear sputum usually develops after a few days. Mild attacks are often unrecognized and dismissed as due to influenza. Recovery is usually complete, but convalescence may be slow. A pandemic of this disease in 1929 was caused by the shipment of 5,000 parrots into Argentina from Brazil for auction. Many of the birds died, and there was considerable human mortality. Mandatory isolation of imported birds for observation has largely controlled this disease in many countries around the world.

       chickenpox (varicella), particularly when it occurs in adults, may affect the lung. Acute lesions may occur in the lung parenchyma, leading to a transient but significant fall in arterial oxygen tension (hypoxemia), occasionally necessitating oxygen therapy. Recovery may be slow but is usually complete, although shadows may remain on a chest radiograph as a result of it.

       whooping cough occurs in epidemic form among children and appears to be linked to the later development of the chronic infective process known as bronchiectasis, which occurs as a result of bronchial damage. In Western countries, both whooping cough and measles (which causes an acute bronchiolitis) have been largely controlled by effective vaccines, although whooping cough sometimes occurs in adults many years after vaccination. In some developing countries, where these vaccines are not consistently available, whooping cough and measles can still be major causes of mortality in children. Mortality is worsened by malnutrition, which reduces resistance to acute respiratory diseases and is present in many children of developing countries.

      The reparative processes in the lung after any viral attack may be quite slow. Apparent clinical recovery may occur relatively quickly and radiographs may show no remaining shadows, yet repair and restitution of the alveolar wall may take several additional weeks. Sometimes a cough persists for two or three months after systemic symptoms have resolved, reflecting continued healing of the bronchi. However, the occurrence of a severe viral infection in childhood may impair subsequent development of the lung or even set the stage for chronic respiratory disease in later life.

Bacterial (bacteria) infections of the respiratory system
 Before effective antibiotics (antibiotic) became available, pneumonia was the respiratory disease responsible for the greatest mortality and consequently was one of the most-feared diseases. Because it frequently led to the death of severely disabled elderly people, it was also known as the “old man's friend.” The most common form of the disease, streptococcal pneumonia (sometimes called pneumococcal pneumonia), is caused by a Streptococcus called Streptococcus pneumoniae (pneumococcus). This form of pneumonia begins abruptly with a high fever and severe malaise followed by natural resolution in survivors after several days or longer. In some cases, infection is followed by complications, such as a lung abscess, pleurisy, or heart failure. Prompt antibiotic therapy controls the acute process within 24 hours in most instances; however, some deaths occur despite appropriate antibiotic therapy. Emergence of antibiotic-resistant S. pneumoniae strains is a growing concern. Streptococcal pneumonia may also occur as a complication of an acute attack of influenza because susceptibility to infection by streptococci is increased. In addition, the much-lowered mortality of influenza can be explained by antibiotics effective against streptococci. Staphylococcal (staphylococcus) pneumonia occurs as an acute illness in small children and may lead to rapid destruction of lung tissue with abscess formation; however, if the acute state is survived, as it usually is with chemotherapy, the lung recovers fully. This type of pneumonia may also occur as a complication of preexisting lung disease of any kind and may follow aspiration of stomach contents into the lung. The development of antibiotic-resistant staphylococci (staphylococcus), such as methicillin-resistant Staphylococcus aureus (MRSA), has meant that this form of pneumonia may be a problem in the hospital environment, complicating other lung diseases or occurring postoperatively.

      Pneumonia due to infection with Klebsiella pneumoniae may be difficult to treat and characteristically may occur as a repetitive series of episodes of pneumonia, each running a rather long course with slow resolution.

      The organism Hemophilus influenzae is commonly isolated from the sputum of patients with chronic bronchitis during acute exacerbations of infection and is an important cause of pneumonia in adults.

       mycoplasma, identified in 1944 as responsible for a group of pneumonias previously thought to be of viral origin, is a member of a group of organisms known as the pleuropneumonia-like organisms and has also been termed the “Eaton agent” after the scientist who first described it. Mycoplasma pneumoniae is the single most common cause of pneumonia in school-age children and young adults. The infection produces soft patchy shadows on the chest radiograph and relatively few signs on physical examination. A nonproductive cough and a fever occur for a few days. Familial spread is common, and disease occurs in epidemic form in young healthy people brought together in clusters, as in military recruit camps and colleges, where a number of outbreaks have been documented. It is not usually a life-threatening disease, but in rare cases it may progress to cause acute respiratory distress syndrome.

      In all these bacterial pneumonias, the diagnosis may be made from the characteristic radiographic pattern, together with isolation from the sputum of the bacterium primarily responsible.

Legionnaires disease (Legionnaire disease)
      In July 1976 an outbreak of severe pneumonia occurred among U.S. veterans attending a convention of the American Legion in Philadelphia. Of the 147 persons admitted to hospitals, 29 died. Identification of the organism responsible (subsequently named Legionella pneumophila) constituted a classic medical detective story. The bacterium had evaded detection before because it does not stain with the usual stains used in sputum examination. Today it is known that this bacterium may grow in air-conditioning systems or on shower heads, and it has been shown to be responsible for sporadic but severe outbreaks of pneumonia, particularly but not exclusively in older people. Fortunately, the bacterium is sensitive to erythromycin and other antibiotics.

Pneumonia in immunocompromised persons
      For some years prior to 1980, it had been known that if the immune system was compromised by immunosuppressive drugs (given, for example, before organ transplantation to reduce the rate of rejection), the patient was at risk for developing pneumonia from organisms or viruses not normally pathogenic. Patients with AIDS may develop pneumonia from cytomegalovirus or Pneumocystis infections, capable of causing invasive pneumonic lesions in the setting of reduced immunity. Such infections are a major cause of illness in these patients, are difficult to treat, and may prove fatal. Infections with fungi (fungus) such as Candida also occur. The diagnosis and management of these cases has become a challenging and time-consuming responsibility for respiratory specialists in locations with large numbers of AIDS cases.

      Of all the lung diseases caused by bacteria, pulmonary tuberculosis is historically by far the most important. Particular features of this dreaded condition include the severe general debilitation and weakness that it may cause; the insidious nature of the onset of its initial symptoms, which may not be pulmonary in nature; the familial tendency; the long-drawn-out course of the disease and the distressing nature of many of its manifestations, particularly severe hemorrhage from the lung and from tuberculous involvement of the brain ( meningitis), or involvement of the adrenal gland leading to adrenal insufficiency ( Addison disease); and, above all, the general inefficacy of medical treatment before effective antibiotic therapy became available. Antibiotics have greatly reduced the mortality from pulmonary tuberculosis in all developed countries, but the decline in mortality began well before their introduction, and it is clear that improved diet and housing were responsible for this. With antibiotic therapy, however, the bacilli quickly disappear from the sputum and the spread of infection is quickly controlled.

      In its classic form, tuberculosis first causes pulmonary inflammation at the apices (upper portions) of the lungs, and it may progress slowly to form a chronic cavity in this region. Secondary infection of the cavity may occur and may be difficult to eradicate. When still active, pulmonary tuberculosis is a constant threat to the patient, because blood-borne spread may occur at any time. Diffuse spread of tuberculosis in the lung (known as miliary tuberculosis) may occur at the onset of the disease. The chest radiograph reveals many small and diffuse shadows. The exact sequence of events that leads to this disseminated form of disease is not understood, but prompt treatment is required to prevent spread to the brain and other organs. Pulmonary tuberculosis remains an important disease.

      Treatment of tuberculosis is based on whether an individual has tuberculosis infection or tuberculosis disease. Treatment of the former is aimed at preventing disease and often involves only one drug, whereas treatment of the latter is aimed at fighting the disease and involves regimens that employ multiple drugs. streptomycin was the first clinically successful antituberculous drug; however, it is only occasionally used today. Most cases of tuberculosis infection are treated with isoniazid (isonicotinic hydrazide). Treatment often lasts for a period of six to nine months. In the case of active tuberculosis disease, the combination of isoniazid, rifampin, pyrazinamide, and ethambutol is often used for a period of six months to a year. There is no one “best” regimen preferable to all others since tubercle bacilli can acquire resistance to most of the antituberculous drugs. The development of resistance to antibiotics is often delayed by the concomitant use of two or more drugs, by continuous treatment without significant interruption until all bacterial growth has ceased, and by the use of bed rest and resectional surgery in a few selected cases. Surgery may be indicated when a chronic cavity has developed.

      The major problem in treating pulmonary tuberculosis is ensuring continued medication and supervision. This may be very difficult in developing countries and in isolated regions of the world. Although the death rate from respiratory tuberculosis in the Western world has fallen greatly since 1900, it remains a serious and difficult problem in many tropical countries and in any population with inadequate medical care and poor diet and hygiene. In addition, pulmonary tuberculosis has reappeared in the West in persons with AIDS, in whom treatment is complicated by diminished immunity.

Q fever
       Q fever is an infection with the pathogenic bacteria Coxiella burnetii. The disease was first described in Queensland, Australia; areas in which Q fever is known to be endemic include Australia, the western United States, Africa, England, and the Mediterranean countries. Animal infection is widespread and involves a large variety of domestic farm animals, particularly cattle and sheep, and some wild animals. Transmission is believed to occur between mammals through ticks (tick) and mice (mouse). Human disease, which is uncommon, is probably acquired through inhalation of infected material. Laboratory workers and employees in slaughterhouses are particularly at risk. Q fever is usually a mild and self-limited disease, requiring only symptomatic treatment.

Allergic (allergy) lung diseases
      There are at least three reasons why the lungs (lung) are particularly liable to be involved in allergic responses. First, the lungs are exposed to the outside environment, and, hence, particles of foreign substances such as pollen may be deposited directly in the lungs; second, the walls of the bronchial tree contain smooth muscle that is very likely to be stimulated to contract if histamine is released by cells affected by the allergic reaction; and, third, the lung contains a very large vascular bed, which may be involved in any general inflammatory response. It is therefore not surprising to find that sensitivity phenomena are common and represent an important aspect of pulmonary disease as a whole. The most common and most important of these is asthma.

 Asthma is characterized by spasmotic contraction of the smooth muscle of the airways, by increased production of an abnormally viscous mucus by bronchial mucous glands, and, in severe attacks, by airway obstruction from mucus that has accumulated in the bronchial tree. This results in a greater or lesser degree of difficulty in breathing. One approach to classifying asthma differentiates cases that occur with an identifiable antigen, in which antigens affect tissue cells sensitized by a specific antibody, and cases that occur without an identifiable antigen or specific antibody. The former condition is known as extrinsic asthma and the latter as intrinsic asthma. Extrinsic asthma commonly manifests first in childhood because the subject inherits an atopic characteristic: the serum contains specific antigens to pollens, mold spores, animal proteins (protein) of different kinds, and substances from a variety of insects (insect), particularly cockroaches (cockroach) and mites (mite) that occur in house dust. Exacerbation of extrinsic asthma is precipitated by contact with any of the substances to which sensitization has occurred; airway obstruction is often worse in the early hours of the morning. The other form of asthma, intrinsic, may develop at any age, and there may be no evidence of specific antigens. Persons with intrinsic asthma experience attacks of airway obstruction unrelated to seasonal changes, although it seems likely that the airway obstruction may be triggered by infections, which are assumed to be viral in many cases. Asthma attacks may be precipitated by food. For example, in small children, sensitivity to milk may stimulate an attack, and, in some adults, sensitivity to sulfite (sulfite process) compounds found in some foods and wine may stimulate an attack. A subgroup of asthmatics are so sensitive to aspirin (acetylsalicylic acid) that ingestion of this chemical may lead to a life-threatening attack.

      In some people, asthma is acquired as the result of occupational exposure (a special form of intrinsic asthma). Exposure to solder (soldering) resin used in the electronics industry, to toluene diisocyanate (used in many processes as a solvent), to the dust of the western red cedar (in which plicatic acid is the responsible agent), and to many other substances can initiate an asthmatic state, with profound airflow obstruction developing when the subject is challenged by the agent.

      Individuals affected by any type of asthma may exhibit airflow obstruction when given aerosols of histamine or methylcholine at much lower concentrations than provoke airflow obstruction in healthy people. Individuals with asthma may also develop airflow obstruction while breathing cold air or when exercising. These characteristics are used in the laboratory setting to study the airway status of patients.

      The severe acute asthmatic attack is characterized by acute difficulty in breathing in which mucus clogs the air passages in such a way that air can be inspired but not expired. Despite the severe respiratory difficulty, the patient remains fully conscious. The most dangerous form of the condition is known as status asthmaticus. The bronchial spasm worsens over several hours or over the course of an entire day, during which the bronchi become plugged with thick mucus and airflow is progressively more obstructed. The affected person becomes fatigued, arterial oxygen tension falls, and carbon dioxide accumulates in the blood (leading to drowsiness). As a result, the acidity of the arterial blood increases to dangerous levels that could lead to cardiac arrest. Prompt treatment with intravenous corticosteroids and bronchodilators is usually sufficient to relieve the attack, but in some cases ventilatory assistance is required. In a few cases, death from asthma is remarkably rapid. The exact mechanism of death in these cases is not completely understood.

      Asthma may interrupt normal activities and schooling and provide intense or sustained anxiety, especially in individuals who have experienced one or more severe attacks. It tends to diminish in severity with age, and people who had quite severe asthma in childhood may lead normal lives after the age of 20.

      Developed countries around the world are reporting a disturbing increase in the prevalence, severity, and mortality of asthma. For example, today more than 7 percent of children and about 9 percent of adults in the United States suffer from asthma. The reason for this increase is not known; however, evidence points to greater control of ordinary infections during early childhood than was achievable in the 1960s and '70s. For example, children who regularly attend day care or preschool programs acquire more respiratory infections but are less likely to develop asthma later in life. It is possible that frequent infections in early childhood are necessary to “prime” the developing immune system in a way that protects against subsequent development of asthma.

  hay fever is a common seasonal condition caused by allergy to grasses and pollens. It is frequently familial, and the sensitivity is often to ragweed pollen. Conjunctival infection (conjunctivitis) and edema of the nasal mucosa lead to attacks of sneezing. Allergic inflammation and the development of polyps (polyp) in the nasal passages represent a severer form of hay fever that is often associated with asthma.

Hypersensitivity pneumonitis
 Hypersensitivity pneumonitis is an important group of conditions in which the lung is sensitized by contact with a variety of agents and in which the response to reexposure consists of an acute pneumonitis, with inflammation of the smaller bronchioles, alveolar wall edema, and a greater or lesser degree of airflow obstruction due to smooth muscle contraction. In more chronic forms of the condition, granulomas, or aggregations of giant cells, may be found in the lung. Inflammation can lead to widespread lung fibrosis and chronic respiratory impairment.

      One of these illnesses is the so-called farmer's lung, caused by the inhalation of spores from moldy hay (thermophilic Actinomyces (actinomycete)). This causes an acute febrile illness with a characteristically fine opacification (clouding, or becoming opaque) in the basal regions of the lung on the chest radiograph. Airflow obstruction in small airways is present, and there may be measurable interference with diffusion of gases across the alveolar wall. If untreated, the condition may become chronic, with shortness of breath persisting after the radiographic changes have disappeared. Farmer's lung is common in Wisconsin, on the eastern seaboard of Canada, in the west of England, and in France. Education of farmers and their families and the wearing of a simple mask can completely prevent the condition.

      A similar group of diseases occurs in those with close contact with birds (bird). Variously known as pigeon breeder's lung or bird fancier's lung, these represent different kinds of allergic responses to proteins from birds, particularly proteins contained in the excreta of pigeons, budgerigars (parakeets (parakeet)), and canaries (canary).

      An acute hypersensitivity pneumonitis may also occur in those cultivating mushrooms (mushroom) (particularly where this is done below ground), after exposure to redwood sawdust, or in response to a variety of other agents. An influenza-like illness resulting from exposure to molds growing in humidifier systems in office buildings (“humidifier fever”) has been well documented. It is occasionally attributable to Aspergillus, but sometimes the precise agent cannot be identified. The disease may present as an atypical nonbacterial pneumonia and may be labeled a viral pneumonia if careful inquiry about possible contacts with known agents is not made.

Acute diseases of the bronchi
      Acute bronchitis most commonly occurs as a consequence of viral infection. It may also be precipitated by acute exposure to irritant gases, such as ammonia, chlorine, or sulfur dioxide. In people with chronic bronchitis—a common condition in cigarette smokers—exacerbations of infection are common. The bronchial tree in acute bronchitis is reddened and congested, and minor blood streaking of the sputum may occur. Most cases of acute bronchitis resolve over a few days, and the mucosa repairs itself.

Bronchiolitis
      Bronchiolitis refers to inflammation of the small airways. Bronchiolitis probably occurs to some extent in acute viral disorders, particularly in children between the ages of one and two years, and particularly in infections with respiratory syncytial virus. In some cases the inflammation may be severe enough to threaten life, but it normally clears spontaneously, with complete healing in all but a very small percentage of cases. In adults, acute bronchiolitis of this kind is not a well-recognized clinical syndrome, though there is little doubt that in most patients with chronic bronchitis, acute exacerbations of infection are associated with further damage to small airways. In isolated cases, an acute bronchiolitis is followed by a chronic obliterative condition, or this may develop slowly over time. This pattern of occurrence has only recently been recognized. In addition to patients acutely exposed to gases, in whom such a syndrome may follow the acute exposure, patients with rheumatoid arthritis may develop a slowly progressive obliterative bronchiolitis that may prove fatal. An obliterative bronchiolitis may appear after bone marrow replacement for leukemia and may cause shortness of breath and disability.

      Exposure to oxides of nitrogen, which may occur from inhaling gas in silos (silo), when welding in enclosed spaces such as boilers, after blasting underground, or in fires involving plastic materials, is characteristically not followed by acute symptoms. These develop some hours later, when the victim develops a short cough and progressive shortness of breath. A chest radiograph shows patchy inflammatory change, and the lesion is an acute bronchiolitis. Symptomatic recovery may mask incomplete resolution of the inflammation.

      An inflammation around the small airways, known as a respiratory bronchiolitis, is believed to be the earliest change that occurs in the lung in cigarette smokers (smoking), although it does not lead to symptoms of disease at that stage. The inflammation is probably reversible if smoking is discontinued. It is not known whether those who develop this change (after possibly only a few years of smoking) are or are not at special risk of developing the long-term changes of chronic bronchitis and emphysema.

      Bronchiectasis is believed to usually begin in childhood, possibly after a severe attack of pneumonia. It consists of a dilatation of major bronchi. The bronchi become chronically infected, and excess sputum production and episodes of chest infection are common. In some cases, clubbing (swelling of the fingertips and, occasionally, of the toes) may occur. The disease may also develop as a consequence of airway obstruction or of undetected (and therefore untreated) aspiration into the airway of small foreign bodies, such as parts of plastic toys.

      Bronchiectasis may also develop as a consequence of inherited conditions, of which the most important is the familial disease cystic fibrosis. Cystic fibrosis is due to the production of an abnormal protein called cystic fibrosis transmembrane conductance regulator, or CFTR. This protein normally serves as a channel for the transport of chloride into and out of cells and regulates the activity of other transport channels, including sodium channels. Defects in the transport of chloride, sodium, and other ions (ion) result in unusually high levels of ions inside the cells of the lungs. Thus, fluids are absorbed into the cells from the airways, causing the airways to become dehydrated and impairing the ability of the lungs to clear foreign materials and debris. The most important consequence of cystic fibrosis, apart from the malnutrition it causes, is the development of chronic pulmonary changes, with repetitive infections and bronchiectasis as characteristic features. This condition does not progress to pulmonary emphysema but rather causes obliteration and fibrosis of small airways and dilation and infection of the larger bronchi. Thick, viscid secretions in the bronchial tree are difficult to expectorate.

      Management of the condition includes antibiotics to fight lung infections, medications to dilate the airways and to relieve pain, enzyme therapy to thin the mucus, and postural drainage and percussion to loosen mucus in the lungs so it can be expelled through coughing. These therapies, in addition to others, have helped control pulmonary infections and have markedly improved survival in affected persons, many of whom, who would formerly have died in childhood, now reach adult life.

Chronic obstructive pulmonary disease
 Chronic obstructive pulmonary disease (COPD) refers broadly to a group of conditions that cause irreversible respiratory impairment by increasing obstruction to airflow through the bronchi of the lungs. This condition occurs most commonly in current or former regular cigarette smokers (smoking) and is present in moderate to severe form in 80 million people worldwide. In 2005 the World Health Organization reported that 5 percent of all deaths around the world were due to COPD.

      COPD typically has two components which may be present to varying degrees: chronic obstructive bronchitis and pulmonary emphysema. Individuals who predominantly have emphysema experience symptoms that differ in detail from those who predominantly have chronic bronchitis; however, both disorders contribute to shortness of breath during exercise and to general disability.

Chronic bronchitis
      The chronic cough and sputum production of chronic bronchitis were once dismissed as nothing more than “smoker's cough,” without serious implications. But the striking increase in mortality from chronic bronchitis and emphysema that occurred after World War II in all Western countries indicated that the long-term consequences of chronic bronchitis could be serious. This common condition is characteristically produced by cigarette smoking. After about 15 years of smoking, significant quantities of mucus are coughed up in the morning, due to an increase in size and number of mucous glands lining the large airways. The increase in mucous cells and the development of chronic bronchitis may be enhanced by breathing polluted air (particularly in areas of uncontrolled coal burning). The changes are not confined to large airways, though these produce the dominant symptom of chronic sputum production. Changes in smaller bronchioles lead to obliteration and inflammation around their walls. All these changes together, if severe enough, can lead to disturbances in the distribution of ventilation and perfusion in the lung, causing a fall in arterial oxygen tension and a rise in carbon dioxide tension. By the time this occurs, the ventilatory ability of the patient, as measured by the velocity of a single forced expiration, is severely compromised; in a cigarette smoker, ventilatory ability has usually been declining rapidly for some years. It is not clear what determines the severity of these changes. Some people can smoke for decades without evidence of significant airway changes, whereas others may experience severe respiratory compromise after 15 years or less of exposure.

Pulmonary emphysema
 This irreversible disease consists of destruction of alveolar walls. It occurs in two forms, centrilobular emphysema, in which the destruction begins at the centre of the lobule, and panlobular (or panacinar) emphysema, in which alveolar destruction occurs in all alveoli within the lobule simultaneously. In advanced cases of either type, this distinction can be difficult to make. Centrilobular emphysema is the form most commonly seen in cigarette smokers, and some observers believe it is confined to smokers. It is more common in the upper lobes of the lung (for unknown reasons). By the time the disease has developed, some impairment of ventilatory ability has probably occurred. Panacinar emphysema may also occur in smokers, but it is the type of emphysema characteristically found in the lower lobes of patients with a deficiency in the antiproteolytic enzyme known as alpha-1 antitrypsin. Like centrilobular emphysema, panacinar emphysema causes ventilatory limitation and eventually blood gas changes. Other types of emphysema, of less importance than the two major varieties, may develop along the dividing walls of the lung (septal emphysema) or in association with scars from other lesions.

      A major step forward in understanding the development of emphysema followed the identification, in Sweden, of families with an inherited deficiency of alpha-1 antitrypsin, an enzyme essential for lung integrity. Members of affected families who smoked cigarettes commonly developed panacinar emphysema in the lower lobes, unassociated with chronic bronchitis but leading to ventilatory impairment and disability. Intense investigation of this major clue led to the “protease-antiprotease” theory of emphysema. It is postulated that cigarette smoking either increases the concentration of protease enzymes released in the lung (probably from white blood cells (leukocyte)) or impairs the lung's defenses against these enzymes or both. Although many details of the essential biochemical steps at the cellular level remain to be clarified, this represents a major step forward in understanding a disease whose genesis was once ascribed to overinflation of the lung (like overdistending a bicycle tire).

      Chronic bronchitis and emphysema are distinct processes. Both may follow cigarette smoking, however, and they commonly occur together, so determination of the extent of each during life is not easy. In general, significant emphysema is more likely if ventilatory impairment is constant, gas transfer in the lung (usually measured with carbon monoxide) is reduced, and the lung volumes are abnormal. Development of high-resolution computerized tomography has greatly improved the accuracy of detection of emphysema. Some people with emphysema suffer severe incapacity before the age of 60; thus, emphysema is not a disease of the elderly only. An accurate diagnosis can be made from pulmonary function tests, careful radiological examination, and a detailed history. The physical examination of the chest reveals evidence of airflow obstruction and overinflation of the lung, but the extent of lung destruction cannot be reliably gauged from these signs, and therefore laboratory tests are required. (For more information about the methods of detection of lung diseases, see above Methods of investigation (respiratory disease).)

      The prime symptom of emphysema, which is always accompanied by a loss of elasticity of the lung, is shortness of breath, initially on exercise only, and associated with loss of normal ventilatory ability and increased obstruction to expiratory airflow. The expiratory airflow from a maximum inspiration is measured by the “forced expiratory volume in one second,” or FEV1, and is a predictor of survival of emphysema. Chronic hypoxemia (lowered oxygen tension) often occurs in severe emphysema and leads to the development of increased blood pressure in the pulmonary circulation, which in turn leads to failure of the right ventricle of the heart. The symptoms and signs of right ventricular failure include swelling of the ankles ( edema) and engorgement of the neck veins (vein). These are portents of advanced lung disease in this condition. The hypoxemia may also lead to an increase in total hemoglobin content and in the number of circulating red blood cells (erythrocyte), as well as to psychological depression, irritability, loss of appetite, and loss of weight. Thus, the advanced syndrome of chronic obstructive lung disease may cause such shortness of breath that the afflicted person has difficulty walking, talking, and dressing, as well as numerous other symptoms.

      The slight fall in ventilation that normally accompanies sleep may exacerbate the failure of lung function in chronic obstructive lung disease, leading to a further fall in arterial oxygen tension and an increase in pulmonary arterial pressure.

      Unusual forms of emphysema also occur. In one form the disease appears to be unilateral, involving one lung only and causing few symptoms. Unilateral emphysema is believed to result from a severe bronchiolitis in childhood that prevented normal maturation of the lung on that side. “Congenital lobar emphysema” of infants is usually a misnomer, since there is no alveolar destruction. It is most commonly caused by overinflation of a lung lobe due to developmental malformation of cartilage in the wall of the major bronchus. Such lobes may have to be surgically removed to relieve the condition. Bullous emphysema can occur in one or both lungs and is characterized by the presence of one or several abnormally large air spaces surrounded by relatively normal lung tissue. This disease most commonly occurs between the ages of 15 and 30 and usually is not recognized until a bullous air space leaks into the pleural space, causing a pneumothorax.

      Up to the time of World War II, cancer of the lung was a relatively rare condition. The increase in its incidence in Europe after World War II was at first ascribed to better diagnostic methods, but by 1956 it had become clear that the rate of increase was too great to be accounted for in this way. At that time the first epidemiological studies began to indicate that a long history of cigarette smoking was associated with a great increase in risk of death from lung cancer. By 1965 cancer of the lung and bronchus accounted for 43 percent of all cancers in the United States in men, an incidence nearly three times greater than that of the second most common cancer (of the prostate gland) in men, which accounted for 16.7 percent of cancers. In 1964 Smoking and Health: Report of the Advisory Committee to the Surgeon General of the Public Health Service (United States) concluded categorically that cigarette smoking is causally related to lung cancer in men. Since then, many further studies in diverse countries have confirmed this conclusion.

      The incidence of lung cancer in women began to rise in 1960 and continued rising through the mid-1980s. This is believed to be explained by the later development of heavy cigarette smoking in women compared with men, since women greatly increased their cigarette consumption during World War II. By 1988 there was evidence suggesting that the peak incidence of lung cancer due to cigarette smoking in men may have been passed. The incidence of lung cancer mortality in women, however, is increasing.

      The reason for the carcinogenicity (carcinogen) of tobacco smoke is not known. Tobacco smoke contains more than 60 carcinogenic compounds, including harmful nitrosamines and polycyclic aromatic hydrocarbons. In addition to its single-agent effects, cigarette smoking greatly potentiates the cancer-causing proclivity of asbestos fibres, increases the risk of lung cancer due to inhalation of radon daughters (products of the radioactive decay of radon gas), and possibly also increases the risk of lung cancer due to arsenic exposure. People who do not smoke but who live or work with smokers and who therefore are exposed to secondhand tobacco smoke have an increased risk for lung cancer.

      Because lung cancer is characterized by different types of tumours, because it may be located in different parts of the lung, and because it may spread beyond the lungs at an early stage, the first symptoms noted by the patient vary. These symptoms may include a persistent cough, blood staining of the sputum, a pneumonia that does not resolve fully with antibiotics (antibiotic), or shortness of breath due to a pleural effusion. A physician may discover distant metastases in bone tissues or in the brain that cause symptoms unrelated to the lung. Lymph nodes (lymph node) may be involved early, and enlargement of the lymph nodes in the neck may lead to a chest examination and the discovery of a tumour. In some cases a small tumour metastasis in the skin, abnormal mental function or behaviour, jaundice from liver dysfunction, or sensory changes in the legs from peripheral neuropathy (damage to nerves outside the central nervous system (nervous system, human)) may be the first sign of the disease. In other cases, only a general feeling of malaise, unusual fatigue, or seemingly minor symptoms may serve as the first indication. In addition, some affected individuals experience clubbing (swelling) of the fingers and toes, an unusual sign that may disappear after surgical removal of the tumour. Lung cancer may develop in an individual who already has chronic bronchitis and who therefore has had a cough for many years.The diagnosis often depends on securing tissue for histological examination, although in some cases this entails removal of the entire neoplasm before a definitive diagnosis can be made.

      Survival from lung cancer has improved only slightly since the mid-1970s, when the U.S. government greatly increased funding for cancer research in an effort to find a cure for the disease. Early detection with routine chest radiographs has been attempted, and large-scale trials of routine sputum examination for the detection of malignant cells have been conducted, but neither screening method has been shown thus far to affect long-term mortality. Attention has been turned to prevention (preventive medicine) by every means possible. Foremost among them are efforts to inform the public of the risk and to limit the advertising of cigarettes. Steps have been taken to reduce asbestos exposure, both in the workplace and in public and private buildings, and to control air pollution. The magnitude of the contribution of air pollution to the incidence of lung cancer is not known with certainty.

      Persons exposed to radon are at risk for lung cancer. The hazard from exposure was formerly thought to be confined to uranium miners, who, by virtue of their work underground, encounter high levels of these radioactive materials. However, significant levels of radon have been detected in houses built over natural sources, and, with increasingly efficient insulation of houses, radon may reach concentrations high enough to place the occupants at risk for lung cancer. Major regional variations in the natural distribution of radon occur, and it is not yet possible to quantify precisely the actual magnitude of the risk. In some regions of the world (such as the Salzburg region of Austria), levels are high enough that radon exposure is believed to account for the majority of cases of lung cancer in nonsmokers.

      Workers exposed to arsenic in metal-smelting operations, and the community around the factories from which arsenic is emitted, have an increased risk for lung cancer. Arsenic is widely used in the electronics industry in the manufacture of various products, including microchips and semiconductors (semiconductor), and careful surveillance of this industry has helped prevent future disease.

      Some types of lung cancer are unrelated to cigarette smoking. Alveolar cell cancer is a slowly spreading condition that affects men and women in equal proportion and is not related to cigarette smoking. Pulmonary adenocarcinoma of the lung also has a more equal sex incidence than other types; although its incidence is increased in smokers, it may also be caused by other factors.

      It is common to feel intuitively that one should be able to apportion cases of lung cancer among discrete causes, on a percentage basis. But in multifactorial disease, this is not possible. Although the incidence of lung cancer would probably be far lower without cigarette smoking, the contribution of neither this factor nor any of the other factors mentioned can be precisely quantified. Treatment of most forms of lung cancer may involve chemotherapy, radiation therapy, or surgery (see Lung cancer: Treatment (lung cancer)).

Occupational (occupational disease) lung disease

Silicosis and black lung disease
       silica dust produces a distinctive reaction in the lung that eventually leads to the development of masses of fibrous tissue and distinctive nodules of dense fibrosis, which, by contracting, distort and damage the lung. silicosis is a hazard in any occupation in which workers are exposed to silica dust, particularly rock drilling above or below ground, quarrying, or grinding with a wheel containing silica. Cases have also been reported in dental technicians, who use the material ground into a fine powder. Silicosis is usually fairly easy to detect on radiographs, and in its later stages it causes considerable shortness of breath and reduction of the vital capacity (a maximal breath). Sandblasting without respiratory protection is exceedingly dangerous, and fatal cases of acute silicosis caused by unprotected sandblasting have been reported. The dangers of silica are generally well recognized, and better protection has reduced the incidence of this condition. The disease may advance, with increasing disability, for years after the person has stopped inhaling the dust.

       coal dust alone, even if its silica content is very low, causes a distinctive pattern of change in the lung known as coalworker's pneumoconiosis (also called black lung). Initially the dust is deposited in the terminal bronchioles, where it causes a fibrotic reaction. At this stage there is little disability, but later the disease may progress to a more-generalized form, and in some instances large masses of fibrotic tissue form in the lung. This condition, known as progressive massive fibrosis, is usually associated with severe disability and the risk of secondary heart failure. It is not clear whether this stage is more likely to develop if pulmonary tuberculosis is superimposed on the respiratory damage caused by coal dust inhalation. There is no curative treatment for silicosis or black lung disease.

Asbestosis and mesothelioma
      The widespread use of asbestos as an insulating material during World War II, and later in flooring, ceiling tiles, brake linings, and as a fire protectant sprayed inside buildings, led to a virtual epidemic of asbestos-related disease 20 years later. The first disease recognized to be caused by asbestos was asbestosis, which produces characteristic changes in the lungs that can be identified in chest X-rays and that can impair lung function at an early stage. Later it was discovered that exposure to much less asbestos than was needed to cause asbestosis led to thickening of the pleura, and, when both cigarette smoking and asbestos exposure occurred, there was a major increase in the risk for lung cancer. The risks from smoking and from significant asbestos exposure are multiplicative in the case of lung cancer. A malignant tumour of the pleura known as mesothelioma is caused almost exclusively by inhaled asbestos. Often a period of 20 years or more elapses between exposure to asbestos and the development of a tumour.

      As far as is known, all the respiratory changes associated with asbestos exposure are irreversible. Malignant mesothelioma is rare and unrelated to cigarette smoking, but survival after diagnosis is less than two years. In most cases, thickening of the pleura is not associated with disturbance of lung function or with symptoms of exposure to asbestos, although in occasional cases pleuritis is very aggressive and thus may produce symptoms. It is not yet understood exactly why asbestos devastates the tissues of the lungs. Asbestos has been suspected to play a role in stimulating certain cellular events, such as the generation of harmful reactive molecules and the activation of damaging inflammatory processes. These events could contribute to the scarring and fibrosis that are characteristic of inhalation of asbestos fibres. Not all types of asbestos are equally dangerous; the risk of mesothelioma in particular appears to be much higher if crocidolite, a blue asbestos that comes from South Africa, is inhaled than if chrysotile is inhaled. But exposure to any type of asbestos is believed to increase the risk of lung cancer, especially when associated with cigarette smoking. While the removal of asbestos from buildings has greatly alleviated the risk of exposure to asbestos for many people, inhalation of asbestos remains a significant risk for the workers removing the material. All industrialized countries have imposed strict regulations for handling asbestos, and the workforce is generally aware of the material's dangers.

      There is no curative therapy for asbestosis or mesothelioma. Treatment is aimed at managing symptoms, preventing infections, and delaying disease progression. Individuals with asbestosis often receive annual vaccinations against influenza and pneumococcal pneumonia. In some cases, aerosol medications that thin mucous secretions and oxygen that is supplied by a portable tank are necessary to maintain adequate oxygen intake. In other cases, lung transplantation is required. Individuals with mesothelioma often undergo chemotherapy and radiation therapy, which may prolong survival for a short period of time.

Respiratory toxicity of glass and metal fibres
      The increasing use of man-made mineral fibres (as in fibreglass and rock wool) has led to concern that these may also be dangerous when inhaled; present evidence suggests that they do increase the risk of lung cancer in persons occupationally exposed to them. Standards for maximal exposure have been proposed.

      The toxicity of beryllium was first discovered when it was widely used in the manufacture of fluorescent light tubes shortly after World War II. In susceptible individuals, beryllium causes the formation of granulomas in the lung and alveolar wall thickening, often with considerable disability as a result. Although beryllium is no longer used in the fluorescent light industry, it is still important in the manufacture of metal alloys and ceramics, and new cases of beryllium poisoning are occasionally reported.

Byssinosis and related diseases
      It is not only inorganic minerals and dusts that may affect the lung. The dust produced in the processing of raw cotton, flax, or hemp may cause chronic obstructive lung disease. However, this does not have a characteristic pathology, and it does not give rise to emphysema. It is unclear whether the dust from the fibres alone or the combination of cigarette smoke and fibre dust is particularly dangerous. The disease that results is known as byssinosis, or “brown lung.” Workers in cotton plants in England used to complain of “Monday morning fever” and were found to suffer an easily measurable decrement in ventilatory function when they returned to work after spending a weekend away from the plant. The active particle or contaminant in the cotton dust that is responsible for the syndrome appears to be an endotoxin produced by bacteria in the fibres of cotton.

      The dust from western red cedar may cause occupational asthma, and dust from the redwood and other trees (tree) may cause an acute hypersensitivity pneumonitis. Workers in the sugarcane industry may be affected by a similar syndrome, known as bagassosis; sisal workers also develop airflow obstruction.

Respiratory toxicity of industrial chemicals (chemical industry)
       toluene diisocyanate, used in the manufacture of polyurethane foam, may cause occupational asthma in susceptible individuals at very low concentrations; in higher concentrations, such as may occur with accidental spillage, it causes a transient flulike illness associated with airflow obstruction. Prompt recognition of this syndrome has led to modifications in the industrial process involved.

      Although the acute effects of exposure to many of these gases and vapours are well-documented, there is less certainty about the long-term effects of repeated low-level exposures over a long period of time. This is particularly the case when the question of whether work in a generally dusty environment has contributed to the development of chronic bronchitis or later emphysema—in other words, whether such nonspecific exposures increase the risk of these diseases in cigarette smokers.

      Many chemicals can damage the lung in high concentration: these include oxides of nitrogen, ammonia, chlorine, oxides of sulfur, ozone, gasoline vapour, and benzene. In industrial accidents, such as occurred in 1985 in Bhopal, India, and in 1976 in Seveso, near Milan, people in the neighbourhood of chemical plants were acutely exposed to lethal concentrations of these or other chemicals. The custom of transporting dangerous chemicals by rail or road has led to the occasional exposure of bystanders to toxic concentrations of gases and fumes. Although in many cases recovery may be complete, it seems clear that long-term damage may occur.

Disability and attribution of occupational lung diseases
      Occupational lung diseases are of social and legal importance. In such cases, respiratory specialists must assess the extent of an individual's disability and then form an opinion on whether an individual's disability can be attributed to an occupational hazard. Pulmonary function testing and tests of exercise capability provide a good indication of the impact of a disease on the physical ability of a patient. However, it is much more difficult to decide how much of a patient's disability is attributable to occupational exposure. If the exposure is historically known to cause a specific lesion in a significant percentage of exposed persons, such as mesothelioma in workers exposed to asbestos, attribution may be fairly straightforward. In many cases, however, the exposure may cause only generalized pulmonary changes or lung lesions for which the precise cause cannot be determined. These instances may be complicated by a history of cigarette smoking. Physicians asked to present opinions on attributability before a legal body frequently must rely on the application of probability statistics to the individual case, a not wholly satisfactory procedure.

Miscellaneous conditions of the respiratory system

Idiopathic pulmonary fibrosis
      Idiopathic pulmonary fibrosis is also known as cryptogenic fibrosing alveolitis. This is a generally fatal lung disease of unknown cause that is characterized by progressive fibrosis of the alveolar walls. The disease most commonly manifests between the ages of 50 and 70, with insidious onset of shortness of breath on exertion. A dry cough is common as well. Sharp crackling sounds, called rales or “Velcro crackles,” are heard through a stethoscope applied to the back in the area of the lungs. Computerized tomography (CT) imaging shows fibrosis and cysts (cyst) that characteristically form in a rim around the lower outer portions of both lungs. In addition, pulmonary function testing shows a reduction in lung volume. Lung biopsies (biopsy) confirm the diagnosis by showing fibrosis with a lack of inflammation.

      The disease causes progressive shortness of breath with exercise and ultimately produces breathlessness at rest. Hypoxemia (decreased levels of oxygen in the blood) initially occurs with exercise and later at rest and can be severe. Some individuals have clubbed fingertips and toes. The average duration of survival from diagnosis is four to six years; however, some people live 10 years or longer. Aside from administration of supplemental oxygen, there is no effective treatment. Some individuals may benefit from single or double lung transplantation (see above Lung transplantation (respiratory disease)).

       sarcoidosis is a disease of unknown cause characterized by the development of small aggregations of cells, or granulomas, in different organs; the lung is commonly involved. Other common changes are enlargement of the lymph glands at the root of the lung, skin changes, inflammation in the eye (eye, human), and liver dysfunction; occasionally there is inflammation of sheaths, leading to signs of involvement in the affected area. The kidney is not commonly involved, but some changes in blood calcium levels occur in a small percentage of cases. In most cases the disease is first detected on chest radiographs. Evidence of granulomas in the lung may be visible, but often there is little interference with lung function. The disease usually remits without treatment within a year or so, but in a small proportion of cases it progresses, leading finally to lung fibrosis and respiratory failure. The granulomatous inflammation in sarcoidosis can be controlled by long-term administration of a corticosteroid such as prednisone.

Eosinophilic granuloma
      Also known as pulmonary histiocytosis X, this disease causes granulomas associated with eosinophil cells, a subgroup of the white blood cells (leukocyte). It sometimes also causes lesions in bone. Eosinophilic granuloma is a lung condition that may spontaneously “burn out,” leaving the lung with some permanent cystic changes. Its cause is not known; however, the incidence is greatly increased in cigarette smokers.

 Pulmonary alveolar proteinosis is a disease of unknown cause characterized by accumulation in the alveolar spaces of surfactant. Small amounts of this lipid- and protein-rich fluid normally line the surfaces of the alveoli, reducing surface tension and thereby keeping the air spaces open. Buildup of this liquid within the air spaces interferes with gas exchange and causes progressive shortness of breath. The only effective treatment of this disease is whole-lung lavage. Under general anesthesia (anesthetic), the bronchus leading to one lung is isolated, and that lung is filled with sterile salt water. Drainage of the fluid removes some of the excess surfactant. Flooding and drainage are repeated up to 20 or 30 times until little or no more surfactant is removed. Then on another day the opposite lung is treated. Whole-lung lavage may be required at 6- to 12-month intervals for several years before complete remission occurs.

Immunologic conditions
      The lung is often affected by generalized diseases of the blood vessels (blood vessel). Wegener granulomatosis, an acute inflammatory disease of the blood vessels believed to be of immunologic origin, is an important cause of pulmonary blood vessel inflammation. Acute hemorrhagic pneumonitis occurring in the lung in association with changes in the kidney is known as Goodpasture syndrome. The condition has been successfully treated by exchange blood transfusion, but its cause is not fully understood. Pulmonary hemorrhage also occurs as part of a condition known as pulmonary hemosiderosis, which results in the accumulation of the iron-containing substance hemosiderin in the lung tissues. The lung may also be involved in a variety of ways in the disease known as systemic lupus erythematosus, which is also believed to have an immunologic basis. Pleural effusions (pleural effusion) may occur, and the lung parenchyma may be involved. These conditions have only recently been recognized and differentiated; accurate diagnosis has been much improved by refinements in radiological methods, by the use of pulmonary function tests, and especially by improvement in thoracic surgical techniques and anesthesia that have made lung biopsy much less dangerous than it formerly was.

      The common condition of rheumatoid arthritis may be associated with scattered zones of interstitial fibrosis in the lung or with solitary isolated fibrotic lesions. More rarely, a slowly obliterative disease of small airways (bronchiolitis) occurs, leading finally to respiratory failure.

Radiation damage
      The lung may be damaged by radiation therapy in the treatment of cancer of the breast and other conditions. About three weeks or so after the end of the treatment, a pneumonitis may develop in the underlying lung, signaled by an unproductive cough. The condition may resolve, but in a few cases the lung becomes fibrotic and contracts to a small fraction of its normal volume. There is considerable individual variation in the response to the same dose of radiation.

Circulatory disorders
 The lung is commonly involved in disorders of the circulation. The most important and common of these is blockage of a branch of the pulmonary artery by blood clot, which has usually formed in the veins (vein) of the legs or of the pelvis. The resulting pulmonary embolism leads to changes in the lung supplied by the affected artery. When severe, these changes are known as a pulmonary infarction (lung infarction). The consequences of embolism range from sudden death, when the infarction is massive, to an increased respiratory rate, slight fever, and occasionally some pleuritic pain over the site of the infarction. An individual is at an increased risk for pulmonary embolism whenever his or her circulation is sluggish. This occurs most often during a postoperative period when the affected individual is immobilized in bed. Early mobilization after surgery or childbirth is considered an important preventive measure. Repetitive pulmonary emboli may lead to chronic pulmonary thromboembolism, in which the pressure in the main pulmonary artery is persistently increased. Over time, a clot is replaced with an adherent fibrous material in the pulmonary arteries, causing shortness of breath on exertion and, ultimately, right ventricular heart failure. The obstructing lesions can be surgically removed in some instances, thereby relieving symptoms of breathlessness.

      In primary pulmonary hypertension, a condition of unknown origin, a marked increase in pulmonary arterial pressure occurs as a result of progressive narrowing and obliteration of small pulmonary arteries. Primary pulmonary hypertension leads to enlargement of the heart and eventual failure of the right ventricle of the heart, usually after increasing disability with severe shortness of breath. In addition to chest X-rays and basic pulmonary function tests, a diagnosis of pulmonary hypertension is often confirmed following an electrocardiogram (EKG) to assess electrical function of the heart, an echocardiogram to determine whether the heart is enlarged and to evaluate the flow of blood through the heart, and cardiac catheterization to measure pressure in the pulmonary artery and right ventricle of the heart.

      Treatment of primary pulmonary hypertension is aimed at alleviating symptoms. Because of the variability in physiological response to certain drugs and because of the progressive nature of the disease, affected individuals require careful, long-term evaluation and treatment. While some medications such as calcium channel blockers may be taken orally, others such as prostacyclin are given by continuous intravenous infusion supplied through a portable battery-powered pump. Prostacyclin can sometimes be given in oral or inhaled forms. In some cases, lung transplantation is necessary.

      Congestion of the lungs (pulmonary edema (lung congestion)) and the development of fluid in the pleural cavity, with consequent shortness of breath, follows left ventricular failure, usually as a consequence of coronary arterial disease (coronary heart disease). When the valve between the left atrium of the heart and the left ventricle is thickened and deformed by rheumatic fever ( mitral stenosis), chronic changes develop in the lung as a result of the increased pressure in the pulmonary circulation. These changes contribute to the shortness of breath and account for the blood staining of the sputum.

Acute respiratory distress syndrome of adults
      Bacterial or viral pneumonia, exposure of the lung to gases, aspiration of material into the lung (including water in near- drowning episodes), or any generalized septicemia (blood poisoning) or severe lung injury may lead to sudden, widespread bilateral lung injury. This syndrome is known as acute respiratory distress syndrome of adults. It was recognized as “shock lung” in injured soldiers evacuated by helicopter to regional military hospitals during the Vietnam War. Many causes of respiratory distress syndrome of adults have been identified. Acute respiratory distress syndrome carries about a 50 percent mortality. Life-support treatment with assisted ventilation rescues many patients, although superimposed infection or multiple organ failure can result in death. Recovery and repair of the lung may take months after clinical recovery from the acute event.

 The disastrous fog and attendant high levels of sulfur dioxide and particulate pollution (and probably also sulfuric acid) that occurred in London in the second week of December 1952 led to the deaths of more than 4,000 people during that week and the subsequent three weeks. Many, but not all, of the victims already had chronic heart or lung disease. Prize cattle at an agricultural show also died in the same period as a result of the air pollution. This episode spurred renewed attention to this problem, which had been intermittently considered since the 14th century in England, and finally the passage of legislation banning open coal burning, the factor most responsible for the pollution. This form of pollution, common in many cities using coal as heating fuel, is associated with excess mortality and increased prevalences of chronic bronchitis, respiratory tract infections in the young and old, and possibly lung cancer. Today many industrial cities have legislation restricting the use of specific fuels and mandating emission-control systems in factories.

      In 1952 a different kind of air pollution was characterized for the first time in Los Angeles. The large number of automobiles in that city, together with the bright sunlight and frequently stagnant air, leads to the formation of photochemical smog. This begins with the emission of nitrogen oxide during the morning commuting hour, followed by the formation of nitrogen dioxide by oxygenation, and finally, through a complex series of reactions in the presence of hydrocarbons (hydrocarbon) and sunlight, leads to the formation of ozone and peroxyacetyl nitrite and other irritant compounds. Eye irritation, chest irritation with cough, and possibly the exacerbation of asthma occur as a result. Modern air pollution consists of some combination of the reducing form consequent upon sulfur dioxide emissions and the oxidant form, which begins as emissions of nitrogen oxides. Ozone is the most irritant gas known. In controlled exposure studies it reduces the ventilatory capability of healthy people in concentrations as low as 0.12 part per million. These levels are commonly exceeded in many places, including Mexico City, Bangkok, and São Paulo, where there is a high automobile density and the meteorologic conditions favour the formation of photochemical oxidants. Although acute episodes of communal air exposure leading to demonstrable mortality are unlikely, there is much concern over the possible long-term consequences of brief but repetitive exposures to oxidants and acidic aerosols (aerosol). Such exposures are common in the lives of millions of people, and the impact of these exposures is an area of intense scientific investigation.

      The indoor environment can be important in the genesis of respiratory disease. In developing countries, disease may be caused by inhalation of fungi from roof thatch materials or by the inhalation of smoke when the home contains no chimney. In developed countries, exposure to oxides of nitrogen from space heaters or gas ovens may promote respiratory tract infections in children. Inhalation of tobacco smoke in the indoor environment by nonsmokers impairs respiration, and repeated exposures may lead to lung cancer. A tightly sealed house may act as a reservoir for radon seeping in from natural sources.

      Acute carbon monoxide poisoning is a common and dangerous hazard. The British physiologist John Scott Haldane (Haldane, John Scott) pioneered the study of the effects of carbon monoxide at the end of the 19th century, as part of his detailed analysis of atmospheres in underground mines (mine). Carbon monoxide is produced by incomplete combustion, including combustion of gas in automobile engines, and for a long period it was a major constituent of domestic gas made from coal (its concentration in natural gas is much lower). When the carbon monoxide concentration in the blood reaches 40 percent (that is, when the hemoglobin is 40 percent saturated with carbon monoxide, leaving only 60 percent available to bind to oxygen), the subject feels dizzy and is unable to perform simple tasks; judgment is also impaired. Hemoglobin's affinity for carbon monoxide is 200 times greater than for oxygen, and in a mixture of these gases hemoglobin will preferentially bind to carbon monoxide; for this reason, carbon monoxide concentrations of less than 1 percent in inspired air seriously impair oxygen-hemoglobin binding capacity. The partial pressure of oxygen in the tissues in carbon monoxide poisoning is much lower than when the oxygen-carrying capacity of the blood has been reduced an equivalent amount by anemia, a condition in which hemoglobin is deficient. The immediate treatment for acute carbon monoxide poisoning is assisted ventilation with 100 percent oxygen.

      The carbon monoxide inhaled by smokers who smoke more than two packs of cigarettes a day may cause up to 10 percent hemoglobin saturation with carbon monoxide. A 4 percent increase in the blood carbon monoxide level in patients with coronary artery disease is believed to shorten the duration of exercise that may be taken before chest pain is felt.

David V. Bates John Hansen-Flaschen

Additional Reading
The human respiratory system is described in David V. Bates, Peter T. Macklem, and Ronald V. Christie, Respiratory Function in Disease: An Introduction to the Integrated Study of the Lung, 2nd ed. (1971), a detailed text on impairment of lung function caused by disease; and Robert G. Fraser et al., Diagnosis of Diseases of the Chest, 2nd ed., 4 vol. (1977–79), with vol. 1 also available in a 3rd ed. (1988). H. Corwin Hinshaw and John F. Murray, Diseases of the Chest, 4th ed. (1980), is a general textbook covering diagnosis and treatment of chest diseases; see also J.G. Scadding and Gordon Cumming (eds.), Scientific Foundations of Respiratory Medicine (1981). Steven E. Weinberger, Principles of Pulmonary Medicine (1986), is an introductory text in which respiratory pathophysiology is considered from the clinical vantage. Comprehensive texts include Gordon Cumming and Stephen J. Semple, Disorders of the Respiratory System, 2nd ed. (1980); John Crofton and Andrew Douglas, Respiratory Diseases, 3rd ed. (1981); and Ian R. Cameron and Nigel T. Bateman, Respiratory Disorders (1983). Alfred P. Fishman (ed.), Pulmonary Diseases and Disorders, 2nd ed., 3 vol. (1988), provides a comprehensive overview of pathophysiology as related to clinical syndromes. See also John F. Murray and Jay A. Nadel (eds.), Textbook of Respiratory Medicine (1988); and William M. Thurlbeck (ed.), Pathology of the Lung (1988).David V. Bates Alfred P. Fishman

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Universalium. 2010.

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