Emphysema can be classified according to the site of damage:
IN CENTRI-ACINAR EMPHYSEMA distension and damage of lung tissue is concentrated around the respiratory bronchioles, whilst the more distal alveolar ducts and alveoli tend to be well preserved. This form of emphysema is extremely common; when of modest extent, it is not necessarily associated with disability. Severe centri-acinar emphysema is associated with substantial airflow limitation.
PAN-ACINAR EMPHYSEMA is less common. Here, distension and destruction appear to involve the whole of the acinus, and in the extreme form the lung becomes a mass of bullae. Severe airflow limitation and VA/Q mismatch occur. This type of emphysema occurs in a) antitrypsin deficiency.
IN IRREGULAR EMPHYSEMA there is scarring and damage affecting the lung parenchyma patchily without particular regard for acinar structure. Emphysema can lead to expiratory airflow limitation and air trapping. The loss of lung elastic recoil results in an increase in TLC while the loss of alveoli with emphysema results in decreased gas transfer.
VIQ mismatch occurs partIy because of damage and mucus plugging of smaller airways from chronic bronchitis and also because of the rapid expiratory closure of the smaller airways due to loss of elastic recoil from emphysema. This leads to a fall in Pa02 and a subsequent rise in Pac02.
CO2 is normally the major stimulant of the respiratory centre. In the face of a prolonged high Pac02 this sensitivity is dirnirIished and hypoxaemia becomes the chief drive to respiration. In this situation an attempt to abolish hypoxaemia by administration of oxygen can result in an increase in Pac02 by decreasing the respiratory drive.
Bronchoalveolar washes have shown that smokers have neutrophil granulocytes present within the lumen of the lung that are absent in non-smokers. Additionally, the small airways of smokers are infiltrated by granulocytes. These granulocytes are capable of releasing elastases and proteases, which possibly help to produce emphysema. It is suggested that an imbalance between protease and antiprotease activity may produce the damage. a)-Antitrypsin is a major serum antiprotease which can be inactivated by cigarette smoke (see below).
The hypertrophy of mucous glands in the larger airways is thought to be a direct response to persistent irritation resulting from the inhalation of cigarette smoke. The smoke has an adverse effect on surfactant, favouring overdistension of the lungs.
Infections are frequent and are often the precipitating cause of acute exacerbations of the disease. However, the role of infection in the development of the progressive airflow limitation that characterizes disabling chronic bronchitis and emphysema is far less clear. Nevertheless, release of enzymes from the excess neutrophil granulocytes found in infections probably adds to the lung damage.
Antitrypsin is produced in the liver, secreted into the blood and diffuses into the lung. Here it functions as an anti protease that inhibits neutrophil elastase, a proteolytic enzyme capable of destroying alveolar wall connective tissue. More than 75 allelles of ai-antitrypsin gene have been described. The three main phenotypes are MM (normal), MZ (heterozygous deficiency) and ZZ (homozygous deficiency); these groups are defined by the serum level of (XI-antitrypsin. About 1 child in 5000 in Britain is born with the homozygous deficiency, but not all develop chest disease. Those who do develop breathlessness under the age of 40 years have radiographic evidence of basal emphysema and are usually, but not always, cigarette smokers. Hereditary ‘deficiency of alantitrypsin accounts for about 2% of emphysema cases and a few develop liver disease.
The characteristic symptoms of chronic bronchitis and emphysema are cough with the production of sputum, wheeze and breathlessness following many years of a smoker’s cough. Colds seem to ‘go down to the chest’ and frequent infective exacerbations occur, giving purulent sputum. Symptoms can be worsened by factors such as cold, foggy weather and atmospheric pollution. With advanced disease, breathlessness becomes severe even after mild exercise such as dressing.
In mild disease there are no signs apart from ‘wheeze’ throughout the chest. In severe disease, the patient is tachypnoeic, with prolonged expiration. The accessory muscles of respiration are used and there may be intercostal indrawing on inspiration and pursing of the lips on expiration. Chest expansion is poor, the lungs are hyperinflated, and there is loss of the normal cardiac and liver dullness.
The ‘pink puffer’ is always breathless and is not usually cyanosed. Rarely oedema or heart failure may be seen. The ‘blue bloater’ is oedematous, deeply cyanosed, with hypoventilation and often little respiratory effort. These patients are likely to have hypercapnia, which gives the following physical findings:
• Peripheral vasodilatation
• A bounding pulse
• Later, a coarse flapping tremor of the outstretched hands
More severe hypercapnia leads to:
• Progressive drowsiness and coma with papilloedema
There is often considerable overlap between these two clinical patterns.