Lung compliance is a measure of the ease of expansion of the lungs and thorax, determined by pulmonary volume and elasticity. A high degree of compliance indicates a loss of elastic recoil of the lungs, as in old age or emphysema. Decreased compliance means that a greater change in pressure is needed for a given change in volume, as in atelectasis, edema, fibrosis, pneumonia, or absence of surfactant. Dyspnea on exertion is the main symptom of diminished lung compliance.
Emphysema/COPD may be associated with an increase in pulmonary compliance due to the loss of alveolar and elastic tissue. Low compliance indicates a stiff lung and means extra work is required to bring in a normal volume of air. This can occur when lungs become fibrotic, lose their distensibility and become stiffer. In a highly compliant lung, as in emphysema, the elastic tissue is damaged by enzymes. These enzymes are secreted by leukocytes (white blood cells) in response to a variety of inhaled irritants, such as cigarette smoke. Patients with emphysema have a very high lung compliance due to the poor elastic recoil, they have no problem inflating the lungs but have extreme difficulty exhaling air. In this condition, extra work is required to get air out of the lungs. Compliance also increases with increasing age due to a degenerative loss of elastic recoil that accompanies aging.
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