ABSTRACT
Disorders of the pleural space, thoracic skeleton or inspiratory muscles prevent full expansion of the lungs and produce a similar functional picture of extrapulmonary volume restriction. Respiratory muscle weakness is considered in Chapter 20, Section 20.2. In addition to failure to distend the lungs fully, there are secondary functional effects on the lungs themselves. The effects of lung compression by a distorted thoracic cage, of microatelectasis owing to lack of a periodic full inspiration, or of mismatching of ventilation and perfusion, therefore often contribute to the results of conventional tests. In general the integrity of the alveolar-capillary membrane is retained, but impairment of full expansion may reduce carbon monoxide diffusing capacity (DLCO) to some extent, while KCO is typically high normal or frankly increased due to mismatching of alveolar and pulmonary capillary blood volumes (see Chapter 3, Section 3.1.2).
