Cardiopulmonary limitations to exercise in restrictive lung disease

Restrictive lung disease encompasses a large and diverse group of disorders characterized by a diminished lung volume. These disorders exhibit common pathophysiologic features including abnormal gas exchange caused by loss of functioning alveolar-capillary unit, abnormal respiratory muscle energetic s caused by altered mechanical ventilatory function, and secondary hemodyna mic and cardiac dysfunction. Impaired gas exchange is the most prominent ex ercise abnormality in interstitial lung disease and eventually develops in other causes of lung restriction as well. Measurements of diffusing capacit y (DL,,) and alveolar-arterial oxygen tension gradient during exercise are more sensitive detectors of disease than measurements at rest. Excessive de ad space ventilation is common in pulmonary parenchymal, pleural, and thora cic diseases, leading to a higher minute ventilation and ventilatory work d uring exercise. The associated increase in the metabolic energy requirement of respiratory muscles may exceed 50% of available total body oxygen deliv ery and result in insufficient energy delivery to nonrespiratory muscles th at sustain locomotion. Pulmonary arterial hypertension develops secondarily to an increased pulmonary vascular resistance. In addition, diastolic fill ing of the ventricles during exercise may be restricted by pulmonary fibros is or anatomical restriction of the pleura and thorax, contributing to seco ndary cardiac dysfunction. Examples of heart-lung interaction are illustrat ed by the patient after unilateral pneumonectomy. These pathophysiologic ch anges help explain why functional disability in these patients is often out proportion to the impairment in lung function.