The Diffusing Capacity of the Lungs at Rest and on Exercise in Adults with Atrial or Ventricular Septal Defect

Diffusing capacity at rest and on exercise was measured in 25 patients with atrial or ventricular septal defect and in 40 normal subjects. In the normal subjects, diffusing capacity was significantly higher in men than in women. In men, there was a significant decrease in diffusing capacity on exercise with increasing age due, possibly, to subclinical emphysema. In most of the patients, the diffusing capacity was within normal limits irrespective of pulmonary vascular resistance, pulmonary blood flow or degree of dyspnea. The few patients with high diffusing capacity all had normal pulmonary vascular resistance. Patients who had abnormally high minute volumes usually achieved only normal diffusing capacities. This suggested that a true diffusion defect is present in many patients, but that it is masked by compensatory hyperventilation.