The pulmonary diffusing capacity in congenital and rheumatic heart disease.

THE pulmonary diffusing capacity (DL ) is a measureinent relating the flow rate of gas across the alveolo-capillary membrane to the pressure gradient. Estimation of DL is possible by a variety of teehuies.l and in recent years it has resulted in a materially improved understanding of diffuse pulmonary parenchymal disease2 and of the effect of chronic airway obstruction on the pulmonary parenchyma.'4 The effect of heart disease otn DL has not received comparable attention; however, Carroll, Cohnl, and Riley5 have demonstrated that DL at rest may or may not be reduced in disease of the mitral valve. Classically, the magnitude of DL has been considered to depend upon the' permeability of the diffusing surface and the amount of area available for diffusion. In addition, Roughton and Forster6 have recently shown that DL is also affected by the volume of blood in ventilated pulmonary capillaries. It would seem reasonable that both the surface area and capillary blood volume might be increased under conditions of passive congestion, as iii nitral valve disease, and under conditions of increased pulmonlary blood flow (pulmonary hypereinia) such as occurs in septal defects ith left-to-riglht shunts. Also, it might be