Functional Significance of a Low Pulmonary Diffusing Capacity for Carbon Monoxide*

Abstract

Theoretically a low pulmonary diffusing capacity can limit maximal O2 intake during exercise, causing arterial O2 saturation to fall sharply as O2 intake increases. The purpose of this investigation was to determine whether measurements of CO diffusing capacity can be used in accord with presently accepted theory to predict the limits that a low diffusing capacity imposes on O2 consumption. Pulmonary capillary blood flow, volume (Vc), and membrane diffusing capacity for CO (Dmco)were measured at full inspiration both at rest and exercise in 6 patients suspected of having alveolar capillary block. Predictions were made of how arterial O2 saturation should fall as O2 consumption increases. The patients were exercised up to the heaviest work load tolerated and the relationship between O2 consumption and arterial O2 saturation was measured. At heavy exercise O2 consumption approached the upper limits predicted from DmCO and Vc, though arterial O2 saturation began to fall at lower O2 consumptions than predicted. Experimental data and theory indicate that maximal O2 intake at sea level should not be limited significantly by diffusion until DMco is less than 520% predicted normal. Theory indicates that above 10,000 feet diffusing capacity will be an important limit to O2 consumption even in normal subjects.