Relationship Between Cardiac Output and Apparent Diffusing Capacity of the Lung in Normal Men During Treadmill Exercise

¹ From the Departments of Environmental Medicine and Medicine, The Johns Hopkins University, Baltimore, Maryland

Two methods for estimating pulmonary diffusing capacity (steady-state O₂ method of Lilienthal and Riley and steady-state CO method of Filley) were compared by applying them simultaneously in three normal young men at different levels of treadmill exercise with and without hypoxia. Cardiac output (indicator dilution) was estimated at the same time. Subjects were studied at each of four levels of exercise producing oxygen uptakes between 1 and 2 /l/min. and cardiac outputs between 9 and 22 l/min. Diffusing capacity for CO increased progressively with increasing exercise in all subjects. It was higher during hypoxia than during air-breathing at the same level of exercise, as were total ventilation, tidal volume and cardiac output. This increase in Dco cannot be explained solely on the basis of increased affinity of hemoglobin for CO during hypoxia. Diffusing capacity for oxygen increased progressively until the cardiac output reached about 15/l/min. but did not increase significantly with further increase in cardiac output. The reasons for the difference in the shape of the curve of diffusing capacity versus cardiac output in the case of CO as opposed to O₂ remain obscure but are believed to be related primarily to differences in the weighting of the many factors involved.