Functional Significance of a Low Pulmonary Diffusing Capacity for Carbon Monoxide

Diffusing capacity of the lungs imposes a theoretical limit to oxygen consumption, causing oxygen saturation of arterial blood to fall sharply if this limit is approached (1). The diffusing surface of the normal lung is so large, however, that at sea level oxygen capacity of the blood and the cardiac output rather than diffusing capacity create the major bottleneck to oxygen transport (2). Diffusion becomes an important limit only at high altitudes (3) or when diffusing capacity is reduced sufficiently by disease to cause alveolar capillary block (4). Diffusing capacity of the lungs usually is measured with respect to CO (DLco) rather than oxygen (DLO2). Yet little information exists regarding how low DLCO must be before alveolar capillary block is manifest. Recent work of Roughton and Forster (5) and of Staub, Bishop, and Forster (6) defines the theoretical relationship between DLco and DLO2 allowing translation of CO diffusing capacity into terms of oxygen transport. Thus we should be able to state more explicitly the functional significance of a low CO diffusing capacity. Our purpose has been to predict the restriction in maximal oxygen transport implied by a low DLco and then to check the prediction by experimental measurement.