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

Diffusing capacity of the lungs imposes a theo- retical limit to oxygen consumption, causing oxy- gen 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).