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

Abstract

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 and oxygen transfer into pulmonary capillary blood. The red cell as well as the pulmonary membrane offers resistance to CO and oxygen transfer into blood (5, 6); the total resistance (1/DL) is the sum of the membrane and red cell resistances:

Exercise Limitation Following Extensive Pulmonary Resection*

“…gen saturation of arterial blood can be explained by the venous admixture caused by anatomical shunts (Table V), but during exercise, arterial oxygen saturation fell more than can be explained by the measured anatomical shunt. Even in the presence of pulmonary disease, ventilation probably becomes more uniform with respect to perfusion during exercise (18), and hence this discrepancy between measured and calculated saturation probably results from impaired diffusion (15). The fall in arterial oxygen saturation with exercise was most pronounced in AB and RH, the patients who had the least remaining lung.…”

Maximal oxygen consumption in patients with lung disease.

“…Some results in eight of these patients (footnoted in Table II) have been reported in earlier studies from this laboratory (4,6). Training and exercise during the studies were performed either on a motor-driven treadmill or on a Monark bicycle ergometer (Monark-Crescent AB, Varberg, Sweden) with patients breathing room air (Plo2 -150 mm Hg) or 100%o oxygen from a reservoir bag.…”

“…The time interval (t) required for the oxygen saturation (So2) of blood passing through the lung capillaries to rise between that in mixed venous and end-capillary blood can be estimated as follows (3,4):…”

“…The Fick equation states that V02 = Qc(Sc'o2-SVo2)(cap)/100 (4) Combining equations 3 and 4 V02 = Vc(Sc'2-SVo2)(cap)/1OOt…”

“…The total lung capacity and subdivisions of the lung volume were determined from the spirometry and the functional residual capacity determined by the volume plethysmograph method of DuBois et al (9). The diffusing capacity of the lung and pulmonary capillary blood flow were determined at rest by a single-breath method described in detail elsewhere (4,10). At peak exercise DLco and Qc were measured either by the breath-holding method or by a modification of the rebreathing method described by Lawson (11).…”