Maximal Diffusing Capacity of the Lung for Carbon Monoxide*

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Abstract
The purpose of this investigation was to determine how much the apparent CO [carbon mon-oxide] diffusing capacity (DlcO) increases from rest to peak exercise and whether it reaches a plateau with increasing work load. Pulmonary capillary blood flow and apparent CO diffusing capacity were measured in 5 normal adults and in 5 normal children by a breath-holding technique both at rest and during exercise and repeated at 2 different alveolar oxygen tensions so that the true membrane diffusing capacity and pulmo-nary capillary blood volume could be determined by the Roughton-Forster method. DLCO kept rising as work load was increased until the pulmo-nary blood flow and oxygen consumption stopped going up. DLCO in-creased principally because of a 2 fold increase in pulmonary capillary blood volume during exercise. Membrane diffusing capacity increased only about 20% above the resting value. In 3 patients with mitral stenosis pushed to their peak work loads pulmonary capillary blood volume rose to the normal level during exercise but not above the normal upper limit. This suggests that maximal distensibility of the capillary bed in normal subjects is reached or closely approached at peak work loads. The maximal pulmonary capillary blood volumes measured in these subjects agree closely with anatomical estimates of the maximal volume capacity of the pulmonary capillary bed.