Maximal Diffusing Capacity of the Lung for Carbon Monoxide*

Abstract

During exercise pulmonary diffusing capacity for carbon monoxide and oxygen increases because the pulmonary capillary bed expands (1). It seems reasonable that there should be an upper limit to this expansion at which the diffusing ca- pacity reaches maximum. The apparent oxygen diffusing capacity (DLO2) has been noted to ap- proach a plateau or upper limit as the work load increases (2, 3), but a similar plateau for CO dif- fusing capacity has never been clearly demon- strated (1, 4) perhaps because it has not been measured at heavy enough work loads. Thus our purpose was to determine how high CO diffusing capacity can go as exercise work load increases and to see whether it reaches a plateau before the maximal tolerated work load is achieved.