Effect of static or slowly flowing blood on carbon monoxide diffusion in dog lungs.

During temporary left pulmonary artery occlusion (TLPAO) in dogs, blood in pulmonary capillaries downstream from the occlusion is static or flowing slowly. In such areas, the uptake of carbon monoxide (CO) and diffusing capacity (DLCO) should decrease with time as carboxyhemoglobin concentration increases. We measured DLCO during exhalation of five sequential breaths in anesthetized dogs using a modification of a technique recently described in our laboratory (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43: 617--625, 1977). During TLPAO, the decrease in DLCO was greatest at low lung volumes, suggesting that the occluded lung was emptying later in exhalation, a conclusion supported by measurements of DLCO during TLPAO with the right mainstem bronchus occluded. In addition. DLCO decreased with each breath as the backpressure to diffusion increased in static capillary blood. Inhalation of 4% CO accelerated the rate of decrease in DLCO. Measurement of DLCO during exhalation over multiple breaths may help detect pulmonary vascular obstruction.