Role of reaction resistance in limiting carbon monoxide uptake in rabbit lungs

Abstract

The contribution of reaction resistance to overall resistance to pulmonary carbon monoxide (CO) uptake [Dl CO/(ΘCO ⋅ Vc), where Dl CO is lung CO diffusing capacity, ΘCO is CO uptake conductance of erythrocytes, and Vc is pulmonary capillary blood volume] was determined in 10 anesthetized, paralyzed, and artificially ventilated rabbits. On the basis of the classical double-reciprocal equation of F. G. W. Roughton and R. E. Forster ( J. Appl. Physiol. 11: 290–302, 1957), Dl CO/(ΘCO ⋅ Vc) was obtained by solving the relation Dl CO/(ΘCO ⋅ Vc) = 1 − 2/(Dl NO/Dl CO), where Dl NO/Dl COrepresents the ratio between the respective single-breath diffusing capacities (Dl) of nitric oxide (NO) and CO pulmonary capillary blood. The lungs of eight rabbits were inflated, starting from residual volume, by using 55 ml of indicator gas mixture (0.2% CO and 0.05% NO in nitrogen). Dl values were calculated by taking the end-tidal partial pressures of CO and NO as analyzed by using a respiratory mass spectrometer. The overall value was Dl CO/(ΘCO ⋅ Vc) = 0.4 ± 0.025 (mean ± SD). Because of the use of O2-free indicator gas mixtures, the end-tidal O2 partial pressures were ∼21 Torr. In one other rabbit, the application of 0.2% CO and 0.001% NO yielded Dl CO/(ΘCO ⋅ Vc) = 0.39; in the tenth rabbit, however, inspiratory volume was varied, and an identical value was found at functional residual capacity. We conclude that the contribution of reaction resistance to overall resistance to pulmonary CO uptake is independent of the inspiratory NO concentration used, including, with respect to the pertinent literature, the conclusion that in rabbits, dogs, and humans this contribution amounts to 40% when determined at functional residual capacity.