Comparison of steady state pulmonary diffusing capacity estimates for O 2 and CO in dogs

Abstract

In view of the fact that the inhomogeneity effects on pulmonary diffusing capacity (D l) estimates are quite different for O 2 and for CO, simultaneous determinations of steady-state D l CO and D l O 2 were attempted and compared. To this end, pulmonary gas exchange was measured in 17 anethetized and atificially ventilation dogs, in hypoxia with and without carbon monoxide in inspired gas (F i O 2 = 0.12, F i CO = 0.0009 to 0.0016). The diffusing capacity estimates were computed by tow conventional procedures, the firs (Dapp) taking into account the mean alveolar partial pressures and the second (D vda) the ideal alveolar partial pressures. It was found that the presence of COHb in blood, inevitable in the steady-state D l CO procedure, leads to a marked underestimation of D l O 2 ; therefore D l CO values could only be adequately compared to D l O 2 values obtained in the absence of CO from inspired gas. These D l O 2 were 18.5 and 33.4 ωmol·min −1·kg −1 for the mean Dapp O 2 and D vda O 2 , respectively, whereas the D l CO values obtained after 15 to 25 min CO inspiration were 30.0 and 83.4 ωmol·min −1·Torr −1·kg −1 for the mean Dapp CO and the mean D vda CO, respectively. The salient feature is that with both procedures the mean value of D l CO estimate is higher than the corresponding D l O 2 estimate. This finding suggests that in anesthetized, arttificially ventilated dogs Dapp O 2 estimates obtained by steady-state procedures in hypoxia are largely influenced by inhomogeneity effects and of limited value for assessment of the diffusing properties of the lung. Dapp CO and D vda O 2 are also affected by inhomogeneities but to a lesser degree.