The blood transfer conductance for CO and NO

Nitric oxide was introduced over 30 years ago as a test gas for alveolar capillary diffusion. As for CO its transfer has been interpreted according to the Roughton Forster relationship: 1/DL=1/DM+1/θVc. There has been disagreement, since the first measurements of DLNO, over whether θ_NO is infinite and thus DLNO=DMNO. There is overwhelming in vitro evidence that θ_NO is finite yet several groups (Coffman et al., 2017; Tamhane et al., 2001) use an infinite value in vivo. They also assume that DMNO is greater than twice DMCO, making DMCO less than that predicted by the physical laws of diffusion. Finally some (Coffman et al., 2017) recommend use of Reeve and Park's value for θ_CO (Reeves and Park, 1992; Coffman et al., 2017) rather than Forster's (Forster, 1987). Their grounds for doing so are that the combination of an infinite theta NO, an empirical value for DMNO/DMCO (>2.0) and Reeve and Park's θ_CO gives a value of DMCO (using a combined DLNO-DLCO analysis) which agrees with the DMCO value calculated separately by the classical two-stage oxygen technique of Roughton and Forster. In this paper we examine whether there are physiological reasons for assuming that DMNO is over twice DMCO in vivo. We are critical of Reeves and Park's estimate for the 1/θ_CO-PO₂ relationship. We review in vitro estimates of θ_CO in the light of Guenard et al.'s recent in vivo estimate.