In vivo estimates of NO and CO conductance for haemoglobin and for lung transfer in humans

• Description of an original in vivo method applied to humans to determine the blood conductances of two gases, CO and NO.
• Estimate of membrane lung conductance in the diffusion model of Roughton and Forster is close to morphometric one.

Membrane conductance (Dm) and capillary lung volume (Vc) derived from NO and CO lung transfer measurements in humans depend on the blood conductance (θ) values of both gases. Many θ values have been proposed in the literature. In the present study, measurements of CO and NO transfer while breathing 15% or 21% O2 allowed the estimation of θNO and the calculation of the optimal equation relating 1/θCO to pulmonary capillary oxygen pressure (PcapO2).In 10 healthy subjects, the mean calculated θNO value was similar to the θNO value previously reported in the literature (4.5 mmHg min−1) provided that one among three θCO equations from the literature was chosen. Setting 1/θCO = a·PcapO2 + b, optimal values of a and b could be chosen using two methods: 1) by minimizing the difference between Dm/Vc ratios for any PcapO2, 2) by establishing a linear equation relating a and b. Using these methods, we are proposing the equation 1/θCO = 0.0062·PcapO2 + 1.16, which is similar to two equations previously reported in the literature. With this set of θ values, DmCO reached the morphometric range.