Calculating alveolar capillary conductance and pulmonary capillary blood volume: Comparing the multiple-and single-inspired oxygen tension methods

Key elements for determining alveolar-capillary membrane conductance (Dm) and pulmonary capillary blood volume (Vc) from the lung diffusing capacity (D_L) for carbon monoxide (D_LCO) or for nitric oxide (D_Lno) are the reaction rate of carbon monoxide with hemoglobin (θ_CO) and the Dm_CO/D_{L NO} relationship (α-ratio). Although a range of values have been reported, currently there is no consensus regarding these parameters. The study purpose was to define optimal parameters (θ_CO, α-ratio) that would experimentally substantiate calculations of Dm and Vc from the single-inspired O₂ tension [inspired fraction of O₂ (Fi_O2)] method relative to the multiple-Fi _O2 method. Eight healthy men were studied at rest and during moderate exercise (80-W cycle). Dm and Vc were determined by the multiple-Fi_O2 and single-Fi_O2 methods (rebreathe technique) and were tabulated by applying previously reported θ_CO equations (both methods) and by varying the a-ratio (single-Fi_O2 method) from 1.90 to 2.50. Values were then compared between methods throughout the examined α-ratios. Dm and Vc were critically dependent on the applied θ_CO equation. For the multiple-Fi_O2 method, Dm was highly variable between θ_CO equations (rest and exercise); the range of Vc was less widespread. For the single-Fi_O2 method, the θ_CO equation by Reeves and Park (1992) combined with an α-ratio between 2.08 and 2.26 gave values for Dm and Vc that most closely matched those from the multiple-Fi_O2 method and were also physiologically plausible compared with predicted values. We conclude that the parameters used to calculate Dm and Vc values from the single-Fi _O2 method (using D_LCO and D _LNO) can significantly influence results and should be evaluated within individual laboratories to obtain optimal values.