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

Abstract

Key elements for determining alveolar-capillary membrane conductance (Dm) and pulmonary capillary blood volume (Vc) from the lung diffusing capacity (Dl) for carbon monoxide (DlCO) or for nitric oxide (DlNO) are the reaction rate of carbon monoxide with hemoglobin (θCO) and the DmCO/DlNOrelationship (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 (θCO, α-ratio) that would experimentally substantiate calculations of Dm and Vc from the single-inspired O2tension [inspired fraction of O2(FiO2)] method relative to the multiple-FiO2method. Eight healthy men were studied at rest and during moderate exercise (80-W cycle). Dm and Vc were determined by the multiple-FiO2and single-FiO2methods (rebreathe technique) and were tabulated by applying previously reported θCOequations (both methods) and by varying the α-ratio (single-FiO2method) 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 θCOequation. For the multiple-FiO2method, Dm was highly variable between θCOequations (rest and exercise); the range of Vc was less widespread. For the single-FiO2method, the θCOequation 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-FiO2method and were also physiologically plausible compared with predicted values. We conclude that the parameters used to calculate Dm and Vc values from the single-FiO2method (using DlCOand DlNO) can significantly influence results and should be evaluated within individual laboratories to obtain optimal values.