Optimizing the calculation of D_M,CO and V_C via the single breath single oxygen tension DL_CO/NO method

Alveolar-capillary membrane conductance (D_M,CO) and pulmonary-capillary blood volume (V_C) are calculated via lung diffusing capacity for carbon monoxide (DL_CO) and nitric oxide (DL_NO) using the single breath, single oxygen tension (single-FiO₂) method. However, two calculation parameters, the reaction rate of carbon monoxide with blood (θ_CO) and the D_M,NO/D_M,CO ratio (α-ratio), are controversial. This study systematically determined optimal θ_CO and α-ratio values to be used in the single-FiO₂ method that yielded the most similar D_M,CO and V_C values compared to the 'gold-standard' multiple-FiO₂ method. Eleven healthy subjects performed single breath DL_CO/DL_NO maneuvers at rest and during exercise. D_M,CO and V_C were calculated via the single-FiO₂ and multiple-FiO₂ methods by implementing seven θ_CO equations and a range of previously reported α-ratios. The RP θ_CO equation (Reeves, R.B., Park, H.K., 1992. Respiration Physiology 88 1-21) and an α-ratio of 4.0-4.4 yielded D_M,CO and V_C values that were most similar between methods. The RP θ_CO equation and an experimental α-ratio should be used in future studies.