The blood transfer conductance for nitric oxide: Infinite vs. finite θ_NO

Whether the specific blood transfer conductance for nitric oxide (NO) with hemoglobin (θ_NO) is finite or infinite is controversial but important in the calculation of alveolar capillary membrane conductance (Dm_CO) and pulmonary capillary blood volume (V_C) from values of lung diffusing capacity for carbon monoxide (DLCO) and nitric oxide (DLNO). In this review, we discuss the background associated with θ_NO, explore the resulting values of Dm_CO and V_C when applying either assumption, and investigate the mathematical underpinnings of Dm_CO and V_C calculations. In general, both assumptions yield reasonable rest and exercise Dm_CO and V_C values. However, the finite θ_NO assumption demonstrates increasing V_C, but not Dm_CO, with submaximal exercise. At relatively high, but physiologic, DLNO/DLCO ratios both assumptions can result in asymptotic behavior for V_C values, and under the finite θ_NO assumption, Dm_CO values. In conclusion, we feel that the assumptions associated with a finite θ_NO require further in vivo validation against an established method before widespread research and clinical use.