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

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