Hypothesis: Why θNO could be finite in vitro but infinite in vivo

Abstract There is controversy as to whether the lung Diffusing Capacity for Nitric Oxide (DLNO) is a direct measure of DM in the Roughton-Forster equation or whether θNO is finite and DM is greater than DLNO. Despite in vitro evidence that θNO is finite, some groups believe that it is infinite in vivo and that DMNO/DMCO (α) is greater than predicted by the combined Fick/Graham law of Gas Diffusion through a membrane. We here present a hypothesis applying the fundamental rules of combined diffusion and chemical reaction to a red cell to explain (i) why θNO could be finite in vitro but effectively infinite in vivo and (ii) why ∝ could appear greater than predicted. DLNO would mainly reflect the conductance of the alveolar capillary membrane with a smaller contribution from plasma and minimal contribution from the outermost layers of the red cell. If this hypothesis is correct DMCO and Vc could not be obtained from a combined DLNO and DLCO manoeuvre since these variables would differ for NO and for CO.