Steady-state measurement of NO and CO lung diffusing capacity on moderate exercise in men

Using a rapidly responding nitric oxide (NO) analyzer, we measured the steady-state NO diffusing capacity (DL(NO)) from end-tidal NO. The diffusing capacity of the alveolar capillary membrane and pulmonary capillary blood volume were calculated from the steady-state diffusing capacity for CO (measured simultaneously) and the specific transfer conductance of blood per milliliter for NO and for CO. Nine men were studied bicycling at an average O(2) consumption of 1.3 +/- 0.2 l/min (mean +/- SD). DL(NO) was 202.7 +/- 71.2 ml. min(-1). Torr(-1) and steady-state diffusing capacity for CO, calculated from end-tidal (assumed alveolar) CO(2), mixed expired CO(2), and mixed expired CO, was 46.9 +/- 12.8 ml. min(-1). Torr(-1). NO dead space = (VT x FE(NO) - VT x FA(NO))/(FI(NO) - FA(NO)) = 209 +/- 88 ml, where VT is tidal volume and FE(NO), FI(NO), and FA(NO) are mixed exhaled, inhaled, and alveolar NO concentrations, respectively. We used the Bohr equation to estimate CO(2) dead space from mixed exhaled and end-tidal (assumed alveolar) CO(2) = 430 +/- 136 ml. Predicted anatomic dead space = 199 +/- 22 ml. Membrane diffusing capacity was 333 and 166 ml. min(-1). Torr(-1) for NO and CO, respectively, and pulmonary capillary blood volume was 140 ml. Inhalation of repeated breaths of NO over 80 s did not alter DL(NO) at the concentrations used.