PULMONARY DIFFUSING CAPACITIES FOR OXYGEN-LABELED CO2 AND NITRIC-OXIDE IN RABBITS

We determined the pulmonary diffusing capacity (DL) for O-18-labeled C O2 ((CO2)-O-18) and nitric oxide (NO) to estimate the membrane compone nt of the respective gas conductances. Six anesthetized paralyzed rabb its were ventilated by a computerized ventilatory servo system. Single -breath maneuvers were automatically performed by inflating the lungs with gas mixtures containing 0.9% (CO2)-O-18 Or 0.05% NO in nitrogen, with breath-holding periods ranging from 0 to 1 s for (CO2)-O-18 and f rom 2 to 8 s for NO. The alveolar partial pressures of (CO2)-O-18 and NO were determined by using respiratory mass spectrometry. DL was calc ulated fi-om gas exchange during inflation, breath hold, and deflation . We obtained values of 14.0 +/- 1.1 and 2.2 +/- 0.1 (mean value +/- S D) ml.mmHg(-1).min(-1) for DLC18O2 and DLNO, respectively. The measure d DLC18O2/DLNO ratio was one-half that of the theoretically predicted value according to Graham's law(6.3 +/- 0.5 vs. 12, respectively). Ana lyses of the several mechanisms influencing the determination of DLC18 O2 and DLNO and their ratio are discussed. An underestimation of the m embrane diffusing component for CO2 is considered the likely reason fo r the low DLC18O2/DLNO ratio obtained.