Sloping alveolar plateaus of CO2, O2, and intravenously infused C2H2 and CHClF2 in the dog.

To investigate the role of the various mechanisms assumed to contribute to the slope of the alveolar plateau, two test gases exhibiting identical solubility but two-fold differing diffusivity, acetylene (C2H2) and chlorodifluoromethane (Freon 22, CHClF2), dissolved in saline were intravenously infused in 10 anesthetized, paralyzed, artificially ventilated dogs (mean body mass, 18 kg). The partial pressures of C2H2, CHClF2, CO2 and O2 during a constant-flow single-breath washout maneuver were recorded by mass spectrometry and analyzed in terms of slope of the alveolar plateau (phase III) and series (Fowler) dead space. The slope of the alveolar plateau (S) was determined as the relative alveolar slope normalized to mixed-expired partial pressure and referred to expired volume (VE), S(V) = delta PE/(PE - PI)/delta VE or expiration time (tE), S(t) = delta PE/(PE - PI)/delta tE (subscripts I, E, and E refer to inspired, instantaneous expired and mixed-expired gas, respectively). The effects of expiratory flow rate (VE), and time of breath-hold (BH) were studied with reference to control conditions (VI = 0.5 L.sec-1, VE = 0.1 L.sec-1, VI = 50% and VE = 75% of volume at FRC, BH = 0 sec). In control conditions, the following significantly different S(V) values (units: L-1), grouped in ascending order, were obtained (means +/- SD): CO2, 0.83 +/- 0.26; C2H2, 0.93 +/- 0.18; CHClF2, 1.00 +/- 0.20; O2, 1.07 +/- 0.29. The mean C2H2/CHClF2 ratio for S(V), 0.94 (SD +/- 0.03), was statistically different from unity. In line with model calculations, the experimental findings suggest that three mechanisms contribute to the sloping alveolar plateaus: 1, continuing gas exchange during expiration; 2, ventilation-perfusion inequality combined with sequential emptying; 3, intrapulmonary diffusion limitation.