Helium and SF6 washout from dog lungs during high-frequency ventilation

Simultaneous washout of He and SF6 was studied in anesthetized paralyzed dogs (mean body weight 19 kg) subjected to high-frequency ventilation at varying frequencies (10-40 Hz), stroke volumes (20-40 ml), lung volumes (0.8-1.2 L) and fresh gas flow rates (7-13 L/min). The washout curves could be analyzed into three exponential components for both test gases. The rate constants of the intermediate and slow components were slightly but significantly higher for He than for SF6 while the fast component was the same for the two test gases. The data were analyzed on the basis of a series lung model with a dead space compartment and two serially arranged alveolar compartments. The He/SF6 ratio of the effective conductances for gas transfer between the alveolar compartments averaged 1.15 +/- 0.08 (SD). Since this ratio is much closer to unity, predicted for convective transport, than 6 to 7, predicted for diffusive transport, it is concluded that during high-frequency ventilation gas transport in peripheral airways occurs by both convection and diffusion, convection being quantitatively more important.