Multiple breath washout of He and SF6 in panting dogs

Pulmonary gas transport mechanisms in panting were studied by multiple breath washout of two poorly soluble inert gases of similar solubility but different diffusivity (He and SF6). The experiments were performed in 6 chronically tracheotomized conscious dogs (mean body weight 31.0 kg) which, upon exposure to elevated room temperature, were enforced to thermal panting (mean breathing frequency 288/min). After equilibration of lung gas with 1% He and 1% SF6 followed by changeover to test gas-free air, end-tidal gas concentrations during multiple breath washout were recorded by mass spectrometry. The washout time course was analyzed into 3 exponential components. The initial fast component was considered to be in part determined by the transient response of the measuring system, whereas the intermediate and the slow component could be attributed to lung washout. The mean He/SF6 ratio of medium and slow rate constants was 1.06 and 1.13, respectively (both values differing from 1.0 at P less than 0.001). It is concluded that gas transport in dog lungs during panting was mainly determined by convection, diffusion-dependent processes being discernible but playing a minor role.