Pulmonary gas exchange during high-frequency ventilation in dogs

Steady-state gas exchange was studied in anesthetized paralyzed dogs subjected to high-frequency ventilation (HFV) to determine the effects of oscillation frequency (f 10-40 Hz) and stroke volume (Vs 20-40 ml) on gas exchange efficiency. Alveolar gas fractions were measured by rapid withdrawal of lung gas while oscillations and the bias flow circuit were interrupted. Alveolar ventilation (VA) calculated from steady-state CO2 elimination and FACO2 was found to exhibit a frequency-dependent maximum. Similarly, PaCO2 increased and PaO2 decreased again when f exceeded a critical level. VA was 13% of total ventilation (f . Vs) at 10 Hz (40 ml Vs) decreasing to about 4% at 40 Hz. Blood - gas differences for O2 (PA - Pa) were about 6 torr and close to zero for CO2 (Pa - PA). Lung volume assessed from He wash-out was increased with f and Vs. O2 uptake and CO2 elimination were significantly enhanced by HFV as a function of Vs. Because the effective VT was essentially unknown, the apparent gas exchange inefficiency at high f and Vs values may be due to the functional characteristics of the apparatus rather than a general feature of HFV.