CO2 elimination and airway opening pressure during high frequency oscillation in dogs

CO2 elimination (VCO2) and pressure at the airway opening (Pao) were measured over a wide range of frequencies (f) (5-40 Hz) and tidal volumes (VT) (1-45 ml) during high frequency oscillatory ventilation (HFO) in 11 anesthetized, paralyzed dogs. CO2 elimination was found to depend primarily on the product off and VT which is consistent with theoretical predictions of gas exchange by augmented dispersion. Airway opening pressures during HFO were frequency dependent. At frequencies near 5 Hz Pao was lower, while at frequencies above 5 Hz Pao was higher than Pao during conventional mechanical ventilation (12 cm H2O). The minimum Pao (5 cm H2O) achieved while maintaining VCO2 equivalent to normal metabolic CO2 production occurred when VT was near that of the dead space volume and frequency was near 5 Hz. At lower VT, higher frequencies were required to maintain VCO2 and this increased frequency resulted in an increase in Pao amplitude (10-15 cm H2O).