ALVEOLAR PRESSURE AND TIDAL VOLUME DURING HIGH FEEQUENCY VENTILATION

Abstract

Alveolar pressures and tidal volumes have not been quantified during high frequency ventilation. We have measured dynamic and mean pressures at the airway opening, trachea, and alveoli as well as delivered volumes and blood gases in vivo in closed-chest adult rabbits ventilated at rates of 2-37.5 Hz with the Emerson flow-interrupting high frequency ventilator. Alveolar pressure was measured by opening the chest, gluing an adapter to the visceral pleural surface, puncturing the pleura and lung surface through the adapter, closing the chest, and attaching a pressure transducer. Delivered volume was measured with a pressure plethysmograph. As frequency increased, alveolar pressure swings decreased from 26 to 8% of pressure swings at the airway opening, and 100 to 42% of those in the trachea. Delivered volume decreased from 5.3±0.5 to 0.6±0.1 ml/kg as frequency increased. Respiratory system impedance fell initially with increasing frequency, then tended to remain relatively stable with no resonance seen. Lung tissue plus chest wall impedance behaved as an elastance and fell as frequency increased. Muscle paralysis had no effect on alveolar pressure swings, but excision of the rib cage resulted in a decrease. Mean pressures were equal at the airway opening, trachea and alveoli at all frequencies. The clinical implication of these results is that if gas exchange can be maintained with constant tracheal pressure swings, alveolar barotrauma may be minimized by choosing the highest possible frequency for ventilation. (Supported in part by HL 27372.)