Tracheal pressure and impedance as determinants of gas exchange during high frequency ventilation

This work explores the use of catheter pressure transducers to measure time averaged, mean squared pressure (MSP) in the trachea of anesthetized animals during high frequency ventilation using a prototype Emerson oscillator with a 25 ml stroke volume at 15 and 34 Hz. Tidal volume and MSP were varied by shunting a portion of the ventilator output for the study of the effect of driving pressure on ventilation. A given level of ventilation was accomplished at a lower driving pressure at the lower frequency. Both ventilation as assessed by a modified nitrogen washout procedure and gas exchange were related to MSP and oscillatory frequency by the substitution of MSP/Z2 for alveolar ventilation in the equations of Jaeger et al. (1984). The technique and equations describing it were validated by the prediction of impedance changes resulting from shifts in oscillatory frequency. The calculated impedances matched experimentally measured values found in dogs by other investigators. Catheter pressure transducers are thus shown to be a useful tool for quantifying high frequency ventilation when coupled with a knowledge of respiratory impedance. The technique offers advantages over existing techniques, the most notable being ease of use due to the catheters' exceptional frequency response.