Resonant amplification of delivered volume during high-frequency ventilation

Abstract

The volume of gas delivered from a high-frequency ventilation (HFV) circuit was measured with an ultrasonic flowmeter. The measurements were done in vitro (20-liter air-filled glass bottle) and in vivo (9 anesthetized dogs lying supine) at oscillation frequencies ranging from 4 to 23 Hz and stroke volumes of the pump ranging from 36 to 150 ml. We varied (1) the length and diameter of the tube connecting the pump with the endotracheal tube, (2) the length and diameter of the bias outflow tube, (3) the diameter of the endotracheal tube, and (4) the stroke volume of the pump. Both in vitro and in vivo, there was resonant amplification of the delivered gas volume; i.e., the delivered gas volume exceeded the stroke volume at certain frequencies. Altering the dimensions of connecting tube, endotracheal tube, bias outflow tube, or stroke volume, i.e., changing the resistance to gas flow, gas compliance, and/or gas inertance in these elements, altered the ratio of gas delivered to stroke volume that could be predicted by an electric analog. These data indicate that the delivered gas volume during HFV depends critically on the configuration of the HFV circuit, the size of the endotracheal tube, the oscillation frequency, and the pump stroke volume. Knowledge of the delivered gas volume during HFV and appreciation of the phenomenon of resonant amplification of the delivered gas volume will permit a more accurate description of factors contributing to gas transport during HFV.