Gas movement during jet ventilation

Young, John Duncan (1991) Gas movement during jet ventilation. University of Southampton, Doctoral Thesis.

Abstract

A method was developed to measure the exhaled tidal volume and the volumes of gas entrained split out of the airway during jet ventilation. The method proved very accurate when tested on bench models. The method was used to determine the changes in these gas volumes during jet ventilation in the dog at a constant arterial carbon dioxide tension and respiratory rates from 12 to 200 breaths per minute. Tidal volume and split volume decreased with increasing respiratory rate, and entrained volume remained a constant fraction of tidal volume. The pattern of the change in tidal and minute ventilation with increasing ventilatory rate indicated the physiological deadspace was constant. A mathematical model to describe the maximum pressure generated by a jet in the airway was developed using fluid mechanical theory and proved accurate when tested on bench models. When the mathematical model was extended to cover all of the ventilatory cycle it accurately predicted tidal volume on bench testing, but became increasingly inaccurate as respiratory rate increased when compared with in vivo experiments. This was attributed to a reduction in dynamic compliance with increasing ventilatory rate. The end-tidal to arterial carbon dioxide tension difference was examined during large tidal volumes delivered during brief pauses in high frequency jet ventilation. The difference was minimal when the timing and volume of these `deep breaths' caused no change in alveolar ventilation. This was confirmed on a mathematical model using a stepwise approach to examine carbon dioxide transfer during one respiratory cycle.

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