Fluid dynamic factors in tracheal pressure measurement

Abstract

Because tracheal pressure measurement generally involves the use of a cannula or an endotracheal tube, fluid dynamic factors may cause a considerable artifact. A theoretical explanation is presented of the observed apparent paradox in which the resistance of a tracheal cannula or an endotracheal tube in isolation exceeded the resistance of the airways plus the cannula or the tube in situ. By estimating the viscous dissipation and the kinetic energy change in a conduit with sudden variation of cross-sectional area, a predictive model is derived. The predictions are verified by a series of in vitro experiments with both steady and oscillatory flows. The experiments show that the pressure that is recorded from the sidearm of a tracheal cannula or endotracheal tube contains an error which, in general, increases with the mean Reynolds'' number of the through flow and which also depends on the diameter ratio between the trachea and the tube or cannula, the position of the pressure tap, and the frequency of ventilation. When feasible, direct measurement with a needle in the trachea is suggested as a way to avoid the possible artifacts arising from the use of a side tap of the cannula. Theoretical considerations and in vitro and animal experiments indicate that adding a properly chosen expansion to the tracheal cannula makes it possible to alter inspiratory and expiratory pressures selectively. This device may be useful in breathing control studies.