Clinical evaluation of tracheal pressure estimation from the endotracheal tube cuff pressure.

OBJECTIVE: Air flow through an endotracheal tube causes a pressure drop across the tube. This pressure drop creates a difference between air pressure measured in the trachea and the pressure measured in the breathing circuit, which can lead to errors when calculating pulmonary mechanics and when setting ventilators. We have developed a method of estimating tracheal pressure from the pressure in the endotracheal tube cuff and tested this system in clinical trials.

METHODS: Pressure measurement ports were placed between the Y piece of the ventilator circuit and the ETT connector, in the trachea at the carinal end of the ETT, and in the ETT cuff inflation line. Tracheal pressures and cuff pressures were found at end-inspiration and end-expiration (no flow states) and used to define a linear relationship between cuff pressure and tracheal pressure. Using the estimated tracheal pressure (Ptrach) and the measured pressure at the Y piece of the breathing circuit (PY), the pressure drop across the ETT was found as a function of flow through the tube. Tracheal pressure was then calculated from the flow-dependent pressure drop and PY. Tests of this system were performed in six patients in the operating room and six patients in the intensive care unit.

RESULTS: The flow-based tracheal pressure estimates were within 0.7 +/- 0.4 cm H2O of actual tracheal pressure (mean +/- SD). At peak inspiratory pressure the difference averaged 0.5 +/- 0.3 cm H2O. The difference between our estimate of tracheal pressure and actual tracheal pressure was always less than 1 cm H2O.

CONCLUSION: The flow-based tracheal pressure estimates were accurate during intermittent spontaneous breathing, but not during spontaneous breathing or with a poorly inflated cuff. The estimates were more immune to noise than the cuff-based estimates of tracheal pressure. The estimates of tracheal pressure measured from the ETT cuff should be accurate enough for clinical use in the operating room.