CLINICAL-EVALUATION OF TRACHEAL PRESSURE ESTIMATION FROM THE ENDOTRACHEAL-TUBE CUFF PRESSURE

Objective. Air flow through an endotracheal tube causes a pressure dro p across the tube. This pressure drop creates a difference between air pressure measured in the trachea and the pressure measured in the bre athing circuit, which can lead to errors when calculating pulmonary me chanics and when setting ventilators. We have developed a method of es timating tracheal pressure from the pressure in the endotracheal tube cuff and tested this system in clinical trials. Methods. Pressure meas urement ports were placed between the Y piece of the ventilator circui t 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 press ures were found at end-inspiration and end-expiration (no flow states) and used to define a linear relationship between cuff pressure and tr acheal pressure. Using the estimated tracheal pressure (P-trach) and t he measured pressure at the Y piece of the breathing circuit (P-Y), th e pressure drop across the ETT was found as a function of how through the tube. Tracheal pressure was then calculated from the flow-dependen t pressure drop and P-Y. Tests of this system were performed in six pa tients in the operating room and six patients in the intensive care un it. Results. The flow-based tracheal pressure estimates were within 0. 7 +/- 0.4 cm H2O of actual tracheal pressure (mean +/- SD). At peak in spiratory pressure the difference averaged 0.5 +/- 0.3 cm H2O. The dif ference between our estimate of tracheal pressure and actual tracheal pressure was always less than 1 cm H2O. Conclusion. The flow-based tra cheal pressure estimates were accurate during intermittent spontaneous breathing, but not during spontaneous breathing or with a poorly infl ated cuff. The estimates were more immune to noise than the cuff-based estimates of tracheal pressure. The estimates of tracheal pressure me asured from the ETT cuff should be accurate enough for clinical use in the operating room.