Abstract
BACKGROUND: Airway pressure is usually measured by sensors placed in the ventilator or on the ventilator side of the endotracheal tube (ETT), at the Y-piece. These remote measurements serve as a surrogate for the tracheal or alveolar pressure. Tracheal pressure can only be predicted correctly by using a model that incorporates the pressure at the remote location, the flow through the ETT, and the resistance of the ETT if the latter is a predictable function of Y-piece flow. However, this is not consistently appropriate, and accuracy of prediction is hampered. METHODS: This in vitro study systematically examined the ventilator pressure in dependence of compliance of the respiratory system (C(RS)), inspiratory time, and expiratory time during pressure-controlled ventilation by using a small intratracheal pressure sensor and a mechanical lung simulator. Pressures were measured simultaneously at the ventilator outlet, at the Y-piece, and in the trachea during pressure-controlled ventilation with a peak inspiratory pressure of 20 cm H(2)O and a PEEP of 5 cm H(2)O while changing C(RS) (10, 30, 60, 90, and 100 mL/cm H(2)O) and varying inspiratory time and expiratory time. RESULTS: Tracheal pressures were always lower (maximum 8 cm H(2)O during inspiration) or higher (maximum 4 cm H(2)O during expiration) than the pressures measured proximal to the ETT if zero-flow conditions were not achieved at the end of the breathing cycles. CONCLUSIONS: Dependent on C(RS) and the breathing cycle, tracheal pressures deviated from those measured proximal to the ETT under non-zero-flow conditions. Intratracheal pressure and pressure curve dynamics can differ greatly from the ventilator pressure, depending on the ventilator setting and the C(RS). The small pressure sensor may be used as a measurement method of tracheal pressure via integration onto an ETT.