The effect of time and warming on breathing circuit compliance

BACKGROUND: Some microprocessor-controlled ventilators correct the gas volume delivered to the patient to compensate for volume lost due to the compliance of the breathing circuit (C(BC)). Because C(BC) is usually calculated only when the circuit is initially placed in use, changes in C(BC) that occur over time for various reasons may result in inaccurate correction of delivered tidal volume (V(T)). The purpose of this study was to determine whether C(BC) changed with time. MATERIALS AND METHODS: We measured the compliance of 3 different types of breathing circuits (BCs) attached to a ventilator set at V(T) = 1.0 L, f = 12, V̇(max) = 60 L/min, and F(IO₂) = 0.21. After an initial measurement of compliance, the BC were heated to 34°C inside an isolette, connected to a test lung, and ventilated for 48 hours. The compliance of the test lung was adjusted so that peak inspiratory pressure was 50 cm H₂O. Five samples of each BC were tested. Compressible volume was measured at 1, 3, 6, 12, 24, and 48 hours by injecting air from a calibration syringe into the BC (with all ports capped) until pressure reached 100 cm H₂O. The exhalation valve was opened and the released volume was measured with a calibration analyzer. The study was then repeated with the circuits maintained at room temperature. Data were analyzed using analysis of variance and Turkey's honestly significant differences tests with the level of significance set at 0.05. RESULTS: There was no significant difference in C(BC) of the Hudson ethylene vinyl acetate circuit (mean [SD] 1.50 [0.08] mL/cm H₂O, p = 0.26) and Puritan-Bennett polyethylene circuit (1.47 [0.06] mL/cm H₂O, p = 0.24) over 48 hours. C(BC) increased from 2.31 (0.06) mL/cm H₂O to 2.52 (0.06) mL/cm H₂O during the first hour in the Hudson kraton circuit, and the difference was significant (p < 0.001, 1 hour vs time zero); there was no significant difference after the 1 hour measurement (p = 0.67, 1 hour vs all subsequent time periods). There was no significant difference in calibration of the unheated circuits. CONCLUSION: Although we found a statistically significant increase in the compliance of the Hudson kraton circuit, there were no clinically important changes in the compliance of any of the breathing circuits over a 48-hour period. It may not be necessary to recalculate breathing circuit compliance for the purpose of correcting V(T) delivered to the patient after the initial setup.