Estimation of tracheal pressure and imposed expiratory work of breathing by the endotracheal tube, heat and moisture exchanger, and ventilator during mechanical ventilation.

BACKGROUND: The resistance of the endotracheal tube (ETT), the heat and moisture exchanger (HME), and the ventilator may affect the patient's respiratory status. Although previous studies examined the inspiratory work of breathing (WOB), investigation of WOB in the expiratory phase is rare. We estimated tracheal pressure at the tip of the ETT (Ptrach) and calculated expiratory WOB imposed by the ETT, the HME, and the expiratory valve. We examined imposed expiratory WOB in patients under a continuous mandatory ventilation (CMV) mode and during spontaneous breathing trials (SBTs). We hypothesized that imposed expiratory WOB would increase with heightened ventilatory demand.

METHODS: We measured airway pressure (Paw) and respiratory flow (V). We estimated Ptrach using the equation Ptrach = Paw - K1 × V(K2) - 2.70 × V(L/s)(1.42). K1 and K2 were determined by the inner diameter (ID) of the ETT. Imposed expiratory WOB was calculated from the area of Ptrach above PEEP versus lung volume. We examined imposed expiratory WOB and imposed expiratory resistance in relation to mean expiratory flow.

RESULTS: We examined 28 patients under CMV mode, and 29 during SBT. During both CMV and SBT, as mean expiratory flow increased, imposed expiratory WOB increased. The regression curves between mean expiratory flow (x) (L/s) and imposed expiratory WOB (y) (J/L) were y = 1.35x(0.83) (R(2) = 0.79) for 7 mm ID ETT under CMV, y = 1.12x(0.82) (R(2) = 0.73) for 8 mm ID ETT under CMV, y = 1.07x(1.04) (R(2) = 0.85) for 7 mm ID ETT during SBT, and y = 0.84x(0.93) (R(2) = 0.75) for 8 mm ID ETT during SBT. Levels of imposed expiratory WOB were affected by ETT diameter and ventilator mode. The reason for increasing imposed expiratory WOB was an increase in expiratory resistance imposed by the ETT and HME.

CONCLUSIONS: Under mechanical ventilation, imposed expiratory WOB should be considered in patients with higher minute ventilation.