The effect of Airway Leak on Tidal Volume during Pressure- Or Flow-Controlled ventilation of the Neonate: A Model Study

Abstract

Leaks associated with uncuffed endotracheal tubes (ETTs) complicate mechan-ical ventilation of the neonate. The variable nature of leaks and the possibility of different modes of ventilation suggest an optimal strategy for stability of tidal volume (VT). This study evaluated the effect of leak on Vr during pressure-con-trolled ventilation (PCV) or flow-controlled ventilation (FCV). METHOD: We used a lung model consisting of an elastic bellows (compliance = 1 mL/cm H2O) attached to a 3.0-mm ETT with a 3-way stopcock. One port of the stopcock was attached to a fixed-orifice leak (% leak = 19-34% depending on ventilator set-tings). A ventilator generated either constant inspiratory pressure (as in PCV) or constant inspiratory flow (as in FCV) at the same Vr. Inspired and expired volumes were measured with a neonatal monitor. Calibration was verified with a syringe. The effect of leak on exhaled tidal volume (VE) was defined as: $\%\Delta {\text{V}}_{\text{E}}=\frac{{\text{V}}_{\text{E}}\left(\text{leak off}\right)-{\text{V}}_{\text{E}}\left(\text{leak}\text{on}\right)}{{\text{V}}_{\text{E}}\left(\text{leak}\text{off}\right)}\times 100$ Large (≈15 mL) and small (≈7 mL) Vrs were used with long (0.8 s) and short (0.3 s) inspiratory times (Tis). Each combination was evaluated 3 times (n = 3). Differences in %△VE were evaluated with unpaired t tests. A theoretical model was derived to generalize results. RESULTS: PCV resulted in a significantly smaller leak effect for all conditions except small Vr and short TIs. The largest difference was seen with small Vrs and long TIs (leak effect for FCV = 44% vs PCV = 18%). The theoretical model predicts that for FCV, % AVE increases as TIs increases. For PCV, % AVE at first increases as TIs increases and then decreases to a constant value. For both FCV and PCV, gas exchange will prob-ably be most stable with short TIs in the presence of a leak. CONCLUSION: An optimum strategy for dealing with intermittent airway leaks might be short TIs and relatively large Vrs independent of mode. At small Vrs and long TIs, ventilation should be more stable with PCV. [Respir Care 1996;41(8):728-735]