Accurate, noninvasive quantitation of expiratory gas leak from uncuffed infant endotracheal tubes

Accurate measurement of expiratory gas leak from uncuffed infant endotracheal tubes is an important requirement for evaluating the rates of metabolic gas exchange by indirect calorimetry in neonates receiving mechanical ventilation. The present study reports a new, noninvasive technique for the efficient collection and analysis of expiratory gases leaked from a closed-circuit metabolic monitoring system recently described. A loose-fitting face mask is placed over the infant's endotracheal appliance and air is entrained from the mask cavity at 100 mL/min for CO2 analysis by infrared capnometry. In vitro calibration of this apparatus demonstrates a relative error of less than 5% of simulated endotracheal tube leak. In vivo application to metabolic rate assessment (using the MGM Jr. metabolic cart) in 12 intubated, ventilated infants ranging from 1.56 to 4.07 kg study weight demonstrated endotracheal tube leaks from 0.49 to 7.40% of net CO2 production, which ranged 10.67 to 70.91 mL/min (or mean 8.22 +/- 0.93 SEM mL/kg/min). The magnitude of tube leakage of mixed expiratory gases could not be predicted from endotracheal tube diameter, ventilator settings, or infant activity or posture. As new instrumentation becomes available to measure the rates of metabolic gas exchange in intubated infants, systematic evaluation of uncuffed endotracheal tube gas leaks becomes critical.