Effects of Endotracheal Tube Leaks on Functional Residual Capacity Determination in Intubated Neonates

Abstract

Summary: The present study evaluates a new closed circuit helium (He) dilution technique for determination of endotracheal (ET) tube leakage and functional residual capacity (FRC) in neonates with ET tubes. By analytically relating the fall in He concentration due to mixing with that due to leakage, it is possible to predict the final equilibration concentration of He and, therefore, correct for ET tube leaks. The system (120 ml) contains an air pump, He meter, breathing bag in cylinder, a strip chart readout, and solenoid valve. Continuous positive airway pressure (CPAP) or ventilator pressure can be applied during testing. FRC measurements were performed on 13 neonates (700–4500 g) on CPAP with ET tubes. Leak rates were significantly higher (P < 0.001) on 3 cm H₂O CPAP compared to 0 cm H₂O CPAP. The mean measured FRC was 53.5 ml at 3 cm H₂O and 46.3 ml at 0 cm H₂2O CPAP. If gas leakage had not been considered in FRC calculations, the error in FRC could have been as high as 39% at 3 cm H₂O CPAP and 18% at 0 cm H₂O CPAP. Speculation: The present, technique provides a high degree of accuracy for determining FRC and ET tube leakage in critically ill infants on CPAP. Although measurements were not conducted on CPAP levels above 3 cm H₂O, it is possible that higher levels of pressure may result in even greater leakage. Under these conditions FRC could only be determined accurately with this method.