A Method for Measuring Functional Residual Capacity in Neonates with Endotracheal Tubes

This study evaluates a new 60-s closed circuit helium (He) dilution technique for determination of functional residual capacity (FRC) in intubated neonates independent of small gas leaks present around uncuffed endotracheal (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 value. Continuous positive airway pressure (CPAP) or ventilator pressure can be applied during testing. One hundred in vitro measurements of FRC ranging from 5-100 cc in both leak and nonleak models were performed and were accurate to within ±7.8% standard deviation. Functional residual capacity measurements were also performed in 30 infants (weight 600-4400 gm).