Oxygen wash-in method for monitoring functional residual capacity

Atelectasis, pulmonary edema, fibrosis, pneumothorax, and mucous plug airway obstruction all result in reduced lung volume. The oxygen (O2) wash-in method provides a way to monitor routinely the functional residual capacity (FRC) in the ICU without disconnecting the patient from the ventilator and without additional personnel or instrumentation. This method is a modification of an open-circuit nitrogen (N2) wash-out procedure and requires a computer-based respiratory monitoring system with a fast response O2 analyzer and respiratory flowmeter. FRC is computed after a 20% or greater change in the ventilator FIO2 setting. The accuracy and reproducibility of the method were evaluated using artificial lungs, normal subjects, and postcardiac surgery patients. FRC estimates by O2 wash-in and helium dilution were highly correlated, with r = 0.97 and a regression slope and zero intercept of 1.06 and -0.13, respectively. The FRC difference between 23 repeated trials in 18 postcardiac surgery patients was 70 +/- 160 ml (mean +/- SD).