Measurement of functional residual capacity by sulfur hexafluoride in small-volume lungs during spontaneous breathing and mechanical ventilation.

We modified a sulfur hexafluoride (SF6) washout technique to allow functional residual capacity (FRC) determinations in small-volume lungs both during spontaneous breathing and controlled mechanical ventilation. This method facilitates measurements in subjects who attempt spontaneous breaths between ventilator-generated breaths. We wished to confirm the accuracy and precision of the measurements and the method's sensitivity to change. The method uses a pneumotach together with a fast, mainstream infrared SF6 sensor mounted between the endotracheal tube and the ventilator circuit. A low flow of pure SF6 is delivered into the constant gas flow of the ventilator circuit to wash in tracer gas at a concentration of less than 2%. The flow signal and the instantaneous SF6 concentration is processed on-line by a computer. The calibration of the SF6 sensor's nonlinear signal and the ability of the flow sensor to reflect flow values precisely near zero flow had a major impact on the accuracy of the FRC estimate. This accuracy was tested by comparing measured FRC values with a dummy lung's true FRC that was varied from 7 to 70 mL. The comparison differed by 0.7 +/- 3.2% (mean +/- SD; range, -5.1 to 7.8%). As a measure of reproducibility (precision) across 20 FRC determinations in five adult rabbits, the average coefficient of variation was 1.7% (range, 0.57 to 4.33%) during continuous positive airway pressure and 1.98% (range, 0.35 to 3.81%) during controlled mechanical ventilation. The method proved sensitive to changes in FRC related to changes in airway pressure.(ABSTRACT TRUNCATED AT 250 WORDS)