[Measurement of functional residual capacity by sulfur hexafluoride washout]

An open circuit tracer gas washout method for measurement of functional residual capacity (FRC) during mechanical ventilation is described and tested. The system employed a piezo-valve for dispensing sulfur hexafluoride (SF6) gas, a fast response infrared SF6 analyzer, Servo 900-C ventilator and a computer. The piezo-valve unit delivers SF6 into the airway in proportion to instantaneous inspiratory flow so that inspiratory SF6 concentration was held constant regardless of the inspiratory flow pattern. The washin concentration was approximately 0.6% which was so low that the supply of other gases was hardly influenced. The amount of SF6 at the end of a washin was calculated during washout by signals of expired SF6 concentration and expired flow. The results obtained in model lungs were accurate and reproducible; mean difference (limits of agreements) was 14 +/- 48ml (M +/- 2SD) in the range between 0.5 and 3.0 liters. Comparison with helium dilution methods in 24 healthy subjects gave a regression equation: y = 0.98x + 56, r = 0.98. The mean difference of the values between the two methods was 21 +/- 142 ml (M +/- 2SD). The authors conclude that the system is quite useful in determining FRC of the patients under mechanical ventilation.