Measurement of functional residual capacity during mechanical ventilation by simultaneous exchange of two insoluble...

A precise method for rapid measurement of functional residual capacity (FRC) during mechanical ventilation that uses the simultaneous exchange of argon and nitrogen is described. Circuit leaks were immediately recognizable upon completion of a run, and pneumotachygraphic inaccuracies due to turbulent flows, changes in viscosity, and time delays between pneumotachygraph and mass spectrometer signals were avoided. For 166 duplicate determinations, the first measurement of FRC differed from the second by 0.5 +/- 0.5 per cent (mean +/- SE). The technique does not affect pulmonary gas exchange. During 35 consecutive determinations of FRC (with an inspired oxygen of 50 per cent), mixed expired oxygen and carbon dioxide tensions varied less than 7 and 1.5 torr, respectively.