A Simplified Method to Determine Functional Residual Capacity during Mechanical Ventilation

Abstract

We describe a nonrecirculating helium-rebreathing method providing rapid and simple measurement of the functional residual capacity (FRC) in spontaneously breathing subjects and patients receiving mechanical ventilation. Results of triplicate determinations in 24 normal subjects revealed a repeatability of +/- 5.2 percent of the measured FRC, results similar to those predicted for these subjects. Results in six additional subjects were not significantly different from those obtained with a standard method using helium equilibration in the pulmonary function laboratory. Triplicate determinations in eight patients after cardiac surgery demonstrated similar repeatability. In 22 other patients studied after open-heart surgery, the FRC fell more in those who could not be weaned than in those who could be weaned from the respirator. Since an increase in FRC is the goal of therapy with positive end-expiratory pressure, this method should be useful in determining the efficacy of a particular level of positive end-expiratory pressure.