A practical procedure for measuring functional residual capacity during mechanical ventilation with or without PEEP.

The measurement of functional residual capacity (FRC) in patients receiving mechanical ventilation may provide valuable data in the assessment and management of acute respiratory failure. Previous descriptions of apparatus and techniques for FRC measurement have either been inapplicable to patients receiving positive end-expiratory pressure (PEEP), or insufficiently detailed to allow convenient duplication in the clinical setting. The authors describe a helium rebreathing method for bedside determination of FRC which can be performed during ventilation with PEEP and which is applicable in patients with prolonged equilibration times. The method is both reproducible in patients (variation from mean FRC: +/- 2.2%) and accurate (coefficient of variation from in vitro FRC of 3000 ml: +/- 1.7%). The apparatus and assembly are described in detail and require only components which are readily available commercially, so that they may be applicable to clinical use in a general hospital.