Measurement of functional residual capacity by helium dilution during partial support ventilation: in vitro accuracy and in vivo precision of the method

OBJECTIVE: Measurement of functional residual capacity (FRC) during controlled and especially during assisted ventilation remains a challenge in the physiological evaluation of ventilated patients. To validate a bag-in-box closed helium dilution technique allowing measurements both during pressure-controlled (PCV) and pressure-support ventilation (PSV). DESIGN AND SETTING: Experimental study on lung models containing different volumes, and measurements in patients in the intensive care unit of a university hospital. In patients measurements were performed in duplicate during controlled and assisted ventilation. PATIENTS: Thirty-three patients (aged 57+/-17 years) mechanically ventilated with PCV and PSV. MEASUREMENTS AND RESULTS: In the lung model assessment of accuracy showed an overall mean difference between FRC measurements and lung model volume of 0.5% (2 SD 5.7%). In patients assessment of repeatability showed a bias between duplicate FRC measurements of -1+/-70 ml (95% CI -141 to +139 ml). The coefficient of variation was of 3.2% for all measurements with a comparable repeatability in PSV and PCV mode (coefficient of variation of 3.4 and 3.2%, respectively). During the rebreathing period a small reduction in tidal volume (-8.5+/-5.4%) and mean airway pressure (-2.3+/-4.7%) was observed with only a 0.3 cmH2O mean increase in PEEP and no change in respiratory rate and I/E ratio. CONCLUSIONS: This specifically designed closed helium dilution bag-in-box technique allows accurate FRC measurement with good repeatability during both partial PSV and PVC without exposing patients to disconnection and changes in PEEP.