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