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).


METHODS
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.


METHODS
Thirty-three patients (aged 57+/-17 years) mechanically ventilated with PCV and PSV.


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.
Di Marco
Fabiano
F
Università degli Studi di Milano, II Clinica di Malattie dell'Apparato Respiratorio, Ospedale San Paolo, Via A. di Rudinì 8, 20142, Milan, Italy. fabiano.dimarco@unimi.it
Rota Sperti
Lidia
L
Milan
Barbara
B
Stucchi
Riccardo
R
Centanni
Stefano
S
Brochard
Laurent
L
Fumagalli
Roberto
R
D000368:Aged; D005260:Female; D005652:Functional Residual Capacity / Q000502:physiology*; D006371:Helium / Q000008:administration & dosage*; D006785:Hospitals, University; D006801:Humans; D066298:In Vitro Techniques; D007362:Intensive Care Units; D007558:Italy; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D012121:Respiration, Artificial / Q000379:methods*
D006371:Helium
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