• Hermann Heinze, Beate Sedemund-Adib, Matthias Heringlake, Torsten Meier, and Wolfgang Eichler.
• Department of Anesthesiology, University of Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany. hermannheinze@uk-sh.de
• Resp Care. 2010 May 1;55(5):589-94.

BackgroundMeasurement of functional residual capacity (FRC) is now possible at bedside, during mechanical ventilation.ObjectivesTo determine the relationship of measured absolute and relative predicted FRC values to oxygenation and respiratory-system compliance, and to identify variables that influence FRC in ventilated patients after cardiac surgery.MethodsWe retrospectively analyzed data from 99 patients ventilated after cardiac surgery. Each patient underwent an alveolar recruitment maneuver and was then ventilated with a positive end-expiratory pressure of 10 cm H2O and a tidal volume of 6-8 mL/kg predicted body weight. We measured quasi-static 2-point compliance of the respiratory system, FRC (with the oxygen-wash-out method), PaO2, and fraction of inspired oxygen (F(I)O2). We indexed the FRC values to predicted FRC reference values from sitting and supine healthy volunteers.ResultsCorrelation analyses revealed no meaningful association between FRC and PaO2/F(I)O2 (r2 0.20, P < .001). There was a moderate association between absolute FRC and respiratory-system compliance (r2 0.50, P < .001). Indexing the absolute measured FRC values to the predicted FRC values did not improve the correlation. We conducted multiple linear regression analyses of height, weight, age, sex, presence of mild chronic obstructive pulmonary disease, minute volume, and peak inspiratory pressure during ventilation, and revealed weight, minute volume, and peak inspiratory pressure (r2 = 0.65) as independent covariates of FRC.ConclusionsIndexing the measured FRC values to the predicted supine and sitting FRC values does not improve the association between PaO2/F(I)O2 and respiratory-system compliance. In mechanically ventilated patients after cardiac surgery, FRC is influenced more by the ventilator settings than by physiologic variables (as in spontaneously breathing persons).

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