Relationship between functional residual capacity, respiratory compliance, and oxygenation in patients ventilated after cardiac surgery

BACKGROUND: Measurement of functional residual capacity (FRC) is now possible at bedside, during mechanical ventilation. OBJECTIVES: To 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. & METHODS: We 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 (FIO2). We indexed the FRC values to predicted FRC reference values from sitting and supine healthy volunteers. RESULTS: Correlation analyses revealed no meaningful association between FRC and PaO2 /FIO2 (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. CONCLUSIONS: Indexing the measured FRC values to the predicted supine and sitting FRC values does not improve the association between PaO2/FIO2 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).