Assessment of the respiratory exchange ratio in mechanically ventilated patients by a standard anaesthetic gas analyser.

Background: The respiratory exchange ratio (R) is the CO2 production divided with O2 consumption. R is an essential factor included in several formulas during routine blood gas analysis. Instant and individual measurement of R may be of particular interest to improve the evaluation of each single patient. A standard anaesthetic gas analyser has been recommended for measurement of R among spontaneously breathing healthy subjects, but there is no experience using this method among mechanically ventilated critically ill patients. This study validates the assessment of R by a Brüel & Kjaer gas analyser (B & K) during positive pressure ventilation of intensive care patients.

Methods: The B & K sampled gas from 11 mechanically ventilated patients over a period of 5 min. The recordings of end-tidal values of O2 and CO2 based on fractions (RF) allowed for calculation of RF by the alveolar equation solved for R. The continuous recordings of corresponding values were depicted into an O2-CO2 diagram. A developed computer program calculated estimates of R as the slope of the regression lines related to the full cycle (Rfull) and the expiratory phase only (Rexp). Corrected values of the full respiratory cycle (Rfull*) were also calculated assuming changes of CO2 and O2 volumes during gas exchange. The different estimates of R were compared with simultaneous measurement of a Deltatrac indirect calorimeter (Rdelta).

Results: Ten values of RF were within the expected interval of 0.72 < R < 1.00. For the full respiratory cycles, the mean R-value was 0.94 +/- 0.07 and for the expiratory phase the mean R-value was 0.82 +/- 0.08. The O2-CO2 diagram appeared as a convexo-convex loop during each cycle. The agreement of Rexp and Rdelta (Rexp-Rdelta: 0.01 +/- 0.13) were good.

Conclusion: This study demonstrates that gas measurements by a standard anaesthetic gas monitor can be used for determination of R, and thereby we present an alternative to R calculation made by the Deltatrac monitor.