• Stéphane Bar, Charles Grenez, Maxime Nguyen, Bruno de Broca, Eugénie Bernard, Osama Abou-Arab, Belaid Bouhemad, Emmanuel Lorne, and Pierre-Grégoire Guinot.
• From the Anaesthesiology and Critical Care Department, Amiens University Hospital, Amiens (SB, CG, BdB, OA-A, EL) and Anaesthesiology and Critical Care Department, Dijon University Hospital, Dijon, France (MN, EB, BB, P-GG).
• Eur J Anaesthesiol. 2020 Nov 1; 37 (11): 1050-1057.

BackgroundThe respiratory exchange ratio (RER), defined as the ratio of CO2 production (VCO2) to O2 consumption (VO2), is reported to be a noninvasive marker of anaerobic metabolism. The intubated, ventilated patient's inspired and expired fractions of O2 and CO2 (FiO2, FeO2, FiCO2 and FeCO2) are monitored in the operating room and can be used to calculate RER.ObjectiveTo investigating the ability of the RER to predict postoperative complications.DesignAn observational, prospective study.SettingTwo French university hospitals between March 2017 and September 2018.PatientsA total of 110 patients undergoing noncardiac high-risk surgery.Main Outcome MeasuresThe RER was calculated as (FeCO2 - FiCO2)/(FiO2 - FeO2) at five time points during the operation. The primary endpoint was at the end of the surgery. The secondary endpoints were systemic oxygenation indices (pCO2 gap, pCO2 gap/arteriovenous difference in O2 ratio, central venous oxygen saturation) and the arterial lactate level at the end of the surgery. Complications were classified according to the European Peri-operative Clinical Outcome definitions.ResultsPostoperative complications occurred in 35 patients (34%). The median [interquartile range] RER at the end of surgery was significantly greater in the subgroup with complications, 1.06 [0.84 to 1.35] than in the subgroup without complications, 0.81 [0.75 to 0.91], and correlated significantly with the arterial lactate (r = 0.31, P < 0.001) and VO2 (r = -0.23, P = 0.001). Analysis of the area under the receiver operating characteristic curve for the predictive value of RER for postoperative complications revealed a value of 0.77 [95% confidence interval (CI) 0.69 to 0.88, P = 0.001]. The best cut-off for the RER was 0.94, with a sensitivity of 71% (95% CI 54 to 85) and a specificity of 79% (95% CI 68 to 88).ConclusionAs a putative noninvasive marker of tissue hypoperfusion and anaerobic metabolism, the RER can be used to predict complications following high-risk surgery.Trial RegistrationClinicalTrials.gov identifier: NCT03471962.

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