Predicting postoperative complications with the respiratory exchange ratio after high-risk noncardiac surgery

BACKGROUND The respiratory exchange ratio (RER), defined as the ratio of CO 2 production (VCO 2 ) to O 2 consumption (VO 2 ), is reported to be a noninvasive marker of anaerobic metabolism. The intubated, ventilated patient's inspired and expired fractions of O 2 and CO 2 (FiO 2 , FeO 2 , FiCO 2 and FeCO 2 ) are monitored in the operating room and can be used to calculate RER. OBJECTIVE To investigating the ability of the RER to predict postoperative complications. DESIGN An observational, prospective study. SETTING Two French university hospitals between March 2017 and September 2018. PATIENTS A total of 110 patients undergoing noncardiac high-risk surgery. MAIN OUTCOME MEASURES The RER was calculated as (FeCO 2 − FiCO 2 )/(FiO 2 − FeO 2 ) at five time points during the operation. The primary endpoint was at the end of the surgery. The secondary endpoints were systemic oxygenation indices (pCO 2 gap, pCO 2 gap/arteriovenous difference in O 2 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. RESULTS Postoperative 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 VO 2 ( 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). CONCLUSION As a putative noninvasive marker of tissue hypoperfusion and anaerobic metabolism, the RER can be used to predict complications following high-risk surgery. TRIAL REGISTRATION ClinicalTrials.gov identifier: NCT03471962.