An elevated respiratory quotient predicts complications after cardiac surgery under extracorporeal circulation: an observational pilot study

Following cardiac surgery, hyperlactatemia due to anaerobic metabolism is associated with an increase in both morbidity and mortality. We previously found that an elevated respiratory quotient (RQ) predicts anaerobic metabolism. In the present study we aimed to demonstrate that it is also associated with poor outcome following cardiac surgery. This single institution, prospective, observational study includes all those patients that were consecutively admitted to the intensive care unit (ICU) after cardiac surgery with cardiopulmonary bypass, that had also been monitored using pulmonary artery catheter. Data were recorded at admission (H0) and after one hour (H1) including: oxygen consumption (\({\text{VO}}_{2}\)), carbon dioxide production (\({\text{VCO}}_{2}\)), RQ (\({\text{VO}_{2}}/{\text{VCO}_{2}}\)), lactate levels and mixed venous oxygen saturation (\({\text{SvO}}_{2}\)). The primary endpoint was defined as mortality at 30 days. Comparison of the area under the curve (AUC) for receiver operating characteristic curves was used to analyze the prognostic predictive value of RQ, lactate levels and \({\text{SvO}}_{2}\), in terms of patient outcome. We studied 151 patients admitted to the ICU between May 2015 and February 2016. Seventy eight patients experienced a worse than expected outcome in the post-operative period, and among those seven died. RQ at H1 in non-survivors (\(0.83\pm 0.08\)) was higher than in survivors (\(0.75\pm 0.09\); p = 0.02). The AUC for RQ to predict mortality was 0.77 (IC_95% [0.70–0.84]), with a threshold value of 0.76 (sensitivity 64%, specificity 100%). By comparison, the AUC for lactate levels was significantly superior (AUClact 0.89, IC_95% [0.83–0.93], p = 0.02). In this study, elevated RQ appeared to be predictive of mortality after cardiac surgery with CPB.