A comparison of postoperative blood lactate concentrations and kinetics in cardiac surgical patients being and not being administered metformin

Aim. Early discontinuation of metformin before cardiac surgery is advised by several national societies but no hard evidence exist supporting this practice. This precaution is mostly extrapolated by data on different clinical settings. The aim of this study is to investigate the impact of preoperative metformin use on lactate concentrations and lactate clearance during the first postoperative day after cardiac surgery.Methods. Among 367 consecutive patients who underwent elective on-pump cardiac surgery from January 2019 to October 2019, 109 were diabetics, 74 of which were treated with metformin. Data on lactate concentrations and clearance during the first postoperative day were prospectively collected on arrival, H6, H12 and H24 in the ICU and compared. A subgroup analysis focusing only on diabetic patients was also performed. Repeated measures multivariate analysis of variance (MANOVA) was used to investigate the data based on group, time and their interaction effects.Results. Lactate concentrations were the same for both groups upon arrival in the ICU. Interestingly, metformin users had lower lactate concentrations than non-users on the following measurements (p = 0.003 at 6 h and p = 0.01 at 24 h). No significant interaction was found between the two groups (p = 0.76). No difference was found between the two groups in terms of lactate clearance (p = 0.53). In the subgroup analysis no difference was found between metformin users and non-users neither on lactate concentrations (p = 0.61) nor on lactate clearance (p = 0.86).Conclusions. In a post cardiac surgery ICU setting, using metformin up until the night before surgery was not associated with increased postoperative lactate concentrations or impaired lactate clearance.