Background: Myocardial ischemia–reperfusion during cardiac surgery is associated with complex metabolic alterations that reflect both myocardial injury and recovery. This prospective study aimed to characterize intraoperative lactate, pH, and bicarbonate dynamics in paired coronary sinus (CS) and peripheral blood samples and to explore whether these responses could be integrated into a composite marker of reperfusion-related metabolic burden (RMB).

Methods: We prospectively studied 101 patients undergoing cardiac surgery with cardiopulmonary bypass. Simultaneous blood samples from the CS and peripheral circulation were obtained before aortic cross-clamping (T0), immediately after declamping (T1), and 10 min after reperfusion (T2). Temporal changes and transmyocardial gradients of lactate, pH, and bicarbonate were analyzed. RMB was calculated from standardized changes in these variables. As an exploratory analysis, associations with in-hospital mortality were evaluated using receiver operating characteristic (ROC) analysis.

Results: Significant temporal variations in metabolic parameters were observed, with the most pronounced transmyocardial disturbances occurring at aortic declamping. Lactate demonstrated the largest gradient during early reperfusion, accompanied by transient acidosis and bicarbonate consumption. The RMB framework integrated these coordinated metabolic responses into a single measure of reperfusion stress. In exploratory outcome analyses, higher RMB values were observed among non-survivors, with the largest observed AUC for peripheral RMB measured 10 min after reperfusion (AUC 0.87). However, these estimates were based on only seven deaths and should be considered hypothesis-generating.

Conclusions: Paired CS and peripheral sampling revealed dynamic metabolic adaptations during myocardial ischemia–reperfusion. The exploratory RMB framework integrates coordinated metabolic changes into a single descriptive measure of reperfusion-related stress. Its observed associations with in-hospital mortality remain preliminary and require confirmation through external validation and evaluation in larger independent prospective cohorts.