Association of Perioperative Shock Indices with In-Hospital Mortality and Postoperative Morbidity After Coronary Artery Bypass Grafting: A Retrospective Cohort Study

Background/Objectives: Shock index (SI) and modified shock index (MSI) are simple hemodynamic indices derived from routinely available vital signs, but their perioperative prognostic relevance in coronary artery bypass grafting (CABG) remains incompletely characterized. This study evaluated perioperative changes in SI, MSI, and their age-adjusted derivatives and explored their associations with in-hospital mortality and postoperative outcomes after elective isolated CABG. Methods: This retrospective single-center study included 245 adults who underwent elective isolated CABG between 1 January 2023, and 1 May 2025. SI, MSI, Age-SI, and Age-MSI were calculated at seven perioperative time points. The primary outcome was in-hospital mortality; prespecified secondary outcomes included AKI, stroke, ICU length of stay, and mechanical ventilation duration, while additional postoperative morbidity outcomes were explored. Discrimination was assessed using receiver operating characteristic analysis. Exploratory models evaluated post-cardiopulmonary bypass (post-CPB) SI and MSI in combination with EuroSCORE II, with Firth penalized regression and bootstrap internal validation used as sensitivity analyses because of the limited number of deaths. Results: In-hospital mortality occurred in 12/245 patients (4.9%). Post-CPB measurements were available in 243 patients, including 10 non-survivors. Among the evaluated time points, the strongest observed mortality discrimination occurred post-CPB: SI had an AUC of 0.798 (95% CI 0.621–0.974) and MSI an AUC of 0.793 (95% CI 0.606–0.980). After adjustment for EuroSCORE II, both post-CPB SI (OR 1.30 per 0.1-unit increase, 95% CI 1.09–1.56; p = 0.004) and MSI (OR 1.23 per 0.1-unit increase, 95% CI 1.07–1.41; p = 0.003) remained associated with mortality. Addition of either index increased the apparent AUC from 0.798 to 0.830 and improved model fit; however, the absolute increase in discrimination was modest, and bootstrap confidence intervals for ΔAUC included zero. Firth penalized regression yielded similar effect estimates, and optimism-corrected AUCs were 0.824 for EuroSCORE II + SI and 0.829 for EuroSCORE II + MSI. Age-adjusted indices showed exploratory associations with several postoperative morbidity outcomes. Conclusions: Post-CPB SI and MSI were associated with in-hospital mortality among patients who reached the post-CPB assessment point and remained associated after adjustment for EuroSCORE II. Although their addition improved model fit, incremental improvement in discrimination was modest and uncertain. These readily available indices may serve as supportive markers of perioperative hemodynamic instability rather than standalone prognostic tools. Given the limited number of mortality events, these findings should be considered exploratory and require external validation.