Aims <p>The stress hyperglycemia ratio (SHR) has been associated with adverse outcomes in patients with cardiovascular diseases. Recent studies have also linked higher SHR to increased mortality after cardiac surgery, including analyses using the MIMIC-IV database, and in patients undergoing CABG for acute myocardial infarction. Therefore, the present study should be viewed as an incremental and exploratory analysis rather than as a wholly novel investigation. However, the dose–response pattern and clinically relevant risk thresholds of SHR in an undifferentiated extracorporeal circulation (ECC)-assisted open-heart surgery population remain incompletely characterized. Given the uncertain perioperative timing of glucose measurements in MIMIC-IV, SHR was interpreted as an aggregated prognostic marker rather than a direct measure of intraoperative or immediate postoperative metabolic stress. This study aims to investigate the association between SHR and short-term mortality risk in this patient cohort.</p> Methods <p>A retrospective cohort study was conducted by analyzing data from 1,221 patients who underwent extracorporeal circulation during open-heart surgery, obtained from the MIMIC-IV (version 3.1) database. Patients were divided into quartiles based on SHR levels. Cox proportional hazards models, including a segmented variant, were used to evaluate the association between SHR and 180-day mortality. Glucose and HbA1c values were obtained from available laboratory records within the first 24&#xa0;h of ICU admission; however, precise timing relative to surgery, ECC initiation, or ICU arrival could not be standardized. Important intraoperative parameters, including cardiopulmonary bypass duration and aortic cross-clamp time, were not available as reliable structured variables and therefore could not be adjusted for. In addition, a substantial proportion of potentially eligible patients were excluded because SHR could not be calculated owing to missing glucose or HbA1c measurements, which may have introduced selection bias.</p> Results <p>Within the study cohort, 63 patients (5.16%) died within 180&#xa0;days. Following multivariable adjustment, SHR showed a nonlinear, threshold-like association with 180-day mortality, with an estimated inflection point at 0.97. The increase in mortality risk was more clearly observed at SHR levels above this point, whereas the association below this point was not statistically significant. Therefore, the spline-derived point should be interpreted as exploratory rather than as a clinically validated cutoff or treatment target. Compared to participants with SHR levels below the inflection point, those with higher SHR levels exhibited a fourfold increased risk of 180-day mortality (HR 4.62; 95% CI 2.67–7.95).</p> Conclusion <p>Our findings indicate that an elevated SHR, measured by the glucose/HbA1c ratio, is associated with an increased risk of short-term mortality in patients undergoing extracorporeal circulation during open-heart surgery. Because of the retrospective design, lack of precise perioperative glucose timing, substantial exclusion of patients with missing glycemic data, uncertain completeness of post-discharge mortality ascertainment, and inability to adjust for key intraoperative variables, these findings should be interpreted as prognostic and hypothesis-generating rather than causal. The observed nonlinear pattern may help refine risk stratification, but the proposed inflection point requires validation in prospective studies with standardized perioperative glucose monitoring.</p>