Association of Admission Hyponatremia With Clinical Severity and In-Hospital Outcomes in ST-Elevation Myocardial Infarction: A Single-Centre Retrospective Observational Study

Introduction Hyponatremia, defined as a serum sodium concentration below 135 mEq/L, is the most common electrolyte disorder encountered in clinical practice. In the context of ST-elevation myocardial infarction (STEMI), haemodynamic stress, neurohormonal activation, and diuretic use may predispose patients to hyponatremia. The precise relationship between hyponatremia and clinical severity in STEMI remains incompletely characterised, particularly in South Asian populations. The objective of the study is to determine the prevalence of hyponatremia in patients admitted with STEMI and to evaluate its association with markers of disease severity and in-hospital mortality. Methods A single-centre retrospective observational study reviewed 196 consecutive STEMI admissions. Hyponatremia was defined as serum sodium <135 mEq/L on admission. Clinical severity was assessed using the Killip classification, left ventricular ejection fraction (LVEF), cardiogenic shock, and ICU length of stay. In-hospital mortality was the primary outcome. Comparisons used the independent samples t-test, chi-square test, and one-way ANOVA, with Spearman rank correlation and parsimonious logistic regression as secondary analyses. Results Hyponatremia was identified in 54 of 196 patients (27.6%). Binary hyponatremia classification was not significantly associated with categorical severity endpoints or mortality; however, lower serum sodium levels demonstrated a weak but significant inverse correlation with ICU stay duration (Spearman r = −0.156; p = 0.030). The left circumflex artery (LCX) was significantly more frequent as the culprit vessel in hyponatraemic patients (11/54, 20.4% vs 6/142, 4.2%; p = 0.002). Serum sodium levels demonstrated statistically significant variation across Killip categories without a clear monotonic severity gradient (one-way ANOVA: F = 5.830; p = 0.001). Hyponatremia was not an independent predictor of in-hospital mortality on logistic regression (OR 1.08; 95% CI 0.39-3.02; p = 0.884). Conclusion Hyponatremia affects 54 of 196 patients (27.6%) in this STEMI cohort. Binary hyponatremia classification was not significantly associated with categorical severity endpoints or in-hospital mortality; however, lower serum sodium levels demonstrated a weak but significant inverse correlation with ICU stay duration, and LCX-territory infarcts were more prevalent among hyponatraemic patients. Prospective multicentre studies are warranted.