Long-Term Clinical Relevance of Hyponatremia Identified During Acute Phase of Myocardial Infarction

Background/Objectives: Hyponatremia is associated with increased mortality in the general hospital population. We sought to investigate whether hyponatremia affects the long-term survival of patients following a myocardial infarction (MI) in both ST-segment elevation (STEMI) and non-ST elevation (NSTEMI) presentations. Methods: In this study, 862 MI patients who were hospitalized between 2012 and 2017 were retrospectively followed-up within the median time period of 41.9 [28.2–73.5] months. All participants were assigned to a hyponatremic or normonatremic group with hyponatremia defined as a sodium level of less than 135 mEq/L on admission. Results: In the acute phase of an MI, hyponatremia was diagnosed in 31 (3.6%) patients. The patients with hyponatremia were less often male (38.7 vs. 70.4%, p < 0.001), and less frequently had Killip class I (63.3 vs. 80%) but more often had Killip class IV on admission (16.7 vs. 4.2%, p = 0.024) and more often had a history of impaired renal function (32.3 vs. 15.5%, p = 0.013) than those with normonatremia. Hyponatremic patients had higher troponin T levels on admission by 75.1% (p = 0.003), a higher isoenzyme MB of creatine kinase level by 34.4% (p = 0.006), and lower hemoglobin (by 8.5%, p = 0.001) levels as compared to the normonatremia group. Long-term mortality was significantly higher in the patients with hyponatremia versus normonatremia (18 [58.1%] vs. 243 [29.2%], log-rank p < 0.001). This was driven by differences in the NSTEMI population (65 vs. 30.5%, p < 0.001). By a Cox proportional hazard regression analysis, hyponatremia was associated with a higher long-term mortality (hazard ratio [HR] of 2.222, a 95% confidence interval [CI] of 1.309–3.773, and p = 0.003). Conclusions: Hyponatremia rarely identified in acute phase of MI was associated with higher long-term mortality, particularly in the NSTEMI population.