Heat-associated hyponatremia and its outcomes in very old inpatients in northern Germany

Background Hyponatremia is frequent in very old patients ≥ 80 years (VOP). Data for hyponatremia associated with moderate heat in VOP are scarce. We aimed to provide epidemiological data, to identify risk factors of hyponatremia and to present outcomes in VOP after hot and cold days in northern Germany. Methods Of 22,085 VOP admitted unplanned to the departments of internal medicine and trauma surgery in 2011–2023, we studied those admitted 1–2 days after hot ( N = 308) and cold days ( N = 253) in June-August of these years (57.9% female, median age 86 years). Heat was defined as daily mean temperatures ≥ 95th (24.4 °C), cold as ≤ 5th percentile (12.3 °C) in June-August from 2011–2023. Primary outcome was hyponatremia on admission. Secondary outcomes were risk factors of hyponatremia and the frequency of major adverse hyponatremia events until day 30 (MAHE30, intensive care unit and/or long hospital stay, mortality and/or discharge to a care facility). Results In 561 patients, hyponatremia occurred more often after hot than cold days (36.7% vs. 15.0%; P < 0.001). Hyponatremia was predominately mild, acute or acute on chronic and isotonic in patients admitted after both hot and cold days. Hyponatremia rates increased by 50% after 3 consecutive hot days. Heat was a strong risk factor of hyponatremia. In 151 hyponatremic patients, MAHE30 occurred more often after hot compared to cold days (55.6% vs. 31.6%; P = 0.008). Heat was identified as a strong risk factor also of MAHE30 in hyponatremic patients. Conclusion Moderate heat in northern Germany is associated with an increased rate of hyponatremia and is a strong risk factor of hyponatremia in VOP. After hot days, hyponatremic VOP are at an increased risk for severe associated morbidity and mortality.