Although heatstroke is often associated with dehydration, the clinical significance of serum sodium abnormalities in patients with heat-related illness during heat wave has been poorly documented.

We evaluated 1263 patients (age, 82 ± 15 years; body temperature, 40.1°C + 1.2°C) admitted to emergency departments during the August 2003 heat wave in Paris, having a core temperature greater than 38.5°C and measurement of serum sodium concentrations. Patients were classified according to our previously described risk score of death.

Hyponatremia (<135 mmol/L) was reported in 409 (32%) and hypernatremia (>145 mmol/L) in 220 patients (17%). One-year survival was significantly decreased in patients with hypernatremia (45%; P = .004) but not in those with hyponatremia (58%; P = .86) as compared with patients with serum sodium concentration in the reference range (57%). Using Cox regression, only hypernatremia was an independent prognostic factor (hazard ratio, 1.35; 95% confidence interval, 1.09-1.36) when risk score was taken into account. Using logistic regression, 2 variables were independently associated with hyponatremia (heatstroke severity score and blood urea nitrogen–creatinine ratio <100). Conversely, 5 variables were independently associated with hypernatremia (living in an institution, dementia, serum creatinine >120 μmol/L, a blood urea nitrogen–creatinine ratio >100, and absence of long-term diuretic intake).

Serum sodium abnormalities are frequently observed in patients with a nonexertional heatstroke during heat wave; however, only hypernatremia should be considered as an independent risk factor of death. Rapid measurement of serum sodium concentration is mandatory to appropriately guide electrolyte resuscitation.