Hypernatremia of critical illness

Hypernatremia of critical illness is an common and important complication that affects close to 7% of intensive care unit (ICU) patients. The term describes the development of hypernatremia that is not present at admission but occurs during the patient’s illness in the ICU. Given that hypernatremia appears preventable by the administration of electrolyte-free water, either enterally or intravenously, it is surprising that this condition should develop in the ICU where monitoring of serum sodium is universal and frequent. In this article, we review the epidemiology of this condition, the pathophysiological background behind the development of such ICU-acquired hypernatremia and the risk factors that predispose to its development. In particular, we focus on the role of urinary free water losses associated with the catabolic state of patients and the large amounts of urea excreted in the urine. We further discuss the role of diarrhea and fever in exacerbating such free water losses. We identify the importance of diuretics in this setting, where they often fail to achieve natriuresis and further increase free water losses. We highlight the importance of estimating urinary free water losses by measurement of urinary sodium and potassium. We explain the misleading nature of urinary osmolarity measurements in a setting where osmolarity and tonicity are dissociated by the presence of urea in urine. Finally, we discuss the importance of prevention or rapid correction of hypernatremia and the evidence supporting particular rates of correction.