Sodium dominates the chemistry of the body’s extracellular fluid, accounting for roughly ninety percent of its osmolality. The plasma sodium concentration therefore serves as a sensitive barometer of the balance between sodium and water intake and excretion. Hypernatremia, defined as a plasma sodium concentration exceeding 145 millimoles per liter, arises whenever there is a net gain of sodium, a deficit of free water, or a combination of both, and it is invariably accompanied by hyperosmolality. The review also highlights a subtlety that laboratory reports obscure: because only about ninety-three percent of plasma volume is actually water, the physiologically active sodium concentration is slightly higher than the measured value. In hyperglycemic patients, osmotic water shifts from inside cells to the extracellular space can dilute the measured sodium and mask the true severity of the disturbance.

Under normal circumstances, the body defends its osmolality with remarkable precision. Osmoreceptors in the hypothalamus detect even small rises in plasma osmolality and trigger two compensatory responses: the release of antidiuretic hormone from the posterior pituitary, which increases water reabsorption in the collecting tubules of the kidney, and the activation of thirst. Antidiuretic hormone secretion begins when plasma osmolality exceeds approximately 275 to 285 milliosmoles per kilogram, while thirst is triggered at a slightly higher threshold. Hypovolemia independently stimulates hormone release, and expanded blood volume mildly suppresses it. Crucially, even when the kidney achieves maximal urinary concentration, it cannot offset insensible water losses without additional intake, making an intact thirst response indispensable. Intensive care dismantles this defense: sedation, intubation, and impaired consciousness silence thirst, creating a dysregulated regulatory loop in which patients cannot signal their own dehydration.

The scale of the problem is substantial. ICU-acquired hypernatremia affects between six and forty-seven percent of patients depending on the definition, timing of diagnosis, and clinical setting, and multiple cohorts have linked it independently with increased mortality. The elderly are particularly vulnerable because aging raises the osmotic threshold for thirst and reduces baroreceptor sensitivity. The causes divide into two broad categories: free water deficits caused by reduced intake or excessive losses, and positive sodium balance caused by sodium administration or impaired sodium excretion. Non-renal water losses include vomiting, diarrhea, insensible losses, and drainage from surgical sites, while renal losses can follow osmotic diuresis driven by non-reabsorbed solutes such as glucose, mannitol, or urea, or impaired urinary concentrating ability in diabetes insipidus, tubular dysfunction, or recovering acute kidney injury.

Among the underappreciated culprits, the authors single out urea-mediated osmotic diuresis. Increased urea generation associated with burn injury, sepsis, polytrauma, high-protein feeding, or recovery from acute kidney injury elevates urine osmolality and can quietly increase electrolyte-free water losses. Exogenous glucocorticoids, widely used in septic shock, may compound the problem by enhancing urea-mediated osmotic diuresis and possibly suppressing antidiuretic hormone secretion. On the sodium side of the ledger, inappropriate renal retention can result from mineralocorticoid excess, including glucocorticoid therapy, while in shock states the expected suppression of the renin-angiotensin-aldosterone system may be blunted by hemodynamic instability and sympathoadrenergic activation, perpetuating sodium retention. Notably, studies in polytrauma patients have observed reduced urinary sodium and chloride excretion before hypernatremia developed, suggesting that impaired natriuresis is not merely a consequence but may be an early warning sign.

Iatrogenic sodium loading deserves equal scrutiny. Every sodium-containing intravenous fluid contributes to the daily sodium load regardless of its tonicity, and therapeutic hypertonic saline can raise serum sodium rapidly. Less obvious contributors include certain antibiotics such as fosfomycin, trisodium-citrate anticoagulation during continuous renal replacement therapy, sodium bicarbonate, enteral and parenteral nutrition, and drug-dilution fluids prepared in 0.9 percent saline, which contains 154 millimoles of sodium per liter. Even potassium administration, particularly during correction of major potassium deficits, can raise serum sodium by causing intracellular water shifts. These insights reframe hypernatremia prevention as a matter of auditing the total daily sodium burden rather than simply reacting to a laboratory number.

The physiological consequences extend well beyond the laboratory values. Hypernatremia impairs insulin-mediated glucose uptake and glucagon-dependent glucose release, contributing to hyperglycemia. In the central nervous system, osmotic fluid shifts can shrink brain cells and damage cerebral vessels, potentially producing neurological deficits or delirium. Experimental data further suggest that hyperosmolality may impair myocardial contractility through the induction of proinflammatory cytokines, hinting that elevated osmolality could compound the cardiovascular dysfunction already present in critically ill patients. These systemic effects help explain the consistent association between hypernatremia and death across observational cohorts, though the authors are careful to note that association does not prove causation.

Diagnostically, the review prescribes a structured approach anchored in assessment of volume status and urine production. Calculation of the free water deficit and free water clearance helps determine the appropriate volume and rate of fluid replacement, with the free water deficit computed from total body water and the difference between the current sodium concentration and a target of 140 millimoles per liter. In critically ill patients, the authors argue that electrolyte-free water clearance, which incorporates urinary sodium and potassium concentrations, may more accurately reflect ongoing water losses than conventional free water clearance, particularly when urea- or glucose-driven osmotic diuresis is suspected. This distinction matters because a patient losing electrolyte-free water through an osmotic diuresis needs a very different fluid strategy than one retaining sodium.

Treatment, the review emphasizes, must target the underlying cause rather than applying a one-size-fits-all water infusion. In hypovolemic patients, resuscitation with isotonic fluids takes priority, with free water added only once hemodynamic stability is achieved. In euvolemic or hypervolemic patients with adequate urine output, the focus shifts to quantifying and reducing the daily sodium load. Replacing 0.9 percent saline drug solvents with glucose-based solutions, adopting low-sodium maintenance fluid strategies, and limiting fluid creep have all been shown to lower the incidence of hypernatremia, although drug compatibility and stability constraints must be considered when changing diluents. Perhaps most provocatively, the long-standing teaching that serum sodium should be corrected at a rate below 0.5 millimoles per liter per hour has been challenged by a large analysis suggesting that faster correction may be safe and associated with shorter hospitalization and decreased mortality, a finding likely to spark debate among nephrologists and intensivists.

Pharmacological options remain limited but evolving. Hydrochlorothiazide was tested as an adjunct for ICU-acquired hypernatremia in a single small randomized controlled trial that showed no significant effect. A separate trial of forty fluid-overloaded ICU patients found that adding indapamide to furosemide produced larger natriuresis, 210 versus 119 millimoles of sodium, with similar urine volumes, and the furosemide-only group, unlike the combination group, experienced a rise in serum sodium after twenty-four hours. Mineralocorticoid receptor antagonists can induce natriuresis and represent a potential therapy, though clinical data are lacking. Animal work has shown that free water improves sodium mobilization in furosemide-treated pigs after a hyperosmotic sodium load, supporting combined strategies, while renal replacement therapy remains an option when free water administration risks fluid overload. For diabetes insipidus, management depends on etiology: nephrogenic cases call for correcting electrolyte disturbances and reversible causes, whereas central diabetes insipidus typically responds to desmopressin. The authors conclude that hypernatremia in the critically ill demands a thorough evaluation of fluid status, electrolytes, and urine chemistry, grounded in a clear understanding of the underlying physiology, if outcomes are to improve.

Subject of Research: ICU-acquired hypernatremia and its mechanisms, diagnosis, and management in critically ill patients

Article Title: Hypernatremia in the critically Ill: beyond free water replacement

Article References: Paraskevas, T., Ostermann, M., & Joannidis, M. (2026). Hypernatremia in the critically Ill: beyond free water replacement. Intensive Care Medicine. https://doi.org/10.1007/s00134-026-08566-4

Image Credits: AI Generated

DOI: 10.1007/s00134-026-08566-4

Keywords: hypernatremia, ICU-acquired hypernatremia, sodium regulation, antidiuretic hormone, osmoreceptors, free water deficit, osmotic diuresis, electrolyte-free water clearance, fluid management, critical care, diabetes insipidus, intensive care medicine