• Definition: Serum [Na] > 145 mEq/L
• Is ALWAYS assoc with HYPERTONICITY. It is possible, though, to have hypertonicity without hypernatremia
• Generally, will not develop in an alert person with an intact thrist mechanism and access to water
• Mechanisms which protect against hypernatremia:
• (1) Thirst: activated at P_osm > 290
• (2) ADH secretion: activated at P_osm>280

• (1) Loss of pure water:
• Diabetes insipidus: inability of the kidney to concentrate the urine appropriately due to either absence or deficiency of ADH (central) or unresponsiveness of the kidney to the effects of ADH (nephrogenic)
• Urine: inappropriately dilute, decreased Na⁺ concentration, polyuria
• Diabetes insipidus (CENTRAL): deficiency of ADH (vasopressin) secretion
• Etiology:
• truama
• hypoxic encephalopathy
• vascular lesions
• neoplasms (primary or metastatic)
• encephalitis
• meningitis
• sarcoidosis
• Diabetes insipidus (NEPHROGENIC): inadequate kidney response to ADH
• Etiology:
• congenital
• hypercalcemia
• hypokalemia
• drugs (lithium, demeclocycline)
• Sjorgren’s syndrome
• amyloidosis
• osmotic diuretics
• loop diuretics
• sickle cell anemia
• chronic renal failure
• pregnancy
• fever
• hyperventilation
• Ý ambient temp
• (2) Gain of solute limited to ECF:
• Accidental or deliberate NaCl ingestion
• Sea water ingestion
• NaHCO₃ administration (post-cardiac arrest)
• (3) Loss of hypotonic fluids:
• loss of water and solute (but more water than solute)
• GI losses
• diuretics
• excessive sweating
• burns
• (4) Essential hypernatremia:
• ADH turns on at a higher than usual P_osm (280)

• All causes of hypernatremia are generally associated with either an abnormal mental status, and impaired thirst drive or an inability to get to water (i.e. too old, too sick, too young, NPO, vomiting)
• Generally not hypotensive if there is a pure water loss Þ most loss is from interstitial fluid, not plasma!
• Urine osmolality generally 2x> than that of plasma except when the cause is diabetes insipidus
• Moderate azotemia (uremia) - Ý BUN
• Infants generally have CNS symptoms: lethargy, irritability coma, Ý muscle tone
• Long term neuro deficits: subdural hematoma, adults may not have marked CNS symptoms until [Na] > 170 mEq/L
• Mechanism: Na⁺ concentration Ý as water is lost. Water shifts out of cells to establish osmotic equilibrium Þ brain cells shrink Þ dehydrated, shrunken brain "hangs" by the meninges in the skull Þ tearing of bridging veins Þ intracranial bleeding

• Reason for water loss or sodium gain
• Ý insensible loss (fever, tachypnea)
• Sweat losses
• Diarrhea
• Renal water loss (>3Liters/24hrs of dilute urine)
• Administration of hypertonic Na solutions (iatrogenic)
• Reason for inadequate water intake reardless of source of water loss
• Impaired thirst
• Altered mental status
• primary neurological disorder (stroke, infection, tumor)
• no access to water
• Is polyuria present? Urine volume > 3L/24hr
• Urine osmolality > 300mOsm/L (osmotic diuresis): urea, glucose, mannitol, saline
• Urine Osm <150mOsm/L (diabetic insipidus)
• Response to vasopressin:
• No response: nephrogenic diabetes insipidus
• Urine Osm increases to >300mOsm/L: central diabetes insipidus

Treatment – Replacement of water deficit!!

• Calculate water deficit: H₂O deficit= Total Body Water x ([Na _measured] - [Na _desired])/[Na_desired]
• Total body water – 0.6 x weight (kg)
• Remember ongoing losses of water in the urine
• Calculate urinary electrolyte free water clearance based on urine volume and urine [Na] and [K]
• Electrolyte clearance = [U Na⁺ + U K⁺] V/ [Plasma Na⁺ + Plasma K⁺]
• free water clearance =V- electrolyte clearance
• Remember ongoing insensible losses: 500cc/day if afebrile
• Ask "is there a solute loss as well?"
• Go slowly: replace 50% of the water deficit over 24-48 hours. Hypernatermia results in dehydration of the brain cells and subsequent decrease in their size. Over time, cells make idiogenic osmoles from intracellular proteins Þ intracellular tonicity Ý Þ water shifts back into cells (normalizing their sizes).
• If water is given too fast to correct deficit Þ induce further brain swelling Þ fatal!