Hypernatremia

Definition

• A serum sodium above 145mmol/L, usually indicative of a free-water deficit

Free Water Deficit

$$ FWD = \text{free water deficit} \ TBW = \text{total body water} $$

$$ TBW_{man} = 0.6 \times weight \ TBW_{woman} 0.5 \times weight $$

$$ FWD_total = \frac{Na_{current} - 140}{140} \times TBW \ FWD_{24h} = \frac{10}{Na_{current} - 10} \times TBW $$

Differential Diagnosis

• Hypervolemic (increased salt intake)
  • Hypertonic saline or bicarb
  • Salt-water drowning
• Eu- or hypovolemic (increased free water loss)
  • Appropriate urinary concentration, with osmolality >600-800 mOsm/L
    • Insensible losses (fever, exercise, heat, burns)
    • Gastrointestinal water loss (diarrhea)
    • Remote renal water loss
  • Inappropriate urinary dilution, with osmolality <300-600 mOsm/L
    • Urine osmoles > 750 mOsm/d
      • Osmotic diuresis (e.g. mannitol)
    • Urine osmoles ≤ 750 mOsm/d: diabetes insipidus
      • ddAVP increases urine osmolality
        • Central diabetes insipidus
      • ddAVP does not increase urine osmolality
        • Nephrogenic diabetes insipidus

Investigations

• Electrolytes & creatinine
• Urine lytes, osmolality, to rule out diabetes insipidus
• Follow serum and urine lytes and osmolality daily or more frequently

Management

• Treat underlying cause, and correct free water deficit
• Correction rate
  • No more than 10 mmol daily, in general
  • Usually corresponds to 1-2x maintenance fluids of 1/2 NS
  • If acute <48h, can correct by up to 1mmol/h (24mmol daily)
• Monitor urine and sodium lytes, urine osmolality, and urine output while correcting
  • If correcting too quickly, switch fludis
• In cases of diabetes insipidus
  • For central DI: ddAVP
  • For nephrogenic DI from lithium: amiloride 2.5-10mg/d