II. Pathophysiology

  1. Decreased Total Body Water (TBW)
  2. Normal Total Body Sodium
  3. Normal Extracellular Fluid

III. Causes: Extra-renal Water Loss

  1. Findings
    1. Urine Osmolality increased
  2. Causes
    1. Skin losses
    2. Respiratory losses
      1. Mechanical Ventilation or Hyperventilation
    3. Iatrogenic Example of excess Sodium administration
      1. Febrile, tachypneic patient
      2. Hypotonic insensible loss replaced with 0.9% saline
    4. Rhabdomyolysis
      1. Damaged cells extract water from ECF
    5. Sickle Cell Anemia

IV. Causes: Renal Water Loss

  1. Central Diabetes Insipidus
    1. Secondary to CNS injury
    2. Desmopressin results in return of urinary concentrating function
  2. Nephrogenic Diabetes Insipidus
    1. Desmopressin does not result in improvement of urinary concentrating function
    2. Results from nephrotoxic medications (e.g. Amphotericin, Lithium, demeclocycline)

V. Management

  1. Mild to moderate Hypernatremia
    1. Increase free water intake
  2. Sodium correction (moderate to severe Hypernatremia)
    1. Calculate Free Water Deficit
    2. Replace Free Water Deficit with D5W over 48 hours
    3. Correction rate
      1. Acute: 1 mEq/hour
      2. Chronic: 0.5 mEq/hour (do not decrease Sodium >8-10 mEq in 24 hours)
    4. Monitor electrolytes closely while administering D5W
      1. Serum Sodium
      2. Serum Osmolality
        1. Do not decrease faster than 1-2 mOsm/kg water/hour
    5. Initial: Restore extracellular fluid volume
      1. Administer Normal Saline (0.9%)
    6. Next: Correct Serum Sodium
      1. Administer half Normal Saline (0.45%)
  3. Treat underlying renal causes
    1. Losses from fever or Mechanical Ventilation
  4. Treat underlying renal causes
    1. Central Diabetes Insipidus
      1. Replace ADH (Desmopressin)
    2. Nephrogenic Diabetes Insipidus
      1. Treat primary problem (e.g. withdraw offending agent)

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