Urine Studies
Urine Osmolality | Urine Sodium | ||
Hypovolemia | CSW | > 100 mOsm/kg | > 40 mEq/L |
Low solute intake/GI losses | < 100 mOsm/kg | < 40 mEq/L | |
Euvolemia | SIADH | > 100 mOsm/kg | > 40 mEq/L |
Polydipsia | < 100 mOsm/kg | < 40 mEq/L | |
Hypervolemia | CHF/Cirrhosis | > 100 mOsm/kg | < 40 mEq/L |
AKI | < 100 mOsm/kg | > 40 mEq/L |
Impact of Diuretics
- Administration of loop or thiazide diuretics will alter interpretation of UNa
- Expect UNa to be high after diuretic dosing
- Fractional excretion of urea can be helpful to interpret sodium handling in the presence of diuretics
- Urea re-absorbed proximal to site of action of diuretics
- Low FeUrea (<55%) suggests appropriate sodium retention
- High FeUrea (>55%) suggests inappropriate sodium retention
Management
Severe Symptoms
- Seizures, coma, altered mental status possible when sodium very low (< 120 mEq/L)
Give 3% hypertonic saline regardless of etiology
Can bolus 100-250 mL over 15 minutes or start infusion at 25-50 mL/hr
Hypovolemic Hyponatremia
- Isotonic saline functions both as solute replacement and volume replacement
- Normal saline contains 154 mEq/L of sodium, equating to osmolality of 308 mOsm/L
- Administration will replace sodium and volume losses
- Eventually, volume will be restored and volume-related ADH release will be turned off
- Important to correct at a safe level, 6-8 mEq/L/day
Volume resuscitated after 2 liters Volume resuscitated after 5 liters
Hypervolemic Hyponatremia
- Volume restriction
- Symptoms very rare, so aggressive volume restrictions often not needed
- Progression to significant hyponatremia often a very chronic process
- Combination of loop diuresis and ACE inhibitor effective at improving sodium
Other Less Common Causes of Hyponatremia
- Adrenal insufficiency
- Suspect in hypotensive, hypokalemic, hyponatremic patients
- Hydrocortisone supplementation
- Hypothyroidism
- Rare, responds well to LT4
- Hypokalemia
- Reduction in total body potassium will lead to osmotic shift of sodium
- Potassium repletion always indicated when treating hyponatremia