Pathophysiology:
Hyponatremia is a disorder of water imbalance
Hyponatremia means excess of body water compared to total body sodium and potassium. Â Also likely to involve ADH (vasopressin), the hormone involved in water balance.
Total osmolality: concentration of all solutes in a given weight of water (mOsm/kg) regardless of whether or not osmoles move across biological membranes
Effective osmolality: number of osmoles that contribute to water movement between intracellular and extracellular compartments
Most hyponatremia is from low effective osmolality (hypo-osmotic/hypotonic hyponatremia), but occasionally, hyponatremia occurs in isotonic or hypertonic serum because other active osmoles are present.
ACUTE vs CHRONIC: Â
Is there a documented change in sodium within 48 hours?
Basis: when there is rapid hyponatremia (often from excess water), brain takes about 24-28 hours to reduce number of osmotically active particles within its cells to restore brain volume to normal (ie, avoid swelling)
Role of vasopressin (anti-diuretic hormone):
osmo-receptors in hypothalamus sense effective osmolality; baroreceptors in carotid sinus, left atrium, aortic arch will sense what the effective volume is, based on the input from the above receptors, body increase/inhibit ADH secretion accordingly.
How to evaluate hyponatremia
- Assess Volume status: Is the patient volume overloaded, depleted, or euvolemic?
- Assess Osmolality (hyper, iso, hypo): Is the blood concentrated?
For hypotonic hyponatremia, continue to 3 rd step - Assess Urinary sodium excretion and FeNa %: Is the urine concentrated?
*Remember VOU – volume status, osmolality, and urine studies
Will Fluid restriction work ?
If you decide that patient is Euvolemic or hypervolemic, and would like to try fluid restriction, use Urine ( Na + K ) to evaluate the probability of responsiveness to fluid restriction.
If Serum Na > Urine (Na + K), it means patient's kidney is still able to clear FREE water. Â And thus patient's hyponatremia will response to fluid restriction
If Serum Na < Urine (Na + K), it means patient's kidney cannot clear FREE water and patient need more solute. Â Patient need increase salt intake (IV v.s. PO)
Hyponatremia v.s. solute intake
Minimal urine osmolarity ~ Â 50 mOsm/kg of water
If a person take 650 mOsm daily, one can drink up-to 650/50 (minimal urine osmolarity, i.e. maximal free water excretion) = 13 L of water without causing hyponatremia.
If a person has potomania (beer drinker's hyponatremia), Â only intake about 200 mOsm daily. Â Any water intake > 200/50 = 4L will cause hyponatremia.
Edited by Dr. Tan Nguyen, The God of Jacobi RISC
Â