Tolvaptan, a vasopressin V2 receptor antagonist (vaptans), is specifically used in hyponatremia. Which type of hyponatremia does it NOT effectively treat?
- A Euvolemic SIADH (inappropriate ADH secretion with normal volume status)
- B Hypervolemic hyponatremia from congestive heart failure
- C Hypovolemic hyponatremia (true sodium depletion with volume depletion) — tolvaptan causes aquaresis without replacing lost sodium, potentially worsening volume status ✓
- D Hypervolemic hyponatremia from cirrhosis with ascites
Explanation
Tolvaptan is a selective V2 vasopressin receptor antagonist that blocks ADH action on collecting duct principal cells, promoting free water excretion (aquaresis) without sodium loss. It is effective for euvolemic hyponatremia (SIADH) and hypervolemic hyponatremia (heart failure, cirrhosis) — conditions where ADH is inappropriately elevated relative to osmolality. Hypovolemic hyponatremia is a contraindication: in this condition, high ADH is an appropriate compensatory response to volume depletion. Blocking V2 receptors would cause further free water loss, worsening volume depletion, potentially triggering reflex tachycardia, hypotension, and further RAAS activation. Treatment of hypovolemic hyponatremia requires isotonic saline to restore volume.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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