What is the mechanism of vasopressin in critical care?

• A. V1 receptor–mediated vasoconstriction; adjunct to catecholamines, may reduce norepinephrine requirements
• B. Increases heart rate
• C. Causes vasodilation via V2
• D. Blocks renin release

Answer: (A)

Vasopressin works in critical care by activating V1 receptors on vascular smooth muscle to produce vasoconstriction. This raises systemic vascular resistance and mean arterial pressure, making it valuable in vasodilatory shock (like septic shock) when blood pressure is low despite fluids and catecholamines. As an adjunct to catecholamines, it can allow using lower doses of norepinephrine, potentially reducing side effects from high-dose catecholamines while still maintaining perfusion.

It’s helpful to remember that the V2 receptor handles water reabsorption in the kidneys, not vasoconstriction, so the vasopressor effect isn’t due to V2. Vasopressin doesn’t primarily act by blocking renin release, and its main hemodynamic action isn’t to increase heart rate.