Which vasopressor lowers systemic vascular resistance at low doses and increases it at high doses, with increased cardiac output at all doses and decreased renal blood flow?

Which vasopressor lowers systemic vascular resistance at low doses and increases it at high doses, with increased cardiac output at all doses and decreased renal blood flow?

• A. Dopamine
• B. Norepinephrine
• C. Epinephrine
• D. Isoproterenol

Answer: (C)

Epinephrine fits this pattern because its effects depend on dose. At low doses it mainly activates beta-2 receptors in peripheral vessels, causing vasodilation and a drop in systemic vascular resistance. As the dose rises, alpha-1 receptor stimulation becomes dominant, leading to vasoconstriction and an increase in systemic vascular resistance. Cardiac output rises at all doses due to beta-1 receptor stimulation increasing heart rate and contractility. Renal blood flow decreases because of alpha-1–mediated vasoconstriction of the renal arterioles and the body's tendency to divert blood away from the kidneys during sympathetic activation. This combination—low-dose SVR decrease, high-dose SVR increase, elevated CO across doses, and reduced renal perfusion—best matches epinephrine. Dopamine tends to increase renal flow at low doses, norepinephrine maintains high SVR across doses, and isoproterenol causes vasodilation with increased renal flow, not the described pattern.

Epinephrine fits this pattern because its effects depend on dose. At low doses it mainly activates beta-2 receptors in peripheral vessels, causing vasodilation and a drop in systemic vascular resistance. As the dose rises, alpha-1 receptor stimulation becomes dominant, leading to vasoconstriction and an increase in systemic vascular resistance. Cardiac output rises at all doses due to beta-1 receptor stimulation increasing heart rate and contractility. Renal blood flow decreases because of alpha-1–mediated vasoconstriction of the renal arterioles and the body's tendency to divert blood away from the kidneys during sympathetic activation. This combination—low-dose SVR decrease, high-dose SVR increase, elevated CO across doses, and reduced renal perfusion—best matches epinephrine. Dopamine tends to increase renal flow at low doses, norepinephrine maintains high SVR across doses, and isoproterenol causes vasodilation with increased renal flow, not the described pattern.