Which drug is used as a nonselective beta-agonist to increase heart rate and cardiac output?

• A. Norepinephrine
• B. Dobutamine
• C. Isoproterenol
• D. Propantheline

Answer: (C)

Nonselective stimulation of beta receptors in the heart and vessels increases heart rate and cardiac output. Isoproterenol activates both beta-1 receptors in the heart (raising heart rate and contractility) and beta-2 receptors in vascular smooth muscle (causing some vasodilation). This combination leads to a noticeable increase in heart rate and overall cardiac output, which is why it fits as the nonselective beta-agonist option.

Norepinephrine mainly drives alpha-1–mediated vasoconstriction with some beta-1 activity; the strong rise in blood pressure can trigger reflex mechanisms that blunt or limit heart rate increase, so it’s not the best choice for increasing heart rate and CO via beta stimulation. Dobutamine is predominantly a beta-1 agonist (with some beta-2 effects) and mainly boosts contractility with a smaller effect on heart rate, so it’s not nonselective. Propantheline is an antimuscarinic; it alters parasympathetic tone rather than acting on beta receptors, so it doesn’t work by beta receptor stimulation.