A patient chronically on high-dose propranolol is scheduled for general anesthesia. Which agent should be administered with caution?

Administering phenylephrine with caution is warranted for a patient who has been chronically on high-dose propranolol. Propranolol is a non-selective beta-blocker that primarily functions by reducing heart rate and myocardial contractility, and it also leads to vasodilation in some vascular beds.

Phenylephrine is a selective alpha-1 adrenergic agonist that causes vasoconstriction and is often used to increase blood pressure during anesthesia. In a patient who is reliant on beta-blockade from propranolol, the vasoconstriction caused by phenylephrine could lead to hypertension due to the unopposed alpha-1 stimulation. Since the patient's vascular response is altered by the high-dose beta-blocker, the use of phenylephrine may lead to significant increases in systemic vascular resistance, which could risk inducing cardiovascular complications like reflex bradycardia and further blood pressure alterations.

Thus, this makes phenylephrine an agent that should be approached with caution in such scenarios, ensuring that cardiovascular stability is maintained during anesthesia.