A patient with cardiac disease takes metoprolol. What is the principal concern when administering a local anaesthetic containing epinephrine?

Correct answer: B. A potential interaction between the beta blocker and the vasoconstrictor

• A. Beta blockade does not reduce the anaesthetic efficacy of the local anaesthetic molecule.
• B. CORRECT as the keyed concern. The classic and clinically important interaction is between NON-SELECTIVE beta blockers such as propranolol and nadolol and epinephrine: beta-2 receptors are blocked, leaving alpha-1 vasoconstriction unopposed, which can produce a marked hypertensive response with reflex bradycardia.
• C. Methaemoglobinaemia is a concern with prilocaine and benzocaine, not with beta blockers.
• D. Amide anaesthetic metabolism is hepatic, but metoprolol does not meaningfully impair it.

Note: ⚑ Important nuance the source did not draw out: METOPROLOL IS CARDIOSELECTIVE (beta-1 selective), so the unopposed alpha effect is much less pronounced than with a non-selective agent. The dangerous interaction is with PROPRANOLOL, NADOLOL, TIMOLOL and other non-selective blockers. With any of them, limit epinephrine to about 0.04 mg (roughly 2 cartridges of 1:100,000), aspirate carefully, inject slowly, and monitor blood pressure. Cardioselectivity is also dose-dependent and diminishes at higher doses.