Pharmacology · single question
A patient taking PROPRANOLOL (beta-blocker) is given lidocaine with epinephrine. What drug interaction occurs?
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Correct answer: B. Propranolol increases lidocaine toxicity by reducing cardiac output and competing for hepatic enzymes
✓ B) Propranolol increases lidocaine toxicity via two mechanisms.
Beta-blockers + amide LA (lidocaine): (1) Propranolol decreases cardiac output → decreased hepatic blood flow → REDUCED lidocaine metabolism → increased plasma lidocaine levels → toxicity risk; (2) Propranolol and lidocaine compete for the SAME hepatic CYP enzymes (CYP2D6, 1A2) → less lidocaine metabolism → prolonged action and toxicity. Additionally, non-selective beta-blockers (propranolol) + epinephrine = hypertensive crisis (epinephrine's beta-2 vasodilatory effect is blocked, leaving unopposed alpha-1 vasoconstriction). Use CARDIOSELECTIVE beta-blockers (atenolol) or limit epi.
Beta-blockers + amide LA (lidocaine): (1) Propranolol decreases cardiac output → decreased hepatic blood flow → REDUCED lidocaine metabolism → increased plasma lidocaine levels → toxicity risk; (2) Propranolol and lidocaine compete for the SAME hepatic CYP enzymes (CYP2D6, 1A2) → less lidocaine metabolism → prolonged action and toxicity. Additionally, non-selective beta-blockers (propranolol) + epinephrine = hypertensive crisis (epinephrine's beta-2 vasodilatory effect is blocked, leaving unopposed alpha-1 vasoconstriction). Use CARDIOSELECTIVE beta-blockers (atenolol) or limit epi.