Beta-Blocker and Calcium Channel Blocker Overdose
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Direct answer
Beta-blocker and calcium channel blocker overdose causes bradycardia, heart block and shock by suppressing cardiac pacing and contractility. Beta-blocker toxicity mainly produces bradycardia and cardiogenic shock, while calcium channel blocker toxicity can cause vasodilatory shock with hyperglycaemia. Treatment depends on the agent and includes glucagon for beta-blockers, calcium salts, high-dose insulin euglycaemia therapy and vasopressors.
What you must remember
- Beta-blocker toxicity: bradycardia, hypotension, heart block and cardiogenic shock; hypoglycaemia is possible, and sotalol additionally prolongs the QT interval.
- Lipophilic agents: propranolol, with membrane-stabilising effects at high doses, can cause seizures and QRS widening in addition to shock.
- Glucagon: raises cyclic AMP through receptors that beta-blockers do not block, so it is the first-line antidote for beta-blocker-induced shock.
- Dihydropyridines: amlodipine and nifedipine cause peripheral vasodilatory shock with reflex tachycardia, with relatively little bradycardia.
- Non-dihydropyridines: verapamil and diltiazem cause bradycardia, AV block and negative inotropy, which is the pattern that suits calcium therapy.
- Hyperglycaemia: calcium channel blocker overdose impairs insulin release from pancreatic beta cells, so raised glucose is a diagnostic clue.
- High-dose insulin euglycaemia: insulin with dextrose improves myocardial glucose use and contractility, and glucose and potassium must be checked closely.
- Calcium salts: given intravenously to overcome channel blockade, often through a central line for sustained treatment.
A stepwise approach to shock in these overdoses
Both drug classes lead to shock, but the agent in the history decides which antidote comes first.
- Support the circulation: give intravenous fluids and try atropine for bradycardia, although atropine is often ineffective in beta-blocker toxicity.
- Pace early if needed: transcutaneous or transvenous pacing is used when bradycardia or heart block does not respond to drugs.
- Give glucagon for beta-blockers: an infusion is titrated to the heart rate and blood pressure response, since glucagon acts independently of beta receptors.
- Give calcium for channel blockers: intravenous calcium salts are given early, followed by high-dose insulin euglycaemia therapy if the shock persists.
- Add vasopressors: norepinephrine, epinephrine or vasopressin support blood pressure in vasoplegic or cardiogenic shock.
- Monitor and escalate: glucose and potassium are checked frequently during insulin therapy, and lipid emulsion or specialist referral is considered for refractory shock.
Reassess the rhythm, blood pressure and glucose after every intervention, because the picture changes quickly. A patient who responds to glucagon or calcium still needs observation, since the effect can wear off while the drug persists, especially with extended-release products.
Where the exam tries to catch you
Most lost marks come from matching the wrong antidote to the drug class.
- Trap: treating hyperglycaemia in a calcium channel blocker overdose as diabetes Correct: calcium channel blockade impairs insulin release, so the raised glucose is a toxic clue and insulin therapy is what helps.
- Trap: expecting atropine to correct beta-blocker bradycardia Correct: atropine is often ineffective, so glucagon and pacing are the mainstays.
- Trap: assuming amlodipine overdose causes marked bradycardia Correct: dihydropyridines cause peripheral vasodilatory shock with reflex tachycardia, whereas verapamil causes bradycardia and AV block.
- Trap: starting high-dose insulin without monitoring Correct: insulin therapy requires close glucose and potassium checks, because hypoglycaemia and hypokalaemia can follow.
Frequently asked questions
Why is glucagon used in beta-blocker overdose?
Glucagon is used in beta-blocker overdose because it raises cyclic AMP through receptors that beta-blockers do not block. This increases heart rate and contractility, which makes it the first-line antidote for beta-blocker-induced shock.
Why does calcium channel blocker overdose cause hyperglycaemia?
Calcium channel blocker overdose causes hyperglycaemia because the drug impairs calcium-dependent insulin release from pancreatic beta cells. The raised glucose is a diagnostic clue and also a sign that high-dose insulin therapy may help.
What is high-dose insulin euglycaemia therapy?
High-dose insulin euglycaemia therapy gives insulin with dextrose to improve myocardial glucose use and restore contractility in calcium channel blocker or beta-blocker shock. Blood glucose and potassium need close and frequent monitoring, because hypoglycaemia and hypokalaemia can develop.
How do dihydropyridine and non-dihydropyridine overdoses differ?
Dihydropyridines such as amlodipine cause peripheral vasodilatory shock with reflex tachycardia, whereas verapamil and diltiazem cause bradycardia, heart block and negative inotropy. The clinical pattern guides whether vasopressors or calcium and insulin therapy come first.
Why is atropine often ineffective in beta-blocker bradycardia?
Atropine is often ineffective in beta-blocker bradycardia because the slowing comes from beta-receptor blockade rather than vagal tone. Glucagon and cardiac pacing are therefore the mainstays, with atropine tried but not relied upon.