Core65: Drug-induced adverse reaction: bradycardia / AV block

🫀 High-yield / 要点：Drug-induced bradycardia and AV block are caused by drugs that slow SA/AV nodal conduction or increase vagal tone. Classic causes: β-blockers, verapamil/diltiazem, digoxin, amiodarone.

Definitions / 定義

• Bradycardia:
  • Resting heart rate < 60 bpm.
• AV block:
  • Impaired conduction of the electrical signal between atria and ventricles.

Types of AV block / 房室ブロックの種類

Type	Definition	Key ECG idea
1st degree	Delay in AV nodal conduction	PR > 200 ms
2nd degree	Some impulses fail to conduct	Skipped beats
Mobitz I	Progressive delay before dropped beat	PR gradually lengthens, then one beat is skipped
Mobitz II	More severe conduction failure	PR unchanged before skipped beat
3rd degree	Complete AV dissociation	No conduction from atria to ventricles

Drug causes / 原因薬

Drug class	Examples
Anticholinesterases	Neostigmine, pyridostigmine, physostigmine, donepezil, rivastigmine, galantamine, edrophonium
α2 agonists	Clonidine, guanfacine
β-blockers	Propranolol, esmolol, bisoprolol, labetalol, metoprolol, nadolol, nebivolol
Class III antiarrhythmics	Amiodarone, sotalol, ibutilide, dofetilide
Non-dihydropyridine calcium channel blockers	Diltiazem, verapamil
Cardiac glycosides	Digoxin
Anesthetics	Propofol, bupivacaine

Clinical idea / 臨床的ポイント

• Suspect drug-induced bradycardia/AV block when new slow pulse, dizziness, syncope, or conduction disturbance appears after starting one of these drugs.
• Risk rises when multiple AV nodal blockers are combined.

Remember / 覚え方

• β-blocker + verapamil/diltiazem + digoxin = classic bradycardia/AV block drugs
• Mobitz II and 3rd degree block are more serious
• Drug combinations can worsen conduction block