Class II & III Antiarrhythmics

Rapid Review·Cardiovascular

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PHARMACOLOGY

T1Must know

FA P327-328

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Focus on

Beta-blockers as antiarrhythmics, and the potassium channel blockers led by amiodarone.

Key takeaways

Class II, the beta-blockers

Drugs

metoprolol, propranolol, esmolol, atenolol, timolol, carvedilol.

Mechanism

reduce SA and AV nodal activity by lowering cAMP and calcium currents, and suppress abnormal pacemakers by reducing the slope of phase 4, as shown on the class II pacemaker action potential. The AV node is particularly sensitive, which is why the PR interval lengthens. Esmolol is very short acting.

Use

SVT, ventricular rate control in AF and flutter, and prevention of ventricular arrhythmia post-MI.

Adverse

impotence, exacerbation of COPD and asthma, bradycardia, AV block, heart failure, sedation and sleep disturbance, and masking the signs of hypoglycemia.

Metoprolol can cause dyslipidemia. Propranolol can worsen vasospastic angina.

Overdose treatment, by "GAS"

Glucagon, Atropine, Saline. Glucagon works because it bypasses the blocked adrenergic receptor and drives its own G-protein-coupled receptor on the myocyte, restoring rate and contractility.

Class III, the potassium channel blockers

Drugs

Amiodarone, Ibutilide, Dofetilide, Sotalol. "AIDS."

Mechanism

they lengthen AP duration, ERP, and the QT interval, visible on the class III action potential.

Use

atrial fibrillation and flutter, plus ventricular tachycardia for amiodarone and sotalol.

Torsades

sotalol causes torsades and excessive beta blockade; ibutilide causes torsades.

Which class III causes torsades
Drug	Torsades risk
Sotalol	Yes, plus excessive beta blockade
Ibutilide	Yes
Amiodarone	Low, despite lengthening the QT. The notable exception

Amiodarone deserves its own list

Three organs

pulmonary fibrosis, hepatotoxicity, and hypo- or hyperthyroidism, the last because amiodarone is 40% iodine by weight.

As a hapten

corneal deposits and blue-gray skin deposits with photodermatitis.

Also

neurologic effects, constipation, bradycardia, heart block, and heart failure.

Monitoring

check PFTs, LFTs and TFTs before and during use.

Why it is different

it is lipophilic and has class I, II, III and IV effects all at once, and it carries a low torsades risk despite prolonging the QT.

Amiodarone's organ toxicities, and the three sets of labs
Organ	Toxicity	Monitor
Lung	Pulmonary fibrosis	PFTs
Liver	Hepatotoxicity	LFTs
Thyroid	Hypo- OR hyperthyroidism (40% iodine by weight)	TFTs
Eye	Corneal deposits	Slit lamp
Skin	Blue-gray deposits with photodermatitis	Clinical

The four classes side by side

The four antiarrhythmic classes
Class	Blocks	Drugs	Main effect	Primary use
I	Na+ channels	IA quinidine, procainamide, disopyramide; IB lidocaine, mexiletine; IC flecainide, propafenone	Reduce the slope of phase 0	Varies by subclass
II	Beta receptors	Metoprolol, propranolol, esmolol, atenolol, carvedilol	Reduce the slope of phase 4, lengthen PR	SVT, rate control, post-MI
III	K+ channels	Amiodarone, ibutilide, dofetilide, sotalol (AIDS)	Lengthen AP duration, ERP and QT	AF and flutter, VT
IV	Ca2+ channels	Diltiazem, verapamil	Slow conduction, lengthen ERP and PR	Rate control in AF and flutter

What each class does to the ECG (QRS from Na+ block, PR from Ca2+ block or sympathetic block, QT from K+ block)
Drug	QRS	PR	QT
Class IA (procainamide)	Prolonged	No change	Prolonged
Class IB (lidocaine)	No change	No change	No change
Class IC (flecainide)	Prolonged	No change	Prolonged
Class II (metoprolol)	No change	Prolonged	No change
Amiodarone	Prolonged	Prolonged	Prolonged
Sotalol	No change	Prolonged	Prolonged
Other class III (dofetilide)	No change	No change	Prolonged
Class IV (verapamil)	No change	Prolonged	No change

Which three sets of labs does amiodarone demand, and what single fact explains the thyroid one?

PFTs, LFTs, TFTs. It is 40% iodine by weight, so it can push the thyroid either way.

How it's tested

Amiodarone is 40% iodine by weight, which is why it causes both hypo- and hyperthyroidism, and why the drug demands monitoring of PFTs, LFTs and TFTs. The other reliable point is that beta-blockers other than carvedilol and labetalol must not be given alone in pheochromocytoma, since blocking beta leaves alpha-mediated vasoconstriction unopposed. Amiodarone is also the exception among class III drugs in rarely causing torsades despite prolonging the QT.

Go deeper

First Aid 2026 — CV/Pharmacology (p.327-328) · B&B — Class II & III antiarrhythmics · Mehlman — HY Cardio (amiodarone toxicity, beta-blockade in pheochromocytoma)

High-yield images2

Class II (beta-blocker) effect on the pacemaker action potential: the slope of phase 4 spontaneous depolarization is reduced so threshold is reached later and the rate falls, and repolarization at the AV node is prolonged, which is what lengthens the PR interval.

Class III (potassium channel blocker) effect on the ventricular action potential: phase 0 is unchanged, but repolarization through the potassium current is markedly prolonged relative to the untreated grey trace, lengthening the action potential duration, the effective refractory period, and the QT interval.

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What are the 4 typical class III antiarrhythmic drugs?

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