Advanced Pharmacology · Topic 7 of 13

Antiarrhythmic Drugs

Pharmacology · rhythm drugs

Antiarrhythmic Drugs

Match the drug to the rhythm and the heart: adenosine for regular narrow SVT, amiodarone or procainamide for VT — and every antiarrhythmic can also cause arrhythmias.

Key numbers

Adenosine 6 mg, then 12 mg6 mg → 12 mg

Arrest: amiodarone 300 mg, then 150 mg300 mg → 150 mg

Amiodarone with a pulse: 150 mg/10 min150 mg / 10 min

01 · GiveAdenosine 6 mg, then 12 mg

For stable regular narrow-complex SVT after vagal maneuvers fail, give adenosine 6 mg by rapid IV push with an immediate saline flush; if the rhythm persists, give 12 mg.

02 · GiveArrest: amiodarone 300 mg, then 150 mg

For shock-refractory VF or pulseless VT, give amiodarone 300 mg IV/IO bolus, then 150 mg; lidocaine 1–1.5 mg/kg, then 0.5–0.75 mg/kg, is the alternative.

03 · Watch forAmiodarone toxicity: lungs, thyroid, liver, eyes, skin

Long-term amiodarone causes pulmonary toxicity (up to 17%, fatal in about 10%), hepatotoxicity, hypo- or hyperthyroidism, corneal deposits and optic neuropathy, photosensitivity and blue-gray skin, bradycardia and QT prolongation.

What it is

Core idea

Vaughan Williams: Na, beta, K, Ca blockersVaughan Williams classes: I sodium channel blockers (Ia procainamide, Ib lidocaine, Ic flecainide/propafenone), II beta-blockers, III potassium channel blockers (amiodarone, sotalol, dofetilide, ibutilide), IV diltiazem and verapamil.

Diagnosis & monitoring

Work-up

Start sotalol inpatient on telemetry, dose by CrClStart or restart sotalol in hospital for at least 3 days with continuous ECG monitoring, set the dosing interval by creatinine clearance, and reduce or stop it if the QT reaches 500 ms.

Correct K and Mg; check QTc firstBefore starting amiodarone, sotalol or another QT-prolonging antiarrhythmic, obtain a baseline ECG and correct hypokalemia and hypomagnesemia; class Ia and III drugs can cause torsades de pointes.

Amiodarone baseline: CXR, PFTs, TFTs, LFTsBefore long-term amiodarone, obtain a chest x-ray, pulmonary function tests with diffusion capacity, thyroid tests and transaminases; repeat the chest x-ray every 3–6 months, TSH every 6 months, and regular eye exams.

Ibutilide: telemetry for 4 hours afterIbutilide converts recent-onset AF or flutter but can cause torsades: monitor the ECG continuously during and for at least 4 hours after the infusion, and avoid it with a long QT or low LVEF.

Management

Orders

Amiodarone with a pulse: 150 mg/10 min must knowFor VT with a pulse, give amiodarone 150 mg IV over 10 minutes (repeat if VT recurs), then 1 mg/min for 6 hours, then 0.5 mg/min; hypotension is its most common IV adverse effect.

Procainamide 20–50 mg/min to a set endpointProcainamide 20–50 mg/min IV until the arrhythmia stops, hypotension develops, the QRS widens more than 50% or 17 mg/kg is given, then 1–4 mg/min; avoid with a prolonged QT or heart failure.

Red flags & don'ts

Avoid harm

Pre-excited AF: no AV-nodal blockers avoidIn pre-excited AF (WPW), adenosine, beta-blockers, diltiazem, verapamil, digoxin and IV amiodarone can precipitate VF; cardiovert if unstable, otherwise give IV procainamide or ibutilide.

Procainamide: lupus, agranulocytosis, torsades complicationProcainamide causes hypotension during infusion, a positive ANA and drug-induced lupus, agranulocytosis (about 0.5%) and torsades; its active metabolite NAPA is renally cleared and accumulates in kidney failure.

Lidocaine toxicity is neurologic complicationLidocaine (class Ib) acts on ventricular, not atrial, arrhythmias; toxicity is neurologic — tremor, paresthesias, drowsiness, delirium, seizures — and it undergoes extensive hepatic metabolism, so reduce the dose after 24 hours.

No sotalol with long QT, low K, asthma avoidSotalol (beta-blocker plus class III) is contraindicated with bradycardia, second- or third-degree AV block, long QT (baseline QT > 450 ms for AF), potassium < 4 mEq/L, decompensated heart failure and asthma.

Amiodarone: cut warfarin by one-third to one-half interactionAmiodarone inhibits warfarin and digoxin metabolism: reduce warfarin by one-third to one-half and follow the INR, halve the digoxin dose, and limit simvastatin to 20 mg daily.

Amiodarone thyroid disease: 37% iodine complicationAmiodarone is about 37% iodine and causes thyroid dysfunction in 15–20%: treat hypothyroidism with levothyroxine without stopping amiodarone; hyperthyroidism may be iodine-induced (thionamide) or destructive thyroiditis (prednisone).

No class IC drugs with structural heart disease avoidFlecainide and propafenone raise mortality after MI and in significant structural heart disease or HFrEF; when used, pair them with an AV-nodal blocker to prevent 1:1 atrial flutter conduction.

No verapamil or diltiazem in wide-complex tachycardia avoidVerapamil and diltiazem are harmful in wide-complex tachycardia (they can precipitate VF and shock) and in AF with LVEF under 40% or decompensated heart failure because of negative inotropy.

Adenosine: avoid in high-grade block and bronchospasm avoidAdenosine is contraindicated in second- or third-degree AV block or sick sinus syndrome without a pacemaker and avoided in active bronchospasm; dipyridamole, heart transplant and central lines potentiate it, while theophylline and caffeine blunt it.

Amiodarone or procainamide?

Commonly confused

Class	III (with I, II and IV effects)
Stable VT dose	150 mg over 10 min, then 1 mg/min
Stop or avoid when	Hypotension, bradycardia, AV block
Pre-excited AF	Harmful (blocks the AV node)
Long-term toxicity	Lung, thyroid, liver, eyes, skin

More detail

Reference

Show 7 reference facts

Give IV amiodarone through a central venous catheter when possible, with an in-line filter; peripheral concentrations above 3 mg/mL cause a high rate of phlebitis.

Starting sofosbuvir-based hepatitis C therapy in a patient on amiodarone has caused symptomatic bradycardia requiring pacemakers; avoid combining amiodarone with other QT-prolonging drugs.

Dofetilide is contraindicated with a prolonged QTc or reduced GFR; dronedarone should not be used with heart failure or permanent AF.

Torsades is not treated with class Ia, Ic or III drugs; give magnesium sulfate 2 g IV, and lidocaine (class Ib), which shortens the QT, may help drug-induced torsades.

Digoxin slows AV-nodal conduction through vagal effects and is contraindicated in manifest WPW; its toxicity is covered on the Digoxin Toxicity sheet.

Adenosine has a half-life under 10 seconds; expect flushing, chest discomfort, dyspnea and a few seconds of asystole before conversion, and warn the patient beforehand.

Avoid combining amiodarone, procainamide and sotalol for VT (additive proarrhythmia); amiodarone also raises procainamide, quinidine and flecainide levels, so reduce those doses by 30–50%.

Pearl

Amiodarone works on almost every rhythm and harms almost every organ: lungs, thyroid, liver, eyes and skin.

Sources · Merck Manual Professional — Medications for Arrhythmias (Vaughan Williams table) · AHA 2025 ACLS algorithms poster set (Adult Cardiac Arrest; Adult Tachyarrhythmia With a Pulse — doses/details), international edition · 2025 AHA Guidelines for CPR and ECC — Part 9: Adult Advanced Life Support (Circulation 2025; doi 10.1161/CIR.0000000000001376), cpr.heart.org text · Nexterone (amiodarone HCl) premixed injection prescribing information — DailyMed · Pacerone (amiodarone HCl) tablets prescribing information — DailyMed · Procainamide hydrochloride injection prescribing information (Hospira) — DailyMed · Betapace / Betapace AF (sotalol) prescribing information — DailyMed · Adenosine injection (antiarrhythmic) prescribing information — DailyMed · Merck Manual Professional — Reentrant (Paroxysmal) Supraventricular Tachycardias (PSVT) · Amiodarone and thyroid dysfunction (review). Acta Clin Croat 2022;61(2) · Joglar JA et al. 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation. Circulation 2024;149:e1–e156 · Merck Manual Professional — Atrial Fibrillation · Merck Manual Professional — Torsades de Pointes Ventricular Tachycardia · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).