Diagnostics & Procedures · Topic 16 of 16

Cardioversion & Defibrillation

Procedures · electrical therapy

Cardioversion & Defibrillation

Synchronize shocks for unstable tachycardias with a pulse, defibrillate VF and pulseless VT, and anticoagulate before elective cardioversion of AF lasting 48 hours or more.

Key numbers

Sync: 100 J narrow/VT; 200 J AF/flutter100 J · ≥ 200 J

VF/pVT: biphasic 120–200 J or 360 J monophasicbiphasic 120–200 J

Anticoagulate 3 weeks or TEE first≥ 48 h → 3 wk or TEE

01 · Don't confuseSync with a pulse; unsync for VF

Synchronized cardioversion times the shock to the QRS, avoiding the vulnerable T wave that can induce VF; use it for organized tachycardias with a pulse. Defibrillate (unsynchronized) VF, pulseless VT and polymorphic VT.

02 · The numberSync: 100 J narrow/VT; 200 J AF/flutter

2025 AHA initial synchronized energies: narrow-complex tachycardia 100 J, monomorphic VT 100 J, atrial fibrillation at least 200 J biphasic, atrial flutter 200 J — escalate if the first shock fails.

03 · The ruleAnticoagulate 3 weeks or TEE first

Before elective cardioversion of AF or flutter lasting ≥ 48 hours, give 3 weeks of uninterrupted therapeutic anticoagulation or exclude left atrial thrombus by TEE; continue anticoagulation for at least 4 weeks afterward.

Diagnosis & monitoring

Work-up

VF/pVT: biphasic 120–200 J or 360 J monophasic must knowDefibrillate VF or pulseless VT with one shock — biphasic at the manufacturer dose (typically 120–200 J; device maximum if unknown) or monophasic 360 J — then resume compressions immediately.

Do right now

In this order

First moves

Unstable tachycardia with a pulse: cardiovert now

→

Sync with a pulse; unsync for VF

→

Sync: 100 J narrow/VT; 200 J AF/flutter100 J · ≥ 200 J

→

Re-check sync markers before every shock

Why each step

Unstable tachycardia with a pulse: cardiovert nowIf a tachycardia causes hypotension, acute altered mental status, shock, ischemic chest pain or acute heart failure, perform synchronized cardioversion immediately, giving procedural sedation when time and mental status allow.

Sync with a pulse; unsync for VFSynchronized cardioversion times the shock to the QRS, avoiding the vulnerable T wave that can induce VF; use it for organized tachycardias with a pulse. Defibrillate (unsynchronized) VF, pulseless VT and polymorphic VT.

Sync: 100 J narrow/VT; 200 J AF/flutter2025 AHA initial synchronized energies: narrow-complex tachycardia 100 J, monomorphic VT 100 J, atrial fibrillation at least 200 J biphasic, atrial flutter 200 J — escalate if the first shock fails.

Re-check sync markers before every shockConfirm the sync marker sits on each QRS before every synchronized shock: defibrillators differ after a shock — some stay synchronized, others revert to unsynchronized mode — risking a shock on the T wave.

Management

Orders

Polymorphic VT: unsynchronized, high-energy shockSustained polymorphic VT cannot be synchronized reliably because each QRS differs: give an immediate unsynchronized shock at high (defibrillation, maximum manufacturer) energy.

Unstable preexcited AF: cardiovert, no nodal blockersPreexcited AF (WPW: fast, irregular, wide, varying QRS) must not get AV-nodal blockers, which can precipitate VF: cardiovert it if unstable; give IV procainamide or ibutilide if stable.

Elective: fasted, sedated, airway readyFor elective cardioversion, have the patient fast 6–8 hours and give brief general anesthesia or deep sedation (eg, propofol, or midazolam with fentanyl), with airway equipment and trained personnel at the bedside.

Energy and pad pressure matter more than positionAnterolateral or anteroposterior pads are both acceptable; for AF, biphasic waveforms, higher fixed energy (≥ 200 J) and manual pressure on the pads raise success, while pad position has no proven effect.

Pacemaker or ICD: pads away, interrogate afterWith a pacemaker or ICD, place the pads well away from the generator (anteroposterior works well) and check device function, thresholds and leads after cardioversion, because shocks can transiently alter pacing impedance and sensing.

Red flags & don'ts

Avoid harm

No elective cardioversion in digoxin toxicity avoidAvoid elective cardioversion in digitalis toxicity, which can provoke resistant ventricular fibrillation, and when left atrial thrombus is present.

The rules

Know these

AF under 48 h: decide by stroke riskAF under 48 hours: consider TEE when CHA₂DS₂-VASc ≥ 2 and 3 weeks of anticoagulation were not given; with a score of 0–1 and AF under 12 hours, imaging or anticoagulation beforehand has uncertain benefit.

Long-term anticoagulation follows stroke riskRestoring sinus rhythm does not remove the need for anticoagulation: long-term anticoagulation after cardioversion follows the CHA₂DS₂-VASc score, not the rhythm (see Atrial Fibrillation).

Shock reentry, not sinus or automatic rhythmsCardiovert only when the tachycardia itself causes instability: sinus tachycardia from sepsis or bleeding needs the cause treated, and automatic tachycardias respond poorly because the automatic rhythm returns.

Synchronized cardioversion or defibrillation?

Commonly confused

Rhythms	Organized tachycardia with a pulse: SVT, AF, flutter, monomorphic VT
Timing	Delivered on the QRS, avoiding the T wave
Initial energy	100 J narrow or monomorphic VT; ≥ 200 J AF; 200 J flutter
Before the next shock	Re-check that the sync markers are on

More detail

Reference

Show 5 reference facts

Complications are usually minor (atrial and ventricular premature beats, muscle soreness, skin burns); myocyte injury and electromechanical dissociation are more likely with poor LV function or multiple shocks.

After a failed biphasic shock, later shocks use the same or higher energy; monophasic defibrillation shocks stay at 360 J. Most VF seen after a shock is recurrence rather than shock failure.

For VF persisting after 3 or more shocks, the usefulness of vector change and double sequential defibrillation is not established (2025 AHA); double synchronized cardioversion for AF is likewise of uncertain usefulness.

Electrical cardioversion restores sinus rhythm in about 90% of AF but recurrence is common; it works less well with long-standing AF or a left atrium > 5 cm, and antiarrhythmic pretreatment helps maintain sinus rhythm.

Urgent synchronized cardioversion is recommended for new-onset AF or flutter with rapid ventricular rates in acute coronary syndrome when there is hemodynamic compromise, ongoing ischemia or inadequate rate control.

Pearl

Organized with a pulse: sync it. Pulseless or chaotic: shock it unsynchronized.

Sources · 2025 AHA Guidelines for CPR and ECC — Part 9: Adult Advanced Life Support (Circulation 2025; doi 10.1161/CIR.0000000000001376), cpr.heart.org text · AHA 2025 Interim Training Materials — ACLS Provider Manual changes (synchronized cardioversion energies) · Merck Manual Professional — Direct Current (DC) Cardioversion-Defibrillation · Merck Manual Professional — Atrial Fibrillation · Joglar JA et al. 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation. Circulation 2024;149:e1–e156 · Monsieurs KG, Coady EM. Variation in defibrillation modes after synchronized cardioversion: a potential hazard. Resuscitation 1997 (abstract via Europe PMC) · Sucu M, Davutoglu V, Ozer O. Electrical cardioversion. Ann Saudi Med 2009;29:201–206 · Nguyen ST et al. Techniques improving electrical cardioversion success for patients with atrial fibrillation: a systematic review and meta-analysis. Europace 2023;25:318–330 · Manegold JC et al. External cardioversion of atrial fibrillation in patients with implanted pacemaker or cardioverter-defibrillator systems. Eur Heart J 2007;28:1731–1738 (abstract via Europe PMC) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).