Synchronized Cardioversion

Synchronized cardioversion shocks on the R wave to convert unstable tachycardia with a pulse; it is time-critical when the patient is deteriorating.

Synchronized cardioversion is a direct-current shock timed to the QRS. It breaks the re-entrant circuit that keeps the arrhythmia going. You use it when the tachycardia is making the patient unstable. Emergency use means unstable tachycardia with a pulse. Elective AF cardioversion is a hospital decision, and this page does not cover it.

How It Works

• Synchronization: The device finds each R wave and fires during the QRS. That keeps the shock out of the vulnerable T-wave window.
• Mechanism: Enough myocardium depolarizes at once to end the circuit. That circuit can be AF, flutter, SVT or monomorphic VT.
• Energy: Regular narrow-complex and monomorphic VT need less energy than AF or flutter. The 2025 AHA values assume a US-approved biphasic device.
• Atrial stunning: The atria are left stunned after conversion. That raises thromboembolic risk, and it is the reason anticoagulation continues after AF cardioversion.

Where It Fits

Where does this sit in the adult tachycardia algorithm? Unstable tachycardia with a pulse goes to synchronized cardioversion. Stable tachycardia gets drugs or vagal maneuvers first. Pulseless tachycardia gets CPR and defibrillation.

For stable regular narrow-complex tachycardia, vagal maneuvers come first, then adenosine. Adenosine is contraindicated in asthma or active bronchospasm (2025 AHA Part 9). Need rate control instead? IV diltiazem or verapamil is COR 2a for this rhythm. Infuse them slowly, because hypotension is common. Avoid them if systolic heart failure is suspected (2025 AHA Part 9, supportive text). Do not give AV-nodal blockers such as diltiazem or verapamil to wide-complex tachycardia of uncertain type.

Indications

• Unstable tachycardia with a pulse: hypotension, altered mental status, ischemic chest pain, acute heart failure or shock.
• Atrial fibrillation or flutter with rapid ventricular response causing instability.
• Narrow-complex regular tachycardia causing instability, when it does not respond to first-line therapy.
• Monomorphic VT with a pulse causing instability.
• Pediatric unstable SVT or VT with a pulse and poor perfusion; stable pediatric SVT gets vagal maneuvers and adenosine first.
• Pre-excited AF (irregular wide-complex tachycardia with an accessory pathway) that is unstable: this one gets synchronized cardioversion. Do not give any of these: IV amiodarone, digoxin, nondihydropyridine calcium channel blockers and beta-blockers (2025 AHA Part 9, COR 3: Harm, LOE C-LD).

Absolute Contraindications

• Pulseless rhythm: No pulse means CPR and defibrillation. Cardioversion is off the table.

Relative Contraindications & Cautions

• AF of 48 hours or longer, or unknown duration, without anticoagulation: The clot risk is real. Elective cardioversion needs 3 weeks of uninterrupted anticoagulation or imaging to exclude thrombus. Unstable? Do not delay.
• Digoxin toxicity: A shock can trigger VF here. Avoid elective cardioversion. If the patient is unstable, use the lowest effective energy and have defibrillation ready.
• Sinus tachycardia or multifocal atrial tachycardia: Not a shock indication. Treat the cause.
• Electrolyte abnormality: When the patient is stable, fix the potassium and magnesium first.
• Pregnancy and implanted devices: Neither one is a contraindication. Details are under Special Populations.

Abort / Stop Criteria

• Pulse loss or VF during setup: stop and start CPR and defibrillation per algorithm.
• Polymorphic VT or VF after a shock: unsynchronized defibrillation.

Team Roles During the Procedure

• Operator: reads the rhythm, sets the energy, places pads and presses SHOCK.
• Monitor and recorder: confirms SYNC markers, reads the rhythm and records times, energies and shocks.
• Airway and oxygen: keeps the airway open and ventilation ready during sedation.
• Medication clinician: gives and labels sedation, reads back doses and concentrations.
• Safety call: announces "charging" and "stand clear" and checks that no one touches the patient or bed.

Rhythm and Stability

• Confirm the rhythm: Pull a 12-lead or a monitor strip. You're looking for a narrow or wide-complex tachycardia with a pulse. A wide, irregular rhythm may be pre-excited AF. Avoid AV-nodal blockers and IV amiodarone.
• Duration: Pin down the AF onset time. At 48 hours or more, or with an unknown onset, the anticoagulation requirement changes for elective cases.
• Instability: Hypotension, altered mental status, ischemic chest pain or signs of acute heart failure all count. The 2025 AHA Part 9 gives SBP less than 80 mm Hg or altered mentation as an example that calls for immediate synchronized cardioversion in wide-complex tachycardia.

Physiologic Risk

Look at BP, SpO₂, mental status and lung sounds. With hypotension or shock, cardioversion becomes urgent. If labs are available, check potassium and magnesium. Abnormal values raise the arrhythmia risk.

Site / Device Check

• Implanted device: note any pacemaker or ICD generator and keep pads away from it.
• Transdermal patches: remove medication patches from the pad sites.
• Skin: dry the chest and remove jewelry.

Pad position is the only real choice on this procedure. Anterolateral is the standard. Anteroposterior is the alternative, under the device or local protocol.

Anterolateral Pad Placement

1. Place the sternal pad: right upper chest, below the clavicle.
2. Place the apex pad: left lateral chest, in the midaxillary line.
3. Check spacing: pads must not touch each other or any gel bridge.
4. Connect and confirm: attach the pad leads and check the impedance reading on the device.

Anteroposterior Pad Placement

1. Place the anterior pad: front of the chest over the heart.
2. Place the posterior pad: back below the left scapula.
3. Check spacing and device distance: same rules as anterolateral.

Pediatric

Small children need pediatric-sized pads, or a dose attenuator where the device calls for one. Pad size is device-specific, so the device instructions have the final word.

Equipment

• Defibrillator: with SYNC function and a visible sync indicator.
• Pads: adult and pediatric sizes, with a spare set.
• Monitoring: continuous ECG, SpO₂, BP and EtCO₂ if sedated.
• Sedation: per the medication table, drawn up and labeled.
• Airway: bag-valve mask, suction and oxygen.
• Backup: defibrillator on DEFIB mode, crash cart in reach.

Sizing

• Adult: adult pads and the energies in Key Numbers.
• Pediatric: pediatric pads or attenuator per device; energy by weight (see Special Populations).

Patient Positioning & Preparation

• Position: supine with the chest exposed and the airway reachable.
• Access: IV access in place before sedation.

These are typical adult and pediatric references. Verify them against current drug labeling. Sedate whenever feasible. The 2025 AHA electrical cardioversion algorithm (box 4) lists effective regimens that have included a sedative such as diazepam, with or without an analgesic such as fentanyl. Many experts recommend anesthesia when it is readily available. AHA gives no sedation doses for cardioversion, so this page lists none. Don't mistake the etomidate induction dose for a sedation dose. It is not one for this procedure.

Energy Settings

Adult energies come from the 2025 AHA Part 9 electrical cardioversion algorithm and recommendations. Pediatric energies come from the 2025 AHA Part 8 recommendations and the PALS tachyarrhythmia algorithm.

Sedation Agents

Phase 1: Prepare

1. Assess and confirm: Confirm an unstable tachycardia with a pulse and identify the rhythm as narrow or wide complex.
2. Assign roles: Name the operator, the medication clinician and the monitor and recorder.
3. Prepare the defibrillator: Turn it on, attach leads or pads and select the energy for the rhythm.
4. Engage SYNC: Confirm a sync marker on every R wave before charging.
5. Place pads and oxygen: Anterolateral or anteroposterior, away from any device; apply oxygen and suction.

Phase 2: Sedate and Shock

6. Sedate if time allows: If the patient is conscious and the shock will not be delayed, give sedation per the medication table and confirm adequate sedation before shocking.
7. Charge and clear: Move oxygen delivery devices away from the chest (at least 1 m or 3 ft, or per device and local protocol) or remove them. Announce "charging," charge the device, announce "stand clear" and confirm no one touches the patient or bed.
8. Shock: Press SHOCK and hold until the shock is delivered; the device may wait for the next R wave.

9. Reassess rhythm and pulse: Check the monitor and the pulse immediately after each shock.
10. Escalate if not converted: Increase energy per device and protocol, re-engage SYNC, and repeat.

Phase 3: Confirm & Secure

11. Confirm conversion: Obtain a 12-lead ECG and compare it with the baseline.
12. Start aftercare: Anticoagulation for AF or flutter, antiarrhythmic per protocol or cardiology, and continuous monitoring.

Primary Confirmation

Success looks like the expected rhythm on the monitor and on a 12-lead, a palpable pulse and better perfusion (BP and mental status).

Secondary Confirmation

• Improved BP, SpO₂ and mental status.
• Stable EtCO₂ during sedation, with no hypoventilation.
• Rhythm strip after the shock for recurrence.

Ongoing Monitoring

• Continuous ECG, BP and SpO₂ until stable; EtCO₂ during and after sedation.
• 12-lead ECG after conversion and at handoff.
• Reassess pulse and perfusion after every move and every shock.

Pediatrics

• Energy: 0.5–1 J/kg for the first shock, then 2 J/kg if it fails (2025 AHA Part 8: SVT with cardiopulmonary compromise, COR 2a, LOE C-LD; wide-complex tachycardia with cardiopulmonary compromise, COR 2a, LOE C-EO). AHA does not address AF or flutter in children.
• Pads: pediatric pads or an attenuator per device; confirm pad size against the device instructions.
• Sedation: per pediatric protocol; see weight-based dosing.
• Sizing: see equipment sizing.

Pregnancy

• Shock: Pregnancy does not change the method. Cardioversion and defibrillation follow the adult approach when they are indicated.
• Monitors: take external fetal monitors off before the shock. The 2025 AHA Part 10 (COR 3: No Benefit, LOE C-EO) says fetal monitoring should not be done in a pregnant patient in cardiac arrest. It pulls attention from the team and can limit timely defibrillation. Cardioversion gets the same logic. That is clinical teaching, not an AHA statement about cardioversion.

Older Adults and Digoxin

• Ask about digoxin, and look for toxicity on the ECG. If there is toxicity, avoid elective cardioversion. If the patient is unstable, use the lowest effective energy.

Implanted Devices

• Keep pads away from a pacemaker or ICD generator.
• An ICD may have fired. Notify the device team after any shock.

Immediate Aftercare

• Obtain a 12-lead ECG and compare it with the baseline.
• Check BP, SpO₂, mental status and the skin under the pads.
• Monitor until sedation has worn off and the patient is stable.

Ongoing Maintenance

• Anticoagulation: keep it going without a gap for at least 4 weeks after AF or flutter cardioversion (2023 AF guideline). No prior anticoagulation and an urgent cardioversion? Start as soon as possible, and continue for at least 4 weeks. Long-term therapy follows stroke risk (CHA₂DS₂-VASc), not the conversion itself.
• Rhythm control: an antiarrhythmic to hold sinus rhythm is a call for cardiology or protocol.
• Triggers: correct electrolytes, evaluate thyroid disease and ischemia, treat sepsis or other drivers.

Transport Considerations

• Transport on continuous monitoring and reassess after every move.
• Keep the defibrillator and pads in reach until handoff.

Pearls

• Shock on the R wave: SYNC keeps the shock out of the T-wave window, where a shock can trigger VF.
• Energy follows the device: the 2025 AHA values are where you start. Escalate per the device label.
• Antiarrhythmic pretreatment: IV amiodarone has supporting data for pharmacologic AF conversion. Using it before electrical cardioversion depends on the protocol. It is not used in pre-excited AF (2025 AHA Part 9, COR 3: Harm) (see amiodarone).
• Emergency vs elective: Unstable patients get shocked without waiting on anticoagulation. Elective AF cardioversion runs on the anticoagulation pathway.

Common Pitfalls

• Shock without re-engaging SYNC: The device may be unsynchronized, so the shock is not timed to the QRS. Re-engage before every shock.
• Delaying an unstable shock for sedation or labs: Shock first. Sedate only if it doesn't cause a delay.
• Cardioverting stable tachycardia by reflex: Stable means drugs or vagal maneuvers first, not the paddles.
• Trusting a monitor strip alone: One strip doesn't settle it. Confirm with a 12-lead ECG and look at perfusion.