Key Points
- Synchronized cardioversion applies low-energy, R-wave-timed shocks to terminate unstable tachyarrhythmias with a pulse.1
- Unsynchronized defibrillation delivers immediate high-energy shocks for pulseless ventricular tachycardia (VT) or ventricular fibrillation (VF).1
- Biphasic waveforms achieve more than 90% first-shock success for VF at lower energies than monophasic waveforms.2,3
- Survival decreases 7%–10% for every minute of delay in VF. Early, high-quality cardiopulmonary resuscitation (CPR) dramatically improves survival.2
- There are no absolute contraindications, including pregnancy or patients with pacemakers/implantable cardioverter-defibrillators.4,5,8
Introduction
- Sudden cardiac arrest affects approximately 350,000 adults each year in the United States. VF is the initial rhythm in 20%–30% of out-of-hospital arrests.2
- Perioperative arrhythmias may be triggered by electrolyte disturbances, ischemia, anesthetics, or surgical stress, and are diagnosed using electrocardiograms (ECG).6
- External cardioversion and defibrillation are essential interventions for life-threatening arrhythmias in perioperative, emergency, and critical care settings.1
- These techniques deliver transthoracic electrical current to depolarize the myocardium and restore organized electrical activity.1
- Modern portable biphasic defibrillators are standard in hospital automated external defibrillators (AEDs).2,3
- Proper electrode placement (anterolateral or anteroposterior) minimizes transthoracic impedance, or opposition to electrical current.1
Figure 1. Proper pacing pad locations with anterolateral (left) and anterposterior (right) placements.
Source: Source: Ramzy M. Rebel EM. 2022. CC BY NC ND7
Cardioversion vs. Defibrillation
- Electrical cardioversion and defibrillation differ in synchronization mode, energy delivery, and clinical indication.1,2,8
- Synchronized cardioversion delivers R wave–timed shocks to avoid the R on T phenomenon and is used for hemodynamically unstable tachyarrhythmias with a pulse.8
- Defibrillation delivers immediate, unsynchronized shocks for pulseless VT or VF to rapidly depolarize myocardium and restore organized rhythm.1
- Current ACLS guidelines recommend:
- Synchronized cardioversion for unstable tachycardia with a pulse.2,8
- Defibrillation for shockable cardiac arrest rhythms (VF or pulseless VT).2
Key distinctions:
- Cardioversion requires ECG synchronization mode; defibrillation does not.1,8
- If synchronization cannot be achieved or a pulse is lost, treat as defibrillation.2,8
- Biphasic devices are recommended for both interventions.2,3
Table 1. Cardioversion vs. defibrillation1,2,8
Abbreviations: R-on-T, VT, ventricular tachycardia; VF, ventricular fibrillation
Indications
Synchronized Cardioversion
- Hemodynamically unstable supraventricular tachycardia or monomorphic VT with pulse.8
- Signs of hemodynamic compromise: hypotension (<90 mm Hg systolic), chest pain, dyspnea, or altered mental status.8
- Atrial fibrillation or flutter with rapid ventricular response refractory to medications.2
- Perioperative tachyarrhythmias requiring urgent restoration of sinus rhythm.6
Defibrillation
- VF or pulseless VT (both shockable rhythms).2
- Witnessed cardiac arrest when a defibrillator is rapidly available.2
- Refractory VF after standard shocks and antiarrhythmics (double sequential defibrillation investigational).10
Energy Selection
Monophasic Waveforms (rarely used)
- Monophasic waveforms deliver current in a single direction. They typically start at 200J and escalate up to 360J with subsequent attempts at restoring rhythm. First-shock success is around 70-80%, and it carries a higher myocardial injury risk.3,9
Biphasic Waveforms (preferred)
- Biphasic waveforms reverse the direction of current partway through the shock. They start at lower initial energies (120-200J) for VF with greater first‑shock success.2
- There is reduced post‑shock dysfunction as compared with monophasic waveforms.3
- All modern manual defibrillators and AEDs use biphasic technology.1
Table 2. Biphasic Energy Selection for Various Indications1,2,9,10
Abbreviations: SVT, supraventricular tachycardia; VT, ventricular tachycardia; VF, ventricular fibrillation
Factors Determining Success
- Shorter time from collapse to first shock dramatically improves survival.2
- High-quality, uninterrupted CPR maintains perfusion and increases shock success.2
- Low transthoracic impedance improves current delivery; achieve this with firm pad pressure, conductive gel/self-adhesive pads, correct placement, and anteroposterior position when possible.1
- Rapid correction of reversible causes (hypoxia, acidosis, hyperkalemia, hypothermia, etc.) is essential.6
Postshock Care
- If the rhythm remains in VF or no pulse is present, resume high-quality chest compressions immediately and continue for a full two minutes.2
- Administer epinephrine 1 mg IV/interosseous (IO) as soon as possible and repeat every 3-5 minutes.2
- For refractory VF, pulseless persisting after three shocks, administer amiodarone 300 mg IV/IO bolus followed by an additional dose of 150 mg.2
Special Considerations
- Pregnancy: There is no contraindication to cardioversion or defibrillation in pregnant patients; perform left uterine displacement and fetal monitoring if viable.4
Please see the OA summary on CPR in pregnancy for more details. Link - Pacemaker/ICD: Cardioversion or defibrillation is safe if pads are placed more than 8 cm from the generator; consider the anteroposterior position rather than the anterolateral if feasible.2
- Pediatrics: Use pediatric pads or attenuator mode when the patient weighs less than 25 kg or is less than 8 years.11
- Obesity: Anterior-posterior pad placement may reduce impedance.1
- Open-chest surgery: Consider using internal paddles and 5-20 J.9
References
- Goyal A, Chhabra L, Singh B, et al. Defibrillation. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Link
- Panchal AR, Bartos JA, Cabañas JG, et al. Part 3: Adult basic and advanced life support: 2020 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation. 2020;142(16_suppl_2):S366-S468. PubMed
- Faddy SC, Jennings PA. Biphasic versus monophasic waveforms for transthoracic defibrillation in out-of-hospital cardiac arrest. Cochrane Database Syst Rev. 2016;2(2):CD006762. PubMed
- Lucà F, Oliva F, Abrignani MG, et al. The Challenge of Managing Atrial Fibrillation during Pregnancy. Rev Cardiovasc Med. 2023;24(10):279. PubMed
- Elgaard AF, Dinesen PT, Riahi S, et al. External cardioversion of atrial fibrillation and flutter in patients with cardiac implantable electrical devices. Pacing Clin Electrophysiol. 2023;46(2):108-13. PubMed
- Thompson A, Fleischmann KE, Smilowitz NR, et al. 2024 AHA/ACC/ACS/ASNC/HRS/SCA/SCCT/SCMR/SVM guideline for perioperative cardiovascular management for noncardiac surgery: A report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Circulation. 2024;150(19):e351-e442. PubMed
- REBEL EM. Electrode pad placement positions for cardioversion in atrial fibrillation (anterior-lateral vs anterior-posterior). REBEL EM. June 2022. Accessed December 15, 2025. Link
- Goyal A, Singh B, Chhabra L, et al. Synchronized Electrical Cardioversion. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Link
- Tacker WA Jr, Guinn GA, Geddes LA, et al. The electrical dose for direct ventricular defibrillation in man. J Thorac Cardiovasc Surg. 1978;75(2):224-6. PubMed
- Cheskes S, Verbeek PR, Drennan IR, et al. Defibrillation strategies for refractory ventricular fibrillation. N Engl J Med. 2022;387(21):1947-56. PubMed
- Topjian AA, Raymond TT, Atkins D, et al. Part 4: Pediatric basic and advanced life support: 2020 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation. 2020;142(16_suppl_2):S469-S523. PubMed
Other References
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