Defibrillation and synchronised cardioversion

Procedure

An electric shock through pads on the chest resets the heart's rhythm, in cardiac arrest or in a fast rhythm causing harm.

Updated Nov 13, 2026

What it is

Both procedures pass a direct-current shock across the heart through two pads, depolarising a critical mass of myocardium at once so that the sinus node can take over again. Defibrillation is unsynchronised: the shock is delivered the moment the button is pressed, and it is used for the shockable arrest rhythms, ventricular fibrillation and pulseless ventricular tachycardia, as early as possible with high-quality CPR before and straight after. Synchronised cardioversion times the shock to the R wave of the QRS, so that it does not fall on the T wave, the vulnerable period in which a shock can induce ventricular fibrillation. It is used for a tachyarrhythmia with a pulse: urgently when there are adverse features (shock, syncope, myocardial ischaemia or heart failure), and electively for atrial fibrillation or flutter. Modern devices are biphasic, and the energy is the one the device and local protocol set, escalating for further shocks when needed. Pads are placed anterolateral (below the right clavicle and at the left midaxillary line at the V6 level) or anteroposterior, away from an implanted device. A conscious patient needs sedation or anaesthesia, so airway skills must be present. Before elective cardioversion of atrial fibrillation lasting more than 48 hours or of unknown duration, the risk of stroke from left atrial thrombus is managed with prior anticoagulation or a transoesophageal echo, and anticoagulation continues afterwards.

Indications

Ventricular fibrillation and pulseless ventricular tachycardia: immediate defibrillation within ALS.
Unstable tachyarrhythmia with a pulse (broad or narrow): urgent synchronised cardioversion.
Stable ventricular tachycardia that does not respond to drugs: cardioversion under sedation.
Elective cardioversion of atrial fibrillation or flutter for rhythm control.
Public access: an automated external defibrillator (AED) analyses the rhythm and advises a shock.

Contraindications and precautions

Not for asystole or pulseless electrical activity: shocks do not help; continue CPR and treat causes.
Elective cardioversion: unprotected AF over 48 hours, digoxin toxicity or severe hypokalaemia: delay and correct first.
Sinus tachycardia and multifocal atrial tachycardia do not respond to shocks.
Implanted pacemaker or defibrillator: place pads away from the device and check it afterwards.
Safety: dry chest, remove medication patches, oxygen away from the pads, nobody touching the patient or bed.
A conscious patient needs sedation or anaesthesia with someone able to manage the airway.

Equipment

Manual or automated defibrillator with ECG monitoring, sync function and charged battery.
Self-adhesive defibrillation pads (adult and paediatric), razor and towel to dry the skin.
Resuscitation trolley: airway equipment, bag-valve-mask, oxygen, suction and IV access.
Monitoring: blood pressure, pulse oximetry and capnography during sedation.
Sedative drugs, resuscitation drugs and a team member trained in airway management.

Step by step (and why)

Arrest: confirm VF or pulseless VT with CPR running — every pause lowers the chance of survival.
Elective or urgent with a pulse: consent, fasting if possible, anticoagulation checked, IV access and sedation — safety first.
Expose and dry the chest and place pads anterolateral or anteroposterior, away from any device — current must cross the heart.
Cardioversion: select sync and check the markers sit on the R waves, not the T waves — a shock on T can cause VF.
Select the energy the device and protocol set and charge while CPR continues in an arrest — this shortens the pause.
Remove oxygen from the chest, shout «stand clear», check that nobody touches the patient — the shock can harm staff.
Deliver the shock; in an arrest resume CPR at once for two minutes before reassessing — perfusion matters more than a check.
Cardioversion: check rhythm and pulse; if still in the rhythm, re-select sync and escalate as protocol sets.
Afterwards, monitor, record a 12-lead ECG, check the skin and document rhythm, energy and number of shocks.

Defibrillation versus synchronised cardioversion

Feature	Defibrillation	Synchronised cardioversion
Rhythm	VF, pulseless VT	Tachyarrhythmia with a pulse (VT, SVT, AF, flutter)
Timing	Immediate, unsynchronised	Timed to the R wave
Patient	Unconscious, no pulse	Conscious: sedation needed
After the shock	CPR for two minutes at once	Check rhythm and pulse

Complications

Skin burns or erythema under the pads, more with poor contact or repeated shocks.
Post-shock arrhythmias: bradycardia, asystole or VF (especially an unsynchronised shock on a T wave).
Thromboembolism and stroke after cardioversion of AF without adequate anticoagulation.
Complications of sedation: hypoventilation, aspiration and hypotension.
Injury to staff from contact during the shock, and fire risk with oxygen near the pads.

Next step

After return of spontaneous circulation: ABCDE, 12-lead ECG, post-arrest care and a search for the cause.
After cardioversion: monitor until fully awake, record a 12-lead ECG and check electrolytes.
Unsuccessful or recurrent arrhythmia: call a senior or cardiology for antiarrhythmic drugs or another strategy.
Elective AF cardioversion: continue anticoagulation afterwards as guidelines set, regardless of the result.
Have any implanted device interrogated after a shock, and arrange follow-up of the underlying heart disease.

Test yourself

Think of the answer, then tap the question to see it.

What is the difference between defibrillation and synchronised cardioversion?

Defibrillation delivers the shock at once, for VF or pulseless VT; cardioversion times the shock to the R wave, for a tachyarrhythmia with a pulse.

Why is the shock synchronised to the R wave in cardioversion?

To avoid delivering it during the T wave, the vulnerable period of repolarisation, when a shock can trigger ventricular fibrillation.

Which adverse features call for urgent synchronised cardioversion of a tachyarrhythmia?

Shock (hypotension, poor perfusion), syncope, myocardial ischaemia (chest pain, ischaemic ECG) and severe heart failure.

What must be ensured before elective cardioversion of AF lasting more than 48 hours?

Protection against stroke: adequate anticoagulation for several weeks beforehand or a transoesophageal echo excluding left atrial thrombus, with anticoagulation continued afterwards.

Add to my cards (4)

Elsewhere in DurgiPedia

Connections

In Pharmacology6

← All clinical skills

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Defibrillation (StatPearls (NCBI Bookshelf), 2025); Synchronized Electrical Cardioversion (StatPearls (NCBI Bookshelf), 2023).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.