Defibrillation is for patients without a pulse. Cardioversion is for patients who still have one. That single distinction, whether the heart is producing any meaningful output, drives the entire decision. Both procedures deliver an electrical shock to reset the heart’s rhythm, but the circumstances, timing, and technique differ in ways that matter.
The Core Difference: Pulse or No Pulse
Defibrillation treats cardiac arrest. Specifically, it’s used for two rhythms: ventricular fibrillation (VF) and pulseless ventricular tachycardia (pulseless VT). In both cases, the heart’s electrical system has gone haywire. During VF, multiple areas of the heart fire chaotically and independently, producing a quivering motion instead of coordinated contractions. The heart stops pumping blood entirely. Pulseless VT is functionally the same emergency: the ventricles beat so fast and ineffectively that no blood reaches the body. Both are cardiac arrest, and the person is clinically dead without immediate intervention.
Synchronized cardioversion, by contrast, treats patients whose hearts are still beating and producing a pulse, but doing so dangerously fast or irregularly. These patients are conscious (or at least alive), often with a heart rate above 150 beats per minute. Their heart still has an organized electrical pattern, just an abnormal one. The goal is to reset that pattern before the situation deteriorates further.
Why Synchronization Matters
Every heartbeat has a brief vulnerable window during the recovery phase of the electrical cycle (represented by the T-wave on a heart monitor). If a shock lands during this window, it can potentially throw an otherwise beating heart into ventricular fibrillation, turning a bad situation into cardiac arrest. This is sometimes called the R-on-T phenomenon.
Synchronized cardioversion avoids this by having the machine detect the heart’s own electrical peaks and time the shock to land safely between those vulnerable windows. The device reads the rhythm, identifies the right moment, and delivers the shock in sync with the heart’s activity. This is why it’s called “synchronized.”
During defibrillation, synchronization doesn’t matter. The heart is already in VF or pulseless VT, meaning there’s no organized rhythm to accidentally disrupt. The shock fires the instant the button is pressed, because every second of delay reduces the chance of survival.
What Makes a Patient “Unstable”
Cardioversion is reserved for patients with a pulse who are hemodynamically unstable, meaning their fast heart rhythm is causing their body to fail. The specific signs that trigger cardioversion include:
- Hypotension: systolic blood pressure dropping below 90 mmHg
- Altered mental status: confusion, agitation, or decreased consciousness
- Signs of shock: pale, clammy skin with poor circulation
- Ischemic chest pain: the heart muscle itself not getting enough blood
- Acute heart failure: sudden fluid backup into the lungs
If a patient has a fast rhythm but is stable (alert, normal blood pressure, no chest pain), the first approach is typically medication rather than electrical shock. Drugs like adenosine, beta-blockers, or calcium channel blockers can slow or convert the rhythm without sedation or electrical intervention. Cardioversion becomes the treatment when these measures fail or when the patient is too unstable to wait.
Which Rhythms Get Which Treatment
The 2025 American Heart Association algorithm breaks it down clearly. Defibrillation is used exclusively for VF and pulseless VT. Every other fast rhythm with a pulse that’s causing instability gets synchronized cardioversion. That includes atrial fibrillation, atrial flutter, supraventricular tachycardia (SVT), and ventricular tachycardia with a pulse.
One important nuance: if a patient is in polymorphic VT (a chaotic, irregular wide-complex rhythm) with a pulse, many protocols treat it like VF and use unsynchronized defibrillation. The rhythm is so disorganized that the machine often can’t identify a reliable point to synchronize with, and attempting to sync may dangerously delay the shock.
Energy Levels Differ
Defibrillation uses higher energy. For biphasic defibrillators (the modern standard), the initial shock is typically 120 to 200 joules, depending on the manufacturer. If the first shock doesn’t work, subsequent shocks use equal or higher energy, escalating up to the machine’s maximum. Older monophasic devices use 360 joules.
Cardioversion generally starts lower because the heart still has organized activity that’s easier to reset. The AHA’s current recommendations for synchronized cardioversion are:
- Atrial fibrillation: 200 joules
- Atrial flutter: 200 joules
- Narrow-complex tachycardia (SVT): 100 joules
- Monomorphic ventricular tachycardia: 100 joules
If the initial shock fails, energy is increased for the next attempt.
Sedation and Patient Experience
Defibrillation happens during cardiac arrest, so the patient is unconscious. There’s no need for sedation, and there’s no time for it. The priority is getting the shock delivered as fast as possible, ideally within minutes of the arrest, alongside CPR.
Cardioversion is different. The patient is alive and typically conscious, so the shock would be extremely painful without sedation. Short-acting sedatives are given intravenously before the procedure to put the patient into a brief, deep sleep. The shock is delivered while they’re sedated, and most patients wake within minutes with no memory of the event. The AHA guidelines note that sedation should be provided “whenever feasible,” acknowledging that in a rapidly deteriorating patient, the shock may need to come first.
Pad Placement
For both procedures, pads can be placed in two positions: anterior-lateral (one pad on the upper right chest, one on the left side under the armpit) or anterior-posterior (one on the front of the chest, one on the back between the shoulder blades). For defibrillation, either position is acceptable.
For cardioversion of atrial fibrillation specifically, a large multicenter randomized trial published in Circulation found that the anterior-lateral position was significantly more effective than anterior-posterior. Fifty-four percent of patients cardioverted successfully with the first shock using anterior-lateral placement, compared to 33% with anterior-posterior. After the final shock, 93% of the anterior-lateral group converted to normal rhythm versus 85% of the anterior-posterior group. This was a notable finding because anterior-posterior placement had been the traditional recommendation based on older studies that used monophasic (now outdated) technology.
The Quick Decision Framework
In practice, the decision tree is straightforward. First: does the patient have a pulse? If no, and the rhythm is VF or pulseless VT, defibrillate immediately with an unsynchronized shock. If yes, and the patient is unstable with a fast heart rate, deliver a synchronized cardioversion shock. If yes and the patient is stable, try medications first and consider cardioversion only if drugs fail or the patient’s condition worsens.
The one scenario that blurs the line is when a patient with a pulse deteriorates into pulselessness during preparation for cardioversion. At that point, the protocol switches immediately from synchronized cardioversion to unsynchronized defibrillation. Most modern defibrillators have a sync button that can be toggled on or off for exactly this reason.