Defibrillation and cardioversion are not the same thing, though they’re closely related. Both deliver an electrical shock to the heart to correct an abnormal rhythm, and both use the same equipment. The critical difference is timing: cardioversion synchronizes the shock to a specific point in the heart’s electrical cycle, while defibrillation delivers the shock immediately with no synchronization at all. That distinction determines when each one is used, what rhythms they treat, and how much risk is involved.
The Key Difference: Shock Timing
Every heartbeat produces a pattern of electrical activity, and within that pattern is a brief vulnerable window. If a shock lands during that window, it can actually trigger a more dangerous rhythm. Cardioversion avoids this by syncing the shock to a safe point in the cycle called the R-wave, which falls during the heart’s natural refractory period, when the muscle is resetting and can’t be accidentally pushed into a worse rhythm.
Defibrillation skips this synchronization entirely. It fires the moment the button is pressed. This sounds riskier, and it would be if used on a heart that still had an organized rhythm. But defibrillation is reserved for situations where the heart’s electrical activity is already so chaotic that there’s no reliable R-wave to sync to. In ventricular fibrillation, for example, the heart is quivering uselessly rather than pumping. There’s no organized beat to protect, so the shock simply depolarizes the entire heart muscle at once, forcing it to reset.
Which Heart Rhythms Get Which Treatment
The type of abnormal rhythm dictates which approach is used. Cardioversion (the synchronized shock) treats rhythms that are abnormal but still organized enough to have a detectable heartbeat. These include:
- Atrial fibrillation: the most common reason for elective cardioversion, where the upper chambers of the heart quiver rapidly and irregularly
- Atrial flutter: a fast but more regular rhythm in the upper chambers
- Narrow-complex tachycardia: a fast heart rate originating above the ventricles
- Monomorphic ventricular tachycardia: a dangerously fast rhythm from the lower chambers that still has a recognizable, repeating pattern
Defibrillation treats the two rhythms that cause cardiac arrest: ventricular fibrillation (a chaotic, quivering pattern with no effective heartbeat) and pulseless ventricular tachycardia. Polymorphic ventricular tachycardia, where the electrical pattern shifts unpredictably from beat to beat, also requires unsynchronized defibrillation because the machine can’t reliably lock onto a consistent R-wave.
Energy Levels Differ by Situation
Both procedures use the same defibrillator machine, but the energy settings vary. For synchronized cardioversion, current AHA guidelines recommend starting at 200 joules for atrial fibrillation and atrial flutter, while narrow-complex tachycardia and monomorphic ventricular tachycardia start lower at 100 joules. If the first shock doesn’t work, the energy is increased on subsequent attempts.
Defibrillation typically starts at higher energy. With older monophasic devices, the traditional first shock was 200 joules, escalating to 360 joules if the first two shocks failed. Newer biphasic defibrillators, which send current in two directions through the heart, are effective at 200 joules or less. The first biphasic AED approved in the United States was set at 150 to 175 joules for the initial shock.
Elective Cardioversion Requires Preparation
One of the biggest practical differences between these procedures is the setting. Defibrillation is an emergency intervention performed on someone in cardiac arrest, often by bystanders with an AED or by paramedics. There’s no time for preparation because the person has no pulse.
Cardioversion, by contrast, is frequently a planned procedure. When someone with atrial fibrillation is scheduled for cardioversion, the preparation starts weeks in advance. Blood clots can form in the heart’s upper chambers during prolonged irregular rhythms, and the shock of cardioversion can dislodge those clots, causing a stroke. To prevent this, patients typically take blood thinners for at least three weeks before the procedure. If the abnormal rhythm has lasted more than 48 hours, this anticoagulation period is standard practice. The 2023 ACC/AHA guidelines note that even patients with atrial fibrillation lasting less than 48 hours face elevated stroke risk after cardioversion, particularly if they have additional risk factors.
If a blood clot is found on imaging before a scheduled cardioversion, the procedure is called off. The patient continues blood thinners for another three to six weeks, then gets repeat imaging to confirm the clot has dissolved before cardioversion is reattempted. After a successful cardioversion, blood thinners continue for at least four more weeks because the upper chambers of the heart often remain sluggish during this recovery period, keeping clot risk elevated.
Because patients are awake and the shock is painful, elective cardioversion is performed under short-acting sedation. Defibrillation in cardiac arrest requires no sedation because the patient is unconscious.
Electrode Placement Is the Same
Both cardioversion and defibrillation use the same electrode pads placed on the chest. Current guidelines from both American and European cardiology organizations recommend an anterior-posterior configuration: one pad on the front of the chest over the left side, the other on the back just below the left shoulder blade. For best results, both pads should be centered at or within about 5 centimeters above the bottom of the breastbone.
If the first cardioversion attempt fails, one strategy is switching to an anterior-lateral placement (one pad on the right upper chest, one on the left side), escalating the energy, or applying firm external pressure over the pads to improve contact. These troubleshooting steps apply to both procedures.
Why the Terminology Gets Confusing
Part of the confusion comes from medical professionals themselves. Defibrillation is sometimes called “unsynchronized cardioversion,” which frames both as versions of the same intervention, just with different settings. That’s technically accurate. The machine is the same, the pads are the same, and the basic principle (resetting the heart’s electrical activity with a shock) is the same. The operator simply toggles the “sync” mode on or off.
In everyday language, “defibrillation” usually refers to the emergency, unsynchronized shock delivered during cardiac arrest. “Cardioversion” usually refers to the synchronized, often planned shock delivered to someone who still has a pulse but an abnormal rhythm. Both reset the heart, but cardioversion does it with more precision, more preparation, and lower stakes than the life-or-death urgency of defibrillation.