Synchronized cardioversion is a controlled electrical shock delivered to the heart to reset an abnormal rhythm back to normal. The key word is “synchronized”: the device reads the heart’s electrical activity in real time and times the shock to land at a precise, safe moment in the heartbeat cycle. This timing is what separates it from standard defibrillation and makes it safe to use on a heart that still has an organized rhythm.
Why Timing Matters
Your heart’s electrical cycle has a vulnerable window near the end of each beat. If an electrical shock hits during that window, it can actually trigger ventricular fibrillation, a chaotic, life-threatening rhythm. This is called the R-on-T phenomenon. In synchronized cardioversion, the defibrillator detects the R-wave (the largest spike on a heart monitor, marking the start of each heartbeat) and delivers the shock at that exact moment. By locking onto the R-wave, the device ensures the energy lands well away from the vulnerable period, making the procedure far safer.
The machine doesn’t fire the instant you press the button. It waits, sometimes for a fraction of a second, until it detects the next R-wave. This brief delay is normal and intentional.
How It Differs From Defibrillation
Both procedures use electrical energy to correct dangerous heart rhythms, but they’re used in different situations and work differently in practice. Defibrillation delivers an unsynchronized shock, meaning the energy fires immediately without waiting for any particular point in the heartbeat. That’s necessary for ventricular fibrillation and pulseless ventricular tachycardia, rhythms so chaotic that the machine can’t reliably detect an R-wave to sync with.
Synchronized cardioversion, by contrast, is used when the heart still has an identifiable rhythm but that rhythm is abnormal or dangerously fast. The synchronization protects the heart from being accidentally thrown into fibrillation by the very shock meant to help it. Defibrillation also typically uses higher initial energy levels, while cardioversion can often succeed at lower settings.
Which Heart Rhythms It Treats
Synchronized cardioversion is recommended for several specific abnormal rhythms:
- Atrial fibrillation: the most common reason for the procedure. The upper chambers of the heart quiver chaotically instead of contracting in an organized way.
- Atrial flutter: a rapid but more organized rhythm in the upper chambers.
- Supraventricular tachycardia (SVT): an abnormally fast heartbeat originating above the lower chambers.
- Monomorphic ventricular tachycardia with a pulse: a fast rhythm from the lower chambers that still produces a detectable pulse and blood pressure.
If a patient with ventricular tachycardia loses their pulse, becomes unconscious, or has dangerously low blood pressure, the approach switches to unsynchronized defibrillation. The same applies to polymorphic ventricular tachycardia (where the rhythm is irregular and unpredictable), which is treated like ventricular fibrillation with a high-energy unsynchronized shock.
How Effective It Is
Electrical cardioversion terminates atrial fibrillation in over 90% of cases, making it the most reliable method for restoring a normal rhythm. It’s the treatment of choice for patients with new-onset atrial fibrillation or flutter, especially when the abnormal rhythm is causing serious symptoms like low blood pressure, chest pain, or shortness of breath.
For comparison, medications alone convert recent-onset atrial fibrillation in about 50 to 70% of cases, and they rarely work for episodes that have been going on for longer periods. Success rates for electrical cardioversion drop as the duration of the abnormal rhythm increases. Other factors that influence success include the patient’s age, overall heart health, and whether rhythm-stabilizing medications are given beforehand.
Energy Levels Used
The amount of electrical energy is measured in joules and varies depending on the rhythm being treated. Current American Heart Association guidelines recommend:
- Atrial fibrillation: 200 joules
- Atrial flutter: 200 joules
- Narrow-complex tachycardia (SVT): 100 joules
- Monomorphic ventricular tachycardia: 100 joules
If the first shock doesn’t work, the energy is increased in stepwise fashion and the shock is repeated. Atrial flutter and SVT often respond to lower energy levels than atrial fibrillation.
Preparing for the Procedure
If cardioversion is planned (elective rather than emergency), you’ll be asked to fast beforehand, just as you would before any procedure involving sedation. Short-acting sedation is used so you’re unconscious during the shock but wake up quickly afterward. Most elective cardioversions are done as outpatient procedures, meaning you go home the same day.
Blood thinners play a critical role in preparation. If your atrial fibrillation or flutter has lasted more than 48 hours, or if the duration is unknown, guidelines recommend anticoagulation therapy for 3 to 4 weeks before the procedure. This reduces the risk that a blood clot formed in the heart could break loose when the normal rhythm returns and cause a stroke. The same anticoagulation continues for at least 3 to 4 weeks after the procedure as well. In some cases, an imaging study of the heart (specifically looking for clots in the left atrial appendage) is done beforehand. If a clot is found, cardioversion is typically postponed until adequate anticoagulation has been completed.
Risks and Complications
Synchronized cardioversion is a low-risk procedure overall. The most significant concern is stroke or systemic embolism from a dislodged blood clot. A 2025 study of over 2,500 emergency department patients with acute atrial fibrillation or flutter found that the 30-day rate of stroke, systemic embolism, or death was 0.37% among those who underwent cardioversion compared to 0.23% among those who did not. That difference was not statistically significant, and at 7 days the risk was essentially zero in both groups.
Other possible complications include skin irritation or mild burns at the paddle sites, temporary drops in blood pressure from the sedation, and the possibility that the abnormal rhythm returns after initially converting. In rare cases, the shock itself can trigger a different arrhythmia, though the synchronization feature specifically minimizes this risk.
What Recovery Looks Like
After the procedure, you’ll be monitored as the sedation wears off, which typically takes a short time given the fast-acting medications used. Your heart rhythm will be watched on a monitor to confirm the normal rhythm is holding. Most people feel groggy for a brief period and may notice mild soreness on the chest where the paddles were placed. You’ll need someone to drive you home because of the sedation. The skin irritation, if any, usually resolves within a few days. Your doctor will typically continue or adjust your blood-thinning medication and may also prescribe a rhythm-stabilizing drug to help prevent the abnormal rhythm from returning.