Yes, synchronized cardioversion is one of the most effective ways to restore a normal heart rhythm in atrial fibrillation (AFib). Using a stepwise approach, it restores normal sinus rhythm in over 99% of patients acutely, with about 87% converting on the very first shock. The procedure works by delivering a carefully timed electrical shock that resets the heart’s chaotic electrical signals, and it’s considered the treatment of choice when AFib is causing dangerous symptoms.
Why the Shock Must Be Synchronized
Your heart’s electrical cycle has a brief window during each beat where it’s especially vulnerable to disruption. This window corresponds to the T wave on a heart monitor, the moment when the heart muscle is resetting itself electrically. If a shock lands during this period, it can throw the lower chambers of the heart into ventricular fibrillation, a life-threatening cardiac arrest rhythm. This is called the R-on-T phenomenon, and it’s the entire reason synchronization exists.
When a defibrillator is set to “sync” mode, it doesn’t fire the instant you press the button. Instead, it reads the heart’s electrical signal, waits for the next R wave (the tall spike that represents the start of a heartbeat), and delivers the shock at that precise moment. This ensures the energy hits during the safest part of the cardiac cycle, well away from the vulnerable T wave. The result: you reset the abnormal rhythm without accidentally stopping the heart.
When Cardioversion Is Used for AFib
Synchronized cardioversion serves two very different clinical scenarios. In an emergency, when AFib is driving dangerously low blood pressure, chest pain, or signs of heart failure, the 2023 ACC/AHA guidelines call immediate electrical cardioversion the treatment of choice. Speed matters in these cases, and no other intervention works as fast.
In a non-emergency (elective) setting, cardioversion is planned for patients whose AFib hasn’t responded to medications or whose symptoms significantly affect quality of life. The procedure is scheduled, and preparation focuses heavily on preventing blood clots, since restoring a normal rhythm can dislodge clots that formed while the heart was fibrillating.
The Blood Clot Question
When the upper chambers of the heart fibrillate instead of contracting normally, blood can pool and form clots, particularly in a small pouch called the left atrial appendage. Shocking the heart back into rhythm can then push those clots into the bloodstream, causing a stroke. This is the single biggest safety concern with cardioversion, and it dictates the timeline for the entire procedure.
If your AFib has lasted longer than 48 hours, or if no one knows exactly when it started, European and American guidelines recommend at least three weeks of blood-thinning medication before cardioversion. Clinical trials show this approach, combined with continued anticoagulation afterward, reduces the 30-day risk of a clot-related event to less than 1%. You’ll also need to continue blood thinners for at least four weeks after the procedure, because the heart’s upper chambers can be sluggish even after returning to a normal rhythm.
There’s a shortcut for patients who can’t wait three weeks or whose medication adherence is uncertain. A transesophageal echocardiogram (TEE), an ultrasound probe passed down the throat to get a close-up view of the heart, can directly check for clots. If the imaging is clear, cardioversion can proceed without the full three-week anticoagulation window. The risks of the TEE itself are minimal.
What the Procedure Feels Like
Elective cardioversion is brief, typically lasting only a few minutes for the shock itself. You’ll be given intravenous sedation to keep you unconscious during the actual shock, so you won’t feel it. The sedation is short-acting, designed to put you under just long enough for the procedure while keeping you breathing on your own. Most people wake up within minutes and don’t remember the shock at all.
Adhesive electrode pads are placed on your chest (and sometimes your back). The clinician sets the defibrillator to synchronized mode, selects the energy level, and delivers the shock. If the first attempt doesn’t convert your rhythm, the energy is increased and another shock is delivered. With modern biphasic defibrillators, the initial energy setting for AFib is typically 120 to 200 joules. Older monophasic machines start at 200 joules. Each failed attempt is followed by a step up in energy.
How Successful Is It?
Immediate success rates are high. In a large study of 414 patients using a stepwise energy protocol, 87.4% converted to normal rhythm on the first shock. When subsequent shocks at higher energy levels were added, the overall acute success rate climbed to 99.3%. The three patients who didn’t convert had their protocols interrupted for other reasons, not because the shocks themselves failed.
The catch is staying in normal rhythm long-term. AFib has a strong tendency to return, particularly in people with enlarged atria, longstanding AFib, or untreated underlying conditions like sleep apnea or uncontrolled high blood pressure. Many patients who undergo successful cardioversion will eventually need a repeat procedure, ongoing medication, or a catheter ablation to maintain sinus rhythm. Your likelihood of staying in rhythm improves if the underlying triggers are addressed.
Risks and What Can Go Wrong
The most serious acute risk, ventricular fibrillation from an unsynchronized shock, is effectively eliminated by the sync function on the defibrillator. As long as the machine correctly identifies and times the shock to the R wave, this complication is extremely rare. Skin irritation or mild burns at the pad sites can occur. Some patients experience brief drops in blood pressure from the sedation medications.
Stroke from a dislodged clot remains the most consequential risk overall, which is why the anticoagulation protocol exists. Following the recommended blood-thinner schedule before and after the procedure is critical to keeping this risk low.
There’s also a small chance the heart converts briefly and then slips back into AFib within hours or days. This doesn’t mean cardioversion “failed” permanently, but it may indicate that medication or ablation will be needed alongside any future attempts.
After the Procedure
You’ll be monitored on a heart rhythm monitor and pulse oximeter as the sedation wears off. Most patients are observed for one to several hours before discharge, depending on how they respond. Because of the sedation, you won’t be able to drive yourself home. You’ll need to continue your prescribed blood thinner for at least four weeks, even if the procedure was successful and your rhythm feels completely normal. Some patients stay on anticoagulation indefinitely based on their individual stroke risk profile.