What Is Cardioversion for AFib and How Does It Work

Cardioversion is a medical procedure that resets the heart’s rhythm back to normal in people with atrial fibrillation, the most common sustained heart-rhythm disorder. It comes in two forms: electrical cardioversion, which delivers a carefully timed shock to the chest, and pharmacological cardioversion, which uses drugs to chemically coax the heart out of its chaotic rhythm. The procedure itself is fast, but the preparation and follow-up involve anticoagulation, imaging decisions, and a realistic conversation about recurrence that most patients find more complex than they expected.

How Electrical Cardioversion Works

During atrial fibrillation, the upper chambers of the heart fire electrical signals in a rapid, disorganized pattern instead of contracting in an orderly way. Electrical cardioversion interrupts that chaos by sending a brief, controlled burst of direct-current energy through the chest wall. The shock depolarizes the heart muscle all at once, giving the heart’s natural pacemaker (the sinus node) a chance to resume control and restore a normal rhythm.

The timing of that shock is critical. The device is synchronized to fire during a specific part of the heartbeat cycle, landing squarely on the electrical spike that corresponds to the ventricles contracting. Delivering energy at the wrong moment, particularly during the heart’s vulnerable recovery phase, could trigger a dangerous ventricular rhythm. Modern devices handle this synchronization with extremely high reliability. In one analysis of over 242,000 heartbeats and more than 3,700 delivered shocks, synchronization accuracy was 100%, with no episodes of the shock triggering a dangerous ventricular arrhythmia.¹ The rare instances where ventricular fibrillation has been triggered involve inappropriate sensing of the wrong part of the heartbeat, not a failure of the synchronization logic itself.²

The technology has evolved considerably since the early 1960s, when direct-current cardioversion was first introduced as a replacement for alternating-current shocks, which carried a much higher risk of provoking dangerous rhythms. By the late 1990s, the field had shifted from monophasic waveforms to biphasic waveforms, which achieve higher success rates at lower energy levels and cause less damage to heart tissue.³

Why Pad Placement Matters

One detail that makes a measurable difference in whether cardioversion succeeds on the first attempt is where the electrode pads go. The two standard positions are anterior-lateral (one pad on the front of the chest, one on the left side) and anterior-posterior (one on the front, one on the back between the shoulder blades). Multiple trials have found that the anterior-posterior position is more effective. In one randomized trial, the first shock succeeded in about 33% of patients with anterior-posterior pads compared with 54% failure in the anterior-lateral group, a 22-percentage-point difference in first-shock success.⁴ An earlier trial in The Lancet found a similar gap: cardioversion succeeded in 96% of anterior-posterior patients versus 78% of anterior-lateral patients.⁵ If a first attempt fails, switching pad positions and trying again often gets the job done. In the Lancet study, crossover from the anterior-lateral to the anterior-posterior position succeeded in eight of twelve patients who initially failed.

Pharmacological Cardioversion

Not every cardioversion involves a shock. Certain drugs can restore normal rhythm on their own, particularly when AFib is recent-onset (hours to days rather than weeks or months). The approach works differently from electrical cardioversion. Instead of resetting all the heart tissue at once, drugs alter the electrical properties of heart cells to make the chaotic signals unable to sustain themselves.

Vernakalant is one example. It primarily targets potassium and sodium channels that are found mainly in atrial heart cells, which means it can change the electrical behavior of the upper chambers without significantly affecting the ventricles. It works especially well at the rapid firing rates that characterize AFib, because the drug binds to ion channels in a way that becomes more potent at faster rates.⁶ Class IC drugs like flecainide and propafenone are also commonly used for pharmacological cardioversion, both in the hospital and in outpatient settings.

When comparing overall success rates, the two approaches end up in roughly the same range. A meta-analysis of randomized trials found no meaningful difference in the rate of returning to normal rhythm between electrical and pharmacological approaches (about 89% versus 85%), though electrical cardioversion was associated with less risk of low blood pressure afterward.⁷

The Pill-in-the-Pocket Approach

For people who have occasional, well-tolerated episodes of AFib, there is an option that avoids the emergency room altogether. The “pill-in-the-pocket” strategy gives patients a supply of a class IC drug (usually flecainide or propafenone) to take on their own when an episode starts. The first dose is always tested in a monitored clinical setting to make sure the patient tolerates it safely. If that goes well, they carry the medication with them and self-treat future episodes at home.

The landmark study on this approach followed patients for an average of 15 months. Among those who had recurrences, the self-administered drug worked in 94% of episodes, and symptoms resolved in under two hours on average. Emergency room visits and hospitalizations dropped significantly compared with the year before patients started the strategy.⁸ The protocol typically includes taking a heart-rate-slowing drug about 30 minutes before the antiarrhythmic to prevent the AFib from converting into atrial flutter with a dangerously fast heart rate, which is one of the known risks.⁹

Serious side effects during the initial monitored dose are uncommon but real. In one study of 273 patients, significant adverse events occurred in about 3%, including fainting, dangerously slow heart rate, and atrial flutter with rapid conduction. All of these events happened in patients taking the standard single doses of flecainide (300 mg) or propafenone (600 mg).¹⁰ This is exactly why the first dose needs medical supervision: catching that 3% in a controlled environment rather than at home.

Anticoagulation Before and After

Blood-clot risk is the central safety concern around cardioversion, and the anticoagulation timeline depends on how long AFib has been going on. When the atria fibrillate instead of contracting properly, blood can pool and clots can form, particularly in a pouch called the left atrial appendage. If cardioversion then restores a normal contraction, it can dislodge a clot and send it to the brain, causing a stroke.

The standard protocol for AFib lasting more than 48 hours requires at least three weeks of anticoagulation therapy before the procedure. Alternatively, a transesophageal echocardiogram (a specialized ultrasound done through the esophagus to get a close look at the left atrium) can check for clots directly. If no clot is found, cardioversion can proceed sooner, though anticoagulation still starts immediately.¹¹ For AFib lasting less than 48 hours, guidelines are less rigid, but heparin or a similar short-acting blood thinner is typically given before the procedure as a precaution.

In emergencies where the patient is hemodynamically unstable, cardioversion cannot wait three weeks. In that situation, intravenous heparin is given immediately before the shock.¹²

Anticoagulation continues for at least four weeks after cardioversion regardless of whether the heart stays in normal rhythm. The reason is a phenomenon called atrial stunning: even after the rhythm is restored, the atria are mechanically sluggish for a period, meaning they do not contract forcefully enough to prevent blood from pooling.¹³ This temporary weakness is why clots can still form after a successful procedure. For patients who have other stroke risk factors, anticoagulation often continues indefinitely, independent of whether cardioversion was performed.

What Sedation Involves

Electrical cardioversion is painful if you are awake for it, so the procedure is done under brief sedation. Propofol is one of the most commonly used agents. It works quickly, wears off quickly, and patients typically wake up within minutes of the shock. The main side effect is a temporary drop in blood pressure, since propofol relaxes blood vessels. Studies comparing propofol against another sedation agent, etomidate, have found that blood pressure tends to dip lower with propofol, but severe drops leading to dangerous levels are rare and are easily managed when they occur.¹⁴ From the patient’s perspective, the experience is usually brief and uneventful: you go to sleep, the shock happens, and you wake up moments later.

Why AFib Often Comes Back

Cardioversion is very good at restoring normal rhythm in the short term, but keeping the heart in that rhythm long-term is a different challenge. AFib recurrence after cardioversion is common. In a large retrospective study, early recurrence was detected in about a third of patients who had initially successful procedures.¹⁵ The same study found that a long history of AFib (more than five years) and an episode lasting over 30 days were independent predictors of outright failure.

Other factors that raise the odds of recurrence include a large left atrium (a sign of structural remodeling from prolonged AFib), not being on antiarrhythmic medication after the procedure, smoking, a faster resting heart rate, and a history of stroke.¹⁶ One study even found that higher levels of trait anxiety before the procedure were independently associated with early recurrence, alongside left atrial size.¹⁷ The mechanism is not entirely clear, but chronic anxiety may affect autonomic nervous system tone in ways that make the atria more susceptible to fibrillation.

Antiarrhythmic drugs reduce the chance of recurrence but do not eliminate it. A Cochrane review covering multiple drugs found that every analyzed medication reduced AFib recurrence to some degree, with amiodarone showing the strongest effect. Even so, AFib still recurred in 43% to 67% of people taking antiarrhythmics.¹⁸ These drugs also carry their own side effects, and an older systematic review found that class IA drugs (like quinidine) were associated with increased mortality compared with no treatment.¹⁹ Amiodarone, while the most effective at preventing recurrence, carries a well-known burden of long-term toxicity to the thyroid, lungs, and liver. In one head-to-head comparison, amiodarone showed no superior efficacy over flecainide, propafenone, or dronedarone during the first year after cardioversion, which raises the question of whether its side-effect profile is worth accepting as a first choice.²⁰

Why Early Rhythm Control Matters

For years, the dominant question in AFib management was whether it was better to try to maintain normal rhythm (rhythm control) or simply slow the heart rate and accept that the atria will fibrillate (rate control). Older trials found little difference in hard outcomes between the two strategies, which pushed many clinicians toward rate control as the simpler option. More recent evidence has shifted the conversation, particularly when rhythm control is started early.

The EAST-AFNET4 trial, which was stopped early because of clear benefit in the rhythm-control group, found that early rhythm control reduced the combined risk of cardiovascular death, stroke, and heart-failure hospitalization compared with rate control.²¹ A large observational study confirmed the pattern: rhythm control started within six months of AFib diagnosis was associated with a reduced risk of stroke and heart-failure hospitalization. The benefit faded when rhythm control was initiated later. By five years after diagnosis, there was no advantage to starting rhythm control at all.²² The implication for cardioversion is clear: it is likely to deliver the most benefit when used as part of an early rhythm-control strategy rather than as a standalone rescue attempt years into the disease.

How Cardioversion Compares With Catheter Ablation

Cardioversion and catheter ablation are both rhythm-control tools, but they work in fundamentally different ways. Cardioversion resets the rhythm; ablation destroys the tissue generating the abnormal signals in the first place, usually by creating scar lines around the pulmonary veins where most triggers originate. Ablation is a more invasive procedure, often lasting several hours, and targets longer-term freedom from AFib rather than immediate rhythm restoration.

Both procedures cause some degree of atrial stunning, but the effects differ. After ablation, left atrial function tends to be more impaired and takes longer to recover compared with cardioversion. One study found that the atrial pumping function improved from one week to three months after cardioversion, while function after ablation remained lower and had not returned to baseline levels by three months.²³ A separate comparison showed that left atrial function was lowest in patients who had undergone surgical ablation, intermediate in those who had been cardioverted, and highest in healthy controls.²⁴

The stroke risk profile also differs by time. A propensity-matched study of over 24,000 patients found that ablation carried a higher risk of stroke in the first 30 days compared with cardioversion. After that initial window, however, the risk flipped: the ablation group had a lower stroke rate over the longer term.²⁵ This makes intuitive sense. Cardioversion is a reset button that can be pressed again when AFib returns. Ablation aims to prevent AFib from returning at all, so when it works, the long-term stroke risk drops. The procedures are not really competitors; they occupy different places on the treatment timeline, and many patients experience both.

Cardioversion During Pregnancy

AFib during pregnancy is uncommon but does happen, and the question of whether cardioversion is safe for the fetus comes up immediately. The available evidence, drawn from case series and reviews rather than randomized trials, indicates that electrical cardioversion can be performed safely at all stages of pregnancy. The amount of energy reaching the fetus is negligible because the current path runs through the mother’s chest, not through the uterus.²⁶ Standard precautions include continuous fetal heart rate monitoring during and after the procedure, and having the ability to perform an emergency cesarean section if needed, although the need for one has not been reported in the published literature.²⁷ Most antiarrhythmic drugs cross the placenta, which makes electrical cardioversion a particularly attractive option in this population because it avoids fetal drug exposure entirely.

Cost and Emergency Department Management

Where and how cardioversion is performed has a surprisingly large effect on healthcare costs. When cardioversion is done in the emergency department and the patient is discharged the same day, the charges are a fraction of what they are when the patient is admitted without an attempt at cardioversion. One study found median charges of about $5,500 for ED cardioversion and discharge, versus over $23,000 for patients admitted without a cardioversion attempt.²⁸ Another found that the median hospital stay dropped from about 39 hours for admitted patients to 4 hours for those cardioverted and sent home, with median charges roughly a quarter of the admitted group’s.²⁹ These numbers come from the U.S. healthcare system, where admission charges can escalate quickly. The practical takeaway is that for stable patients with new-onset AFib, an emergency department cardioversion followed by same-day discharge is a viable and far less expensive path compared with admission for monitoring alone.