Cardioversion: What Resetting the Heart's Rhythm Actually Involves

The Scene

The following scene is drawn from the composite of patients I have cared for in clinic and on the hospital floor. All identifying details are changed.

He came in at eleven in the morning complaining of fatigue that had started four days earlier. No chest pain. No syncope. His heart rate on arrival was 118 beats per minute, irregular. The ECG showed atrial fibrillation. He had no prior cardiac history. His blood pressure was 138/88. His echocardiogram showed a normal ejection fraction and no left atrial thrombus on transesophageal echo. He had been in AFib for no more than four days by his estimate, and possibly less.

He asked me a simple question: “Can you just shock it back to normal?”

That question, stripped of all its medical complexity, is actually the right question. Electrical cardioversion delivers a precisely timed electrical current through the chest wall to interrupt the chaotic electrical activity of atrial fibrillation and allow the sinus node to resume its role as the heart’s primary pacemaker. It takes approximately 0.5 seconds. When it works, the heart transitions from the irregular, poorly coordinated rhythm of atrial fibrillation to the organized, effective rhythm of sinus rhythm within a single heartbeat.

The complication he did not know about, and the one that every AFib patient needs to understand before cardioversion is discussed, is the clot risk. Atrial fibrillation causes the left atrium and, specifically, the left atrial appendage, to lose coordinated mechanical function. Blood stagnates in that appendage. Thrombus forms. When sinus rhythm is restored, whether spontaneously or by cardioversion, the left atrium begins contracting again. If a thrombus is present, it can be ejected into the systemic circulation. Stroke. The cardioversion itself does not cause this. The restoration of mechanical function does.

This is why the anticoagulation question is inseparable from the cardioversion question, and why the timing of AFib onset matters so much. It is also why the AFFIRM trial’s finding, that rate control was non-inferior to rhythm control in terms of mortality and stroke risk, fundamentally changed how cardiologists think about the benefit of cardioversion as a long-term strategy versus a symptom-management strategy.

At Carle Foundation Hospital in Urbana-Champaign, cardioversion is performed in a monitored setting with sedation, defibrillation backup, and a protocol-driven anticoagulation evaluation that begins before the date is scheduled. The decision to cardiovert is not separate from the decision about anticoagulation. It is the same decision.

What It Is

Cardioversion is the restoration of normal sinus rhythm from a pathological atrial or ventricular arrhythmia using either electrical current or pharmacologic agents. In clinical cardiology, the term almost always refers to atrial fibrillation, though it is also used for atrial flutter, atrial tachycardia, and, in emergencies, ventricular tachycardia with a pulse.

Electrical cardioversion (ECV): A synchronized biphasic DC electrical shock is delivered through two electrode pads placed on the chest wall (or, less commonly, anteroposteriorly). “Synchronized” means the defibrillator detects the QRS complex and delivers the shock during ventricular depolarization, avoiding the vulnerable period during ventricular repolarization (the T wave), where unsynchronized shocks can induce ventricular fibrillation. The energy dose is typically 100 to 200 joules for AFib and 50 to 100 joules for atrial flutter.

Pharmacologic cardioversion: Antiarrhythmic drugs that can restore sinus rhythm include flecainide, propafenone (class IC agents), ibutilide and dofetilide (class III agents), and amiodarone. Pharmacologic cardioversion is less reliably effective than electrical cardioversion for AFib (success rates approximately 50 to 60 percent for recent-onset AFib with flecainide vs 90 to 95 percent for electrical cardioversion) but does not require procedural sedation 5 / Solid 94713-5).

Biphasic versus monophasic waveforms: Modern defibrillators deliver biphasic waveforms (current flows in both directions sequentially). Biphasic cardioversion requires lower energy doses and has higher first-shock success rates than the older monophasic technology 5 / Solid . Monophasic defibrillators are largely obsolete in cardioversion practice.

What cardioversion does not do: It restores the heart rhythm. It does not address the underlying substrate that caused atrial fibrillation. Structural remodeling of the left atrium, fibrosis, inflammation, hypertension, sleep apnea, valvular disease, and other drivers of AFib are unchanged by cardioversion. The arrhythmia tends to recur unless those underlying conditions are addressed. This is the single most important concept for patients to grasp: cardioversion is a reset, not a cure. The reset holds if the conditions that caused the arrhythmia are treated. It does not hold if those conditions persist.

The Mechanism

Atrial fibrillation physiology: Normal cardiac rhythm requires a single dominant pacemaker (the sinus node) generating organized electrical wavefronts that depolarize the atria and ventricles in sequence. AFib occurs when multiple small re-entrant circuits form within atrial tissue, generating chaotic, disorganized atrial electrical activity at a rate of 350 to 600 impulses per minute. The AV node cannot conduct all of these impulses and acts as a filter, producing an irregularly irregular ventricular response at 100 to 160 bpm in untreated AFib.

The substrate for these re-entrant circuits is atrial fibrosis and dilation. As the atrial wall becomes fibrotic (from hypertension, heart failure, mitral valve disease, aging, inflammation), the conduction velocity across the atrium becomes heterogeneous. Areas of slow conduction adjacent to normal-speed tissue create the conditions for micro-reentry. The left pulmonary veins are the primary anatomical triggers: ectopic electrical activity arising from cardiac muscle sleeves that extend into the pulmonary vein ostia initiates and often maintains AFib (Haissaguerre, N Engl J Med. 1998; doi:10.1056/NEJM199809033391003).

How electrical cardioversion works: The DC electrical shock simultaneously depolarizes all myocardial cells within its current field. This mass depolarization extinguishes all existing re-entrant wavefronts simultaneously. All cells are refractory. The sinus node, which has the fastest intrinsic rate of automaticity, fires its next impulse into a fully refractory myocardium and, when refractoriness resolves in an orderly fashion, establishes itself as the dominant pacemaker. Sinus rhythm is restored.

The current is delivered through a biphasic waveform that flows from the anterior patch (right of the sternum or apex) to the posterior patch (posterior chest, below the left scapula) and back. The anteroposterior position concentrates current through the left atrium and is preferred over the traditional anterolateral position for AFib, achieving higher first-shock success rates 5 / Solid 02345-1).

Why AFib recurs after cardioversion: Restoration of sinus rhythm does not reverse atrial remodeling. Electrical remodeling (shortened atrial refractory periods that favor re-entry) takes days to weeks to reverse after cardioversion, a period of vulnerability called the “stunning period.” Structural remodeling (fibrosis, dilation) may not reverse at all if the underlying cause persists. The result is that AFib recurs in approximately 25 percent of patients within one month of cardioversion and in 50 to 70 percent within one year without antiarrhythmic therapy 5 / Solid . Antiarrhythmic drug therapy prescribed after cardioversion (usually flecainide, sotalol, dronedarone, or amiodarone depending on the patient’s comorbidities) reduces but does not eliminate this recurrence risk.

How It Is Used

Timing of AFib onset: the 48-hour rule:

The thromboembolic risk of cardioversion is directly tied to how long the patient has been in AFib.

AFib confirmed to have started within 48 hours: The risk of intracardiac thrombus is low. Anticoagulation with therapeutic heparin or a DOAC should be started immediately, and cardioversion can proceed after ensuring adequate anticoagulation. The 2023 ESC AFib guidelines support this approach 5 / Solid .

AFib of unknown duration or confirmed to have lasted more than 48 hours: Thrombus may be present in the left atrial appendage. Two strategies:

1. Three-week anticoagulation strategy: Therapeutic anticoagulation for at least three weeks before cardioversion (ensuring INR 2.0 to 3.0 for warfarin or uninterrupted therapeutic DOAC), followed by cardioversion, followed by at least four weeks of anticoagulation post-cardioversion. The four-week post-cardioversion period is required because mechanical stunning of the left atrium persists even after electrical sinus rhythm is restored, maintaining thrombus risk until full mechanical function returns.

2. TEE-guided early cardioversion strategy: Transesophageal echocardiography (TEE) is performed before cardioversion to directly visualize the left atrial appendage and exclude thrombus. If no thrombus is found, cardioversion can proceed within 24 to 48 hours, followed by at least four weeks of anticoagulation. This strategy avoids three weeks of pre-cardioversion anticoagulation wait time. The ACUTE trial demonstrated equivalent safety between TEE-guided early cardioversion and the three-week strategy, with similar stroke rates 5 / Solid .

Long-term anticoagulation after cardioversion: The decision about long-term anticoagulation is based on the patient’s stroke risk, calculated using the CHA2DS2-VASc score, not on whether they are in sinus rhythm. If a patient has a CHA2DS2-VASc score of 2 or more (in men) or 3 or more (in women), anticoagulation should continue even if cardioversion was successful and sinus rhythm is maintained. AFib is a paroxysmal condition, and silent, undetected AF recurrences can cause stroke. Stopping anticoagulation because cardioversion succeeded is one of the most common and consequential management errors in AFib care 5 / Solid .

The Evidence

AFFIRM Trial

The AFFIRM trial (Atrial Fibrillation Follow-up Investigation of Rhythm Management) is the most cited evidence base for the rate-control-versus-rhythm-control debate in AFib. Published in the New England Journal of Medicine in 2002, the trial enrolled 4,060 patients with persistent or paroxysmal AFib and at least one stroke risk factor and randomized them to a rhythm control strategy (cardioversion plus antiarrhythmic drugs to maintain sinus rhythm) versus a rate control strategy (rate-controlling medications without systematic cardioversion) 5 / Solid .

Results:

• All-cause mortality: 23.8 percent (rhythm control) vs 21.3 percent (rate control); p = 0.08 (non-significant trend favoring rate control)
• Hospitalization: significantly higher in rhythm control group
• Stroke: similar in both groups, but critically, most strokes in the rhythm control group occurred after anticoagulation had been stopped, in patients who were believed to be in sinus rhythm

The AFFIRM trial did not show that rhythm control was superior to rate control for the primary outcome of mortality. This was a paradigm-shifting finding. The implication was not that cardioversion is useless, but that cardioversion alone, without sustained anticoagulation and attention to the underlying AFib substrate, does not translate into mortality benefit.

The critical lesson from AFFIRM: stroke risk in AFib is driven by the patient’s CHA2DS2-VASc score, not by their current rhythm status. A patient with a CHA2DS2-VASc score of 4 who is in sinus rhythm after cardioversion is still a high-stroke-risk patient. Anticoagulation continues.

EAST-AFNET 4 Trial

The EAST-AFNET 4 trial, published in 2020, challenged the AFFIRM conclusion in an important way. The trial enrolled 2,789 patients with early AFib (first diagnosed within the prior 12 months) and randomized them to early rhythm control versus rate control. The primary outcome was a composite of cardiovascular death, stroke, worsening heart failure, or ACS 5 / Solid .

Results:

• Primary outcome: 3.9 percent per year (early rhythm control) vs 5.0 percent per year (rate control)
• Hazard ratio: 0.79 (95% CI 0.66 to 0.94; p = 0.005)
• Absolute risk reduction: 1.1 percent per year
• Number needed to treat: approximately 91 patients per year to prevent one primary outcome event

The EAST-AFNET 4 trial rekindled interest in rhythm control, but with an important qualification: benefit was seen in early AFib (within 12 months of diagnosis), not in longstanding AFib. The mechanism is likely early prevention of atrial remodeling. This trial does not contradict AFFIRM; rather, it adds the temporal dimension. Early rhythm control, including early cardioversion, in newly diagnosed AFib may reduce downstream cardiovascular events. Rhythm control in longstanding AFib is primarily about symptom management.

Cardioversion and Stroke Risk

The clinical trials of cardioversion-associated stroke date to the pre-anticoagulation era, when the stroke rate immediately after cardioversion was approximately 1 to 3 percent. With adequate anticoagulation (either three-week pre-cardioversion or TEE-guided exclusion of thrombus plus four-week post-cardioversion anticoagulation), the periprocedural stroke rate with cardioversion is below 0.5 percent 5 / Solid .

Antiarrhythmic Drug Maintenance After Cardioversion

The ATHENA trial demonstrated that dronedarone reduced hospitalizations for cardiovascular causes and cardiovascular death in patients with paroxysmal or persistent AFib (HR 0.76; 95% CI 0.69 to 0.84) 5 / Solid . Dronedarone is contraindicated in heart failure with reduced ejection fraction and permanent AFib (the PALLAS trial showed harm). The choice of antiarrhythmic drug for rhythm maintenance after cardioversion depends on the patient’s EF, coronary artery disease status, and other comorbidities.

Success Rates of Cardioversion

First-shock success rates with modern biphasic electrical cardioversion are 85 to 95 percent for AFib of recent onset (less than one week). For longstanding persistent AFib (more than one year), first-shock success rates drop to 50 to 70 percent and recurrence within 24 hours after successful cardioversion is more common 5 / Solid . Predictors of failure include left atrial diameter greater than 5.5 cm, longstanding AFib, obesity, and high thoracic impedance.

The Patient Experience

The Day of the Procedure

Patients fast for at least 4 to 6 hours before cardioversion because conscious sedation or propofol-based moderate sedation is used for the procedure. Full anesthesia is not required; most patients receive a brief intravenous agent (commonly propofol, etomidate, or midazolam plus fentanyl) that produces 3 to 5 minutes of sedation. The procedure itself takes less than one minute.

Two large adhesive electrode pads are placed on the chest, one anterior (right parasternal or left lateral precordium) and one posterior (below the left scapula). An anesthesiologist or procedural nurse administers sedation. The defibrillator is charged. The shock is delivered. The patient feels nothing. They typically wake within 30 to 60 seconds wondering why everyone is watching them.

Post-procedure monitoring continues for 30 to 60 minutes. A 12-lead ECG confirms sinus rhythm. Vital signs are monitored. Patients are discharged home with a driver.

What Your Cardiologist Will Not Have Time to Explain

Most patients hear “cardioversion” and think “defibrillation for cardiac arrest.” These are related but distinct procedures. Defibrillation for ventricular fibrillation is unsynchronized, high-energy, and urgent. Cardioversion for AFib is synchronized, planned, and performed under sedation. The patient is not in cardiac arrest. The shock is not felt.

What is also frequently not explained:

• The procedure can fail on the first attempt and a second higher-energy shock may be needed
• If cardioversion restores sinus rhythm today, the average patient will be back in AFib within six to twelve months without antiarrhythmic medication
• The fact that you are in sinus rhythm after cardioversion does not mean your anticoagulation can stop if your CHA2DS2-VASc score is 2 or above
• Skin redness or mild irritation under the electrode pads is common and resolves within a day

Sex Differences

Women with AFib tend to present later in the disease course and with more symptomatic burden than men, yet are less frequently referred for rhythm control strategies including cardioversion 5 / Solid . Women have higher rates of antiarrhythmic drug side effects and proarrhythmia (particularly QT prolongation with class III agents), which influences antiarrhythmic drug selection after cardioversion. Women also have higher baseline stroke risk for the same CHA2DS2-VASc score, which means anticoagulation decisions after cardioversion need to account for sex-specific risk. Post-cardioversion antiarrhythmic management in women requires particular attention to baseline QTc and renal function before initiating sotalol or dofetilide.

Geographic Access in Illinois

Cardioversion is performed at multiple cardiology programs across Illinois. At Carle Foundation Hospital in Urbana-Champaign, electrical cardioversion is available with same-day TEE if needed, allowing the TEE-guided early cardioversion strategy for patients without a confirmed 48-hour onset window. Northwestern Medicine Bluhm Cardiovascular Institute in Chicago and Rush University Medical Center offer full AFib management programs that include cardioversion within integrated rate-rhythm-anticoagulation management pathways. OSF Saint Francis Medical Center in Peoria provides electrical cardioversion services with cardiology hospital coverage.

Decisions and Trade-Offs

Should I Have a Cardioversion or Accept Rate Control?

This decision turns on symptoms, AFib duration, and underlying heart disease.

Rhythm control (cardioversion) is preferable when:

• AFib is newly diagnosed (within 12 months, per EAST-AFNET 4 data)
• The patient is symptomatic with AFib (fatigue, dyspnea, palpitations that impair quality of life)
• Heart failure with reduced ejection fraction is present and tachycardia-mediated cardiomyopathy is suspected
• The patient is young with a low burden of structural heart disease, where long-term sinus rhythm maintenance is more achievable

Rate control is preferable when:

• AFib is longstanding (greater than one year) and the patient is minimally symptomatic
• Multiple cardioversions have failed with rapid recurrence
• The substrate for AFib (significant atrial fibrosis, severe mitral disease, very large left atrium) makes sustained sinus rhythm maintenance unlikely

Neither strategy eliminates the need for anticoagulation in high-risk patients.

Cardioversion Versus Catheter Ablation for AFib

Catheter ablation (pulmonary vein isolation) is the most effective rhythm control strategy for paroxysmal AFib and is increasingly used for persistent AFib. In patients who have had multiple cardioversions with rapid recurrence, catheter ablation addresses the pulmonary vein triggers and atrial substrate, producing more durable sinus rhythm maintenance than cardioversion alone 5 / Solid . The CASTLE-AF trial showed that catheter ablation reduced all-cause mortality and heart failure hospitalizations in patients with AFib and heart failure with reduced ejection fraction compared with medical therapy. Cardioversion is appropriate as a first step or as a bridge to ablation; ablation addresses the substrate that cardioversion cannot.

The Anticoagulation Decision: When Is It Safe to Stop?

Stopping anticoagulation after successful cardioversion is appropriate only when all of the following are true:

• The patient has a CHA2DS2-VASc score of 0 (men) or 1 (women) — genuinely low risk
• At least four weeks of post-cardioversion anticoagulation have been completed
• The patient has had no prior strokes, TIA, or thromboembolism

For any patient with a score above these thresholds, anticoagulation should continue indefinitely regardless of rhythm status. The reasoning: AFib is rarely truly cured by cardioversion or even by ablation. Asymptomatic recurrences are common, detected only by monitoring. Stopping anticoagulation in a patient who feels well and is believed to be in sinus rhythm is the pathway to the stroke that AFFIRM showed was just as common in the rhythm control arm as the rate control arm.

The Three Questions Every Patient Should Ask

1. “How long do I need to be on anticoagulation after cardioversion?” The honest answer depends on your CHA2DS2-VASc score. If your score is 2 or above, anticoagulation should continue indefinitely, whether you stay in sinus rhythm or not. The four-week minimum post-cardioversion anticoagulation period is the floor, not the ceiling.

2. “What is the likelihood that my AFib will come back?” Without antiarrhythmic drug therapy, approximately 25 percent of patients recur within one month and 50 to 70 percent within one year. With antiarrhythmic drug therapy, rates are lower but recurrence remains common, particularly with longstanding or persistent AFib. Ask specifically about the expected recurrence rate given your AFib duration and left atrial size.

3. “If cardioversion works today but AFib comes back repeatedly, what is the next step?” The answer should be catheter ablation, which addresses the substrate that cardioversion cannot. Repeated cardioversions without addressing the underlying driver of AFib is a cycle rather than a treatment plan.

The SDE Synthesis

Cardioversion occupies a specific position in AFib management: it is the fastest way to restore sinus rhythm when the clinical indication is clear, and it is appropriate as a first-line strategy for newly diagnosed, symptomatic AFib. But the cardioversion conversation cannot happen without the anticoagulation conversation, and neither can happen without the substrate conversation.

The SDE Foundations framework positions cardioversion as one component of a rhythm management strategy, not an isolated procedure. A patient who comes in for cardioversion needs to understand three things before the procedure is scheduled:

First, the anticoagulation timeline. Whether it is three weeks of pre-cardioversion anticoagulation or same-day TEE, there is a sequence that must be followed, and the four weeks post-procedure are non-negotiable for most patients.

Second, the recurrence risk and what determines it. Left atrial size, AFib duration, presence of hypertension, obesity, sleep apnea, alcohol use, thyroid disease — these are the drivers of recurrence. Cardioversion in the presence of untreated contributing factors is a temporary fix. The SDE Audit, when applied to an AFib patient, includes a structured review of all modifiable contributors to AF burden: blood pressure control, weight, sleep, alcohol, thyroid function, and cardiac structure.

Third, the ablation pathway. For patients with recurring symptomatic AFib, catheter ablation is the more durable rhythm control strategy. The CASTLE-AF data establish ablation as the preferred strategy in AFib with reduced EF. For patients without reduced EF but with recurrent symptomatic AFib, ablation offers a 50 to 70 percent long-term freedom from AFib at five years, compared with much lower rates for antiarrhythmic drug maintenance after cardioversion. The SDE clinical pathway routes recurring AFib patients toward ablation evaluation, not repeated cardioversion, after the first or second recurrence.

Cardioversion services are available at Carle Foundation Hospital in Urbana-Champaign with integrated AFib management including anticoagulation protocols and ablation referral pathways. Northwestern Medicine Bluhm Cardiovascular Institute in Chicago operates one of the highest-volume AFib programs in the Midwest, offering same-day TEE, cardioversion, and ablation consultation within a single-day AFib evaluation visit. SDE Cohort patients with AFib receive integrated rhythm and anticoagulation management as part of their longitudinal cardiovascular plan.

Paired Foundations Articles:

• PROC-007: Cardiac Ablation (pulmonary vein isolation as the durable rhythm control strategy)
• PROC-006: Watchman (LAA occlusion for patients who cannot take long-term anticoagulation)
• PROC-016: ICD and Pacemaker Implantation (device-based therapy in AFib with structural disease)
• PROC-008: EP Study (mechanism characterization before ablation planning)