Synchronized cardioversion is used to treat organized cardiac rhythms that have a detectable R wave on an electrocardiogram, including atrial fibrillation, atrial flutter, supraventricular tachycardia, and monomorphic ventricular tachycardia with a pulse. The procedure delivers a precisely timed electrical shock to reset the heart’s electrical activity, and the key word is “synchronized,” because the timing of that shock relative to the heart’s own electrical cycle determines whether the treatment helps or makes things worse. Not every fast or abnormal rhythm qualifies, and the distinction between rhythms you cardiovert and rhythms you defibrillate is one of the most consequential calls in emergency medicine.
Why Synchronization Matters
Every heartbeat produces a predictable electrical pattern. The tall spike on the monitor, the R wave, corresponds to the main contraction of the ventricles. Shortly afterward comes the T wave, which represents the ventricles resetting for the next beat. That T-wave window is called the vulnerable period of repolarization, and delivering a shock during it can throw the heart into ventricular fibrillation, a chaotic, pulseless rhythm that is immediately life-threatening.1Annals of Emergency Medicine. Synchronized cardioversion of unstable supraventricular tachycardia resulting in ventricular fibrillation
A synchronized cardioverter solves this by detecting the R wave and timing its shock to land on that peak, safely away from the vulnerable period. This is what separates cardioversion from defibrillation. In defibrillation, the shock fires the instant you press the button, because the rhythm is already so disorganized (ventricular fibrillation or pulseless ventricular tachycardia) that there is no R wave to sync with. Cardioversion, by contrast, is for rhythms that still have recognizable electrical structure. In a documented case, a cardioverter mistakenly synchronized to the T wave instead of the R wave and triggered ventricular fibrillation, which illustrates exactly why proper sensing is critical.1Annals of Emergency Medicine. Synchronized cardioversion of unstable supraventricular tachycardia resulting in ventricular fibrillation
Atrial Fibrillation
Atrial fibrillation is by far the most common rhythm treated with electrical cardioversion. In AFib, the upper chambers of the heart fire chaotically instead of contracting in an organized pattern, which leads to an irregular and often fast heartbeat. Cardioversion can restore normal sinus rhythm when medication alone has not worked, when the patient is becoming hemodynamically unstable, or when a rhythm-control strategy is preferred over simply slowing the rate.
The main complication everyone worries about is stroke. When the atria fibrillate rather than contract, blood can pool and form clots, particularly in a small pouch called the left atrial appendage. If cardioversion snaps the atria back into coordinated motion, any clot sitting there can be ejected into the bloodstream and travel to the brain. For this reason, conventional guidelines call for at least three weeks of anticoagulation before elective cardioversion, followed by four weeks of anticoagulation afterward.2PubMed. Role of transesophageal echocardiography-guided cardioversion of patients with atrial fibrillation An alternative approach uses transesophageal echocardiography (a camera passed down the esophagus to image the heart from behind) to look directly for clots. If no clot is found, cardioversion can proceed sooner.2PubMed. Role of transesophageal echocardiography-guided cardioversion of patients with atrial fibrillation
One study raised an important caution about relying too heavily on a short course of blood thinners. Among patients who had visible clots or spontaneous echo contrast (a “smoke-like” swirling of blood suggesting stasis) on imaging before starting direct oral anticoagulants, a clot was still present in about 70% of those with a baseline thrombus after three weeks of treatment.3PubMed Central. Are Three Weeks of Oral Anticoagulation Sufficient for Safe Cardioversion in Atrial Fibrillation? That finding underscores why the decision about when to cardiovert is never purely about rhythm; it is inseparable from clot risk management.
Atrial Flutter
Atrial flutter is AFib’s more organized cousin. Instead of the chaotic electrical activity of fibrillation, flutter involves a rapid but regular circuit looping through the atria, typically producing a characteristic “sawtooth” pattern on the ECG. Cardioversion works even better for flutter than for AFib, and it requires less energy to do the job.4EP Europace. Cardioversion of atrial fibrillation and atrial flutter revisited: current evidence and practical guidance for a common procedure
In a large analysis of over a thousand shocks, the final-shock success rate for atrial flutter was about 98%, compared with 88% for AFib.5PubMed Central. Association between transthoracic impedance and electrical cardioversion success with biphasic defibrillators The cumulative energy needed was also substantially lower. With modern biphasic defibrillators, flutter typically required a cumulative energy of around 108 joules on average, compared to roughly 199 joules for AFib.6American Heart Journal. Comparative efficacy of monophasic and biphasic waveforms for transthoracic cardioversion of atrial fibrillation and atrial flutter
Despite its reputation as a milder arrhythmia, atrial flutter still carries a real stroke risk around cardioversion. Studies have confirmed that “atrial stunning,” a temporary paralysis of the atrial appendage after the shock, also occurs in flutter patients, though to a lesser degree than in AFib.7PubMed. Left atrial appendage “stunning” after electrical cardioversion of atrial flutter During this stunned period, blood flow in the appendage slows dramatically, creating conditions where clots can form even after the rhythm has been corrected. Anticoagulation around flutter cardioversion is therefore treated with similar caution as for AFib.
Supraventricular Tachycardia
SVT is an umbrella term for several fast rhythms that originate above the ventricles, most commonly involving a short-circuit loop in or near the AV node. The typical episode comes on suddenly, with heart rates often between 150 and 250 beats per minute, and in many cases the patient feels terrible but remains hemodynamically stable. For stable patients, the first-line approach is not a shock at all. Vagal maneuvers (bearing down as if straining, or applying a cold stimulus to the face) and adenosine, a drug that briefly interrupts conduction through the AV node, terminate most SVT episodes without any electricity.8Pediatric Emergency Care. Emergency Department Management of the Pediatric Patient With Supraventricular Tachycardia
Synchronized cardioversion enters the picture when the patient is unstable: low blood pressure, altered consciousness, chest pain suggesting ischemia, or signs of heart failure. In those situations, there is no time to cycle through medications. The energy required for SVT cardioversion is generally lower than for AFib, often starting at 50 to 100 joules with biphasic devices. Because SVT has a very regular, well-organized rhythm, the defibrillator can lock onto the R wave reliably, which makes synchronization straightforward.
Ventricular Tachycardia with a Pulse
Monomorphic ventricular tachycardia, a fast, regular rhythm originating in the ventricles, is probably the scariest rhythm that still qualifies for synchronized cardioversion. It qualifies because the patient still has a pulse, meaning the heart is pumping enough blood to maintain some circulation, and because the rhythm is organized enough to have an identifiable R wave. The distinction between “tolerated” and “untolerated” VT is clinically meaningful. In one study, the vast majority of patients with untolerated VT (those who needed cardioversion within about two and a half minutes) showed abnormal blood-pressure responses, compared with about a third of patients whose VT was better tolerated hemodynamically.9PubMed. Vasovagal Responses to Human Monomorphic Ventricular Tachycardia
If a patient with monomorphic VT is relatively stable, clinicians may try intravenous medications first (amiodarone, procainamide, or lidocaine depending on the setting). But the moment the blood pressure drops or consciousness wavers, synchronized cardioversion becomes urgent. The energy levels are higher than for atrial rhythms, typically starting at 100 joules and escalating. An important nuance: if the VT degenerates into ventricular fibrillation or if the monitor cannot reliably identify the QRS complex, the sync mode must be turned off and unsynchronized defibrillation used instead.
Pre-Excited Atrial Fibrillation and WPW Syndrome
Wolff-Parkinson-White syndrome involves an extra electrical pathway between the atria and ventricles. Most of the time, WPW causes episodes of regular SVT that respond to conventional treatment. But when a WPW patient develops atrial fibrillation, the situation becomes dangerous because the accessory pathway can conduct the chaotic atrial impulses directly to the ventricles at extremely high rates, far faster than the normal AV node would allow. This is called pre-excited atrial fibrillation, and it can deteriorate into ventricular fibrillation.
The critical management point is what you must not give these patients. Adenosine, the standard SVT drug, is contraindicated because it blocks conduction through the AV node while leaving the accessory pathway wide open, potentially accelerating conduction to dangerous ventricular rates.10PubMed Central. Pre-Excited Atrial Fibrillation in Wolff-Parkinson-White (WPW) Syndrome: A Case Report and a Review of the Literature Case reports have documented exactly this scenario: a patient given adenosine for a presumed narrow-complex SVT who then developed pre-excited atrial fibrillation.11PubMed. Pre-excited atrial fibrillation triggered by intravenous adenosine: a commonly used drug with potentially life-threatening adverse effects In one series, two patients who received adenosine for narrow-complex tachycardia subsequently developed AFib with fast ventricular rates and required emergency electrical cardioversion.12Indian Pacing and Electrophysiology Journal. Incidence, clinical, electrophysiological characteristics and outcomes of patients with Wolff-Parkinson-White syndrome and atrial fibrillation
Current guidelines recommend synchronized DC cardioversion as a first-line treatment when a pre-excited AFib patient is hemodynamically unstable.10PubMed Central. Pre-Excited Atrial Fibrillation in Wolff-Parkinson-White (WPW) Syndrome: A Case Report and a Review of the Literature For stable patients, certain antiarrhythmic drugs that slow conduction through the accessory pathway (procainamide, ibutilide, or flecainide) are preferred over AV-node blockers. But because the rhythm can deteriorate unpredictably, the threshold for proceeding straight to cardioversion is low.
Rhythms You Do Not Cardiovert
Equally important to knowing what to cardiovert is knowing what never gets the sync button. Ventricular fibrillation and pulseless ventricular tachycardia are treated with unsynchronized defibrillation, not cardioversion, because these rhythms have no reliable R wave to lock onto. Attempting to synchronize on a chaotic rhythm would cause the device to search endlessly for an R wave while the patient remains pulseless.
Polymorphic ventricular tachycardia, especially the form known as torsades de pointes, occupies a unique position. Its QRS complexes constantly change shape and axis, making reliable R-wave detection nearly impossible, so it too is treated with unsynchronized defibrillation if the patient is pulseless or deteriorating. For torsades specifically, intravenous magnesium is a first-line pharmacological treatment. A scoping review found that magnesium resolved torsades in about 78% of patients treated, with no serious magnesium-related side effects, though roughly one in five patients progressed to ventricular fibrillation during treatment, highlighting the need to have a defibrillator charged and ready.13PubMed Central. Safety and Efficacy of Intravenous Magnesium for Torsade de Pointes ― A Scoping Review ― The underlying cause of torsades, often a prolonged QT interval from medications or electrolyte abnormalities, also needs to be corrected, or the rhythm will keep recurring regardless of how many shocks are delivered.
Pharmacological Cardioversion as an Alternative
Not every cardioversion involves electricity. Pharmacological cardioversion uses intravenous drugs to chemically restore normal rhythm, and it is most commonly employed for recent-onset atrial fibrillation. The practical appeal is obvious: no sedation is required, it can be done in a monitored bed, and patients generally prefer it to being shocked. The downside is that it is less reliable. In a trial comparing two commonly used drugs, flecainide converted about 56% of recent-onset AFib cases and ibutilide about 50%, with no significant difference in adverse events between the two.14PubMed. Flecainide versus ibutilide for immediate cardioversion of atrial fibrillation of recent onset
Pharmacological cardioversion works best when AFib has been present for less than about 48 hours. The longer the atria have been fibrillating, the more the atrial tissue remodels electrically, making it progressively harder for drugs to reset the rhythm. After several days of AFib, electrical cardioversion has a significantly better success rate and is usually preferred. The same anticoagulation rules apply regardless of whether the cardioversion method is chemical or electrical.
Pad Placement and Energy Selection
Where you stick the pads matters more than most people realize. A randomized trial comparing anterior-lateral placement (one pad on the front right chest, one under the left armpit) with anterior-posterior placement (one pad on the sternum, one on the back between the shoulder blades) found a substantial difference in first-shock success for AFib. The anterior-posterior position succeeded on the first shock in about 54% of patients compared to 33% for anterior-lateral, a 22-percentage-point gap.15PubMed. Anterior-Lateral Versus Anterior-Posterior Electrode Position for Cardioverting Atrial Fibrillation By the final shock attempt, though, the gap narrowed: 93% of the anterior-lateral group eventually converted compared with 85% of the anterior-posterior group. The anterior-posterior position gets more current through the atrial tissue on the first try, but repeated shocks with anterior-lateral placement can catch up.
Modern defibrillators use biphasic waveforms, which deliver current in both directions. Compared to older monophasic devices, biphasic shocks require substantially less cumulative energy. For AFib, monophasic devices needed an average of about 554 joules of cumulative energy versus roughly 199 joules with biphasic shocks. For flutter, the figures were about 251 versus 108 joules.6American Heart Journal. Comparative efficacy of monophasic and biphasic waveforms for transthoracic cardioversion of atrial fibrillation and atrial flutter In practice, this means clinicians today typically start AFib cardioversion at 120 to 200 joules biphasic and can go lower for flutter or SVT.
Sedation for Electrical Cardioversion
Electrical cardioversion is painful, though the shock lasts only milliseconds. Patients are given brief procedural sedation, putting them under just long enough for the shock to be delivered. The two most commonly used agents are propofol and etomidate. Both provide rapid onset of unconsciousness and quick recovery, but they have different side-effect profiles. Propofol tends to drop blood pressure more, which is a concern in patients whose hemodynamics are already borderline. Etomidate is gentler on blood pressure but is sometimes associated with brief involuntary muscle movements (myoclonus) that can be disconcerting.16PubMed Central. Cardioversion: What to choose? Etomidate or propofol
Respiratory depression is the main risk with either drug. Studies comparing the two have generally found both to be safe for this purpose, with brief episodes of apnea that resolve quickly and rarely require anything beyond a jaw-thrust maneuver or brief bag-mask ventilation.17PubMed Central. Propofol for Sedation for Direct Current Cardioversion Adding an opioid like fentanyl can provide additional pain control and allow lower doses of the sedative, but it also increases the risk of airway complications. Most cardioversions are performed with an anesthesiologist or experienced emergency physician managing the airway.
What Happens to the Heart After Cardioversion
Restoring sinus rhythm is not the end of the story. After cardioversion of atrial fibrillation or flutter, the atria do not immediately return to full mechanical function. This temporary sluggishness, called atrial stunning, can last days to weeks. Imaging studies have shown that left atrial appendage blood-flow velocity drops sharply after cardioversion of atrial flutter, and markers of blood stasis increase significantly in that same window.18PubMed. Reversal of atrial mechanical stunning after cardioversion of atrial arrhythmias This paradox, that clot risk actually rises temporarily right after the rhythm is fixed, is the main reason anticoagulation must continue for at least four weeks post-cardioversion even when the procedure is successful.19PubMed. Prevalence of thrombus, spontaneous echo contrast, and atrial stunning in patients undergoing cardioversion of atrial flutter
Beyond stroke risk, there is a psychological dimension that often goes unaddressed. A study of patients presenting for cardioversion or ablation found that roughly 17% had moderate to severe depressive symptoms and about 30% reported significant state anxiety around the time of their procedure.20PubMed Central. Exploring depressive symptoms and anxiety among patients with atrial fibrillation and/or flutter at the time of cardioversion or ablation Higher levels of depression and anxiety correlated with worse quality-of-life scores related to their arrhythmia. For patients undergoing repeated cardioversions, which is common since AFib recurs frequently after initial restoration of sinus rhythm, the cumulative emotional burden deserves more clinical attention than it typically gets.
Cardioversion in Pregnancy
Arrhythmias can emerge or worsen during pregnancy because of the increased blood volume, hormonal changes, and altered cardiac output that come with carrying a fetus. When a pregnant patient develops a hemodynamically significant tachyarrhythmia, electrical cardioversion is considered safe at any gestational age. The electrical current delivered during cardioversion does not reach the fetus in clinically meaningful amounts because it travels between the two pads on the mother’s chest.
That said, it is not entirely without risk. A multicenter study of direct current cardioversion during pregnancy found that in two out of 29 procedures, urgent delivery was required afterward, and fetal monitoring was performed after only about half of the procedures at gestations beyond 26 weeks.21PubMed. Direct current cardioversion in pregnancy: a multicentre study A literature review also identified five cases of preterm delivery and two cases of maternal-fetal mortality among pregnant women who underwent electrical cardioversion, though in such rare events it is difficult to separate the risks of the procedure from the risks of the underlying arrhythmia itself.22PubMed Central. Efficacy and safety of electrical cardioversion and catheter ablation during pregnancy: a case review and literature analysis The consensus is that cardioversion should be performed when clinically indicated, with fetal monitoring available and facilities ready for emergency cesarean section, particularly in the third trimester.23PubMed Central. Electrical cardioversion during pregnancy: safe or not?
Patients with Pacemakers and Implanted Defibrillators
People with implanted cardiac devices, whether pacemakers, implantable cardioverter-defibrillators (ICDs), or cardiac resynchronization devices, sometimes still need external cardioversion for rhythms their device cannot handle. An ICD, for instance, is designed to detect and treat ventricular tachycardia and fibrillation but typically does not treat atrial fibrillation. When external cardioversion is needed, the main concern is that the electrical energy could damage the device or its leads, or alter its programmed settings.
In a survey of German centers and systematic review, most facilities preferred anterior-posterior pad placement at energies of 150 joules or higher using biphasic waveforms for these patients.24PubMed. Electrical cardioversion of patients with implanted pacemaker or cardioverter-defibrillator: results of a survey of german centers and systematic review of the literature The anterior-posterior position is preferred specifically because it directs current away from the device generator, which is usually implanted in the upper left chest. The device should be interrogated (checked by a technician) before and after the procedure to confirm that its settings have not been altered and that the leads are still sensing and pacing properly. A randomized comparison confirmed that biphasic shocks were effective and safe in this population.25PubMed. External cardioversion of atrial fibrillation in patients with implanted pacemaker or cardioverter-defibrillator systems
Cardioversion in Children
Pediatric cardioversion follows the same principles as in adults but with weight-based dosing. The energy dose matters a great deal in children because their smaller body mass means a given amount of energy has a proportionally larger effect. A study of 40 episodes of atrial arrhythmias in children found that the sweet spot for first-shock success was in the range of 0.5 to 1.0 joules per kilogram of body weight, which converted 88% of cases. Doses below 0.5 J/kg succeeded only 25% of the time, confirming that underdosing is a real risk in pediatric patients. Interestingly, doses above 1.0 J/kg converted only 56% of episodes, suggesting that more energy is not always better and that the response curve has a plateau.
The most common arrhythmias requiring cardioversion in children include SVT that fails to respond to adenosine, atrial flutter (particularly in infants and children after cardiac surgery), and, rarely, ventricular tachycardia. The same synchronization rules apply: organized rhythms get the sync mode, and the switch to unsynchronized defibrillation happens if the rhythm degenerates into ventricular fibrillation. Procedural sedation is typically administered by a pediatric anesthesiologist because airway management in children demands particular expertise.