Is a Cardioversion Dangerous? Evaluating the Risks

Cardioversion is one of the safer procedures in cardiology, but it is not risk-free. In large observational studies, the rate of serious immediate complications sits around 1 to 2 percent, with stroke occurring in roughly 0.3 percent of cases within 30 days. Most of the danger comes not from the shock itself but from what happens afterward, particularly the risk of blood clots dislodging when the heart returns to a normal rhythm. That risk, in turn, depends heavily on whether you are adequately anticoagulated beforehand, which makes preparation as important as the procedure.

What the Complication Numbers Actually Look Like

A study of over 1,000 patients undergoing non-emergency electrical cardioversion found that immediate complications occurred in about 1.4 percent of cases. Within 30 days, roughly 11 percent were rehospitalized for various reasons and about 1.4 percent died, though those deaths were not necessarily caused by the cardioversion itself. The patients in this study were often older and had multiple heart conditions, which inflates the 30-day numbers compared with what a younger, healthier person would face.¹

A separate decade-long emergency department study painted a more reassuring picture for acute cases: among patients cardioverted for new-onset atrial fibrillation, there were no strokes, no major bleeding events, no life-threatening arrhythmias, and no peripheral blood clots. The only physical injury recorded was nine minor skin burns at the electrode site.² The gap between these two datasets makes sense: patients cardioverted quickly for a brand-new arrhythmia are very different from patients with long-standing atrial fibrillation and multiple comorbidities. Your personal risk depends heavily on which group you resemble.

Stroke and Blood Clots After the Shock

The most feared complication of cardioversion is stroke. When the heart flips from atrial fibrillation back to a normal rhythm, any blood clot that formed in the left atrium during the arrhythmia can be ejected into the bloodstream. The FibStroke study identified 100 cardioversions that led to a stroke or transient ischemic attack. Most of these events happened within about two days, and the majority occurred in patients who were not on anticoagulation therapy.³ That study also noted something sobering: about a fifth of the patients who had a post-cardioversion stroke had been considered low risk for stroke based on standard scoring tools. Risk scores help, but they do not catch everyone.

A Finnish registry study found that within 30 days of electrical cardioversion, stroke occurred in about 0.28 percent of patients. The raw numbers initially suggested that acute (urgent) cardioversion carried a higher stroke risk than elective (planned) procedures. But once researchers adjusted for whether patients were on blood thinners, that difference disappeared. The real dividing line was anticoagulation status, not urgency. Patients on either warfarin or newer oral anticoagulants had a substantially lower stroke risk than those who were not anticoagulated.⁴

Why Anticoagulation Is the Single Most Important Safety Measure

Guidelines universally recommend that patients be adequately anticoagulated before elective cardioversion, typically for at least three weeks beforehand and four weeks afterward. Even with proper anticoagulation, the risk is not zero. In one observational study of 296 patients who underwent elective cardioversion while adequately anticoagulated, three patients (about 1 percent overall) still experienced a thromboembolic event, including one ischemic stroke and one case of acute mesenteric ischemia.⁵ One percent is low, but it is not nothing, and it explains why doctors sometimes add an extra layer of screening.

That extra layer is a transesophageal echocardiogram, where a probe is passed down the esophagus to get a close-up view of the left atrial appendage, the small pouch in the heart where clots most often form during atrial fibrillation. If a clot is found there, cardioversion is postponed.⁶ This imaging approach is especially valuable when a patient has not been adequately anticoagulated for the full three weeks, or when other risk factors are present. In one study of over 500 anticoagulated patients referred for cardioversion, clots or dense “smoke-like” echoes in the left atrial appendage were detected in nearly 38 percent, a surprisingly high rate that underscores why imaging catches problems that blood thinners alone sometimes do not.⁷

Atrial Stunning and Why Clots Can Form Even After the Arrhythmia Stops

You might expect that once the heart returns to a normal rhythm, the risk of clots forming would immediately drop. In reality, the opposite happens briefly. After cardioversion, the atrium goes through a period of sluggish mechanical activity called atrial stunning. The atrial muscle contracts weakly or barely at all, allowing blood to pool and potentially clot. Stunning is worst right after the procedure and can persist for days to weeks, depending on how long the arrhythmia lasted beforehand, the size of the left atrium, and whether underlying heart disease is present.⁸ This is the main reason anticoagulation is continued for at least four weeks after a successful cardioversion, even if the heart rhythm looks perfectly normal. The atrium needs time to “wake up” mechanically before the clot risk truly subsides.

Research measuring blood flow velocities in the left atrial appendage has confirmed this phenomenon directly. After termination of an arrhythmia, appendage flow speeds dropped sharply and “spontaneous echo contrast” (the swirling smoke pattern that suggests sluggish blood flow) increased significantly.⁹ In plain terms, the heart was beating normally again but the atrium was barely pumping. This transient vulnerability is one of the less intuitive risks of the procedure and one reason skipping post-procedure anticoagulation is genuinely dangerous.

Arrhythmias Triggered by the Shock Itself

Modern cardioversion devices deliver their shock synchronized to the heart’s own electrical cycle to avoid landing energy during the vulnerable phase of the heartbeat. When synchronization works correctly, dangerous rhythms like ventricular fibrillation are extremely rare. But case reports exist of synchronized shocks still triggering ventricular fibrillation, particularly in unstable patients. These events are uncommon enough to be published as individual case studies rather than appearing at measurable rates in large trials.

A more documented concern is what can happen in the seconds after a successful shock. When the heart returns to sinus rhythm, it sometimes does so at a very slow rate, and that pause or bradycardia can create the conditions for a dangerous arrhythmia called torsades de pointes. This risk is heightened in patients who are also taking drugs that prolong the QT interval, which includes some of the very medications used to maintain sinus rhythm after cardioversion.¹⁰ In a study comparing electrical and pharmacological cardioversion, no deaths, strokes, or sustained dangerous arrhythmias occurred in either group, though a small fraction of patients experienced prolongation of the QT interval past the safety threshold and brief episodes of slow heart rate that resolved on their own within half an hour.¹¹

Slow Heart Rhythms After Cardioversion, Especially in Older Patients

Beyond the immediate post-shock period, cardioversion can unmask an underlying tendency toward slow heart rhythms. This is particularly relevant for older adults. A large study examining the 30-day risk of slow-rhythm events after cardioversion found that the risk climbed steeply with age: roughly 0.5 percent at age 40, about 1.2 percent at 65, around 2.7 percent at 80, and over 5 percent at 90.¹² Some of these patients eventually needed a pacemaker. The underlying issue is that the same degenerative process that allows atrial fibrillation to develop can also impair the heart’s natural pacemaker cells. Cardioversion can expose that weakness once the chaotic atrial rhythm is no longer masking it.

Sedation Risks

Electrical cardioversion is painful, so patients receive short-acting sedation or general anesthesia. The shock itself takes a fraction of a second, but the sedation carries its own set of complications. A Cochrane review examining 23 studies and over 1,200 patients found that the most commonly studied agent was propofol, which was compared against etomidate, midazolam, thiopentone, and sevoflurane. The main complications across all agents were drops in blood pressure and brief periods of apnea (where the patient temporarily stops breathing).¹³ These are typically managed with standard monitoring and are short-lived, but they are the reason cardioversion is performed in a setting with full resuscitation equipment. For most healthy patients, the sedation is uneventful, but in people with severe lung disease, obesity, or a history of difficult airways, the anesthesia component can pose more practical risk than the shock itself.

A meta-analysis comparing electrical cardioversion with drug-based cardioversion for new-onset atrial fibrillation found that the rate of hypotension was actually lower in the electrical group, likely because some pharmacological agents used to convert the rhythm carry their own blood-pressure-lowering effects.¹⁴

Patients with Pacemakers and Defibrillators

People with implanted cardiac devices sometimes worry that an external shock could damage their hardware. The concern is not unfounded in theory: the energy from external paddles passes through the chest and could affect the leads or programming of an implanted device. In practice, though, serious problems are rare. A study of patients with pacemakers and implantable cardioverter-defibrillators found two cases of programming changes and four cases of premature battery depletion, but no deaths related to device malfunction in the year following cardioversion. Minor, temporary changes in device measurements (sensing, pacing thresholds, and impedances) were observed but generally resolved.¹⁵

A randomized trial comparing internal cardioversion (delivered through the device’s own leads) with external cardioversion in patients with implantable defibrillators found no clinically relevant adverse events from either approach. Three patients did have pre-existing silent lead damage that was unmasked by an internal shock, but the leads were already failing before the procedure.¹⁶ Another randomized study found that pacing measurements dipped slightly right after the shock but returned to baseline within a week, and no device or lead dysfunction occurred in any patient.¹⁷ The standard recommendation remains that the device should be interrogated (checked electronically) shortly after the procedure, but the vast majority of patients with implanted hardware come through cardioversion without problems.

Cardioversion During Pregnancy

Arrhythmias during pregnancy are uncommon but not unheard of, and doctors are understandably cautious about any procedure that involves sedation and electrical energy near a fetus. The existing evidence, though drawn from small numbers, is reassuring. A multicenter study of 27 pregnant women who underwent cardioversion found no maternal deaths and a live birth in every case. The median point in pregnancy when cardioversion was performed was 28 weeks. Fetal monitoring was done after the procedure in just under half the cases, and on two occasions urgent delivery was needed afterward.¹⁸ A review of the available literature concluded that cardioversion is safe and effective during pregnancy, provided continuous fetal heart-rate monitoring is used and a team capable of performing an emergency cesarean section is on standby.¹⁹ The main risk is not the shock harming the fetus directly but rather the hemodynamic shifts from sedation and rhythm change potentially causing fetal distress.

Electrode Placement and Body Size

Where the pads are placed on the chest affects both the success rate and the amount of energy needed. The EPIC trial compared anterolateral pad placement (one pad on the front of the chest, one on the side) with anteroposterior placement (front and back). After the first shock, a higher proportion of patients in the anterolateral group converted to sinus rhythm. The benefit of anterolateral placement was especially pronounced in patients with obesity, where the success rate improved by about 15 percentage points compared with the other pad position.²⁰ For non-obese patients, the difference was much smaller. While pad placement is ultimately a clinical decision, this finding matters because failed first shocks often mean escalating energy levels or repeated attempts, which increase sedation time and the chance of skin irritation.

Repeat Cardioversions Over Time

Atrial fibrillation frequently recurs after successful cardioversion, and some patients undergo the procedure multiple times over years. This raises a natural question: does repeated cardioversion pile up risk? A study tracking patients over five years found that those who had more cardioversions also had higher rates of atrial fibrillation hospitalizations and ablation procedures, but stroke rates did not increase with the number of cardioversions. There was a trend toward higher five-year mortality in patients who had undergone more than five cardioversions compared with those who had just one, but this did not reach statistical significance.²¹ The interpretation here requires some caution: patients who need many cardioversions tend to have more severe or treatment-resistant arrhythmias, so the higher hospitalization and mortality rates likely reflect the underlying disease burden rather than cumulative damage from the shocks.

A randomized trial in patients 65 and older compared repeated cardioversion with catheter ablation and with a pacemaker-plus-node-ablation strategy. About 36 percent of patients in the cardioversion arm needed repeat procedures, and 40 percent eventually crossed over to one of the other treatment strategies.²² This high crossover rate suggests that while individual cardioversions are safe, a pattern of frequent recurrence often signals that a different long-term management approach is worth discussing.

QT Prolongation and Drug Interactions

Some of the medications prescribed to keep the heart in sinus rhythm after cardioversion can interact with the procedure in ways that create their own risks. Drugs like dofetilide, sotalol, and amiodarone prolong the QT interval, a measure of electrical recovery time in the heart. When cardioversion is performed while these drugs are on board, the combination of post-shock bradycardia and already-prolonged electrical recovery can set the stage for dangerous arrhythmias. In a study of patients receiving dofetilide during cardioversion, the drug infusion had to be stopped early in four patients because QT prolongation exceeded 500 milliseconds, and one patient developed a brief run of torsades de pointes, though without symptoms.²³ This is one reason cardioversion is typically done in a monitored setting with continuous telemetry for at least a few hours afterward, especially when QT-prolonging drugs are involved.

The Cost Side of the Equation

While not a physical risk, the financial implications of cardioversion shape how patients and clinicians think about the procedure. A study comparing emergency department management strategies for atrial fibrillation found that the median charges for patients who were cardioverted and discharged from the emergency department were about $5,500, versus roughly $23,200 for patients who were admitted to the hospital without an attempt at cardioversion.²⁴ These are charges rather than out-of-pocket costs, and they come from a single U.S. center, but the gap illustrates that a successful cardioversion with same-day discharge can be dramatically less expensive than the alternative of prolonged hospitalization. For patients weighing the risks, it is worth remembering that avoiding cardioversion does not mean avoiding cost or medical complexity. Uncontrolled atrial fibrillation carries its own hospitalizations, medication burdens, and risks.

Pain and Anxiety During the Procedure

Although sedation prevents patients from feeling the shock in the moment, the anticipation of the procedure causes genuine anxiety for many people, especially those who have had a prior cardioversion or who are undergoing internal cardioversion with lower levels of sedation. Research has found that patients who tend toward catastrophic thinking about pain reported higher affective pain ratings from the first shock and increasing fear with subsequent shocks.²⁵ This is not a trivial point for patients who face repeat procedures. Some patients develop a visceral dread of cardioversion that affects their willingness to return for the procedure, which can lead them to live with poorly controlled atrial fibrillation instead. Clinicians who address anxiety proactively with clear communication and adequate sedation planning can make a measurable difference in how patients experience and tolerate the procedure over time.