Synchronized cardioversion is used for four main abnormal heart rhythms: atrial fibrillation, atrial flutter, supraventricular tachycardia (SVT), and monomorphic ventricular tachycardia. The common thread is that these are all fast rhythms where the heart still has an organized electrical pattern and the patient still has a pulse. Whether cardioversion happens urgently or as a planned procedure depends on how the rhythm is affecting the patient.
Why Synchronization Matters
Every heartbeat produces a predictable electrical cycle. The tall spike on a heart monitor, called the R-wave, marks the moment the main pumping chambers contract. Shortly after that contraction, there’s a brief window called the relative refractory period (visible as the T-wave on the monitor) when the heart muscle is resetting and is electrically vulnerable. If an electrical shock hits during that vulnerable window, it can trigger ventricular fibrillation, a chaotic, pulseless rhythm. That would turn a patient who has a pulse into one in cardiac arrest.
Synchronized cardioversion solves this by using a sensor that detects the R-wave and times the shock to land right on it, safely away from the vulnerable period. The machine won’t release energy until it identifies the correct moment. This is also why synchronized cardioversion typically uses less energy than defibrillation: the shock is working with the heart’s existing electrical pattern rather than trying to override total chaos.
The Four Rhythms That Require It
Atrial Fibrillation
In atrial fibrillation (AFib), the upper chambers of the heart quiver rapidly and irregularly instead of contracting in a coordinated way. The resulting heartbeat is fast and uneven. Cardioversion is used when AFib causes significant symptoms or when medications haven’t restored a normal rhythm. For planned cardioversions, timing matters: if AFib has lasted longer than 48 hours (or the duration is unknown), guidelines from the American College of Cardiology recommend at least three weeks of blood-thinning medication beforehand, or an imaging test of the heart to rule out blood clots. This precaution exists because blood can pool and clot in the quivering upper chambers, and restoring a normal rhythm could dislodge a clot.
Atrial Flutter
Atrial flutter is similar to AFib but more organized. The upper chambers beat in a rapid, regular loop pattern, often around 300 beats per minute, though the lower chambers typically respond at a slower rate (commonly 150 beats per minute). Atrial flutter generally responds well to cardioversion, often at lower energy levels than AFib. The same blood clot precautions apply since the upper chambers aren’t contracting effectively in flutter either.
Supraventricular Tachycardia
SVT is an umbrella term for several fast rhythms that originate above the main pumping chambers. The heart rate often jumps suddenly to 150 to 250 beats per minute. Many SVT episodes respond to simpler interventions first, like bearing down (a Valsalva maneuver) or medications given through an IV. Synchronized cardioversion becomes necessary when these measures fail and the patient is showing signs of distress.
Monomorphic Ventricular Tachycardia
Monomorphic ventricular tachycardia (VT) originates in the lower chambers and produces a fast, regular rhythm where each heartbeat looks identical on the monitor. This is the key distinction: the rhythm has a consistent, repeatable pattern, which gives the machine a reliable R-wave to lock onto. As long as the patient has a pulse, synchronized cardioversion is appropriate. If the patient loses their pulse, or if the rhythm is polymorphic VT (where the electrical pattern keeps shifting and looks chaotic), the treatment switches to unsynchronized defibrillation because the machine can’t reliably identify an R-wave to sync with.
When Cardioversion Becomes Urgent
Any of these four rhythms can be managed either as a planned procedure or as an emergency, depending on how the patient is doing. The 2025 AHA tachycardia algorithm identifies five warning signs that call for immediate cardioversion:
- Hypotension: blood pressure dropping low enough to compromise organ function
- Altered mental status: confusion, drowsiness, or unresponsiveness that developed with the fast rhythm
- Signs of shock: pale or clammy skin, weak pulse, poor circulation
- Ischemic chest pain: chest discomfort suggesting the heart muscle isn’t getting enough blood
- Acute heart failure: sudden difficulty breathing, fluid buildup in the lungs
When a heart rate exceeds 150 beats per minute and any of these signs are present, the fast rhythm is the likely cause. In those situations, cardioversion takes priority over medication because restoring a normal rhythm is the fastest way to stabilize the patient. When the heart rate is below 150, it becomes less likely that the rhythm itself is causing the instability, and other causes should be explored first.
What the Procedure Feels Like
For planned cardioversions (most commonly for AFib or flutter), you’ll receive short-acting sedation through an IV so you’re unconscious during the shock. The most widely used sedative for this purpose is propofol, used in roughly 90% of UK hospitals surveyed, sometimes combined with a short-acting pain medication. The sedation wears off quickly, usually within minutes, and most people don’t remember the shock at all. The entire procedure often takes less than 30 minutes, though you’ll be monitored for a period afterward.
In emergency situations where the fast rhythm is immediately life-threatening, sedation is still given when possible, but treatment won’t be delayed if sedation isn’t immediately available.
When Cardioversion Should Be Avoided
One important situation where synchronized cardioversion is dangerous is digoxin toxicity. Digoxin is a medication used to control heart rate, and when levels build up too high, the heart becomes electrically unstable. Delivering a shock in that state can trigger ventricular fibrillation. If digoxin toxicity is suspected, the toxic drug levels need to be addressed first.
Cardioversion is also not appropriate for certain rhythms. Sinus tachycardia, a fast heart rate caused by exercise, fever, dehydration, or anxiety, doesn’t benefit from a shock because the fast rate is the heart’s normal response to a stressor. Treat the underlying cause and the heart rate comes down on its own. And as noted above, polymorphic ventricular tachycardia and any pulseless rhythm require unsynchronized defibrillation, not synchronized cardioversion.