Synchronized cardioversion is used for four specific heart rhythms: atrial fibrillation, atrial flutter, narrow-complex tachycardia (also called SVT), and monomorphic ventricular tachycardia with a pulse. These are all fast heart rhythms where the heart still has an organized electrical pattern that the machine can lock onto. That distinction matters because it separates cardioversion from defibrillation, which is used for chaotic, life-threatening rhythms like ventricular fibrillation.
How Cardioversion Differs From Defibrillation
Both cardioversion and defibrillation deliver an electrical shock to reset the heart, but they work differently. A defibrillator fires the moment you press the button. A cardioversion machine waits. It reads the heart’s electrical signal and times the shock to land on a specific part of the heartbeat cycle, the moment the main pumping chambers contract. This is called “synchronizing” to the QRS complex.
The timing exists for a safety reason. If a shock lands during the brief recovery phase of the heartbeat (the T-wave on a heart monitor), it can actually trigger ventricular fibrillation, a far more dangerous rhythm. Synchronization avoids that window entirely. This is why cardioversion is only possible when the heart still has a recognizable, organized rhythm for the machine to track. In ventricular fibrillation or pulseless ventricular tachycardia, the electrical activity is too chaotic to sync with, so unsynchronized defibrillation is used instead.
Atrial Fibrillation
Atrial fibrillation is the most common rhythm treated with cardioversion. In this rhythm, the upper chambers of the heart quiver rapidly and irregularly instead of contracting in a coordinated way. The result is a fast, irregular pulse that can cause palpitations, shortness of breath, and fatigue. Cardioversion for atrial fibrillation typically starts at 200 joules of energy, and roughly 89% of patients convert back to a normal rhythm.
One important consideration with atrial fibrillation is blood clot risk. When the upper chambers quiver instead of squeezing properly, blood can pool and form clots. If a clot has formed and the heart suddenly returns to a normal rhythm, that clot can be ejected into the bloodstream and cause a stroke. For episodes lasting 48 hours or longer, blood thinners are required before cardioversion can be performed safely. For episodes shorter than 48 hours, the stroke risk is lower (around 0.7% to 0.8%) but not zero, so the decision about blood thinners depends on individual risk factors.
Atrial Flutter
Atrial flutter is a close relative of atrial fibrillation, but the electrical pattern in the upper chambers is more organized, creating a rapid but regular rhythm. It responds very well to cardioversion. In one study of 42 patients cardioverted in the emergency department, 95.2% converted successfully, and nearly all of them converted at 200 joules or less. The same blood-thinning precautions that apply to atrial fibrillation apply to flutter.
Narrow-Complex Tachycardia (SVT)
Narrow-complex tachycardia, often called supraventricular tachycardia or SVT, refers to a group of fast rhythms originating above the main pumping chambers. The heart rate can spike to 150 to 250 beats per minute, causing dizziness, chest tightness, or a pounding sensation. Many SVT episodes respond to simpler interventions first, like bearing down (a Valsalva maneuver) or medications given through an IV. Cardioversion comes into play when those approaches fail or when the fast rate is causing dangerous symptoms. The starting energy for SVT cardioversion is 100 joules.
Monomorphic Ventricular Tachycardia With a Pulse
Ventricular tachycardia (VT) originates in the lower chambers of the heart and produces a fast, wide pattern on the heart monitor. When the rhythm looks uniform and the patient still has a pulse, it is called monomorphic VT with a pulse, and synchronized cardioversion is appropriate. The starting energy is 100 joules.
The “with a pulse” part is critical. If ventricular tachycardia causes the patient to lose their pulse or becomes polymorphic (where the electrical pattern varies beat to beat), the treatment switches to unsynchronized defibrillation. Polymorphic VT looks too irregular for the machine to reliably sync with, and pulseless VT is treated the same as cardiac arrest.
When Cardioversion Is Used Urgently vs. Electively
Cardioversion can be either an emergency procedure or a planned one. The deciding factor is whether the fast rhythm is causing hemodynamic instability, meaning the heart rate is so fast that the body can’t maintain adequate blood flow. Signs that push toward immediate cardioversion include low blood pressure, altered mental status, signs of shock, chest pain from reduced blood flow to the heart, and acute heart failure.
When these danger signs are present, cardioversion happens quickly with sedation given “whenever feasible,” as the American Heart Association puts it. That phrasing is intentional: if a patient is about to lose consciousness from dangerously low blood pressure, waiting for full sedation may not be an option.
Elective cardioversion is a different experience. It is commonly used for atrial fibrillation or flutter that is stable but hasn’t responded to medications, or when restoring a normal rhythm is preferred over simply controlling the heart rate. In these cases, the patient receives short-acting sedation so they are unconscious for the brief moment of the shock. Common sedation options include propofol, etomidate, and midazolam, all of which have been shown to be equally effective for facilitating the procedure. The shock itself takes a fraction of a second, and most patients wake up within minutes with no memory of it.
Rhythms You Do Not Cardiovert
Knowing what not to cardiovert is just as important. Ventricular fibrillation and pulseless ventricular tachycardia are treated with unsynchronized defibrillation because there is no organized rhythm to sync with. Polymorphic ventricular tachycardia (including torsades de pointes) is also defibrillated rather than cardioverted. Sinus tachycardia, a fast heart rate caused by pain, fever, dehydration, or anxiety, is never cardioverted because it is a normal response to an underlying problem, not an electrical malfunction of the heart. Shocking sinus tachycardia would not fix the underlying cause and could be harmful.