Synchronized cardioversion is used when a patient has a rapid abnormal heart rhythm, still has a pulse, but is showing signs of dangerous instability like low blood pressure, altered consciousness, chest pain, signs of shock, or acute heart failure. The key distinction is that the patient has a pulse but is deteriorating because of the rhythm. If the patient is stable, medications are tried first. If the patient has no pulse, the treatment is defibrillation, not synchronized cardioversion.
Why Synchronization Matters
Every heartbeat produces an electrical pattern with distinct phases. The R-wave represents the main contraction of the heart’s lower chambers. The T-wave, which comes shortly after, represents the brief moment when the heart muscle is resetting its electrical charge. During this reset window, the heart is extremely vulnerable. If a shock hits during the T-wave, it can trigger ventricular fibrillation, a chaotic rhythm that is essentially cardiac arrest. This is called the R-on-T phenomenon, and it would turn a patient who has a pulse into one who doesn’t.
Synchronized cardioversion avoids this by using sensors on the defibrillator to detect each R-wave in real time. When the operator presses the shock button, the machine doesn’t fire immediately. Instead, it waits for the next R-wave and delivers the shock at that precise moment, safely away from the T-wave. This timing requires that the heart still has an identifiable, organized rhythm with detectable R-waves, which is why synchronization is only used when a pulse is present.
The Rhythms That Call for It
Synchronized cardioversion applies to a specific set of abnormal fast heart rhythms. These include atrial fibrillation (a chaotic, irregular rhythm originating in the upper chambers), atrial flutter (a rapid but more organized upper-chamber rhythm), narrow-complex supraventricular tachycardia (SVT), and monomorphic ventricular tachycardia with a pulse (a fast rhythm from the lower chambers that still produces blood flow). In all of these, the heart is beating too fast or too disorganized to pump blood effectively, but it is still pumping.
One critical exception: polymorphic ventricular tachycardia, where the electrical pattern is irregular and constantly shifting, is treated with unsynchronized defibrillation even if a pulse is present. The machine may not be able to reliably identify the R-wave in this chaotic pattern, so attempting to synchronize could dangerously delay the shock.
Signs That Make Cardioversion Urgent
Not every fast rhythm needs an electrical shock. Many patients with atrial fibrillation or SVT are uncomfortable but hemodynamically stable, meaning their blood pressure holds and their organs are getting enough blood flow. These patients are typically treated with medications first.
The American Heart Association’s tachycardia algorithm identifies five signs that a rapid rhythm is causing dangerous instability:
- Hypotension: blood pressure too low to support organ function
- Altered mental status: confusion, drowsiness, or unresponsiveness that developed with the rhythm
- Signs of shock: pale or mottled skin, rapid breathing, weak pulses
- Ischemic chest pain: chest discomfort suggesting the heart muscle itself isn’t getting enough blood
- Acute heart failure: sudden fluid buildup in the lungs, severe shortness of breath
If any of these are present and clearly caused by the tachycardia, synchronized cardioversion should not be delayed in favor of medication trials. The rhythm is the problem, and the fastest way to fix it is electrical conversion.
Energy Levels by Rhythm
The 2025 AHA guidelines recommend starting at different energy levels depending on the rhythm being treated. For atrial fibrillation and atrial flutter, the recommended first shock is 200 joules using a biphasic defibrillator. For narrow-complex tachycardia (SVT) and monomorphic ventricular tachycardia, the starting energy is lower at 100 joules.
If the first shock doesn’t convert the rhythm, energy is escalated. A common protocol for atrial fibrillation uses a 200, 300, then 360 joule sequence. In one large study of over 370 patients with atrial fibrillation lasting more than 48 hours, the first 200-joule shock converted 88% of patients. A second shock at 300 joules brought the cumulative success rate to nearly 96%, and a third at 360 joules reached 97%.
The 2025 AHA guidelines specifically note that higher first-shock energy settings of 200 joules or more are preferable to lower settings for atrial fibrillation and flutter. Earlier guidelines favored starting lower out of concern for heart damage, but subsequent studies found no association between starting energies up to 360 joules and cardiac enzyme elevations, post-shock arrhythmias, or other complications. In fact, low-energy shocks were significantly more likely to provoke ventricular fibrillation when cardioverting atrial fibrillation, and to provoke atrial fibrillation when cardioverting flutter.
Biphasic vs. Monophasic Defibrillators
Most modern defibrillators use biphasic waveforms, which send current in two directions through the heart. Older monophasic devices send current in one direction and require substantially more energy to achieve the same result. In a comparison study of atrial fibrillation cardioversion, biphasic shocks achieved 91% cumulative success using a 120 to 200 joule sequence, matching the 90% success rate of monophasic shocks at 200 to 360 joules. The average total energy delivered was 159 joules for biphasic versus 306 joules for monophasic. If you’re using an older monophasic device, higher starting energies are needed, typically beginning at 200 joules for most rhythms and escalating up to 360.
The 48-Hour Rule for Atrial Fibrillation
When atrial fibrillation has been present for more than 48 hours, blood can pool in the upper chambers and form clots. Cardioverting the heart back to a normal rhythm can then dislodge those clots, causing a stroke. Without prior blood-thinning treatment, this stroke risk runs between 5% and 7%.
For atrial fibrillation lasting less than 48 hours, international guidelines support proceeding with cardioversion even without prior anticoagulation. When the arrhythmia has persisted longer than 48 hours, or the duration is unknown, two approaches reduce the clot risk: either three weeks of anticoagulation therapy before the procedure, or an imaging study (transesophageal echocardiogram) to directly check for clots in the heart before shocking. Both approaches have been shown to be effective at reducing post-cardioversion stroke.
After the procedure, anticoagulation should continue for at least four weeks in most patients. Those with ongoing stroke risk factors based on their overall health profile typically need lifelong blood-thinning medication regardless of whether the heart stays in normal rhythm.
This 48-hour consideration applies to elective, planned cardioversions. In an emergency, when a patient with atrial fibrillation is hemodynamically unstable and deteriorating, cardioversion proceeds immediately regardless of arrhythmia duration. Saving the patient’s life takes priority over stroke risk.
Sedation and What the Patient Experiences
Synchronized cardioversion is painful, so conscious sedation is given before the shock whenever the clinical situation allows. Patients receive a short-acting sedative intravenously, enough to make them drowsy or briefly unconscious for the few seconds the shock is delivered. The goal is amnesia for the event and pain control, not deep general anesthesia. In truly unstable patients who are losing consciousness from the arrhythmia itself, the shock may be delivered with minimal or no sedation because the delay could be fatal.
After the Shock
Following cardioversion, patients are monitored with continuous heart rhythm tracking, blood pressure checks, and oxygen level measurement. The sedation wears off over minutes to an hour depending on the medication used. Most patients who undergo elective cardioversion and convert successfully are discharged from the hospital within two to four hours, provided they are fully awake and no complications have occurred. Patients who are started on certain rhythm-controlling medications at the same time may need to stay 24 to 48 hours for additional monitoring, since some of these drugs carry their own risk of triggering arrhythmias during the initial dosing period.