Synchronized cardioversion delivers a controlled electrical shock timed precisely to the heart’s electrical cycle, resetting an abnormal but organized rhythm back to normal. The key distinction from standard defibrillation is that the machine waits for a specific moment in the heartbeat before releasing energy, preventing the shock from triggering an even more dangerous rhythm. The procedure is used for unstable tachycardias where the heart still has an identifiable electrical pattern, such as atrial fibrillation, atrial flutter, supraventricular tachycardia, and monomorphic ventricular tachycardia with a pulse.
Why Synchronization Matters
Every heartbeat produces an electrical signal with distinct phases. The R-wave represents the moment the main pumping chambers contract. The T-wave, which follows shortly after, represents the moment those chambers are recovering and are electrically vulnerable. If a shock lands on the T-wave, it can throw the heart into ventricular fibrillation, a chaotic, life-threatening rhythm where the heart quivers instead of pumping.
Synchronization solves this by making the defibrillator “watch” the heart’s electrical pattern and time its shock to land on the R-wave, safely away from that vulnerable period. This is sometimes called avoiding the “R-on-T phenomenon.” In ventricular fibrillation, though, there is no identifiable R-wave for the machine to lock onto. That’s why synchronization mode must be turned off if the patient deteriorates into V-fib, and unsynchronized defibrillation is used instead.
Step-by-Step Procedure
Before starting, the bedside should have an oxygen saturation monitor, suction device, IV line, and intubation equipment ready. Here’s the sequence:
- Attach monitoring leads. Place the patient on a cardiac monitor so the defibrillator can read the heart’s electrical activity. Ensure a clean, readable tracing with clearly visible QRS complexes.
- Activate sync mode. Press the “sync” or “synchronize” button on the defibrillator. A marker (usually a small dot or arrow) should appear above each R-wave on the monitor. Confirm these markers are tracking consistently before proceeding.
- Select energy level. Dial in the appropriate joules for the rhythm being treated (see energy levels below).
- Apply pads or paddles. Place adhesive electrode pads on the chest in either an anterolateral or anteroposterior position.
- Sedate the patient. If the patient is conscious and time allows, administer procedural sedation through the IV line.
- Clear and shock. Announce “Clear,” verify no one is touching the patient, then press and hold the shock button. The machine will not fire immediately. It waits for the next R-wave, then delivers the shock. This brief delay is normal.
- Reassess the rhythm. Check the monitor. If the rhythm hasn’t converted, re-engage sync mode (many defibrillators automatically revert to unsynchronized mode after each shock), increase the energy, and repeat.
One critical detail that catches people off guard: after each shock, you typically need to re-activate the sync button. Many machines default back to defibrillation mode after delivering a synchronized shock. Forgetting to re-sync before the next attempt means the next shock will be unsynchronized.
Energy Levels by Rhythm
The American Heart Association’s current electrical cardioversion algorithm recommends these initial synchronized energy levels:
- Atrial fibrillation: 200 J
- Atrial flutter: 200 J
- Narrow-complex tachycardia (SVT): 100 J
- Monomorphic ventricular tachycardia: 100 J
An older, widely taught approach starts at 50 J and doubles with each unsuccessful attempt, reaching 200 J after about three shocks. Both strategies are used in practice. For pediatric patients, the AHA recommends starting at 0.5 to 1 joule per kilogram of body weight, increasing to 2 J/kg if the first shock doesn’t work.
If synchronization causes delays and the patient’s condition is deteriorating rapidly, the AHA advises switching immediately to unsynchronized shocks rather than waiting.
Pad Placement
Two standard pad positions are used. Anterolateral placement puts one pad on the upper right chest below the collarbone and the other on the left side of the chest under the armpit. Anteroposterior placement puts one pad on the front of the chest over the sternum and the other directly behind the heart on the back, between the shoulder blades.
There has been debate about which position works better. A systematic review of 11 trials initially suggested anterolateral placement was slightly more effective for atrial fibrillation cardioversion. However, a separate meta-analysis of randomized trials found no meaningful difference in overall success rates between the two positions. Either placement is considered acceptable, and the choice often comes down to patient positioning and clinical preference.
Sedation Before the Shock
Synchronized cardioversion is painful, and conscious patients need sedation before the shock is delivered. The most commonly studied agents include propofol, midazolam, and etomidate, often combined with a short-acting pain medication like fentanyl. The AHA notes that many experts recommend full anesthesia support when readily available.
Propofol is frequently chosen because it acts quickly and wears off fast, though it can drop blood pressure. Midazolam takes a bit longer to reach full effect but causes less blood pressure instability. The specific agent and dose depend on the patient’s hemodynamic status and what’s available. In truly unstable patients where cardioversion can’t wait, sedation should not delay the shock.
Anticoagulation Before Cardioversion
When the heart fibrillates instead of contracting normally, blood can pool and form clots in the upper chambers. Cardioverting the heart back to a normal rhythm can then dislodge those clots, causing a stroke. This is why anticoagulation timing matters.
European Society of Cardiology guidelines recommend at least three weeks of oral anticoagulation before cardioversion for patients whose atrial fibrillation has lasted 48 hours or longer, or when the duration is unknown. The alternative is performing a transesophageal echocardiogram (a specialized ultrasound that looks at the heart from behind, through the esophagus) to rule out clots before proceeding. If no clot is seen, cardioversion can happen sooner.
For atrial fibrillation lasting less than 48 hours, the stroke risk is lower, and cardioversion can generally proceed more quickly, though anticoagulation decisions still depend on individual stroke risk factors.
Success Rates
Synchronized cardioversion successfully restores a normal sinus rhythm in roughly 70% to 75% of patients with chronic atrial fibrillation, based on studies using initial energies of 100 J. Higher starting energies and newer biphasic defibrillators may improve those numbers. Success rates are generally higher for atrial flutter and SVT, which tend to respond to lower energy levels.
Conversion to sinus rhythm doesn’t guarantee the rhythm will stay normal. In one study, 75% of patients who were successfully cardioverted remained in sinus rhythm at follow-up ranging from 6 to 15 months. Maintaining normal rhythm long-term often requires antiarrhythmic medications.
Post-Procedure Monitoring
After cardioversion, patients are monitored with continuous blood pressure measurement and oxygen saturation tracking while the sedation wears off. Most patients who convert successfully and have no complications are discharged 2 to 4 hours after the procedure. If a new antiarrhythmic medication is started at the time of cardioversion, the observation period extends to 24 to 48 hours to watch for medication side effects.
Anticoagulation typically continues for at least four weeks after cardioversion, even if sinus rhythm is restored, because the upper chambers of the heart can remain sluggish for a period after conversion, leaving some residual clot risk.