Synchronized vs. Unsynchronized Cardioversion


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Editor’s Note:

It’s funny how you can go an entire career doing something (CPB) and hear the same thing every day, but actually not evaluate what it means.  Yes, I have taken ACLS and studied the electrophysiology of the heart.

So anyway, we have just taken off the X-Clamp, and the patient is relatively warm, and we decide to cardiovert due to fibrillation.  Pretty standard procedure- we see it more often than not.  At this point the cv surgeon tells the nurse to go with an asynchronous as opposed to synchronous shock mode.  I had a brief moment of clarity… for the first time in 3000+ hearts, I actually think to myself, disassemble the two words “synchronous” and “asynchronous” and realize I really didn’t understand the difference, or what physiologic conditions dictated which mode to use.

I feel foolish in revealing this personal information gap or cluelessness, but I figure there might be a few others out there that may not truly understand this difference.  So bear with me (those perfusion savants out there) and I’ll just go ahead and put down some Cardioversion 101 info here 🙂 

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Synchronized cardioversion is a LOW ENERGY SHOCK that uses a sensor to deliver electricity that is synchronized with the peak of the QRS complex (the highest point of the R-wave). When the “sync” option is engaged on a defibrillator and the shock button pushed, there will be a delay in the shock. During this delay, the machine reads and synchronizes with the patients ECG rhythm. This occurs so that the shock can be delivered with or just after the peak of the R-wave in the patients QRS complex.

Synchronization avoids the delivery of a LOW ENERGY shock during cardiac repolarization (t-wave). If the shock occurs on the t-wave (during repolarization), there is a high likelihood that the shock can precipitate VF (Ventricular Fibrillation).

The most common indications for synchronized cardioversion are unstable atrial fibrillation, atrial flutter, atrial tachycardia, and supraventricular tachycardias. If medications fail in the stable patient with the before mentioned arrhythmias, synchronized cardioversion will most likely be indicated.


Unsynchronized cardioversion (defibrillation) is a HIGH ENERGY shock which is delivered as soon as the shock button is pushed on a defibrillator. This means that the shock may fall randomly anywhere within the cardiac cycle (QRS complex). Unsynchronized cardioversion (defibrillation) is used when there is no coordinated intrinsic electrical activity in the heart (pulseless VT/VF) or the defibrillator fails to synchronize in an unstable patient.

For cases where electrical shock is needed, if the patient is stable and you can see a QRS-t complex use (LOW ENERGY) synchronized cardioversion. If the patient is pulseless, or if the patient is unstable and the defibrillator will not synchronize, use (HIGH ENERGY) unsynchronized cardioversion (defibrillation).

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