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About Synchronized Cardioversion

Synchronized cardioversion can be an effective treatment for certain types of cardiac arrhythmias. It involves the delivery of a low energy shock which is timed or synchronized to be delivered at a specific point in the QRS complex. A synchronized shock is delivered at this precise moment to avoid causing or inducing ventricular fibrillation.

In order to understand why a shock must be synchronized, it is helpful to understand that the cardiac cycle has both a vulnerable and a refractory period.  The refractory period occurs during the QRS complex. The T wave is considered the vulnerable period, especially the middle and second half of the T wave. Because the shock is timed to be delivered during the QRS complex, electrical stimulation is avoided during the vulnerable period, which reduces the risk of inducing ventricular fibrillation.

Synchronized Cardioversion Versus Defibrillation

Both defibrillation and cardioversion impose a therapeutic dose of electrical energy on the myocardium. Defibrillation is used to treat certain types of arrhythmias (ventricular fibrillation and pulseless ventricular tachycardia) while cardioversion is used to treat others (i.e. unstable narrow and wide complex tachyarrhythmias such as atrial fibrillation, atrial flutter and ventricular tachycardia), but there is a very important distinction between the two.

Defibrillation involves the use of a high energy shock which in unsynchronized; that is, the shock is delivered as soon as the shock button is pushed. As mentioned above, it is used to treat conditions such as pulseless monomorphic ventricle tachycardia and ventricular fibrillation.

A synchronized shock is different because it is timed to be delivered at a specific period of time during the cardiac cycle. It is used to treat cardiac arrhythmias such as atrial flutter and atrial fibrillation when medications have failed to convert the rhythm, or when the patient is unstable and the rhythm must be immediately terminated.

Success rates may vary for patients who undergo synchronized cardioversion. According to the National Heart, Lung and Blood Institute, the success rates of cardioversion are 75 percent or greater.


Before performing cardioversion, it is essential that appropriate precautions and steps be taken to increase the likelihood of a positive outcome and decrease the risk of a poor one. The first step involves identifying the patient’s rhythm on the monitor. Take time to obtain a 12-lead if the patient is stable and there is any doubt about the patient’s rhythm.

Since cardioversion is painful, the patient will need to be properly sedated with intravenous medication. Emergency equipment should be made ready, such as a suction device and bag mask device in case manual ventilation is needed. Additional airway management equipment should be easily accessible, such as an oral airway and intubation equipment. Some hospitals also have sedation reversal medications on standby to be used if needed.

If necessary, the patient’s chest hair should be shaved at the site where the electrodes will be placed. Follow hospital policy regarding placing the patient on supplemental oxygen prior to the procedure. Some facilities may also use a CO2 monitor during conscious sedation for cardioversion. If the patient is not placed on oxygen, it should be ready if required.

After an IV has been started, administration of appropriate sedation with short acting medications (usually a muscle relaxant and a narcotic for pain) should take place. Electrodes should be placed below the clavicle on the right side of the chest and about 2 inches below the mid-axillary line beside the nipple on the left side.

The SYNC button on the defibrillator machine should be pressed.  Review the rhythm strip to ensure the R wave is being marked and sensed by the machine. Select the appropriate level of energy and follow standard precautions to “clear” the patient before delivering a shock. Expect a slight delay in the delivery of the shock as the machine times its delivery.

After the shock, reassess the patient’s rhythm. If the patient has not converted and a second shock is indicated, you will again need to push the SYNC button, as the machine will default to defibrillation mode. Follow the procedure detailed above. Continue to monitor the patient’s level of consciousness and vital signs. The patient should be closely monitored until he/she is awake and vital signs are stable.

Troubleshooting and Complications

In some instances, it may be necessary to make a few adjustments if something is not working as it should during a cardioversion. For example, a marker may not appear above the QRS complex because the machine is not sensing it. If this occurs, adjusting the amplitude (or gain) is advised.

Although synchronized cardioversions are often performed without complications, they can occur. One possible complication from a cardioversion is hypoxia. Patients react differently to sedation. That’s why emergency equipment should always be available. In some cases, a patient may not be adequately breathing on their own and may need to be manually ventilated. Cardiac complications can include hypotension and dysrhythmia, including ventricular fibrillation.

Article Sources

National Heart, Lung and Blood Institute. What is a Cardioversion?  Accessed August 2014.

Link, M. Arkins D. 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2010.  Accessed August 2014.