Cardioversion is a medical procedure by which an abnormally fast heart rate (tachycardia) or cardiac arrhythmia is converted to a normal rhythm using electricity or drugs. Synchronized electrical cardioversion uses a therapeutic dose of electric current to the heart at a specific moment in the cardiac cycle.
https://en.wikipedia.org/wiki/Cardioversion
Synchronized cardioversion is shock delivery that is timed with the QRS complex. This synchronization avoids shock delivery during the relative refractory portion of the cardiac cycle, when a shock could produce ventricular fibrillation. The shock dose used for a synchronized shock is lower than that used for defibrillation.
http://www.zoll.com/medical-technology/cardioversion/
Cardioversion can be done using an electric shock or drugs. ELECTRIC CARDIOVERSION. Electric cardioversion may be done with a device that gives off a shock. The device can be placed inside (internal) or the shock can be delivered from the outside (external) the body. External electric cardioversion uses a device called a defibrillator.
https://medlineplus.gov/ency/article/007110.htm
Electrical[edit] To perform synchronized electrical cardioversion, two electrode pads are used (or, alternatively, the traditional hand-held paddles), each comprising a metallic plate which is faced with a saline based conductive gel. The pads are placed on the chest of the patient, or one is placed on the chest and one on the back.
https://en.wikipedia.org/wiki/Cardioversion
A problem with any part of this process can cause an arrhythmia. During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm. Doctors use cardioversion to correct fast or irregular heartbeats. Cardioversion is done two ways: using an electrical procedure or using medicines. For the electrical procedure, your heart is given low-energy shocks to trigger a normal rhythm. You're temporarily put to sleep before the shocks are given. This type of cardioversion is done in a hospital as an outpatient procedure.
http://www.nhlbi.nih.gov/health/health-topics/topics/crv/
Cardioversion can be done using drugs that are taken by mouth or given through an intravenous line (IV). It can take from several minutes to days for this treatment to work. If you are given drugs for cardioversion in a hospital, your heart rate will be regularly checked.
https://medlineplus.gov/ency/article/007110.htm
The R Series supports two types of synchronized cardioversion: Synchronized Cardioversion - The R Series monitors the patient's ECG and synchronizes shock delivery with this ECG source.
http://www.zoll.com/medical-technology/cardioversion/
After the procedure, the patient is monitored to ensure stability of the sinus rhythm. Synchronized electrical cardioversion is used to treat hemodynamically unstable supraventricular (or narrow complex) tachycardias, including atrial fibrillation and atrial flutter.
https://en.wikipedia.org/wiki/Cardioversion
Cardioversion can be done using an electric shock or drugs. Electric cardioversion may be done with a device that gives off a shock. The device can be placed inside (internal) or the shock can be delivered from the outside (external) the body. External electric cardioversion uses a device called a defibrillator.
https://medlineplus.gov/ency/article/007110.htm
1 Synchronized Cardioversion - The R Series monitors the patient's ECG and synchronizes shock delivery with this ECG source.  Remote Synchronized Cardioversion - An external device (such as a patient monitor) monitors the patient’s ECG and provides a synchronization pulse to the R Series’ Sync In/Marker Out connector. The R Series synchronizes shock delivery with these external pulses.
http://www.zoll.com/medical-technology/cardioversion/
when to use synchronized cardioversion
Synchronized cardioversion is use to  treat hemodynamically unstable supraventricular Tachycardia's, including atrial fibrillation and atrial flutter.