Is cardioversion risky? Is it an established procedure?

Shocks the heart. Cardioversion is the process of shocking the heart, usually to get it to stop some type of arrhythmia (or irregular heartbeat) and resume a regular heart beat ... If used when the heartbeat is regular, it can cause an arrhythmia that may be life-threatening, such as ventricular fibrillation ...
Risk of stroke. Cardioversion is often performed for atrial fibrillation. There is a small risk of stroke as the heart is restored to a normal rhythm. This can be minimized either by adequate anticoagulation or transesophageal echocardiography prior to cardioversion.