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. Why Cardioversion Matters Cardioversion may be a necessary procedure when drugs alone have not been able to convert an arrhythmia to a normal heart rhythm. Cardioversion restores the normal heart rate and rhythm, allowing the heart to pump more effectively. The most common use of cardioversion is to treat atrial fibrillation or atrial flutter. But cardioversion may also be used to treat unstable supraventricular tachycardia, which could lead to ventricular fibrillation. How Cardioversion Works with ZOLL R Series Defibrillators By pressing the SYNC soft key, the R Series® defibrillator will enter SYNC mode and the synchronizing circuit within the defibrillator will detect the patient's R waves. When the SHOCK button is pressed and held, the unit discharges with the next detected R wave, thus avoiding the vulnerable T wave segment of the cardiac cycle. When in the SYNC mode, the unit displays downward arrow markers above the ECG trace to indicate the points in the cardiac cycle (R waves) where discharge can occur. 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. 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.