What is synchronized cardioversion?

QRS: I think you mean 'synchronous'. Synchronous cardioversion is when the shock is delivered on the qrs portion of the heart cycle - it is when the heart depolarises.

Additional info: This is important because if the heart is depolarized externally during the wrong portion of the cardiac cycle, it can actually induce worse heart rhythms than it was intended to treat. Synchronized cardioversion is typically used to treat atrial fibrillation, atrial flutter, stable ventricular tachycardia.

Timed shock: This is a cardioversion (shock) given to the heart that is timed to a certain point in the cardiac cycle so that the heart rhythm can be reset without causing further rhythm problems. This is the most common type of shock delivered for rhythms such as atrial fibrillation.