Highly effective. It "sets the clock back to zero" so to speak. It synchronizes all the electrically conducting cells in the heart to begin their cycle at the same instant. It interrupts and stops abnormal rhythms. It doesn't hurt and there are generally no side effects so in that regard, it's better than medicines. The downside is that one must be put to sleep, albeit briefly, for it.
Rebooting the system. Our heart works through electrical pathways. If these pathways get out of sync or mixed up then sometimes cardioversion or defibrillation a.K.A. "reboot of the system" can get the electricity back in sync.
No. A pr interval represents the time of conduction from the time it originates in the sa node, depolarizes the atria causing contraction and reaches the av node. A cardioversion is a procedure to shock the heart back into a normal rhythm and does not correct intrinsic pathway defects. A pacemaker is used for that purpose when necessary, .
No. Cardioversion is for arythmias- like atrial fib/flutter and ventricular fibrillation in emergent situations and special caes nor for prolongred pr intervel.