What are the differences between asynchronous and synchronous cardioversion?

Heart Cycle: Synchronous is when the shock is delivered on the qrs portion of the heart cycle - it is when the heart depolarises. Asynchronous cardioversion can occur anywhere on the heart cycle.

Always synchronous : All cardioversions should be synchronous which means that the delivered electrical impulse is timed to precisely coincide with the instant of electrical activation in the heart as indicated by the qrs complex on the cardiac monitor. Failure to do this can lead to ventricular fibrillation. Defibrillation is asynchronous because there is no qrs and the patient already has vf.