Does the longer the body is being shocked increase or decrease the chance the heart will begin fibrillation?

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The important idea is that defibrillation is most successful when delivered promptly after ventricular fibrillation starts. As time passes, the heart muscle becomes more ischemic and less responsive to a shock because cells run low on energy, acidity rises, and electrolyte imbalances develop. Those changes make it harder for a single defibrillation shock to reset the rhythm and restore a stable heartbeat. So, the chance of returning to a normal rhythm decreases the longer the heart has been in fibrillation before the shock is delivered.