The most common cause of a lethal dysrhythmia is by a PVC hitting the peak of the T wave (known as R-on-T Phenomenon). This causes a pulseless shockable or non-shockable rhythm in at risk patients. 90% of cardiac arrests in adults are shockable (Pulseless VTach/Ventricular Fibrillation). This rhythm started with multifocal PVCs to a wide complex tachycardia (Pulseless VTach). This can only last from a few seconds to a few minutes before it becomes Coarse Ventricular Fibrillation (if no CPR and Defibrillatory shocks were administered). Coarse VFib can only last for about 4-5 minutes before it becomes Fine VFib. From then on, you have seconds for Fine VF to become Asystole.