R on T wave phenomenon occurs in the cardiac cycle. When the ventricles contract they need a certain amount of time to repolarize, before they can contract again, this is called the relative refractory period. If they are stimulated to contract again too soon (R wave falling on T wave) they can be thrown into a chaotic, disorganized rhythm known as ventricular fibrillation. When this happens a "cardiac arrest" has occured and one must be shocked out of it or stay dead.
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