An R on T PVC occurs?

An R on T PVC, or premature ventricular contraction, occurs on or near the T wave of the previous beat. This is significant as the timing of the R wave of the PVC falls on the vulnerable period of the cardiac cycle, which is during the T wave. This can potentially lead to more consequential arrhythmias such as ventricular tachycardia or ventricular fibrillation because the heart is still in a state of repolarization during the T wave.

Choosing this timing for the PVC can disrupt the normal electrical activity of the heart, which is why it is clinically important to recognize R on T phenomenon. In contrast, other timing options, such as those either with the P wave or following normal beats, do not encapsulate the risky nature of a PVC happening during the T wave where it can have adverse effects on cardiac function.