What does the "R on T" phenomenon indicate?

The "R on T" phenomenon occurs when a premature ventricular contraction (PVC) falls on the T wave of the preceding heartbeat, specifically during the vulnerable period of the cardiac cycle. When this happens, it poses a significant risk for triggering dangerous arrhythmias, particularly ventricular fibrillation. This is because the cardiac muscle is in a state of repolarization during the T wave, making it more susceptible to abnormal electrical impulses. The presence of an R wave during this critical time can disrupt the regular heart rhythm, leading to potential hemodynamic instability and an increased likelihood of life-threatening arrhythmias. Understanding this phenomenon is crucial in clinical practice when monitoring patients for signs of acute cardiac events.