What is the "R on T" phenomenon?

The "R on T" phenomenon refers to the occurrence of an ectopic beat, such as a premature ventricular contraction (PVC), occurring on the T wave of a preceding heartbeat. This phenomenon can lead to dangerous arrhythmias, particularly if the ectopic beat occurs during the vulnerable period of the T wave when the heart is in a state of repolarization.

In essence, because the T wave represents the heart's recovery phase after contraction, an early ectopic beat occurring at this time can disrupt the normal electrical cycle, potentially leading to more severe issues like ventricular fibrillation. Understanding the implications of the "R on T" phenomenon is crucial in cardiac rhythm management to prevent serious complications in patients with arrhythmias.

Other options, while they touch on relevant aspects of cardiac physiology or electrical activity, do not accurately define or explain this particular phenomenon, which specifically involves the timing of ectopic contractions in relation to the T wave of the electrocardiogram.