What does an "R-on-T" phenomenon describe?

The "R-on-T" phenomenon specifically refers to a situation where a premature ventricular contraction (PVC) occurs during the T wave of the preceding cardiac cycle. This can be particularly dangerous because it may lead to serious arrhythmias, including ventricular tachycardia or ventricular fibrillation. During the T wave, the heart is in a vulnerable state of repolarization, and the introduction of an early beat can disrupt this process. This scenario highlights the importance of proper timing and coordination in the cardiac cycle, as disturbances can have significant consequences for cardiac function.

The other options do not accurately describe the "R-on-T" phenomenon. Atrial flutter with a rapid ventricular response suggests a different type of rhythm disturbance involving the atria, complete heart block refers to a situation where the electrical signals are completely blocked between the atria and ventricles, and a normal heart rhythm implies there are no abnormalities present. Thus, the focus is on understanding how the "R-on-T" phenomenon results from specific electrical activity that is particularly pertinent in the study of dysrhythmias.