When the R wave of a PVC occurs on the T wave of the preceding beat, the R-on-T phenomenon is present. Any strong aberrant electrical impulse in this phase of the cardiac cycle is likely to cause ventricular depolarization. It is important to note any PVCs that fall within the T wave of the preceding beat. 

When PVCs begin to “creep up” toward the T wave, the R-on-T phenomenon is also considered present. The danger of the R-on-T phenomenon is the possibility that a deadly arrhythmia could result.