• The PVCs occur so early that they fall on the T wave of the preceding beat.
• These PVCs occur during the refractory period of the ventricles, a vulnerable period because the cardiac cells have not fully repolarized.

Rate: Depends on rate of underlying rhythm

Rhythm: Irregular whenever a PVC occurs

P Waves: None associated with the PVC

PR Interval: None associated with the PVC

QRS: Wide (greater than 0.10 sec), bizarre appearance

Clinical Tip: In acute ischemia, R-on-T phenomenon may be especially dangerous because the ventricles may be more vulnerable to ventricular tachycardia (VT), ventricular fibrillation (VF), or torsade de pointes.