Single lead electrocardiogram (ECG) showing an interpolated ventricular premature beat (VPB)

The third beat is a ventricular premature beat (VPB). It is called an interpolated VPB since it does not alter the underlying sinus RR interval.

The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat.