Electrocardiogram (ECG) showing ventricular parasystole

Unifocal parasystolic ventricular premature beats (VPBs) occur at a rate which is slower than the underlying sinus rhythm and manifest different coupling intervals (ie, the distance between the VPB and the prior QRS complex). The ectopic focus does not always activate the ventricular myocardium (and therefore does not produce a VPB on the ECG) since it may arrive at a time when the ventricle is refractory due to activation from the normal conduction pathway. However, the interval between two successive VPBs is always some integer of the underlying rate of the ectopic focus (the interectopic intervals have a common denominator) since the ectopic focus is undisturbed and continues to fire at its own intrinsic rate.

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.