Single-lead electrocardiogram showing ventricular trigeminy

Every third beat is a ventricular premature beat. The coupling interval between the ventricular premature beat and the previous sinus QRS complex is constant (fixed coupling interval). The resulting pattern is referred to as ventricular trigeminy.

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.