When every other beat is a VPB, the rhythm is called bigeminy. When every third beat is a VPB or when each normal beat is followed by two VPB's, the rhythm is called trigeminy. Rarely, when the atria are controlled by the ventricular pacemaker, the P waves are inverted and follow QRS complexes at a constant RP interval. QRS complexes are usually wider than 0.12 sec and bizarre. RR intervals are usually regular but may vary up to 0.03 sec.
Vagal stimulation has no effect on the rate.
An ECG prior to tachycardia showing premature beats of identical configuration favors ventricular origin of the tachycardia. The rhythm pattern is similar to normal sinus rhythm except that the PR interval is prolonged beyond 0.20 sec in adults and 0.18 sec in children. P waves are positive in L2 and negative in AVR and occur regularly at a rate 60-100. QRS complexes may be normal or wide and bizarre if BBB is present. Rhythmic impulses originating in the SA node are intermittently blocked at the AV junction.