Josephson Clinical Cardiac Electrophysiology

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Chapter 6: Ectopic Rhythms and Premature Depolarizations ■

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FIGURE 6.19 Purkinje depolarization with faster antegrade conduction to the ventricles than retrograde conduction to the His bundle. The second complex is a Purkinje extrasystole with retrograde activation of the His bundle (Hr) occurring after the onset of the QRS complex by an interval just shorter than the H-V on the conducted complex. This phenomenon results in a wide, bizarre QRS complex because a great portion of the ven tricles are depolarized by muscle-to-muscle activation before any ventricular depolarization can occur antegradely over the specialized conduction system. A minimum of 35 msec after His bundle depolarization is required for ventricular depolarization via the specialized conduction system to take place. This requirement suggests that the fascicular impulse arises distally and/or that the velocity of retrograde conduction over the fascicle is much slower than the velocity of antegrade conduction. HBE, His bundle electrogram; HRA, high right atrium; T, time line.

RBB extrasystole is much less common, consistent with the fact that the fascicles of the left bundle branch have a faster automatic rate, spontaneously and in response to a variety of perturbations, than the RBB with right or left axis deviation, respectively. 40-42 While these comments regarding short H-V intervals signifying a fascicular origin are valid in normal hearts, occasionally, a true ventricular extrasystole arising in a scarred ventricle can demonstrate a His deflection before the QRS (see Figure 10.9 ).

The therapeutic implications of fascicular rhythms or fas cicular premature depolarizations are not clear, although most physicians regard them as ventricular in origin and treat them as such. It has been suggested that in the setting of myocardial infarction, fascicular depolarizations associated with inferior infarction have an RBB block and left axis deviation morphol ogy because they arise from the posterior-inferior fascicle of the left bundle branch. Those associated with anterior in farction arise from the anterior-superior fascicle and have an

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FIGURE 6.20 Fascicular extrasystole. Leads 1, 2, 3, and V1 are shown with electrograms from the high right atrium (HRA) and His bundle (HBE). During atrial pacing at 900 msec, spontaneous QRS complexes with a right bundle branch block and left axis deviation are induced. Each time this pattern was reproduced, a retrograde His bundle section was observed, just at the onset of the QRS. The QRS width is 120 msec. This fascicular extrasystole was reproducibly initiated by atrial pacing, suggesting triggered activity due to delayed afterdepolarizations as the mechanism of the fascicular complex. The presence of a His deflection before the onset of the QRS as well as the narrowness of the QRS suggests that the origin of the extrasystole is high in the fascicle. T, time line.