Josephson Clinical Cardiac Electrophysiology

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■ Josephson’s Clinical Cardiac Electrophysiology

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FIGURE 6.21 Pseudo-normalization of an LBBB by a fascicular extrasystole. LBBB is present on the first three complexes. A fascicular rhythm (below the site of LBBB) normalizes the QRS. Note a retrograde His does not precede the QRS due to the LBBB, which prevented retrograde conduction over the LBB to the His bundle. When observed on subsequent beats, the His followed the QRS because it was reached by transseptal spread and retrograde activation over the right bundle branch. CS, coronary sinus; d, distal; HRA, high right atrium; LBB, left bundle branch; LBBB, left bundle branch block; p, proximal.

RBB block with a right axis deviation pattern. Although those suggestions seem logical, proof is lacking. Digitalis intoxica tion is also associated with fascicular depolarizations. In our experience, these are invariably from the anterior or poste rior division of the left bundle branch. In addition, they can frequently be initiated by pacing the heart ( Figure 6.20 ). This phenomenon is compatible with triggered activity that is due to delayed afterpotentials, the mechanism postulated for many digitalis-induced arrhythmias. 27,43,44 Such triggered fascicu lar arrhythmias may be encountered in the absence of digi talis and, if incessant, may produce a tachycardia-mediated cardiomyopathy. 45 Early coupled fascicular depolarizations are also impor tant as a cause of triggered ventricular fibrillation in either apparently normal, structurally abnormal or electrically ab normal hearts 46-50 ( Figure 6.22 ). Although the trigger versus substrate contributions are poorly understood, particularly in apparently normal hearts, 46,51 ablation of these fascicular or ventricular depolarizations can prevent recurrent ventricular fibrillation (see Chapter 11).

VENTRICULAR DEPOLARIZATIONS The ECG criteria proposed to distinguish supraventricular depolarizations with aberrations from ventricular depolariza tions are legion. 45, 52-55 Despite the applicability of these criteria to large groups of patients, they may not be valid for a given person. Intracardiac recording is the most accurate diagnostic tool for evaluating the problem. His bundle recordings have demonstrated that bizarre QRS morphologies, A-V dissocia tion, and so on may be encountered with supraventricular im pulses as well as with impulses of ventricular origin. By definition, a ventricular depolarization is one in which no His bundle deflection is seen before the QRS complex in the absence of preexcitation (see Chapter 8). The ventricu lar depolarization may be isolated ( Figure 6.23 ) or in runs, such as an accelerated ventricular rhythm ( Figure 6.24 ) 56 or a sustained ventricular tachycardia (see Chapter 10). Caution must be taken to ensure that the absence of a His bundle deflection is not due to poor placement of the cath eter. From the appearance of a normal His bundle spike

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