Tachycardia- and Bradycardia-Dependent Left Bundle Branch Block Associated with First Degree A-V Block : A Study Using His Bundle Electrogram During Carotid Sinus Compression

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His bundle electrographic examination was performed during carotid sinus compression as well as atrial pacing in a patient who exhibited intermittent left bundle branch block (LBBB) and first degree A-V block. LBBB was observed at the corrected RR intervals below 920 msec and above 1600 msec. The QRS complexes showed normal intraventricular conduction between these 2 values. The critical cycle lengths could not be determined when ordinary RR intervals were used. The HQ interval remained constant at 70 msec regardless of the pattern of intraventricular conduction. It was demonstrated that the patient had both tachycardia-and bradycardia-dependent LBBB with first degree A-V block due to HV block. The authors believe this to be the first case of tachycardia-and bradycardia-dependent bundle branch block studied by His bundle electrography. The important role of His bundle recordings and carotid sinus compression in the diagnosis of rate dependent intraventricular conduction disturbance was stressed and possible mechanisms underlying this rare condition were discussed.