The surface electrocardiogram in complete intra-his heart block

Arrhythmogenic Effects of Cardiac Memory

Cardiac memory is the term used to describe an interesting electrocardiographic phenomenon. Whenever a QRS complex is wide and abnormal, such as during ventricular pacing, the T waves will also be abnormal and will point to the opposite direction of the wide QRS. If the QRS then normalizes, such as after cessation of ventricular pacing, the T waves will normalize as well, but at a later stage. The period of cardiac memory is the phase between the sudden normalization of the QRS and the eventual and gradual return of the T waves to their baseline morphology. Cardiac memory is assumed to be an innocent electrocardiographic curiosity. However, during cardiac memory, reduction of repolarizing potassium currents increases left ventricular repolarization gradients. Therefore, when cardiac memory occurs in patients who already have a prolonged QT interval (for whatever reason), it can lead to a frank long QT syndrome with QT-related ventricular arrhythmias (torsades de pointes). These arrhythmogenic effects of cardiac memory are not generally appreciated and are reviewed here for the first time.

Intra-His bundle block with Wenckebach phenomenon

A 66-year-old lady presented with shortness of breath and a Wenckebach atrioventricular (AV) conduction pattern on the ECG. The electrophysiologic study showed split-His potentials and intra-Hisian Wenckebach. The case highlights the interesting finding of Wenckebach conduction in the His bundle.

Intra-His bundle block corresponds with interruption of the branching portion of the His bundle

We compared His-bundle electrograms with pathological findings of the atrioventricular conduction system in four patients with complete atrioventricular intra-His block with narrow QRS complexes on ECG. Split His electrograms were recorded at the time of electrophysiological study. The patients died from noncardiac causes at 10 days, 1 year, 4 years, and 9 years, respectively, after the pacemaker implantation. Serial sections through the atrioventricular conduction system revealed strictly localized more than 50% reduction of conducting cells replaced by fibrosis at the branching portion of His bundle. The proximal portions of the bundle branches also exhibited decrease of the conducting cells showing a rough positive relation with the patient's age. Therefore, we considered that the H1 spikes seen on His-bundle electrograms originated from the penetrating portion of His, which was virtually intact in our cases, and that the H2 spikes originated from the right side of the distal branching portion of His.

Clinical and electrocardiographic features and long-term results of electrical therapy in patients with isolated His bundle disease

The clinical, ECG, and electrophysiologic findings of 35 consecutive patients with second- and third-degree intra-His block with normal QRS complexes were examined. The follow-up period varied between 12 and 120 months (mean 45). Seventy-seven per cent of the patients were women. Underlying heart disease was present in 43% of the patients. ECGs were characterized by both second-degree type I and type II atrioventricular block, normal or slightly prolonged PR interval of the conducted beats or of the first conducted beat of a Wenckebach sequence, and by subtle changes in the initial forces of the QRS complexes of the escape beats. Electrophysiologic study showed normal sinus and atrioventricular node function and normal infra-His conduction in all patients. In four patients repetitive bradycardia-dependent intra-His block was induced. Thirty-two patients were permanently paced soon after the initial evaluation and three during the follow-up period. Total long-term mortality rate was 23%. None of the patients developed bundle branch block.

Bedside evaluation of atrioventricular block with narrow QRS complexes: usefulness of carotid sinus massage and atropine administration

Second-degree intra-His bundle block is frequently of type I (Wenckebach periods) or 2:1. In this situation, the surface electrocardiogram does not permit distinction between intranodal (atrioventricular [A-V] and subnodal (intra-His) block. This study examined the value of bedside carotid sinus massage and atropine administration in diagnosing the site of block from the standard electrocardiogram in subjects with chronic A-V block and narrow QRS complexes. Fifteen patients had intra-His bundle block and 10 had intranodal block. The combination of two tests correctly located the site of block in 22 subjects, and was noncontributory in 3. Thirteen of the 15 intra-His bundle blocks and 9 of the 10 intranodal blocks were properly identified; in three cases the results were nondiagnostic, but no wrong diagnoses were made. The noninvasive bedside method of carotid sinus massage and the use of atropine permit both the localization and the determination of the type of block in the majority of cases of second degree A-V block and narrow QRS complexes. In a proper clinical context they can obviate the need for invasive electrophysiologic studies.