Tachycardia-dependent versus bradycardia-dependent intermittent bundle-branch block

Complete right bundle branch block (CRBBB) with three different mean frontal plane QRS axes--a case report

A routine ECG of a seventy-year-old man, who had been followed for five years because of complete right bundle branch block (CRBBB) with first-degree atrioventricular (AV) block, showed CRBBB and three different mean frontal plane QRS axes suggesting normal conduction, bradycardia-dependent left anterior hemiblock, and tachycardia-dependent left posterior hemiblock--all within the same tracing. Holter recording demonstrated transient advanced AV block, and a permanent pacemaker was implanted.

Possible role of a ventricular conduction disturbance in the electrogenesis of the ECG-VCG signs of myocardial infarction

The typical QRS patterns of myocardial infarction (MI-QRS) are commonly attributed to myocardial cellular death. However, observation of a transient appearance of MI-QRS during coronary insufficiency, the disappearance of MI-QRS after coronary by-pass surgery and the appearance of MI-QRS after intracranial hemorrhage suggest that a different electrophysiological mechanism may be at work. There is a single convincing explanation for all these observations. It seems possible, at least theoretically, that a localized conduction disturbance can generate or contribute to the generation of the MI-QRS. The results obtained in nine out of 194 cases studied by means of premature right atrial stimulation (PRAS) in our laboratory seem to confirm this hypothesis. In five of them we observed typical MI-QRS in the aberrant beats which were absent in the basal tracings. In the other four cases, MI-QRS which were present in basal tracings disappeared in the aberrant beats. In three of these a reduction in the duration of QRS was also observed, while in the fourth the duration of QRS did not change. In no case could the alterations of QRS (induction or disappearance of MI-QRS) be explained by a classical conduction disturbance, preexcitation or by a premature ventricular beat. While the induction of MI-QRS was clearly due to an aberrant conduction in the supraventricular beats, the disappearance of basal MI-QRS changes in premature supraventricular beats is more difficult to explain. One possible electrophysiological mechanism could be a supernormal phase conduction. If this is the case, the basal MI-QRS could be due to a ventricular conduction disturbance. In conclusion, our results suggest that MI-QRS can be generated, at least in our cases, by a localized conduction disturbance.

A case of coexisting tachycardia- and bradycardia-dependent bilateral bundle branch block

Electrocardiograms and His bundle electrograms were presented from a patient with a rare combination of intermittent bilateral bundle branch block. Critical analyses of R-R intervals and evolution of bundle branch block patterns revealed both tachycardia dependent right bundle branch block and bradycardia dependent left bundle branch block. Evaluation of HV intervals on His bundle electrograms suggested an additional possibility of bradycardia dependent conduction delay in the right bundle branch system.

The anterior displacement of the QRS loop as a right ventricular conduction disturbance. Electrophysiologic and vectorcardiographic study in man

Anterior displacement (AD) of the QRS horizontal loop (Frank VCG method) was induced by programmed right atrial stimulation (PRAS) in 15 cases. When AD occurred we noticed changes of the terminal QRS vectors and of the T loop similar to those observed in incomplete right bundle branch block (RBBB). The increasingly anticipated extrastimuli induced progressively the AD and then progressive degrees of RBBB. The anterior shifting of the efferent limb never appeared after the induction of RBBB. A left conduction disturbance never appeared after the AD. In cases of supposed incomplete left bundle branch block (i.e. left ventricular hypertrophy) the QRS duration decreased when the AD was induced. Therefore, the AD induced by PRAS and probably those observed in some clinical cases are due to a right ventricular conduction disturbance.

Bradycardia-dependent peri-infarction block

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T-wave changes secondary to left anterior hemiblock as shown by study of intermittent and alternating patterns

T-wave changes in spontaneous intermittent and alternating LAH have been described. Attention has been called to the fact that, with the onset of LAH conduction, the T-wave axis shifts oppositely to that of the QRS, thus widening the QRS-T angle in the frontal plane by about 95 degrees. This has the effect of causing T-waves to become upright in leads II, III and AVF, even in those in whom T-waves were previously inverted. This described dicordance of QRS and T-wave axes is considered as evidence that LAH conduction is in fact a true conduction abnormality. As a corollary, an inverted T-wave in leads II, III or AVF in the presence of LAH is a primary abnormality and may be an indication of inferior wall myocardial ischemia or infarction. T-waves are lowered but not inverted in lead I as a result of LAH conduction and precordial leads are variably but not significantly altered.

Electrophysiological studies in patients with rate-related intermittent left bundle-branch block

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Rate dependent aberrancy

Forty patients with "rate dependent aberrancy" (RDA) were studied. This large group of patients permitted a clear definition of the syndrome and recognition of a number of features not previously described. These proved to have a significant bearing on the recognition and differential diagnosis of RDA and non-rate dependent aberrancy.

It was found that a small change in cycle length, perhaps too small to be recognized in the surface ECG, can result in RDA. Consequently, if a critical shortening of cycle length is to be recognized, it is necessary to record not only the onset of aberrant rhythm but also sufficiently long strips with normal intraventricular conduction preceding and following the RDA. In some patients there was no recognizable sudden change in cycle length and the onset of aberrancy was a function of the duration of the accelerated rate. In others only the first cycle of the rhythm with RDA was shortened and the remaining R-R intervals were paradoxically longer than the R-R cycle which initiated the RDA.

The aberrancy in RDA occurred at relatively slow heart rates (in 26 of the 40 patients the rate was below 80), and was frequently independent of any significant changes in the duration of the immediately preceding cycle length. There was a striking prevalence of left bundle branch block, and in 35 of the 40 patients obvious organic heart disease was documented.

Tachycardia-dependent left bundle branch block associated with bradycardia-dependent variable left bundle branch block

This report describes tachycardia- and bradycardia-dependent left bundle branch block in a patient who exhibited normal intraventricular conduction with impulses intermediate in timing. During bradycardia, longer cycles were associated with increasing degrees of incomplete left bundle branch block. These clinical observations support the existence of slow Phase 4 depolarization of latent pacemaker cells as one of the mechanisms for abnormalities of conduction.