BUNDLE-BRANCH BLOCK UNDER VOLUNTARY CONTROL

Case report: transient left bundle branch block associated with ECT

We present the first reported case of transient left bundle branch block (LBBB) occurring during electroconvulsive therapy (ECT). LBBB is an important clinical finding, as it is associated with a significant increase in mortality. Physicians providing ECT should be aware of the significance of new-onset LBBB; it may occur during treatment.

Postoperative episodic left bundle branch block

Transient left bundle branch block (LBBB) associated with physical exertion has been described in patients with and without coronary artery disease. A 64-year-old woman with no history of coronary artery disease underwent Nissen fundoplication under general anesthesia. Preoperatively, an exercise-tolerance test revealed LBBB, without ischemic symptoms. Intraoperatively, the electrocardiogram revealed normal sinus rhythm. Postoperatively, LBBB appeared in the absence of clinical symptoms or physiologic stress. The patient reported several similar episodes previously. Cardiac enzymes were negative. Subsequent electrocardiograms revealed persistence of LBBB. Anesthesiologists should be aware of the possibility of perioperative transient LBBB in the absence of cardiac ischemia.

Laughter-induced left bundle branch block

We present the case of a patient with ischemic heart disease and intermittent left bundle branch block, reproducibly induced by laughter. Following treatment of ischemia with successful deployment of a drug-eluting stent, no further episodes of inducible LBBB were seen. Transient ischemia, exacerbated by elevated intrathoracic pressure during laughter, may have contributed to onset of this phenomenon.

Intraoperative diagnosis of rate-dependent bundle branch block

Rate-dependent left bundle branch block (LBBB) occasionally occurs during anaesthesia when the heart rate exceeds a critical value. While it is usually a benign disorder, it may mask the electrocardiographic manifestations of myocardial ischaemia and the ST-T wave pattern associated with LBBB may be mistaken for those of ischaemia. This case report presents two cases in which rate-dependent LBBB was clearly documented during the perioperative period. It demonstrates the use of pharmacologic agents (e.g., atropine and neostigmine) and physiologic manipulations (e.g., carotid sinus massage) to alter the heart rate and confirm the diagnosis of benign rate-dependent LBBB in the operating room. These interventions should be used with caution in patients who have hypertension, angina, cerebrovascular, or AV node disease or in the setting of myocardial ischaemia or severe bundle branch disease.

Rate-dependent changes in excitability of depressed cardiac Purkinje fibers as a mechanism of intermittent bundle branch block

When the heart rate is accelerated, rate-dependent intraventricular block may occur. This block has been attributed to abnormal action potential prolongation in a diseased conducting pathway. Less often, intraventricular block develops during slowing of the heart rate and has been explained in terms of phase 4 depolarization in potentially automatic cells within the diseased fascicle. We tested these hypotheses in isolated bundles of Purkinje fibers placed in a three-chambered tissue bath. In one group of experiments, conditions of localized injury and depressed excitability were mimicked by superfusing the central segment with sucrose solution. Action potentials were initiated in the proximal segment while the slope of phase 4 of cells in the distal end was controlled by intracellular ramps of current of either polarity. In these preparations, phase 4 depolarization facilitated rather than retarded propagation across the depressed segment, even at takeoff potentials as low as -45 mV. In a second group, depressed excitability was induced by exposing the three fiber segments to Tyrode's solution that contained high concentrations of KCl and CaCl2 or isoproterenol (0.1 microgram/ml). Under these conditions, Purkinje fibers did not undergo phase 4 depolarization and did not generate abnormally prolonged action potentials. These preparations showed a biphasic time dependence of conduction during premature stimulation or in response to changes in the basic cycle length. Conduction impairment and block were manifest at either side of an optimal interval or cycle length. Our results suggest that phase 4 depolarization and abnormally prolonged action potentials are not necessary conditions for intermittent block. Both tachycardia and bradycardia-dependent intraventricular conduction abnormalities may be associated with time-dependent variations in the excitability of depolarized conducting fibers as well as in the amplitude of the slow responses generated by these fibers. These alterations can be explained in terms of regulation of slow inward current by the intracellular calcium concentration.

Electrocardiographic criteria of left ventricular hypertrophy in left bundle-branch block

In order to determine whether the electrocardiographic criteria of left ventricular hypertrophy apply in the presence of left bundle-branch block we studied 79 cases of intermittent left bundle-branch block and compared the QRS voltage and axis before and after its onset. Cases of incomplete left bundle-branch block were excluded. There was a statistically significant correlation between pre- and post-left bundle-branch block values of R or S wave voltage in leads I, V1, V2, V5, and V6, the Sokolow index (R V5 or V6 + S V1), and the QRS axis. There was a statistically significant reduction in R wave voltage in leads I, V5, and V6, an increase in S wave voltage in V1 and V2, and leftward shift of QRS axis, but the Sokolow index remained unchanged, after the onset of left bundle-branch block. The Sokolow criteria for left ventricular hypertrophy apply satisfactorily even in the presence of left bundle-branch block, though specificity is low, but QRS axis is unhelpful.

Intermittent left bundle branch block in the athletic heart syndrome

A 30-year-old man with typical findings of the athletic heart syndrome is presented because of the unique feature of intermittent left bundle branch block. Observations during right atrial pacing and exercise support the concept of autonomic control of intraventricular conduction, both adrenergic and cholinergic.

Direct and imcomplete concealed Wenckebach phenomena in the left bundle-branch system

A case is reported showing a tachycardia dependent Wenckebach phenomenon in both the A-V node and the left bundle branch system. Pacing from the His bundle region induced manifest (direct) and imcompletely concealed (indirect) types of Wenckebach phenomenon within the left bundle branch system.

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.

Atrioventricular block response to exercise and intraventricular conduction at rest

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Bradycardia-dependent bundle-branch block. A critique and proposed criteria

Four cases of bundle-branch block (BBB) occurring with slow heart rates in patients with heart disease are presented. Recognition of bradycardia-dependent BBB is often difficult, and its differentiation from ventricular escape mechanism and from bilateral BBB requires careful analysis. A set of five criteria based on our own observations is presented with a review of the literature. The responsible mechanism for this seemingly paradoxical situation is not known. However, spontaneous depolarization of one or the other of the bundle branches in late stages of diastole appears to be the most plausible explanation.

Incomplete left bundle-branch block. A definite electrocardiographic entity

Rate-dependent intermittent incomplete and complete left bundle-branch block (LBBB) is described in a patient without other evidence of heart disease. Continuous electrocardiographic tracings revealed QRS complexes intermediate in configuration between normal conduction and complete LBBB during the transitional phase from normal to abnormal conduction. This provided an opportunity to study the various grades of incomplete LBBB in man. During the incomplete LBBB phase, the morphology of the QRS complexes bore a close resemblance to the QRS form in the Wolff-Parkinson-White syndrome (type B). The differences between these two entities are emphasized.

Bundle-branch block in acute myocardial infarction

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