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

New-Onset Intermittent Deceleration-Dependent Left Bundle Branch Block Following Induction of General Anesthesia in a Healthy Patient: A Case Report

This case report aims to highlight an atypical presentation of deceleration-dependent aberrancy (DDA) following the induction of general anesthesia in a patient with no known cardiac history. It emphasizes the critical role of intraoperative monitoring and the potential effects of anesthetic agents on the cardiac conduction system. A 46-year-old Hispanic male with no significant past medical or surgical history presented for surgical repair of a comminuted radial fracture. Following anesthesia induction with propofol, midazolam, and fentanyl, he developed a transient left bundle branch block (LBBB) exhibiting deceleration-dependent characteristics. Despite stable hemodynamics, the LBBB pattern appeared at heart rates below 60 beats per minute and resolved with heart rates above 90 beats per minute. This was managed intraoperatively with glycopyrrolate. Postoperative evaluations, including a 12-lead ECG, echocardiogram, and nuclear stress test, indicated normal biventricular function with a small to moderate reversible perfusion defect. The patient did not report cardiac symptoms postoperatively and did not prefer to undergo a coronary angiogram. This report underscores the importance of recognizing rate-dependent LBBB as a potential intraoperative complication, even in patients without pre-existing cardiac conditions. The transient nature of DDA, influenced by anesthetic agents and managed through careful monitoring and pharmacological intervention, highlights the necessity for vigilance in perioperative settings. This case contributes to a growing body of evidence suggesting that anesthetic management may require tailored approaches for patients experiencing or at risk for conduction abnormalities. This case illustrates the complexities of cardiac conduction disturbances such as DDA in the context of general anesthesia, serving as a reminder of the importance of thorough monitoring and the judicious use of rate-modifying drugs. It fosters a deeper understanding of the interaction between anesthesia and cardiac electrophysiology. Further research is needed to explore the mechanisms and management strategies for anesthetic-related cardiac conduction abnormalities.

Challenges in Bradicardias Interpretation

Sinus node dysfunction or atrioventricular blocks are the causes of bradycardias. Diagnosis and management begin with evaluation of patient's hemodynamic status and diagnosis of bradycardia's cause. This is followed by an in depth evaluation of pathophysiology of the arrhythmia, its severity, and likelihood of progression. Implementing emergent measures depends on the presence of subsidiary pace makers maintaining cardiac output. Many of these decisions are greatly helped by 12 lead electrocardiogram, because its tracings are often diagnostic of the cause of the bradycardia and help to assess its persistence and progression and to evaluate the presence and reliability of subsidiary pacemakers.

Use of the Surface Electrocardiogram to Define the Nature of Challenging Arrhythmias

Despite unprecedented advances in technology, the electrocardiogram (ECG) remains essential to the practice of modern electrophysiology. Since its emergence at the turn of the nineteenth century, the form of the ECG has changed little. What has changed is our ability to understand the complex mechanisms that underlie various arrhythmias. In this article, the authors review several important principles of ECG interpretation by providing illustrative tracings. The authors also highlight several important concepts that be can used in ECG analysis. There are several fundamental principles that should be considered in ECG interpretation.

Mouse heart rate in a human: diagnostic mystery of an extreme tachyarrhythmia

We report telemetry recording of an extreme non-fatal tachyarrhythmia noted in a hospitalized quadriplegic male with history of atrial fibrillation where the average ventricular conduction rate was found to be about 600 beats per minute and was associated with transient syncope. A medical literature review suggests that the fastest human ventricular conduction rate reported to date in a tachyarrhythmia is 480 beats per minute. We therefore report the fastest human heart rate noted in a tachyarrhythmia and the most probable mechanism of this arrhythmia being a rapid atrial fibrillation with 1:1 conduction in the setting of probable co-existing multiple bypass tracts.

Unique self-perpetuating cycle of atrioventricular block and phase IV bundle branch block in a patient with bundle branch reentrant tachycardia

Bundle branch reentrant tachycardia (BBRT) is an uncommon form of ventricular tachycardia involving diseased bundle branches. Phase IV bundle branch block also accompanies severe His-Purkinje disease. We present an unusual case of BBRT in a patient who also manifests a unique self-perpetuating cycle of AV block and phase IV left bundle branch block following aortic valve surgery. BBRT was treated successfully by ablation of the right bundle.

Apparent bradycardia-dependent right bundle branch block associated with atrial fibrillation: concealed electrotonic conduction as a possible mechanism

A 79-year-old woman with atrial fibrillation was reported in whom apparent bradycardia-dependent right bundle branch block was suggested. When a conducted supraventricular impulse occurred within a critical period after the preceding conducted impulse, the impulse was blocked in the right bundle branch except when it fell in the supernormal period of the right bundle branch. When the conducted impulse occurred between the critical period and another longer period, it was conducted without bundle branch block. When the impulse occurred beyond that longer period, it was usually blocked in the bundle branch again. However, when the impulse occurred beyond a still longer period, it was conducted without bundle branch block again. These findings suggest that when impulses fell in the right bundle branch shortly after the preceding conducted impulses, they were blocked in both bundle branches; however, it seemed that concealed electrotonic conduction of the blocked impulses affected conduction of the subsequent impulses.

Apparent bradycardia-dependent sinoatrial block associated with respiration

In our previous patients, apparent bradycardia-dependent block has been shown in the atrioventricular (AV) junction and in the accessory pathway. It was suggested that these previous cases were not of true bradycardia-dependent block; namely, that, as a result of periodic increases in vagal tone associated with respiration, conductivity in the AV junction or in the accessory pathway was depressed to a greater degree than automaticity in the sinus node. In the present article, 3 patients with frequent sinoatrial (SA) block were reported. In 1 patient, sinus escape-capture bigeminy caused by SA block was found. In these present patients, when the sinus cycle lengthened, SA block occurred. The purpose of the present article is to show that the patients have apparent bradycardia-dependent SA block, namely, not true bradycardia-dependent SA block. In all patients, the respiration curve was recorded simultaneously with the electrocardiogram. In all patients, during inspiration, the sinus cycle gradually shortened; on the other hand, during expiration, the sinus cycle gradually lengthened, and then a sinus impulse was blocked in the SA junction. These findings suggested that increased vagal tone during expiration depressed conductivity in the SA junction to a greater degree than automaticity in the sinus node.

Bradycardia-dependent activation delay of the atrial tissue in a patient with sick sinus syndrome

Bradycardia-dependent block or activation delay of the atrial tissue has not been documented in the literature. A patient with sick sinus syndrome in whom bradycardia-dependent, as well as tachycardia-dependent, activation delay in the atria was elucidated is reported on. The phenomenon of bradycardia-dependent activation delay was reproducibly demonstrated, both during atrial extrastimulation and incremental atrial pacing from the high right atrium, in the setting of an abnormally prolonged activation interval from the distal stimulating electrodes to the proximal recording electrodes of a quadripolar catheter, which was localized in the high right atrial region. Supernormal conduction seemed to be an unlikely explanation for the observed phenomenon.

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.

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.

The supernormal phase of intraventricular conduction: normalization of intraventricular conduction of premature atrial beats

This report presents the analysis of a case in which numerous atrial extrasystoles, associated with normal intraventricular conduction, occurred during sinus rhythm conducted with right bundle branch block. The wide overlap of coupling intervals of normally conducted extrasystoles and other extrasystoles with right bundle branch block seemed prima facie to negate the possibility of supernormality. Rearrangement of these intervals, however, to allow for the timing of right bundle branch depolarization with respect to the ensuing QRS complex, resulted in a clear cut separation of all normalized complexes from those with right bundle branch block. All normalized extrasystoles were localized in a zone very early in the cycle. This finding, in the presence of atrial extrasystoles with normal P-R intervals, strongly supports the mechanism of supernormality in this case. Alternate explanations do not appear to be tenable.

Transient tachycardia- and bradycardia-dependent left anterior and left posterior hemiblocks. Effects of isoproterenol

Tachycardia- and bradycardia-dependent, left anterior and left posterior hemiblocks as transient phenomena were registered in two patients spontaneously, and especially as a consequence of isoproterenol infusion. A chronic trifascicular type of A-V block was present in the first case, whereas in the second case a bradycardia-dependent left posterior hemiblock was registered during an acute myocardial infarction. In the first patient the isoproterenol effects were: (1) a shortening of the refractoriness and an increase of the conduction velocity in the injured fascicle, (2) an increase in the slope of phase-4 depolarization on the left posterior fascicle, and (3) a presumably shifting toward zero of threshold potential on the left anterior fascicle. Isoproterenol effects disappeared from 30 to 40 min after it was discontinued. In the second case the bradycardia-dependent left posterior hemiblock was registered during very fast heart rates (150 beats min). This finding supports the view that enhanced phase-4 depolarization is the main factor in the development of bradycardia-dependent intraventricular blocks in the course of acute myocardial ischemia.

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.

Fascicular premature beats with narrow QRS in the presence of bundle-branch block

The electrocardiogram of a sixty-two-year-old male revealed right bundle-branch block, left anterior superior fascicular block, and right precordial Q waves consistent with anteroseptal myocardial infarction. Premature beats with narrow QRS (100 ms) occurred intermittently; coupling intervals varied between 560-600 ms. His bundle electrogram confirmed the fascicular origin of the premature beats. During atrial prematuring studies, it was shown that an appropriately timed premature beat could prolong infra-His conduction time, but QRS normalization was not achieved. The possible electrophysiologic mechanisms that could result in a normal QRS complex with fascicular premature beats in this patient are discussed; a new mechanism is suggested.

Direct Wenckebach periods in the left bundle branch during bradycardia-dependent left bundle branch block in a patient with sick sinus syndrome

Direct Wenckebach periods in the left bundle branch with periods of 4:3 and 5:4 conduction alternating with 2:1 left bundle branch block (LBBB) were observed in a patient who was admitted because of congestive heart failure. There was no previous documented LBBB and serial electrocardiographic (ECG) mainfestations afterwards were indicative of sick sinus syndrome. This is probably the first report to describe direct Wenckebach periods in the left bundle branch during bradycardia-dependent (phase-4) LBBB in a patient with sick sinus syndrome.

Electrophysiologic observations in a patient with bradycardia-dependent atrioventricular block

In a patient with atrioventricular (A-V) block distal to the His bundle (H), 1:1 A-V conduction with right bundle branch block and H-V interval of 70 msec was established with atrial pacing at rates of 120 to 150/min, suggesting that the A-V block was bradycardia-dependent. Advanced second degree A-V block distal to the H deflection occurred with atrial pacing at 160/min after completion of A-V nodal Wenckebach periodicity proximal to the H deflection because of the long H-H encompassing the blocked P wave. Atrial extrastimulus testing coupled with sinus rhythm (with A-V block) demonstrated that critical H1-H2 intervals of less than 545 msec allowed conduction to the ventricles. The H2-V2 interval shortened progressively from 290 to 70 msec with shortening of these critical H1-H2 intervals. Atrial extrastimulus testing coupled with an atrial driven cycle lenght of 500 mesc (with intact A-V conduction) revealed block of the H2 deflection with an H1-H2 interval longer than 540 msec. In conclusion, at critical diastolic intervals, impulses were blocked, creating a state of decreased responsiveness. If a cycle length of subsequent impulses was shorter than the critical diastolic blocking interval, membrane responsiveness gradually improved and conduction resumed. If a cycle length of subsequent impulses was longer than the critical blocking diastolic interval, A-V block was sustained. Blocked impulses continually penetrated to the site of block and reset the state of membrane responsiveness.

Recurrent paroxysmal supraventricular tachycardia: a complication of ventricular pacing in a patient with occult Wolff-Parkinson-White syndrome

A 60 year old man suffering from syncope believed to be due to the sick sinus syndrome was treated with a permanent demand ventricular pacemaker. This led to almost continous bouts of paroxysmal supraventricular tachycardia (SVT) over the ensuing two years, mistakenly believed to be part of the sick sinus syndrome. Careful study showed that this man had a type A Wolff-Parkinson-White accessory atrioventricular connection which consistently conducted retrogradely, but only rarely antegradely, during applications of carotid sinus massage. Episodes of SVT were repeatedly induced whenever ventricular-paced impulses captured the atria retrogradely. All episodes of SVT stopped when the ventricular pacemaker was removed. Following insertion of an atrial pacemaker, the patient had no episodes of SVT or syncope over a nineteen month follow-up period. This case illustrates the care required in selecting a proper site for protective pacing in patients who suffer from paroxysmal SVT.

Bradycardia-related (phase 4) left anterior hemiblock

A case is presented in which left anterior hemiblock (LAH) appeared transiently after the pause terminating atrial pacing, after atrial premature beats and during the slowing of sinus rhythm induced by carotid sinus massage. The transient LAH is attributed to phase-4 block, and is reported in order to add weight to the hypothesis that phase-4 block is a significant factor in the genesis of discrete conduction blocks in the human heart.

His bundle electrogram in trifascicular disease: report of a case studied with His bundle electrograms

His bundle electrograms were performed on a 75 year old female with trifascicular block and digitalis induced junctional block. The usefulness of this technique in understanding the patients' electrocardiographic abnormalities and the relationship to phase-3 and phse-4 block is discussed.

Electrocardiographic observations in bradycardia and tachycardia-dependent atrioventricular block. Relationship to supernormal phase of intraventricular conduction

This report describes the clinical course of a patient with bradycardia and tachycardia-dependent atrioventricular block. Bradycardia dependent A-V block (phase 4 block) was transient and precipitated by spontaneous slowing of the sinus rate, atrial and ventricular extrasystoles; The degree of slowing (critical RP interval) required to induce A-V block increased progressively over a three-day period. Bradycardia-dependent A-V block was terminated mostly by critically times spontaneous or paced ventricular escape beats, but normally conducted atrial impulses also appeared to restore A-V conduction on several occasions. The tachycardia-dependent component was manifested by an unusual fatigue phenomenon in the His-Purkinje system seen only at an atrial pacing rate of 150 per minute. These observations document the presence of both bradycardia and tachycardia-dependent A-V block in the presence of a normal H-V time and also illustrate the dynamic nature of both phase 4 block and the period of "supernormal" intraventricular conduction.

Experimental production of rate-dependent bundle branch block in the canine heart

The main ventricular conducting fascicles were slightly injured in anesthetized dogs by gently scratching them with a blunt needle introduced through the ventricular wall. Initially the bundle branch block that resulted was rate independent (stage 1). When conduction returned to normal, both premature atrial and vagal stimulation reproduced the bundle branch block (stage 2). During stage 2 (5-15 minutes), three conduction ranges were documented: an early (phase 3) block range, a late (phase 4) block range, and an intermediate normal conduction range. The normal conduction range was narrow at the beginning but widened progressively, mostly at the expense of the phase 4 block range. Another period during which only phase 4 bundle branch block occurred (stage 3) preceded total normalization (stage 4). The escape beats that arose from the injured fascicle were most abundant and had the shortest coupling during stage 1, they were less common and their coupling became longer during stages 2 and 3, and they disappeared in stage 4. The tachycardia-dependent or phase 3 bundle branch block was related to a prolongation of refractoriness; the bradycardia-dependent or phase 4 bundle branch block was attributed to slight hypopolarization, enhanced spontaneous diastolic depolarization, and a shift in the threshold potential toward zero. These abnormalities were assumed to be secondary to the hypopolarization, which was probably the basic derangement. This hypothesis satisfactorily accounts for the observation that phase 3 and phase 4 bundle branch block commonly coexist in the same injured fascicle.