Functional Bundle Branch Block Research Articles

Physiologic Variants of Cardiac Conduction (Aberration, Gap, Supernormal Conduction).

Wide QRS complexes during supraventricular rhythms can be caused by fixed bundle branch block, functional (intermittent) bundle branch block, preexcitation, or toxic/metabolic causes. Functional bundle branch block can be caused by long-short aberrancy (usually physiologic), or acceleration/deceleration dependent aberrancy (usually pathologic). Electrocardiogram criteria have been proposed to differentiate aberration from ventricular tachycardia; they are not always accurate. The gap phenomenon "paradox" is that with increasingly premature extrastimuli, progressive proximal conduction delay allows time for distal recovery of excitability. Supernormal conduction may explain unusual conduction phenomena in patients with abnormal His-Purkinje function or poorly conducting accessory pathways.

PO-05-226 ORTHODROMIC RECIPROCATING TACHYCARDIA RELYING ON ABERRANT CONDUCTION: NEED FOR A LAGER CIRCUIT

Aberrant conduction during orthodromic reciprocating tachycardia (ORT) can prolong the ventriculo-atrial conduction time, which is found essential for the reentry in certain situations. To investigate the prevalence and different mechanism of aberrancy-dependent ORT. ORT cases relying on aberrancy from 220 cases in our center were searched using the following criteria: (1) Multiple episodes of ORT with aberrant conduction in ECG ; (2) No baseline BBB during sinus rhythm; (3) Absence of narrow QRS form during tachycardia unless isoproterenol used; (4) Restoration of bundle branch conduction giving rise to tachycardia termination or circuit alteration. ECGs and intracardiac tracings during study were carefully examined. 3 patients from 220 patients (1.4%) with ORT showed the phenomenon of dependence on aberrancy. Clinical tachycardia episodes of the patients were short in time. All accessory pathways (APs) were located at anterolateral regions of atrioventricular annulus. None of them had baseline bundle branch block. Creating functional bundle branch block was necessary to induce the tachycardias. In the first patient with ORT and left bundle branch block (LBBB), a late-delivered premature ventricular contraction (PVC) was able to advance the next A and reset the tachycardia, which was diagnostic for ORT with aberrancy. Meanwhile, left bundle branch conduction was restored which narrowed the next QRS complex. Tachycardia was then terminated by the advanced wavefront reaching the ventricular insertion site of AP which was refractory. In the second case, termination of the tachycardia was always preceded by a spontaneous LBBB restoration, with the V and subsequent A at MA advanced, suggesting anterograde block in the AV node. In the other case was, induction required both right BBB and slow pathway conduction. When isoproterenol was given, fast pathway recovered followed by BBB restoration, which made anterograde limb again switched to slow pathway. All pathways were successfully ablated during the procedure. All patients have been free of arrhythmias for at least 6 months. Extra transseptal time caused by aberrancy can be an integral part of ORT, which can explain the infrequent or late onset and unsustainable episodes of ORT in certain patients and is useful in understanding the circuit and localizing the pathway.

Arrhythmia in a COVID-19 patient: A case report

In this study, we considered an 83-year-old male patient admitted to the Al-Zahra hospital emergency department in Isfahan. He complained of fatigue, weakness, headache, and cough. In addition, he had hallucinations and delusions for two days; but he had no fever and chill. His physical examination showed a blood pressure of 170/100 mm Hg, heart rate of 142 beats per minute (bpm), respiratory rate of 23 pbm, oxygen saturation (in room air) of 83%, and oxygen bag reserve mask of 93%. We realized cardiac involvement during hospitalization, including sinus bradycardia, first-degree atrioventricular (AV) block, recurrent premature ventricular from tricuspid ring, atrial tachycardia (AT) rhythm with variable AV conduction block, increased heart rate with functional bundle branch block, and negative troponin. The patient was treated with medicines to control heart rate and admitted to the cardiac care unit (CCU). Next, the patient was intubated due to a worsening lung condition. Afterward, he was admitted to the intensive care unit (ICU) and died the next day. According to the literature, compromised cardiac vascular is the most common complications in hospitalized patients due to COVID-19 infection and has a higher mortality risk. Cardiac arrhythmias are additionally common clinical manifestations. These arrhythmias seem to be caused by inflammatory responses in the myocardium, electrolyte disorders, and hypoxia. Our patient showed that the COVID-19 virus might induce different types of arrhythmias.

Physiologic Variants of Cardiac Conduction (Aberration, Gap, Supernormal Conduction).

Wide QRS complexes during supraventricular rhythms can be caused by fixed bundle branch block, functional (intermittent) bundle branch block, preexcitation, or toxic/metabolic causes. Functional bundle branch block can be caused by long-short aberrancy (usually physiologic), or acceleration/deceleration dependent aberrancy (usually pathologic). Electrocardiogram criteria have been proposed to differentiate aberration from ventricular tachycardia; they are not always accurate. The gap phenomenon "paradox" is that with increasingly premature extrastimuli, progressive proximal conduction delay allows time for distal recovery of excitability. Supernormal conduction may explain unusual conduction phenomena in patients with abnormal His-Purkinje function or poorly conducting accessory pathways.

Case Report: Wide-to-Narrow QRS Tachycardia in a 3-Month-Old Infant

Introduction: It is rare to find that wide QRS tachycardia automatically changes to narrow QRS tachycardia, and it is more difficult to clarify the mechanism.Case Report: A 3-month-old infant with recurrent paroxysmal supraventricular tachycardia underwent transesophageal cardiac electrophysiological examination. The wide QRS tachycardia was induced by atrial RS2 stimulation, and it soon changed to narrow QRS tachycardia automatically. By the accurate measurement of esophageal lead, it was found that the electrocardiogram changes completely conform to Coumel law. The mechanism of wide and narrow QRS tachycardia was orthodromic atrioventricular reentrant tachycardia with or without ipsilateral functional bundle branch block, and the accessory pathway was defined as the left free wall-concealed accessory pathway.Conclusion: Transesophageal cardiac electrophysiological examination can reveal some special electrophysiological phenomena, and its non-invasive nature is especially suitable for infants.

Electrophysiological manifestations of rare supra-ventricular tachycardias with concealed nodo-ventricular fibers.

To clarify the electrophysiological mechanism of supra-ventricular tachycardias (SVT) with concealed nodo-ventricular (NV) fibers. We studied the intra-cardiac electrograms during electrophysiological study (EPS) of three cases of SVT which concerned concealed NV fibers. Electrophysiological maneuvers including right ventricular apex entrainments, RS2 stimuli, adenosine triphosphate injection and so on were done for differential diagnosis before ablation. Among these patients, one had atrio-ventricular nodal reentrant tachycardia (AVNRT) with a bystander NV fiber; the other 2 had NV fiber mediated orthodromic reentrant tachycardias (NVRT). VA dissociation was observed during SVT in all 3 cases with an antegrade His bundle conduction sequence. Ventricular stimulation at His refractory period reset the H-H intervals and the V-V intervals sequentially, suggesting the existence of a retrogradely conductive accessory pathway. Adenosine injection could terminate these tachycardias. The cycle length of an NVRT prolonged during the status of functional right bundle branch block, suggesting that the fiber located on the right side. Multiple QRS fusion morphologies during ventricular entrainments or ventricular stimulation at His refractory period at a fixed position could be observed in these cases. Concealed NV fibers can either mediate orthodromic reentrant tachycardia or be a bystander of AVNRT. V-A dissociation usually occur during such SVTs. Dissociation of H and V due to entrainment of right ventricular apex is a newly discovered maneuver to differentiate AVNRT from NVRT.

The double adenosine test: A simple and non-invasive tip to unmask unapparent pre-excitations at the emergency department

We describe a simple method to unmask unapparent pre-excitations at the emergency room. This method allows a non-invasive and definite diagnosis of accessory pathways capable of anterograde conduction. In patients presenting with paroxysmal SVT, close inspection of the electrocardiogram during sinus rhythm should look for a pre-excitation. Unfortunately, a pre-excitation can be completely unapparent during sinus rhythm despite the presence of an antegradely conducting accessory pathway. This is typically the case in the so-called Mahaim fibers. These bypass tracts give rise to antidromic atrioventricular reentrant tachycardias, in which the bypass tract serves as the anterograde limb of the circuit and the atrioventricular node as the retrograde limb of the reentrant circuit. In these patients, it is easy to make a misdiagnosis of supraventricular tachycardia with functional left bundle branch block. Indeed, due to the slow and decremental conduction properties of these bypass tracts, the baseline ECG is either normal or shows only subtle pre-excitation. Herein, we describe a simple tip, consisting of two successive administrations of adenosine (during ongoing tachycardia [upper panel] and during sinus rhythm [lower panel]), to make a non-invasive and definite diagnosis of this condition at the emergency department. By blocking the anterograde conduction over the atrioventricular node, adenosine infusion during sinus rhythm will indeed result in ventricular activation occurring exclusively over the accessory pathway. Importantly, the use of this simple method to unmask accessory pathways capable of anterograde conduction is not limited to the diagnosis of Mahaim fibers. Indeed, it can be used more generally in patients presenting with paroxysmal SVT whenever the ECG following tachycardia termination does not definitely rule out the presence of an accessory pathway ( Fig. 1 ).

Mechanisms of Supraventricular Tachycardia

Supraventricular tachycardias (SVT) are represented on electrocardiograms (ECG) as narrow QRS tachycardia. They can be wide QRS tachycardia, when functional bundle branch block occurs. Based on ECG appearance, SVT can be classified into short RP tachycardia or long RP tachycardia. According to underlying mechanisms, SVT can be classified into atrioventricular (AV) reciprocating tachycardia (AVRT), AV nodal reentrant tachycardia (AVNRT) or atrial tachycardia (AT). The two most common SVT, including AVNRT and AVRT, are reentrant and include AV node in the tachycardia circuit. In contrast, AT is independent of AV nodal conduction. In the case of AVRT, antegrade conduction proceeds via the AV node while retrograde conduction occurs via the accessory pathway. As ventricles and atria are sequentially activated, the P waves are usually on the ST-T segment. In the case of AVNRT, the presence of dual AV nodal pathways (fast pathway and slow pathway) underlies the mechanism. During common AVNRT, the antegrade limb passes via the slow pathway, while the retrograde limb passes via the fast pathway. As atria and ventricles are activated simultaneously, the P waves are frequently observed as pseudo-R waves in V1 lead. In the case of AT, the mechanism may be abnormal automaticity, triggered activity or reentry. Adenosine (or adenosine triphosphate) is useful to determine the tachycardia as AV nodal dependent or independent. However, a small dose of adenosine peculiarly terminates some AT.

A new criterion to diagnose wide-complex tachycardia: The quest for a simple, efficient diagnostic marker

Wide complex tachycardia (WCT) is a rhythm disturbance with a heart rate 100 beats/min and QRS duration 120 ms. An accurate and timely diagnosis of the underlying rhythm is crucial because the most common cause of WCT is ventricular tachycardia, and inappropriate treatment can lead to devastating consequences. The differential diagnosis of WCT includes supraventricular tachycardia (SVT) with pre-existing or functional bundle branch block (BBB), pre-excitation, drug- and electrolyte-induced QRS widening, and ventricular tachycardia. 1,2 A bewildering number of electrocardiographic (ECG) criteria and other stepwise algorithms have been described for the differential diagnosis of regular WCTs. ECG guidelines using QRS morphology, QRS duration, QRS axis, atrioventricular dissociation, fusion beats, absence of precordial RS complex, QRS concordance, presence of initial R wave in lead aVR, and estimation of initial and terminal ventricular activation velocity ratio (vi/vt) have been reported with varying degrees of sensitivity and specificity. 3-6 Most of the existing morphologic criteria favoring ventricular tachycardia have been noted to be present in a substantial number of patients with intraventricular conduction defect (IVCD) during sinus rhythm as described by Alberca et al. 7 The algorithms are complex, use multiple steps, and often require expert judgment and are still prone to errors. In this edition of the Journal, Pava et al 8 propose a new simple criterion to discriminate VT from SVT in patients with WCT. They analyzed 218 wide complex tachycardia

Functional bundle branch block and orthodromic reciprocating tachycardia cycle length: do not bet on accessory pathway location

To show 2 examples in which the analysis of the effect of bundle branch block on orthodromic reciprocating tachycardia cycle length was misleading. We performed an electrophysiological study in two patients with orthodromic reciprocating tachycardia showing a transition from wide to narrow QRS during tachycardia. Our two cases of left bundle branch block during reciprocating tachycardia using infero-septal pathways show that ventricular to atrial conduction time prolongation may be larger than 30 ms and may be concealed by a shortening of atrial to ventricular conduction time. In the 2 cases, the atrial insertion of the accessory pathway could be successfully ablated from the right atria at the ostium of the coronary sinus. The observation of the association between left bundle branch block and cycle length prolongation during reciprocating tachycardia may be associated with a successful ablation at the ostium of the coronary sinus.

A new algorithm in the differential diagnosis of wide complex tachycardia

A correct electrocardiographic diagnosis of the mechanism of a wide complex tachycardia (WCT) is important when instituting emergent therapy and for long-term prognostic and therapeutic considerations. While any algorithm has the risk of oversimplifying a complex problem, it is absolutely essential to have an initial strategy for the acute evaluation of an arrhythmia. Causes of wide QRS tachycardia include (1) supraventricular tachycardia (SVT) with pre-existing or functional bundle branch block, including sinus tachycardia, atrial tachycardia, atrial flutter, atrial fibrillation (AF) and AV nodal re-entry tachycardia, (2) orthodromic circus movement tachycardia with pre-existing or functional bundle branch block, (3) SVT with conduction over an accessory pathway, (4) Antidromic circus movement tachycardia using an accessory pathway in the anterograde direction and AV node in the retrograde direction, (5) ventricular tachycardia, and (6) ventricular paced rhythm. Ventricular tachycardia is by far the commonest underlying mechanism of WCT.1 The overall goal of any algorithm is to make things simpler yet effective. Most criteria are based on QRS complex duration, QRS axis, concordant pattern, presence of Q-waves, AV dissociation, fusion beats, absence of precordial RS complex, QRS alternans, and presence of multiple wide complex morphologies. Starting in 1978, several criteria have been developed to aid in the diagnosis of WCT. For evaluation of RBBB morphology tachycardias,Wellens et al .2 proposed (1) duration of QRS > 140 ms, (2) left-axis deviation, (3) certain configurational characteristics of QRS, and (4) atrioventricular (A-V) dissociation as the criteria for … *Corresponding author. Tel: +1 617 632 7393; fax: +1 617 632 7620. E-mail address : mjoseph2{at}caregroup.harvard.edu

Transitory or permanent regular wide QRS complex tachycardia induced by atrial stimulation in patients without apparent heart disease. Significance

Objectives. – The purpose of the study was to evaluate the frequency of transitory or permanent bundle branch block (BBB) associated with a paroxysmal tachycardia induced by atrial stimulation in patients without heart disease and its significance. Methods. – Esophageal atrial stimulation was performed in 447 patients suspected to have supraventricular tachycardias (SVT). Sustained regular tachycardia was induced in all of them but three, either in control state (75%) or after administering isoproterenol. In 346 patients, only narrow complex SVTs were induced (77%); in 259 of them, the reentry occurred in the AV node and in remaining patients within a concealed accessory pathway. In 62 patients, a transitory functional BBB was recorded at the onset of the tachycardia (14%). In 33 of them, the reentry occurred in the AV node and in the remaining 29 patients within a concealed accessory pathway. In 36 patients (8%), a permanently wide QRS complex tachycardia was induced. Three patients had also inducible narrow complex SVT. Atrial pacing induced a BBB similar to the aberrancy in tachycardia in 22 patients: the reentry occurred in the AV node in 17 patients, within a concealed accessory pathway in three patients and in a Mahaim bundle in two patients. In other patients, QRS complex remained normal during atrial pacing: all 14 patients had a ventricular tachycardia (VT), either a verapamil-sensitive VT ( n = 7) or catecholamine-sensitive VT ( n = 4) or bundle branch reentry ( n = 3). Followed from 2 to 12 years, the prognosis of these patients was excellent. Conclusion. – Transitory BBB at the onset of an SVT is noted in 14% of the population, is more frequent in patients with accessory pathway reentrant tachycardia, but is helpful for this diagnosis in only 12% of cases. A regular tachycardia with permanent left or right bundle branch morphology induced by atrial stimulation in a patient without heart disease and without BBB during atrial pacing is due to a VT even if this patient has also narrow complex tachycardias. This mechanism does not affect the excellent prognosis of this population.

Quantitative effects of functional bundle branch block in patients with atrioventricular reentrant tachycardia

Changes in the retrograde conduction time (ventriculoatrial [VA]) interval during functional bundle branch block (BBB) have been used to separate septal from free wall accessory pathways (APs), but different values of the VA interval prolongation (ΔVA) have been described in different reports. A total of 95 patients with single nondecremental APs who developed BBB during atrioventricular reentrant tachycardia were studied. Free wall APs were found in 60 patients, and 35 had septal APs. For patients with free wall APs, complete and incomplete BBB ipsilateral to the atrial insertion site of APs were observed in 39 of 60 patients (65%) and 31 of 60 patients (52%), respectively. For patients who had both complete (QRS ≥120 ms) and incomplete (QRS <120 ms) BBB during atrioventricular reentrant tachycardia, ΔVA for patients with complete BBB was significantly greater than in those with incomplete BBB, 59 ± 19 ms versus 30 ± 11 ms, p <0.001. For patients with septal APs and complete and incomplete BBB during tachycardia, the mean ΔVA for those with complete BBB was 31 ± 20 ms and was significantly longer than in patients with incomplete BBB (14 ± 6 ms), p <0.001. There was no significant difference in ΔVA between those with free wall APs and incomplete BBB compared with those with septal APs and complete BBB. The criteria of QRS ≥120 ms associated with ΔVA ≥40 ms served to best separate free wall from septal APs with a sensitivity of 88% and a specificity of 89%. Left anterior fascicular block was associated with marked lengthening of ΔVA for those with left free wall APs, whereas a left posterior fascicular block pattern resulted in a marked increase in the ΔVA for patients with posteroseptal APs. In the absence of fascicular block patterns, a ΔVA ≥40 ms provides strong evidence of a free wall AP, with a sensitivity of 95% and a specificity of 100%. The left posterior fascicle appears to provide predominant innervation of the posterior septum.

Retrograde Migration of the Site of Functional Block:

Functional bundle branch blocks during supraventricular tachycardia have been described, and their sustainment has been attributed to concealed conduction. Such blocks frequently resolve spontaneously, but the electrophysiologic mechanism of resolution has not been well described. This report describes the resolution of functional bundle branch block through proximal migration of the site of block. During electrophysiologic study of a patient with reentrant antidromic tachycardia via an atriofascicular accessory pathway, functional retrograde right bundle branch block could be readily induced following tachycardia initiation with right ventricular apical pacing. Resolution of this block was associated with shortening of the tachycardia cycle length. Electrogram recordings along the right bundle branch during tachycardia determined that resolution of the functional retrograde right bundle branch block was associated with migration of the site of block from the distal to the proximal right bundle. When the site of block was directly at the recording site, both anterograde and retrograde activation of the right bundle was demonstrated. Migration of the site of block is a mechanism of resolution for functional conduction blocks maintained by concealed conduction.

Posteroseptal accessory pathways: an overview of anatomical characteristics, electrocardiographic patterns, electrophysiological features, and ablative therapy.

First, the posteroseptal region of the heart is probably the most complex area among those that harbor AV accessory fibers and a firm grasp of the anatomical characteristics of this region may facilitate achieving a successful AP ablation. Second, there is no sharp demarcation between the posteroseptal area and its surrounding regions including mid-septal, left posterior paraseptal, and right posterior paraseptal locations. Therefore, there are some inevitable overlaps between the electrocardiographic and electrophysiological features of APs located in the posteroseptal region and those areas immediately adjacent to it. Third, in the vast majority of cases, successful ablation can be achieved using a right atrial approach. Therefore, dividing posteroseptal APs into right- or left-sided pathways may only be useful for describing their ECG or electrophysiological characteristics with little or no value in predicting the site of successful ablation. Fourth, it seems advisable to attempt efforts to induce functional bundle branch block during orthodromic tachycardia and assess its effect on the VA interval. Ventriculoatrial interval prolongation due to right bundle branch block strongly favors a right free-wall or anteroseptal AP location. Prolongation of the VA interval by 30 msec or less in response to left bundle branch block is compatible with a posteroseptal location. In this situation, the mapping and ablative efforts should primarily be focused on the right atrial approach, including the terminal coronary sinus. If left bundle branch block causes VA interval lengthening of 30 msec or greater, the AP is most likely in the left free-wall region, including the posterior paraseptal area. Finally, the presence of APs having an intimate relationship with the middle cardiac (posterior interventricular) vein or the coronary sinus pouch, although exceedingly uncommon, should be considered in difficult cases in which radiofrequency applications to the conventional posteroseptal locations are unsuccessful. Such cases may require coronary sinus venography for better visualization and precise mapping of the terminal sinus complex.

Overdrive prolongation of refractoriness and fatigue in the early stages of human bundle branch disease

Objectives. The aim of this study was to assess the response of refractoriness in normal and diseased human bundle branches to changes in cycle length, as well as during a long period of continuous overdrive pacing.Background. The anterograde refractory period of the bundle branches in patients with functional bundle branch block shortens as the rate is increased. The rate-dependent response of refractoriness in diseased bundle branches is quite different. However, this difference has not been precisely delineated, and its physiologic meaning is uncertain.Methods. Refractoriness of the bundle branches was measured by the extrastimulus technique in 16 patients with tachycardia-dependent bundle branch block and 10 patients with functional bundle branch block, both after basic trains of 8 atrial-paced impulses at different cycle lengths and during a 10-min period of continuous overdrive pacing.Results. The baseline refractory period in the bundle branches of patients with functional bundle branch block measured 430 ±32 ms (mean ± SD) and shortened to 368 ± 30 ms at the shortest cycle length. The maximal effect was reached within the 1st min of overdrive pacing. The baseline refractory period of the bundle branches was significantly longer in patients with tachycardia-dependent bundle branch block (611 ± 184 ms) and demonstrated a cumulative overdrive prolongation in 15 (83%) of 18 studies with typical manifestations of fatigue. In two other studies, this occurred only after ajmaline administration.Conclusions. A rate- and time-dependent prolongation of refractoriness frequently occurs in diseased human bundle branches. When absent, this response may be induced under the effects of sodium channel blockers. This would suggest that an abnormality in the recovery from inactivavion of the sodium channel might underlie the early stages of bundle branch disease.

Catheter ablation of left posteroseptal accessory pathways and of long RP′ tachycardias with a right endocardial approach

Fifty-four patients with a posteroseptal accessory connection and symptomatic tachycardias underwent catheter ablation of the anomalous pathway. Eight had the permanent form of reciprocating tachycardias (long RP' tachycardia) and 46 had a left posteroseptal preexcitation marked by a prominent R wave in lead VI. In 14 of 19 patients, ventriculoatrial conduction time during tachycardia lengthened in conjunction with functional left bundle branch block; this behaviour was significantly different from a series of patients with right posteroseptal preexcitation in which functional left bundle branch block lengthened the ventriculoatrial time in only one of 12 patients. A quadripolar electrode catheter was left within the proximal coronary sinus in order to locate the earliest atrial or ventricular activation site. The appropriate bipole was used as the radiographic and electrophysiological reference of the insertion of the accessory pathway. A catheter was then positioned on the septal side of the right atrium, outside the coronary sinus, so that atrial activity during reciprocating tachycardia and ventricular activity during preexcitation were synchronous with or earlier than that recorded within the proximal coronary sinus. Accessory pathway potential was not recorded in any patient. Early ventricular potential occurring --1.5 +/- 8 ms relative to delta wave onset was present at that site. In 38 patients, including 5 with permanent junctional tachycardia, high current (14 mA) pacing yielded direct ventricular paced QRS complexes (no delay spike-QRS) with a morphology similar to left posteroseptal maximal preexcitation. Slight movements of catheter position yielded significantly different pace-maps. One to eight 160 J cathodal shocks (510 +/- 213 J cumulative per patient) were delivered at this site in 61 sessions. Following fulguration, tachycardia recurred without drugs in only one patient over a follow-up period of 20 +/- 13 months. Asymptomatic intermittent preexcitation recurred in two patients. In all patients with long RP' tachycardia, the ablation procedure was successful without the need for drugs or permanent cardiac pacing. A long-term follow-up electrophysiological study in 18 patients demonstrated that conduction through the anomalous pathway was absent in 16 and deeply altered in the two patients with intermittent preexcitation; no tachycardia was inducible in any patient. In conclusion, catheter ablation of left posteroseptal accessory pathways is a feasible procedure using a right atrial approach outside the coronary sinus. This technique is also effective for the treatment of the permanent form of reciprocating tachycardia.

Electrophysiologic mechanisms of functional bundle branch block at onset of induced orthodromic tachycardia in the Wolff-Parkinson-White syndrome. Role of stimulation method.

The mechanisms of aberrant conduction at the onset of induced orthodromic tachycardia in the Wolff-Parkinson-White syndrome were analyzed in 20 consecutive patients in whom this tachycardia was initiated by the atrial (A2) and/or right ventricular (V2) extrastimulus techniques. Of 13 patients in whom orthodromic tachycardia was induced by the A2 method, functional right bundle branch block occurred at tachycardia onset in four (31%) and left bundle aberrancy in two (15%), one of whom also manifested right bundle aberrancy. The occurrence of bundle branch block at the onset of tachycardia was linked to aberrant conduction of the initiating A2 impulse which, in turn, was associated with attainment of relatively short His1His2 intervals within the tachycardia initiation zone. Aberrant conduction of A2 was also more common in patients without manifest preexcitation. In contrast, of 14 patients in whom orthodromic tachycardia was induced by the V2 method, left bundle aberrancy occurred at the onset of tachycardia in 11 (79%), one of whom manifested right bundle branch block as well. Left bundle aberrancy was more likely to occur when the interval from the initiating V2 (or macro-reentrant V3) impulse to the first anterograde His deflection was less than 300 ms. This suggests that left bundle aberrancy at the onset of orthodromic tachycardia induced by the V2 method results from concealed retrograde penetration of the His-Purkinje system, with the left bundle being last to recover. Our findings provide the conceptual basis for a physiologic approach to the deliberate induction of specific types of aberrant conduction at onset of orthodromic tachycardia in patients with Wolff-Parkinson-White syndrome.

Functional bundle branch block: Discordant response of right and left bundle branches to changes in heart rate

Faster heart rates shorten refractoriness more in some tissues than in others. This study investigates whether faster heart rates shorten relative refractoriness more in the right than left bundle branch in humans. Premature atrial stimulation at 2 or more basic cycle lengths was performed in 314 patients with no evidence of atrioventricular conduction system disease. In 10 patients, both functional right and left bundle branch block (BBB) developed with premature atrial stimulation. Functional right BBB occurred at the longer basic cycle length, and functional left BBB at the shorter cycle length in 8 patients. In 2 patients functional right and functional left BBB were present at the same cycle length, but functional left BBB occurred at a shorter premature atrial coupling interval. For all patients, the mean functional right bundle branch relative refractoriness was 438 ms at a basic cycle length of 847 ms, and functional left bundle branch relative refractoriness was 357 ms at a cycle length of 622 ms (p <0.01). The HV interval was 45 ± 15 ms at control and increased with functional left BBB to 77 ± 19 ms (p <0.01), but not with functional right BBB. Thus, relative refractoriness of the right and left bundle branches are rate-dependent and discordant. At longer cycle lengths, relative refractoriness of the right bundle branch is greater than that of the left bundle branch, and at shorter cycle lengths relative refractoriness of the left bundle branch is greater than that of the right bundle branch. The relative refractory period curves “cross over” and can explain the presence of both functional right and left BBB in the same patient.

Effects of alternating cycle lengths on refractoriness of the His-Purkinje system.

The effects of alternating cycle lengths (bigeminal rhythm) on His-Purkinje system refractoriness were studied in 14 patients using His bundle and right bundle recordings. Programmed atrial stimulation at constant cycle length (method I) was scanned using the atrial extrastimulus technique (A2) and compared with an atrial cycle length of identical duration coupled to A2 on alternate beats (method II). The results showed that (a) despite shorter cycle length of the His-Purkinje system with method II due to effect of A2 on atrioventricular nodal conduction (699 +/- 90 vs. 743 +/- 87 ms, P less than 0.001), the relative refractory period of the His-Purkinje system was always longer with method II (463 +/- 52 vs. 440 +/- 43 ms, P less than 0.001). A similar increase also occurred in effective refractory period of the His-Purkinje system; (b) while functional right bundle branch block occurred in eight patients and functional left bundle branch block in two patients with method I, functional right bundle branch block occurred in all 14 patients and left bundle branch block in seven patients with method II; (c) in two patients where both functional right and left bundle branch block occurred with method I this never was manifest at identical degree of abbreviation of His-Purkinje system cycle length but was manifest at identical abbreviation in each of seven patients with method II; (d) site of conduction delay and/or block during functional right bundle branch block was always proximal, i.e., between the His bundle and right bundle recordings with both methods. During method II this resulted in shortening of the subsequent right bundle cycle length relative to the subsequent His bundle (and of necessity left bundle) cycle length. The finding of increased His-Purkinje system refractoriness despite shorter preceding cycle length of the His-Purkinje system during atrial bigeminy has never been previously described and suggests that classical concepts of His-Purkinje system behavior may require revision in this setting. Secondly, during atrial bigeminy the occurrence of alternating functional bundle branch block cannot be accounted for solely by the degree of abbreviation of His-Purkinje system cycle length, but may be explained by a relative shortening of the next ipsilateral bundle branch cycle length in the bundle manifesting block.