Accessory atrioventricular pathways that conduct only in the antegrade direction

Characteristics of antegrade-only accessory pathways in children and adolescents

There is minimal data regarding antegrade-only accessory pathways in young patients. Given evolving recommendations and treatments, retrospective analysis of the clinical and electrophysiologic properties of antegrade-only pathways in patients <21 years old was performed, with subsequent comparison of electrophysiology properties to age-matched controls with bidirectional pathways. Of 522 consecutive young patients with ventricular pre-excitation referred for electrophysiology study, 33 (6.3%) had antegrade-only accessory pathways. Indications included palpitations (47%), chest pain (25%), and syncope (22%). The shortest value for either the accessory pathway effective refractory period or the pre-excited R-R interval was taken for each patient, with the median of the antegrade-only group significantly greater than shortest values for the bidirectional group (310 [280-360] ms versus 270 [240-302] ms, p < 0.001). However, the prevalence of pathways with high-risk properties (effective refractory period or shortest pre-excited R-R interval <250 ms) was similar in both study patients and controls (13% versus 21%) (p = 0.55). Sixteen patients had a single antegrade-only accessory pathway and no inducible arrhythmia. Six patients had Mahaim fibres, all right anterolateral with inducible antidromic reciprocating tachycardia. However, 11 patients with antegrade-only accessory pathways and 3 with Mahaim fibres had inducible tachycardia due to a second substrate recognised at electrophysiology study. These included concealed accessory pathways (7), bidirectional accessory pathways (5), and atrioventricular node re-entry (2). Antegrade-only accessory pathways require comprehensive electrophysiology evaluation as confounding factors such as high-risk conduction properties or inducible Supraventricular Tachycardia (SVT) due to a second substrate of tachycardia are often present.

Arrhythmia diagnosis and management throughout life in congenital heart disease

Arrhythmias, covering bradycardia and tachycardia, occur in association with congenital heart disease (CHD) and as a consequence of surgical repair. Symptomatic bradycardia can occur due to sinus node dysfunction or atrioventricular block secondary to either unrepaired CHD or surgical repair in the area of the conduction system. Tachyarrhythmias are common in repaired CHD due to scar formation, chamber distension or increased chamber pressure, all potentially leading to abnormal automaticity and heterogeneous conduction properties as a substrate for re-entry. Atrial arrhythmias occur more frequently, but ventricular tachyarrhythmias may be associated with an increased risk of sudden cardiac death, notably in patients with repaired tetralogy of Fallot or aortic stenosis. Defibrillator implantation provides life-saving electrical therapy for hemodynamically unstable arrhythmias. Ablation procedures with 3D electroanatomic mapping technology offer a viable alternative to pharmacologic or device therapy. Advances in electrophysiology have allowed for successful management of arrhythmias in patients with congenital heart disease.

Pacing site and bradycardia dependent retrograde conduction block over an atrioventricular accessory pathway

Intermittent retrograde conduction over an accessory pathway (AP) is an uncommon phenomenon. We report a case of Wolff-Parkinson-White syndrome in which retrograde conduction of the bypass tract was present only during right ventricular (RV) outflow tract and left ventricular pacing or during RV inflow tract pacing with a fast pacing rate (pacing site and bradycardia dependent conduction block). The interaction between different branches/fibres of the bypass tract might explain the mechanism of this phenomenon. The AP was successfully ablated at the lateral aspect of mitral valve annulus. It is suggested that careful evaluation by decremental ventricular stimulation should be applied from multiple sites both during baseline study and after apparent AP ablation in order to detect such uncommon cases of intermittent conduction.

Electrophysiologic properties of the atrioventricular node in pediatric patients

OBJECTIVES

The purpose of this study was to characterize anterograde and retrograde properties of the atrioventricular (AV) node in children and to determine the presence of ventriculoatrial (VA) conduction and dual AV node pathways.

BACKGROUND

Although AV node reentry is common in adults, it accounts for 13% of pediatric supraventricular tachycardia (SVT). The age-related changes in the AV node with development are poorly understood. The incidence of dual AV node pathways and VA conduction in the pediatric population is unknown.

METHODS

Electrophysiologic studies were performed in 79 patients with normal hearts and no evidence of AV node arrhythmias. Patients were classified into two groups by age: group I = 49 patients (0.39 to 12.8 years old, mean [+/- SD] age 8.5 +/- 3.6); group II = 30 patients (13.4 to 20.0 years old, mean age 15.6 +/- 1.8).

RESULTS

There was a significant difference (p < 0.05) in the cycle length (CL) at which anterograde AV block occurred between group I (305 +/- 63 ms) and group II (350 +/- 91 ms). Sixty-one percent of children had VA conduction with no age-related differences. There was no significant difference in the mean CL of retrograde VA block (360 ms). The incidence of dual AV node pathways in group I was 15% and 44% in group II (p < 0.05).

CONCLUSIONS

These findings suggest that AV node electrophysiology undergoes maturational changes. The increase in AV node reentrant tachycardia in adults may relate to changes in the relative refractoriness and conduction of the AV node or to differences in autonomic input into the AV node that allow dual pathway physiology to progress to SVT.

Insights into the electrophysiology of accessory pathway-mediated arrhythmias provided by the catheter ablation experience: "learning while burning, part III"

The success of catheter ablation has greatly improved the care of patients with paroxysmal tachycardias and has caused a revolution in the practice of electrophysiology. Some investigators have expressed that concern over procedural success in an increasingly interventional specialty threatens to eclipse attempts to understand the physiology of arrhythmia syndromes. Alternatively, due to the precise and directed nature of the lesions created with radiofrequency energy, catheter ablation procedures have allowed investigation to continue at a more focused level. In this article, the insights provided by the catheter ablation experience into the physiology of arrhythmias mediated by accessory AV pathways will be reviewed. Although the learning process was sometimes delayed by the nearly immediate success of radiofrequency catheter ablation, difficult situations have continued to renew efforts for understanding at a deeper level. Conscious attempts at "learning while burning" will provide the opportunity to investigate aspects of bypass tract physiology that remain incompletely characterized, such as partial response to therapy and late recurrence.

Intermittent preexcitation and mesothelioma of the atrioventricular node: a hitherto undescribed entity

This is the first documented case of spontaneous intermittent preexcitation associated with mesothelioma of the atrioventricular (AV) node. A 66-year-old male with recurrent atrial arrhythmias, palpitations, heart failure, and marked intra-atrial conduction defect that required a pacemaker died during sleep. Electrophysiologic study revealed left free-wall bypass tract with marked intra-atrial conduction defect and prolonged conduction across the bypass tract. With atrial pacing, high degrees of block were noted in the bypass tract. Serial section of the conduction system and both AV rims revealed two left posterior and lateral bypass pathways with patchy areas of fibrosis. A large mesothelioma (benign AV nodal tumor) almost completely replaced the AV node. In addition, there was marked fatty infiltration of the atria. In summary: (1) the intermittent preexcitation with prolonged conduction across the bypass tract and block with atrial pacing were probably related to the incomplete patchy degenerative changes in the bypass tract, and/or almost complete replacement of the AV node by the tumor; (2) the intra-atrial conduction defect was probably related to the replacement of the AV node by mesothelioma and/or the fatty infiltration of the atria; and (3) the paroxysmal atrial arrhythmias probably reflect the marked atrial pathology.

Effect of isoproterenol on accessory pathways without overt retrograde conduction

INTRODUCTION

Absence of overt retrograde accessory pathway conduction may be related to low resting sympathetic tone in patients with apparent unidirectional anterogradely conducting accessory pathways (UACAP).

METHODS AND RESULTS

To test this hypothesis, we studied the effect of isoproterenol on accessory pathway function and tachycardia induction in 18 patients (12 men and 6 women, ages 34 +/- 16 years [mean +/- SD]) with UACAP. After baseline study in the drug-free state, electrphysiologic testing was repeated during infusion of isoproterenol (0.5 to 1.5 micrograms/min, titrated to increase heart rate by 20%). Isoproterenol shortened the anterograde effective refractory period (398 +/- 117 vs 305 +/- 63 msec; P < 0.01; basic drive cycle length 600 msec) of the accessory pathway. However, retrograde accessory pathway conduction and atrioventricular reentrant tachycardia were exposed in only 3 (17%) patients by isoproterenol infusion. All 3 patients with retrograde accessory pathway revealed after isoproterenol had clinically documented tachycardia (supraventricular tachycardia in 2, atrial fibrillation in 1) during exercise, while none of the patients with persistent absence of retrograde accessory pathway conduction had this symptom.

CONCLUSIONS

We conclude that absence of overt retrograde conduction over accessory pathways may be related to low resting sympathetic tone in some individuals. Restoration of retrograde conduction with isoproterenol is unusual and most likely to be observed in patients with clinically documented paroxysmal supraventricular tachycardia related to exercise.

Independent ablation of retrograde and anterograde accessory connection conduction at the atrial and ventricular insertion sites: evidence supporting the impedance mismatch hypothesis for unidirectional block

INTRODUCTION

The impedance mismatch hypothesis has been proposed as one of the mechanisms responsible for accessory connection conduction block.

METHODS AND RESULTS

We describe a case in which retrograde conduction across a posteroseptal accessory connection was ablated at the atrial insertion site in the coronary sinus os, whereas anterograde conduction was ablated at the ventricular insertion site in the left ventricle.

CONCLUSION

The evidence supports the impedance mismatch concept as the cause of unidirectional conduction block and shows that the distal junction between the accessory connection and the myocardium is more vulnerable to ablation.

Relation of syncope in young patients with Wolff-Parkinson-White syndrome to rapid ventricular response during atrial fibrillation

Syncope in patients due to Wolff-Parkinson-White (WPW) syndrome may be related either to a rapid rate of supraventricular tachycardia or to rapid ventricular response over the accessory pathway during atrial fibrillation (AF). From 1982 to 1987, 74 patients less than or equal to 25 years old (mean age 12.6 years) with WPW syndrome on electrocardiogram underwent electrophysiologic study. Of the 74 patients, 14 (19%) had a history of syncope. During electrophysiologic study 9 of 14 patients with syncope had sustained (greater than 5 minutes or requiring termination due to hypotension) AF. Of the remaining 5 patients, 3 had inducible nonsustained AF and 2 had no AF. None of the 60 patients without syncope developed sustained AF; 34 had nonsustained and 26 had no AF. Occurrence of sustained AF had a sensitivity of 64% and specificity of 100% for history of syncope. All patients with syncope and AF (12) had a short RR interval between 2 consecutive preexcited QRS complexes during AF at less than or equal to 220 ms, in contrast to 9 of 34 patients without syncope (p less than 0.001, sensitivity 100%, specificity 74%). No patient with a short RR interval between 2 consecutive preexcited QRS complexes during AF of greater than 220 ms had a history of syncope. Thus, in these young patients with WPW syndrome, occurrence of AF with a rapid ventricular response during electrophysiologic study correlated well with a history of syncope and may be the cause of syncope in most patients. Electrophysiologic study may be helpful in identification of young patients with WPW at risk for syncope.

Use of intracardiac recordings to determine the site of drug action in paroxysmal supraventricular tachycardia

Paroxysmal supraventricular tachycardia most often results from atrioventricular (AV) reentry using an accessory AV pathway (Wolff-Parkinson-White syndrome) or reentry within the region of the AV node. In AV reentry, using an accessory pathway, suppression of the tachycardia may be achieved by depressing either anterograde AV nodal conduction or retrograde accessory pathway conduction. Intracardiac recordings and programmed electrical stimulation have established that beta-adrenergic antagonists and calcium channel blockers principally affect AV nodal conduction (anterograde limb of the reentrant circuit), whereas class IA and IC agents principally affect the accessory AV pathway (retrograde limb). Pharmacologic therapy has been more effective when directed at the limb in which conduction is most marginal at the tachycardia rate (weak limb). In individual patients, intracardiac recordings and programmed electrical stimulation can be used to identify the weak limb, indicating the class of agents most likely to be effective. Specialized techniques allowing direct recording of accessory pathway activation suggest that limitations in accessory pathway conduction may be explained by anatomic impediments. Conduction is most limited at the atrial interface of the accessory pathway in some patients, whereas in others the ventricular interface may be the limiting factor. Class IA and IC agents appear to have the greatest effect at sites where conduction is most tenuous, i.e., at the anatomic impediments. Similar considerations apply to AV nodal reentry. Anterograde slow AV nodal pathway conduction is most often depressed by digitalis preparations, beta-adrenergic antagonists, and calcium channel blockers, whereas retrograde fast AV nodal pathway conduction is more often depressed by class IA and IC agents. Intracardiac recordings and programmed electrical stimulation can also be used in these patients to identify the weak limb and direct pharmacologic therapy. Direct catheter recordings of AV nodal conduction remain elusive, limiting knowledge of the different conduction properties of the anterograde and retrograde limbs and the site(s) of drug action. Studies in progress, comparing the retrograde AV nodal conduction time during tachycardia with that during ventricular pacing at the same rate, suggest that the His bundle may be incorporated in the reentrant circuit in some patients. It appears that verapamil more readily depresses retrograde fast pathway conduction in these patients than in those in whom the His bundle does not form part of the reentrant circuit, but the reasons for this are unknown.

Value of the resting 12 lead electrocardiogram and vectorcardiogram for locating the accessory pathway in patients with the Wolff-Parkinson-White syndrome

The resting 12 lead electrocardiogram and vectocardiogram were reviewed in 47 patients with the Wolff-Parkinson-White syndrome (a) who had pre-excitation on the resting 12 lead electrocardiogram, (b) who had a single anterograde conducting accessory pathway assessed and located during preoperative electrophysiological study and during epicardial mapping at operation, and (c) in whom surgical division of the accessory pathway resulted in loss of pre-excitation. The site of the accessory pathway established during operation was compared with that established by evaluating the polarity of the delta wave and QRS complex on the resting 12 lead electrocardiogram. The electrocardiogram was assessed by the Rosenbaum criteria (Wolff-Parkinson-White type A, left-sided pathway; or type B, right-sided pathway), the Gallagher criteria (atrial pacing resulting in maximal pre-excitation), and the World Health Organisation criteria (a composite of previous studies). The Gallagher and World Health Organisation criteria were derived from patients demonstrating maximal pre-excitation that often required atrial pacing. The present study was designed to determine whether these criteria could be accurately applied to the resting 12 lead electrocardiogram on which the degree of pre-excitation was variable. The Rosenbaum criteria correctly identified a left sided accessory pathway in 26 of 34 patients and a right-sided accessory pathway in nine of 13 patients. The Gallagher and World Health Organisation criteria correctly identified the location in only 15 (32%) of the 47 patients. The resting vectorcardiogram was inaccurate for locating the accessory pathway. Although published criteria are useful for identifying the site of the accessory pathway from an electrocardiogram obtained when rapid atrial pacing is being used to achieve maximal pre-excitation, they are not suitable for identifying the exact site of an accessory pathway from the resting 12 lead electrocardiogram.

Preexcitation syndromes. Mechanisms and management

Ventricular preexcitation occurs when, in relation to atrial events, some or all of the ventricular muscle is activated earlier by the atrial impulse than would be expected if conduction of the impulse activated the ventricles by way of the normal atrioventricular conduction system. The purpose of this article is to review the pathophysiology of the variants of preexcitation and to discuss the therapeutic approach to patients who have tachyarrhythmias.

Basic electrophysiologic mechanisms for the development of arrhythmias. Clinical application

The mechanisms responsible for the genesis of cardiac arrhythmias are frequently divided into categories of disorders of impulse formation, disorders of impulse propagation, or combinations of both. Classification of arrhythmias into these categories is based largely on the results of experimental studies, where the initiation and perpetuation of an arrhythmia can be studied in some detail in a relatively controlled environment.

Bystander accessory pathway during AV node re-entrant tachycardia

Between 1970 and July 1980, wide QRS tachycardia due to re-entry confined to the AV node with bystander involvement of an accessory atrioventricular pathway (AAV) was documented in three of 290 patients with the Wolff-Parkinson-White syndrome studied at Duke Medical Center. In each of the patients, at least one transition between wide and narrow QRS morphology was recorded without change in either the cycle length of tachycardia or the atrial activation sequence. Two of the three patients had a single left-sided AAV (lateral, postero-lateral) showing antegrade conduction only. The third patient had two right-sided AAVs (free wall, septal), each capable of bidirectional conduction. Initiation and termination of repetitive concealed conduction into the ventricular insert of an AAV appeared to be one mechanism determining bystander AAV participation. Documentation of the retrograde sequence of atrial activation during tachycardia, and examination of the effects of interpolated premature depolarizations from both the ventricle and mid-line atrium are the most helpful features in resolving the differential diagnosis of wide QRS tachycardia in patients with W-P-W syndrome.