Selective atrionodal input ablation for induction of proximal complete heart block with stable junctional escape rhythm in patients with uncontrolled atrial fibrillation

Beyond the lens: Unveiling the invisible atrioventricular node in the era of high-density mapping

Numerous studies have clarified the histological characteristics of the area surrounding the atrioventricular (AV) node, commonly referred to as the triangle of Koch (ToK). Although it is suggested that the conduction of electric impulses from the atria to the ventricles via the AV node involves myocytes possessing distinct conduction properties and gap junction proteins, a comprehensive understanding of this complex conduction has not been fully established. Moreover, although various pathways have been proposed for both anterograde and retrograde conduction during atrioventricular nodal reentrant tachycardia (AVNRT), the reentrant circuits of AVNRT are not fully elucidated. Therefore, the slow pathway ablation for AVNRT has been conventionally performed, targeting both its anatomical location and slow pathway potential obtained during sinus rhythm. Recently, advancements in high-density three-dimensional (3D) mapping systems have facilitated the acquisition of more detailed electrophysiological potentials within the ToK. Several studies have indicated that the activation pattern, the low-voltage area within the ToK obtained during sinus rhythm, and the fractionated potentials acquired during tachycardia may be optimal targets for slow pathway ablation. This review provides an overview of the tissue surrounding the AV node as reported to date and summarizes the current understanding of AV conduction and AVNRT circuits. Furthermore, we discuss recent findings on slow pathway ablation utilizing high-density 3D mapping systems, exploring strategies for optimal slow pathway ablation.

New insights into atrioventricular nodal anatomy, physiology, and immunochemistry: A comprehensive review and a proposed model of the slow-fast atrioventricular nodal reentrant tachycardia circuit in agreement with direct potential recordings in the Koch's triangle area

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most frequent regular tachycardia in humans. In this review, we describe the most recent discoveries regarding the anatomical, physiological, and molecular biological features of the atrioventricular junction that could underlie the typical slow-fast AVNRT mechanisms, as these insights could lead to the proposal of a new theory concerning the circuit of this arrhythmia. Despite several models have been proposed over the years, the precise anatomical site of the reentrant circuit and the pathway involved in the slow-fast AVNRT have not been conclusively defined. One possible way to evaluate all the hypotheses regarding the nodal tachycardia circuit in humans is to map this circuit. Thus, we tried to identify the slow potential of nodal and inferior extension structures by using automated mapping of atrial activation during both sinus rhythm and typical slow-fast AVNRT. This constitutes a first step toward the definition of nodal area activation in sinus rhythm and during slow-fast AVNRT. Further studies and technical improvements in recording the potentials of the atrioventricular node structures are necessary to confirm our initial results.

Anatomy for ablation of atrioventricular nodal reentry tachycardia and accessory pathways

The atrioventricular (AV) valve plane and the central septum are of particular importance for electrophysiological diagnosis and interventional therapy of supraventricular tachycardias because accessory electrical connections of various types may be present in addition to the specific conduction system. Although modern 3D electroanatomic reconstruction systems including high-density mapping can be of great assistance, detailed knowledge of the anatomic structures involved, their complex three-dimensional arrangement, and their electrical properties in conjunction with electrophysiological features of supraventricular arrhythmias is essential for safe and efficient electrophysiological treatment. The aim of this article is to present current anatomical, topographical, and electrophysiological findings against the background of historical, seminal, and still indispensable literature.

Atrioventricular junctional ablation: The good, the bad, the better

Background

The management of patients with atrial fibrillation and an abnormally fast ventricular response has been through the use of pharmacologic agents. In those cases where rate control cannot be achieved pharmacologically, a standard approach has been atrioventricular (AV) junctional ablation and ventricular pacemaker implantation to achieve a stable ventricular rate. Long-term ventricular pacing has been shown to result in diminished ventricular function that can lead to heart failure.

Objective

To describe an experimental and clinical study demonstrating a modified form of AV junction ablation.

Methods

Ablation of the slow and fast AV nodal input does not produce AV block. Ablation of the connection between the two induces AV block, leaving the AV node and His bundle intact.

Results

Subsequently the escape heart rate is close to normal and responds well to exercise.

Conclusion

In a clinical study with a 42 month follow-up, the modified procedure resulted in significantly reduced pacemaker dependence and mortality compared to the standard AV ablation procedure.

Ablate and Pace: Is There Still a Role?

Despite the development of newer drugs and procedures to improve rhythm control, there is still a place for ablation of the atrioventricular junction (AVJ) in the management of selected patients with AF who are refractory to medical therapy, to improve quality of life, prevent ventricular dysfunction, and to optimize cardiac resynchronization therapy. We review all aspects of the "ablate and pace" strategy, from its history to patient selection, technique, outcomes and applications, and identify the need for randomized clinical trials to address some of the remaining questions regarding its application in some groups of patients.

Chronic augmentation of the parasympathetic tone to the atrioventricular node: a nonthoracotomy neurostimulation technique for ventricular rate control during atrial fibrillation

INTRODUCTION

The right inferior ganglionated plexus (RIGP) selectively innervates the atrioventricular node. Temporary electrical stimulation of this plexus reduces the ventricular rate during atrial fibrillation (AF). We sought to assess the feasibility of chronic parasympathetic stimulation for ventricular rate control during AF with a nonthoracotomy intracardiac neurostimulation approach.

METHODS AND RESULTS

In 9 mongrel dogs, the small endocardial area inside the right atrium, which overlies the RIGP, was identified by 20 Hz stimulation over a guiding catheter with integrated electrodes. Once identified, an active-fixation lead was implanted. The lead was connected to a subcutaneous neurostimulator. An additional dual-chamber pacemaker was implanted for AF induction by rapid atrial pacing and ventricular rate monitoring. Continuous neurostimulation was delivered for 1-2 years to decrease the ventricular rate during AF to a range of 100-140 bpm. Implantation of a neurostimulation lead was achieved within 37 +/- 12 min. The latency of the negative dromotropic response after on/offset or modulation of neurostimulation was <1 s. Continuous neurostimulation was effective and well tolerated during a 1-2 year follow-up with a stimulation voltage <5 V. The neurostimulation effect displayed a chronaxie-rheobase behavior (chronaxie time of 0.07 +/- 0.02 ms for a 50% decrease of the ventricular rate during AF).

CONCLUSION

Chronic parasympathetic stimulation can be achieved via a cardiac neurostimulator. The approach is safe, effective, and well tolerated in the long term. The atrioventricular nodal selectivity and the opportunity to adjust the negative dromotropic effect within seconds may represent an advantage over pharmacological rate control.

Autonomic control and innervation of the atrioventricular junctional pacemaker

BACKGROUND

The main physiologic function of the AV junction is control of timing between atrial and ventricular excitation. However, under pathologic conditions, the AV junction may become the pacemaker of the heart. Unlike the well-characterized sinoatrial node (SAN), autonomic control of the AV junctional pacemaker has not been studied.

OBJECTIVE

The purpose of this study was to characterize the autonomic control and innervation of the AV junctional pacemaker.

METHODS

The response of rabbit AV junctional pacemaker to autonomic stimulation was investigated using optical mapping, autonomic modulation via subthreshold stimulation (n = 12), and quantitative immunohistochemistry (n = 5), and the density of parasympathetic and sympathetic innervation in optically mapped preparations was quantified.

RESULTS

Subthreshold stimulation applied adjacent to the conduction system in the triangle of Koch autonomically modulates the junctional rate, and parasympathetic and sympathetic components can be separated with atropine and the beta-blocker nadolol. Subthreshold stimulation increased the rate maximally to 2.1 +/- 0.4 times when applied with atropine. Unlike the SAN pacemaker, which shifts significantly in response to autonomic stimulation, the AV junctional pacemaker remains stationary (most often in the inferior nodal extension), moving in only 5% of subthreshold stimulation trials. Staining with tyrosine hydroxylase and choline acetyltransferase revealed heterogeneous innervation within the AV junction.

CONCLUSION

AV junctional rhythm can be autonomically modulated with subthreshold stimulation to produce junctional rates of 145 +/- 16 bpm (cycle length 412 +/- 29 ms), similar to sinus rates in rabbit. Unlike the SAN, the anatomic location of the AV junctional pacemaker is stable during autonomic modulation.

Isolated Atrial Segment Pacing

OBJECTIVES

This study was designed to investigate a practical alternative to His bundle pacing after atrioventricular (AV) junctional ablation by pacing a small area of isolated atrial tissue surrounding the AV node.

BACKGROUND

His bundle pacing is preferred after AV junctional ablation in patients with refractory atrial fibrillation. However, it is technically difficult and not clinically useful at the present time.

METHODS

This study was conducted in an isolated working swine heart model (n = 5), with real-time imaging capabilities. A small area of atrial tissue surrounding the AV node and the His bundle was isolated using sequential radiofrequency ablation lesions.

RESULTS

Complete AV block created by segmental atrial isolation was achieved in 5 of 5 experiments. The isolated atrial segment was bordered by the ablation lines, the tricuspid annulus, and the AV node-His bundle. The AV conduction was characterized using a pacing electrode implanted into the isolated atrial segment. Pacing from the atria, the ventricles, and the isolated atrial segment at different rates confirmed complete bidirectional block between the atria and isolated area, whereas antegrade and retrograde AV nodal conduction between the isolated atrial segment and the ventricles remained intact. Pacing from the isolated area produced minimal changes in systolic left ventricular pressure compared with baseline sinus rhythm (mean -2 mm Hg).

CONCLUSIONS

Isolation of a small area of atrial tissue surrounding the AV node is feasible by transcatheter radiofrequency ablation. This procedure may be a useful alternative to conventional AV junctional ablation because it can create complete AV block, while in effect permitting the equivalent of His bundle pacing after AV junctional ablation.

Chronic atrioventricular nodal vagal stimulation: first evidence for long-term ventricular rate control in canine atrial fibrillation model

BACKGROUND

We have previously demonstrated that selective atrioventricular nodal (AVN) vagal stimulation (AVN-VS) can be used to control ventricular rate during atrial fibrillation (AF) in acute experiments. However, it is not known whether this approach could provide a long-term treatment in conscious animals. Thus, this study reports the first observations on the long-term efficacy and safety of this novel approach to control ventricular rate during AF in chronically instrumented dogs.

METHODS AND RESULTS

In 18 dogs, custom-made bipolar patch electrodes were sutured to the epicardial AVN fat pad for delivery of selective AVN-VS by a subcutaneously implanted nerve stimulator (pulse width 100 micros or 1 ms, frequency 20 or 160 Hz, amplitude 6 to 10 V). Fast-rate right atrial pacing (600 bpm) was used to induce and maintain AF. ECG, blood pressure, and body temperature were monitored telemetrically. One week after the induction of AF, AVN-VS was delivered and maintained for at least 5 weeks. It was found that AVN-VS had a consistent effect on ventricular rate slowing (on average 45+/-13 bpm) over the entire period of observation. Echocardiography showed improvement of cardiac indices with ventricular rate slowing. AVN-VS was well tolerated by the animals, causing no signs of distress or discomfort.

CONCLUSIONS

Beneficial long-term ventricular rate slowing during AF can be achieved by implantation of a nerve stimulator attached to the epicardial AVN fat pad. This novel concept is an attractive alternative to other methods of rate control and may be applicable in a selected group of patients.

Variability of AV nodal potentials recorded, in vivo: direct demonstration of dual AV nodal physiology

OBJECTIVES OF STUDY

We developed a method to record extracellular A-V nodal potentials in the beating dog heart, in vivo.

METHODS

In eleven Na-pentobarbital anesthetized, open-chest dogs, an octapolar electrode catheter (2 mm rings, 2 mm spacing) was inserted through a purse-string suture in the coronary sinus (CS) distal to the ostium and positioned electrographically so that the tip electrode recorded a His bundle (Hb) potential.

RESULTS

Stable recordings of A-V nodal potentials (amplitude, 178 +/- 94 microV; duration 78 +/- 26 msec) were consistently made during sinus rhythm from the second and/or third bipolar pairs of electrodes. Programmed atrial stimulation and vagal stimulation resulted in loss of amplitude and increased duration of the A-V nodal potentials associated with A-H prolongation. In another series of experiments, crushing the sinus node in 6 dogs resulted in AV nodal rhythms with AV nodal potentials of varying amplitudes (132 to 840 microV) and durations (range 25 to 71 msec) as the earliest activation which preceded the Hb, atrial and ventricular deflections. One dog, showing dual AV nodal physiology as documented from the AV nodal function curve, had two distinctly different AV nodal potentials. The low-level, longer duration potentials were associated with longer (slow pathway) A-H intervals; whereas the shorter higher amplitude potentials (fast pathway) showed shorter A-H intervals, each occurring at a critical paced cycle length.

CONCLUSION

We conclude that consistent extracellular AV nodal electrograms can be recorded in vivo although the configuration of these potentials varies depending on heart rate, autonomic stimulation and different arrhythmic conditions such as AV nodal escape rhythms and dual AV nodal physiology.