Atrioventricular Junction Ablation for Heart-Rate Control of Atrial Fibrillation

Introduction

Atrial fibrillation (AF) has become a worldwide health issue not only that it is the most common cardiac arrhythmia seen in clinical practice, but also that it is highly correlated with a significant morbidity and mortality, including the risk for stroke, dementia, heart failure (HF), and overall mortality. Both rate and rhythm control strategies are widely used; however, it is difficult to maintain sinus rhythm for all AF patients. Rate and rhythm control strategies have similar long-term results in patients with AF. The traditional way of attaining ventricular rate control is the use of atrioventricular (AV) nodal blocking agents including beta-blockers, nondihydropyridine calcium-channel blocker, digitalis, and sometimes amiodarone. The adequate rate control was defined in American College of Cardiology/American Heart Association/Heart Rhythm Society (AHA/ACC/HRS) guidelines as a resting rate less than 80 per minute for patients with symptomatic AF and less than 110 per minute for asymptomatic patients with preserved systolic function. In patients with severe symptoms in whom drug therapy fails or drug discontinues as a result of adverse effects, ablation of the AV node and permanent pacing are effective in controlling the ventricular rate. This chapter describes the techniques of radiofrequency (RF) catheter ablation for ventricular rate control in patients with AF. For rhythm control strategy, please see previous Chapter 14, Chapter 15, Chapter 16, Chapter 17, Chapter 18, Chapter 19, Chapter 20 .

Complete Atrioventricular Junction Ablation

Rate reduction, allowing adequate time for ventricular filling and avoiding rate-related ischemia, may result in improved hemodynamics in patients with AF. Therefore complete AV junction ablation provides an effective way to control the ventricular rate during AF. AV junction ablation in conjunction with permanent pacemaker implantation provides highly effective control of heart rate and improves symptoms in selected patients, such as tachycardia-bradycardia syndrome, drug-refractory atrial tachyarrhythmias, and even permanent AF patients with cardiac resynchronization therapy (CRT), who have not received adequate biventricular pacing to improve the heart function as a result of an irregular rapid ventricular rate. This technique has been termed Ablate and Pace. Since 1982 AV junction ablation has remained as an important treatment option for refractory atrial arrhythmias. Early works on AV junction ablation were achieved by direct current (DC) shock. At present, RF energy has completely replaced DC shock as the energy source for catheter ablation of the AV junction. Cryoablation of the AV node has also been reported. However, there is no evidence to demonstrate better clinical outcome by cryoablation than conventional RF.

AV junction ablation could be performed after or before the permanent pacemaker implantation. A frequently used approach is to perform AV junction ablation via femoral vein immediately following permanent pacemaker implantation, but some operators prefer ablation followed by pacemaker insertion since the ablation procedure might damage or dislodge the permanent pacing leads. The latter procedure needs two venous punctures and catheters: one to perform the ablation and one to pace the right ventricle temporarily. However, one study enrolling 70 patients showed that device implantation before RF ablation of the AV junction is feasible and safe. An alternative strategy is to implant the permanent pacemaker for 6 to 8 weeks before the ablation procedure. This approach is safest in terms of acute lead dislodgement and allows proper anticoagulation if a left-sided approach or DC cardioversion is required during the ablation procedure. Recently, AV junction ablation through the superior vena cava has been proposed. Ablation procedure during permanent pacemaker implantation through the pocket by using the same subclavian or axillary vein access could be achieved successfully and safely with a significant reduction in procedure and laboratory occupancy time.

Before AV junction ablation is performed, appropriate ventricular backup pacing by a temporary electrode catheter at the right ventricular (RV) apex or a permanent pacemaker must be ensured. For patients with a permanent pacemaker implanted before ablation, care should be taken to ensure effective pacing during ablation because interaction between RF energy and the pacemaker may occur. The RF skin patch should be placed as far away as possible from the pacemaker generator location. The pacemaker should preferably be set to VOO mode at 40 to 50 beats per minute to avoid oversensing and inappropriate inhibition of pacing during RF energy delivery. If VVI mode is set and if inappropriate inhibition does occur, placing a donut-shaped magnet over the pulse generator could reprogram pacing mode to VOO immediately. Ventricular asystole or extreme bradycardia may occur during RF energy application because of destruction of the AV junction and inhibition of the permanent pacemaker by RF energy. Turning off the RF power supply restores pacemaker activity. After ablation, permanent pacemakers should be interrogated to assess for alterations in pacemaker programming or changes in pacing or sensing thresholds. The pacemaker is often set at a high rate (80–90 beats per minute) to reduce the risk for sudden cardiac death after AV junction ablation (discussed later).

Mapping and Ablation

For the ablation procedure, an approach from the right side is usually tried first. The anatomy of the AV node and conduction system is reviewed in Chapter 21, Chapter 5 . The most common approach is right-sided with access via the femoral vein.

AV junction ablation is performed under the guidance of fluoroscopy and intracardiac electrograms. The ablation catheter is positioned, under fluoroscopy, at the compact node region, which is located at the atrial midseptal region, just proximal and inferior to the His bundle catheter position. In a left anterior oblique view, clockwise twist of the catheter will ensure its contact with the septal aspect of the tricuspid annulus ( Fig. 22.1 ). When positioned over 1 o’clock on the tricuspid valve annulus in the left anterior oblique view, a His bundle electrogram will be clearly observed in between the atrial and ventricular signals. The AV node locates over the apex of the triangle of Koch and does not have a characteristic electrogram. Typically, it is not the size of the His potential, but its relationship to the size of atrial and ventricular electrograms ( Fig. 22.2 ). Ablation of the AV conduction system as proximally as possible would increase the chance of the escape rhythm to emerge. Ablation at the site of maximal His recording often produces right bundle branch block only. Therefore the catheter should be withdrawn toward the atrium to record an A/V ratio of 1:1 or 1:2 and a small His electrogram, usually less than 0.15 mV in amplitude. An atrial position with an A/V ratio of 1:2 to 1:5 favors the His bundle ablation, and a very small or absent atrial signal suggests that the catheter is probably too far into the ventricle and a right bundle branch potential rather than the His bundle is recorded. In addition, the catheter tip may need to be deflected slightly inferiorly to follow the course of the AV conduction system. One report mapped the AV node by using electrograms on both proximal and distal bipoles of the ablation catheter. The report found that the presence of His and atrial electrograms on the distal bipole, and the absence of ventricular electrogram on the proximal bipole, could improve the accuracy of AV junction ablation and avoid right bundle branch injury. Some advocate use of unipolar ablation electrograms to recognize the most proximal His recording. Here, the His potential shows an entirely negative (QS) morphology, reflecting the entire His activation proceeding away from the electrode. During AF, mapping may be complicated by variable atrial electrogram amplitudes and obfuscation of the His electrogram by the continuous atrial activity ( Fig. 22.3 ; Video 22.1 ). If feasible, cardioversion to sinus rhythm may allow better demonstration of the His potential. With standard 4-mm-tip ablation electrodes, RF is delivered for a total duration of 60 seconds at 50 to 60 W with target temperatures of 55°C to 65°C. One study showed that a temperature range of 60°C ± 7°C was required to achieve permanent AV nodal block, whereas at lower temperature, an accelerated junctional rhythm is seen. The development of an accelerated junctional rhythm within 5 seconds and the appearance of AV nodal block within 30 seconds of RF onset were both highly characteristic of successful target sites ( Fig. 22.4 ; see Video 22.1 ). It is highly advisable to carefully map and ensure catheter stability to avoid delivering ineffective lesions. Multiple ineffective deliveries may produce tissue edema and swelling, which obscure the His recording and distance the ablation catheter from the target tissue. Typically, effective AV junction ablation is usually marked by accelerated junctional rhythm followed by slowing of the ventricular response and emergence of a paced ventricular rhythm. It is essential to wait for at least 30 minutes to ensure that the AV block is permanent.

A left-sided retrograde approach via the femoral artery is used if the approach from the right side of the heart is undesirable or unsuccessful, which occurs in about 5% of patients. The left-sided portion of the His bundle emerges on the septum just below the aortic valve ( Fig. 22.5 ). A right-sided His bundle catheter may act as a fluoroscopic reference. The target is a signal with atrial and ventricular potentials and the largest His bundle electrogram. After the aortic valve is crossed with a tight curve on the ablation catheter, the curve can be maintained on the catheter while it is rotated toward the septum and withdrawn to the aortic valve. Alternatively, the ablation catheter can be straightened and directed toward the inferior apical septum, then withdrawn toward the His bundle catheter until the His potential is recorded beneath the noncoronary aortic cusp. A His electrogram is recorded at the site of ablation. The His potential must be differentiated from the left bundle branch recording. In older patients with aortic disease or peripheral arterial disease, the AV node can be approached through a transseptal puncture from the right atrium. The ablation catheter is directed to the left ventricle and placed under the aortic valve. The left-sided His activation should occur essentially at the same time as the right-sided His. The left bundle branch is typically recorded 1 to 1.5 cm inferior to the optimal His bundle recording site. The left bundle branch recording is identified by a potential-to-ventricular electrogram interval of 20 milliseconds or less and an A/V ratio of 1:10 or less. Electrogram recordings and catheter positions for the left conduction system are discussed in Chapter 29 . In rare circumstances in which standard right-sided and left-sided approaches are both unsuccessful, energy delivery in the noncoronary or right aortic cusp where the His bundle potential is recorded may lead to complete AV block.

In patients with preexisting complete bundle branch block, ablation of the contralateral bundle branch results in complete heart block. Mapping and ablation of the bundle branches are described in Chapter 31 . Complete heart block may also result from ablation of both fast and slow pathway inputs to the AV node ( Fig. 22.6 ). The targets for ablation are listed in Box 22.1 .