Advances in Catheter Ablation

Citation:Asia-Pacific Cardiology 2007;1(1):49-51

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Nowadays, radiofrequency (RF) catheter ablation has become the therapy of choice in most tachyarrhythmias. Supraventricular tachycardias such as atrioventricular node re-entry tachycardia (AVNRT) and Wolff–Parkinson–White (WPW) syndrome are curable in 99% of patients, with few complications.1–7 There are two major approaches to the ablation of AVNRT: the anterior–superior approach and the posterior–inferior approach.1–3 The anterior–superior approach with delivery of RF current to the anterior–superior aspect of Koch’s triangle selectively ablates or modifies the retrograde fast pathway conduction. Although effective, it is frequently associated with a higher recurrence rate and a higher complication rate of heart block.

There are three ways of delivering current to the posterior–inferior aspect of Koch’s triangle for selective ablation or modification of the slow pathway conduction: posterior approach, inferior approach and stepwise approach. The posterior approach delivers the current to a site between the ostium of the coronary sinus and the tricuspid annulus.

The inferior approach delivers current to the inferior–mid-septal area of Koch’s triangle. In the stepwise posterior–inferior approach, current is initially delivered to a posterior site near the ostium of the coronary sinus; the target site is then progressively moved anteriorly towards the apex of Koch’s triangle along the tricuspid annulus. All three approaches have good results. However, on rare occasions (in <1% of patients) retrograde or transeptal left atrial approach may be required for ablation of AVNRT.8 Catheter ablation in patients with WPW syndrome is most successful when the accessory pathway (AP) is located in the left side of the heart. However, ablations of right-sided AP, septal AP, multiple APs or even atriofascicular Mahaim pathways are also quite successful.4–7

Special techniques using mapping and delivery of current to the coronary sinus, the cardiac vein, the diverticulum of coronary sinus or epicardium (through percutaneous epicardial puncture technique) may be required for successful ablation in some patients.5,9 Catheter ablation is also very successful in the treatment of isthmus-dependent atrial flutter in which a bi-directional conduction block is created by delivery of the current to the cavotricuspid isthmus.10 Atrial tachycardia arising from a right atrial focus can easily be ablated by delivery of the current to a site with registration of the earliest electrical activation.7 Idiopathic ventricular tachycardia (VT) from the right or left ventricular outflow tract and verapamil-sensitive left ventricular tachycardia can also be eliminated by catheter ablation, with a success rate higher than 90%.11–13

The outflow tract VT may arise from an unusual site such as the aortic cusp, sinus of Valsalva, tricuspid annulus or mitral annulus or epicardium, making ablation difficult. Ablation of VT associated with structure heart disease, especially with previous myocardial infarction, is less successful;14 however, RF ablation is complementary to the implantation of an intra-cardiac defibrillator. In the rare cases of idiopathic ventricular fibrillation provoked by initiating ventricular ectopies arising from the His-Purkinje system,15 RF ablation can be successful.

Ablation of paroxysmal AF was first innovated in the mid-1990s, when rapid firing foci located in the pulmonary veins were found to be the potential source of atrial fibrillation.16 Under such circumstances, the ablation catheter was introduced to the left atrium through transeptal puncture technique, then advanced into the pulmonary vein to map and eliminate these rapid-firing potentials.

This technique was soon replaced by segmental isolation of the pulmonary vein to improve the success rate and prevent pulmonary vein stenosis.17 The technique was further modified with circumferential isolation of the pulmonary veins.18 When these two techniques were compared, the recurrence rate of atrial fibrillation appeared to be higher with segmental pulmonary vein isolation than with circumferential pulmonary vein isolation (33 versus 12%) in a six-month follow up.19 Paroxysmal atrial fibrillation may also arise from foci other than the pulmonary veins, such as the posterior wall of the left atrium, atrial septum and ligament of Marshall, coronary sinus, superior vena cava and inferior vena cava.20,21

Non-pulmonary vein origin of atrial fibrillation may be an important reason for the high recurrence rate of atrial fibrillation (up to 30%) after complete isolation of three to four pulmonary veins. Ablation of chronic atrial fibrillation is more complicated than ablation of paroxysmal atrial fibrillation in that different mechanisms and anatomical substrates may operate in different patients and isolation of the pulmonary vein alone may not be sufficient for the elimination of chronic atrial fibrillation. Linear lesions created in the left atrium encompassing the roof and the posterior wall as well as the mitral isthmus, and/or in the right atrium including the intercaval posterior or septal aspects as well as the cavotricuspid isthmus, may be required.22 Target selection guided by recording of complex fractionated atrial electrograms (CFAE) or identification of the ganglionated plexi has also been shown to improve the success rate in ablation of chronic atrial fibrillation.23,24

Despite the successes in the treatment of the above-mentioned tachyarrhythmias, catheter ablations do have limitations using the traditional fluoroscopic image system and point-to-point activation mapping techniques, especially in rapid and complex tachyarrythmias such as atrial fibrillation, left atrial tachycardia, atypical atrial flutter and non-sustained or haemodynamically unstable rapid ventricular tachycardia. Recent advances in technology with the innovation of 3D navigation–mapping systems have facilitated the ablation of complex tachyarrythmias. The CARTO electroanatomical mapping system (Biosense Webster) provides 3D activation and voltage-amplitude mapping with minimal fluoroscopic exposure.25

The EnSite non-contact mapping system (St Jude Medical) uses a multielectrode array (MEA) probe, an amplifier and a computer workstation to display instantaneous electrical activation mapping. However, the accuracy decreases when the distance of the target to the MEA is beyond 4cm.8

These two mapping systems may better define the critical area of ‘channel’ or ‘isthmus’ of the re-entrant circuit, and thus facilitate the ablation of complex re-entrant atrial tachycardia, atypical atrial flutter or ventricular tachycardia resulting from organic heart disease, previous operation or ablation procedures.26

When these 3D mapping systems are integrated with magnetic resonance imaging (MRI) or computed tomography (CT) images, the anatomical demarcation of cardiac chambers along with the surrounding vital organs and vessels can be clearly defined and the target site can be accurately identified. Complications are then avoided. For example, ablation of atrial fibrillation may be complicated by pulmonary vein stenosis, oesophageal fistula, left atrium–bronchus fistula, phrenic or vagal nerve injury, etc. If the anatomical demarcation of the left atrium, the pulmonary veins and their relationship to the oesophagus, trachea, bronchus, pulmonary artery and aorta could be clearly delineated, these devastating complications might be avoided.

Another novel navigation and mapping system was introduced recently in which the catheters are advanced and positioned using a magnetic navigation system with remote control through a 3D workstation without fluoroscopic exposure.8 If this system is integrated with CT, MRI or 3D rotational angiography and oesophagogram,27 the efficacy of catheter ablation for complex atrial and ventricular tachyarrhythmias may be further facilitated.

Phase-array intra-cardiac echocardiogram is capable of visualising the orifice of pulmonary veins and monitoring the formation of microbubbles with titration of the energy power during isolation of the pulmonary veins, thus preventing the complication of pulmonary vein stenosis. In order to keep up with the advances in the mapping systems, improvements in the design of ablation catheter and the use of alternative energy sources other than RF current are also important.8

The introduction of a circular ring mapping catheter with 10–20 electrodes that is positioned at the ostium of the pulmonary vein has greatly enhanced the mapping of electrical potentials across the pulmonary vein. The use of such catheters guides the selection of the target site for circumferential isolation of the pulmonary vein anatomically.17 The reversibility of the cryoablation technique is useful in the ablation of focal atrial tachycardia or the accessory pathway in which the target site is located in close proximity to the normal conduction system to avoid permanent AV block.28 It is also safer to target the epicardial foci of VT to avoid injury of the coronary artery. Circular balloon cryoablation catheters have also been applied in pulmonary vein isolation; however, the long-term efficacy and complications using this catheter still need to be substantiated. Alternative energy sources such as microwave and laser that are capable of creating deeper and larger lesions may be useful in the ablation of scar-related or deep intramural circuit or foci in VT.29 However, the use of these energy sources is limited by the stiffness of the catheter and the ability of energy titration.

A balloon-shaped catheter for the emission of ultrasound energy has been investigated for pulmonary vein isolation and was found to have a limited success rate in patients with atrial fibrillation, partly due to the mismatch between the balloon shape and the orifice of the pulmonary vein and the inadequate temperature achieved by the ultrasound.30 However, its complications of stroke, pulmonary vein stenosis and phrenic nerve paralysis are not different from those of RF catheter ablation. Although there are no large-scale studies that investigate the advantage of catheter ablation using alternative energy sources, tailored application to target the unusual locations of the focus or circuit is still warranted in individualised patients.

In summary, RF catheter ablation is the current therapy of choice in patients with AVNRT, WPW syndrome, isthmus-dependent atrial flutter and focal tachycardia forms in the right atrium and idiopathic VT. There is also an increasing body of evidence showing that RF catheter ablation decreases mortality and morbidity and increases quality of life in patients with paroxysmal or chronic atrial fibrillation. However, caution should be taken in ablation of atrial fibrillation as complications may lead to fatality.

Catheter ablation in VT associated with structural heart disease, especially with previous myocardial infarction or idiopathic ventricular fibrillation provoked by initiating ventricular ectopies arising from the His-Purkinje system, is currently a complementary therapeutic modality to ICD implantation. Better understanding of electrophysiology with appropriate integration of mapping and navigation systems, improvement in catheter design and current delivery systems and the use of alternative energy sources have all contributed to the advances in ablation technology.


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