Catheter Ablation 

Introduction to Catheter Ablation

Radiofrequency catheter ablation (RFCA) has revolutionized treatment for tachyarrhythmias and has become first-line therapy for some tachycardias. Although developed in the 1980s and widely applied in the 1990s, formalized guidelines for its use in clinical practice were not developed until recently.

History of the procedure

Catheters were first used for intracardiac recording and stimulation in the late 1960s, but surgical treatment for refractory tachyarrhythmias was the mainstay of nonpharmacologic therapy until it was superseded by catheter ablation. The initial energy source used was direct current (DC) from a standard external defibrillator. A shock was delivered between the distal catheter electrode and a cutaneous surface electrode; however, this high-voltage discharge was difficult to control and could cause extensive tissue damage.

Radiofrequency (RF) energy, a low-voltage, high-frequency form of electrical energy familiar to physicians from its use in surgery (eg, electrocautery), quickly supplanted DC ablation. The relative safety of RF energy has contributed to the widespread adoption of catheter ablation as a therapeutic modality.

RF energy produces small, homogeneous, necrotic lesions by heating tissue. Lesion size is influenced, in part, by the length of the distal ablation electrode and the type of catheter (standard vs saline-cooled). With typical power settings and good catheter contact pressure with cardiac tissue, lesions are minimally about 5-7 mm in diameter and 3-5 mm in depth.

Future directions

A curative procedure for atrial fibrillation is a major goal in clinical cardiac electrophysiology. Success has been achieved in patients with paroxysmal lone atrial fibrillation by eliminating conduction from the pulmonary veins to the left atrium, as many of these episodes begin in the pulmonary veins. Other forms of atrial fibrillation may require some degree of substrate ablation (eg, linear transmural lesions in the left atrium).

Techniques are still evolving to address the challenge of a catheter-based cure for all forms of atrial fibrillation. Three-dimensional electroanatomic mapping systems, overlaid on MRI or CT images of the left atrium, can facilitate navigation of the ablation catheter, mapping of ectopic foci and atrial scars, and the assessment of the transmurality of ablation lines.

Intracardiac echocardiography may also be helpful to avoid collateral damage to the pulmonary veins or esophagus, ensure adequate endocardial contact, and to avoid complications from thrombus development.

Alternative energy sources are being investigated in the ablation of atrial fibrillation (eg, balloon-based technologies using cryoablation, ultrasound, and laser). In addition, robotic catheter navigation is now available to deliver RFCA.

Research is also focused on developing better methods and tools for catheter ablation of ventricular tachycardia (VT), and even ventricular fibrillation, in patients with structural heart disease. Epicardial electrophysiology via subxiphoid pericardial puncture is a relatively new frontier, as some tachyarrhythmia substrates (especially VT in nonischemic cardiomyopathy) cannot be reached from the endocardium.

Indications for Catheter Ablation

There are 3 Class I indications for catheter ablation. The first is symptomatic supraventricular tachycardia (SVT) due to atrioventricular nodal reentrant tachycardia (AVNRT), Wolff-Parkinson-White syndrome, unifocal atrial tachycardia, or atrial flutter (especially common right atrial forms). For these conditions, catheter ablation is first-line therapy if that is the patient’s preference.

The second indication is atrial fibrillation with lifestyle-impairing symptoms and inefficacy or intolerance of at least 1 antiarrhythmic agent. Both left atrial ablation for restoration of sinus rhythm and AV junction ablation for rate control are Class I indications, depending on the circumstance.

The third indication is symptomatic ventricular tachycardia (VT). Catheter ablation is first-line therapy in idiopathic VT, if that is the patient’s preference. In structural heart disease, catheter ablation is generally performed for drug inefficacy or intolerance, or as adjunctive therapy in patients with an implantable cardioverter-defibrillator (ICD) who are experiencing frequent ICD discharges.

Uncommon indications for catheter ablation include the following:

• Symptomatic drug-refractory (inefficacy or intolerance) idiopathic sinus tachycardia
• Lifestyle-impairing ectopic beats
• Symptomatic junctional ectopic tachycardia

RFCA has been applied to most clinical tachycardias, even to polymorphic ventricular tachycardia and ventricular fibrillation in preliminary studies. Success rates are highest in patients with common forms of SVT, namely AVNRT and orthodromic reciprocating tachycardia (ORT).

Atrial fibrillation

The simplest catheter ablation procedure performed in patients with atrial fibrillation is RFCA of the AV junction. AV nodal modification is less therapeutic than AV junction ablation and less commonly performed.

Catheter ablation of atrial tissue to cure atrial fibrillation is still evolving. The procedure is technically demanding, more risky, and less successful than AV junction or AV nodal ablation. Nevertheless, the observation of Haissaguerre^[1] and others that pulmonary vein foci can trigger atrial fibrillation has stimulated much additional research, and there is considerable scientific excitement that this common tachyarrhythmia may be amenable to a curative catheter procedure.

The complete surgical Maze procedure (incisions in both atria +/- transmural radiofrequency lesions) is still the most effective technique for potentially curing atrial fibrillation in all comers, regardless of chronicity or whether structural heart disease is present. The best success rates with left atrial catheter ablation are in patients with paroxysmal atrial fibrillation and hearts that are not too structurally abnormal.

No consensus exists on the optimal left atrial ablation technique or what constitutes a clinically successful procedure. Nevertheless, evidence is accruing that catheter ablation for atrial fibrillation is more effective than antiarrhythmic drug therapy, especially in patients who have already failed an antiarrhythmic drug. The 2006 ACC/AHA/ESC guidelines for the management of atrial fibrillation now list catheter ablation as a reasonable strategy for patients with life-style impairing symptoms who have failed at least one antiarrhythmic agent.^[2]

Ventricular tachycardia

Idiopathic ventricular tachycardia (VT) most commonly arises from the right ventricular outflow tract and less commonly originates in the inferoseptal left ventricle near the apex. These forms of VT are amenable to catheter ablation, although success rates are somewhat lower than those for the common forms of SVT.

For patients with stable VT, the potential benefit of catheter ablation before implantation of an ICD was demonstrated in the Ventricular Tachycardia Ablation in Coronary Heart Disease (VTACH) study.^[3] This prospective, randomized, controlled international trial in 104 patients found that time to recurrence of VT or ventricular fibrillation (VF) was longer in the ablation group (median, 18.6 mo) than in the control group (5.9 mo). At 2 years, estimates for survival free from VT or VF were 47% in the ablation group and 29% in the control group.

Contraindications to Radiofrequency Catheter Ablation

Few absolute contraindications to RFCA exist. Left atrial ablation and ablation for persistent atrial flutter should not be performed in the presence of known atrial thrombus. Similarly, mobile left ventricular thrombus would be a contraindication to left ventricular ablation.

Mechanical prosthetic heart valves are generally not crossed with ablation catheters. Reproductive-aged women should not be exposed to fluoroscopy if any possibility exists that they are pregnant.

Anesthesia for Catheter Ablation

The procedure is typically performed under conscious sedation with intravenous tranquilizers and narcotics. General anesthesia is used in children and selected adults.

Preprocedural Considerations

The preprocedural evaluation always includes a thorough history and physical examination, as well as review of electrocardiograms (12-lead, if available) performed during the tachycardia and with the patient in sinus rhythm. At a minimum, preprocedure blood studies typically includes a complete blood cell count and an assessment of renal function and electrolyte levels.

An echocardiogram is frequently obtained to exclude structural heart disease. Other tests that are indicated in specific situations include exercise testing with or without cardiac imaging (especially for exercise-induced tachyarrhythmias), and cardiac catheterization.

The patient should fast overnight before the procedure. Cardiac medications with electrophysiologic effects, such as beta-blockers, calcium channel blockers, digoxin, and class I and III antiarrhythmic drugs, are often tapered and/or discontinued prior to the procedure. Warfarin is typically discontinued for at least a few doses prior to the procedure.

Overview of Technique

Typically, 2-5 electrode catheters are percutaneously inserted via the femoral or internal jugular veins and are positioned within the left heart, right heart, or both. Multiple catheters are needed to induce and map various tachyarrhythmias prior to radiofrequency catheter ablation (RFCA).

Cannulation of the coronary sinus is helpful to exclude left-sided accessory pathways or other left-sided tachyarrhythmia substrates.

For left heart catheterization, 1 of 2 approaches may be taken: transseptal catheterization via the interatrial septum or retrograde catheterization across the aortic valve.

Anticoagulation with intravenous heparin is used to reduce the risk of periprocedural thromboembolism.

Atrial fibrillation

RFCA of the atrioventricular (AV) junction is the simplest catheter ablation procedure performed in patients with atrial fibrillation. AV nodal modification is less often performed.

For catheter ablation of atrial tissue for atrial fibrillation, the most commonly used techniques involve ablation of the muscular connections between the pulmonary veins and the left atrium (pulmonary vein isolation), or a wide circumferential ablation around the pulmonary veins (see image below). The goal is to electrically isolate foci arising from inside the veins, or adjacent to the pulmonary vein ostia, from the rest of the left atrium.

Atrioventricular nodal reentrant tachycardia

In the common form of atrioventricular nodal reentrant tachycardia (AVNRT), the inferior atrionodal input to the atrioventricular (AV) node serves as the anterograde limb (ie, the slow pathway) of the reentry circuit, and the superior atrionodal input serves as the retrograde limb (ie, the fast pathway). Typically, AVNRT can be cured by targeting the slow pathway near the inferior tricuspid valve annulus at the level of the coronary sinus os or somewhat higher.

The risk of iatrogenic heart block with ablation in this region is quite low (1-2%). Targeting the slow pathway is safer than targeting the fast pathway, which is located closer to the compact AV node.

Orthodromic reciprocating tachycardia

In orthodromic reciprocating tachycardia (ORT) the AV node serves as the anterograde limb and an accessory AV connection (ie, the accessory pathway) serves as the retrograde limb (see images below).

Typically, ORT can be cured by targeting the accessory pathway as it crosses the mitral or tricuspid valve annulus.

Unifocal atrial tachycardia

Unifocal atrial tachycardia, which can arise from either atrium, is somewhat more challenging to ablate than the more common forms of generic SVT. For those tachycardias originating from the left atrium, transseptal catheterization via a patent foramen ovale or transseptal puncture is usually required.

Wolff-Parkinson-White syndrome

Typically, a transition from ORT to atrial fibrillation can be the cause of rapid preexcited tachycardia in Wolff-Parkinson-White syndrome. Ablation of the accessory pathway cures the syndrome, eliminating ORT and atrial fibrillation in most instances.

Atrial flutter

Atrial flutter is most commonly due to a large reentrant circuit in the right atrium, involving an isthmus of tissue between the tricuspid valve annulus and the inferior vena cava. Most commonly, reentry proceeds counterclockwise up the atrial septum and down the lateral wall of the right atrium, inscribing inverted (ie, "sawtooth") flutter waves in the inferior leads and upright P waves in V₁ (see images below).

Clockwise reentry using this same circuit can also occur, giving upright P waves inferiorly and inverted P waves in V₁. Linear ablation of the cavotricuspid isthmus cures these common forms of atrial flutter.

Non–isthmus-dependent flutters can occur elsewhere in the right atrium as well as in the left atrium. Left atrial flutters are uncommon, may be difficult to ablate, and generally require a 3-dimensional mapping system to facilitate the procedure.

Overview of Results

Atrial fibrillation

Radiofrequency catheter ablation (RFCA) of the atrioventricular (AV) junction results in excellent rate control, relieves palpitations, and improves functional capacity; however, patients who undergo this procedure require permanent pacemaker implantation to manage the resulting AV block and require warfarin therapy to prevent stroke because the atrial fibrillation itself is not affected. AV nodal modification is less therapeutic than AV junction ablation and may result in late heart block.

Supraventricular tachyarrhythmias

The common forms of supraventricular tachycardia (SVT) (eg, atrioventricular nodal reentrant tachycardia [AVNRT], SVT associated with Wolff-Parkinson-White syndrome) are usually curable with a single procedure; the success rate is typically 90-95%. Cure rates for unifocal atrial tachycardia and common right atrial flutter are somewhat lower but still approach 90%. Recurrent tachyarrhythmias typically occur in the first few months after ablation and may be amenable to cure with a second procedure.

AVNRT is usually amenable to cure with a slow pathway ablation near the inferior atrial septum, where the risk of heart block is 1-2%. In the uncommon circumstances in which ablation near the compact AV node is required (eg, fast pathway for AVNRT, or an accessory pathway in a para-Hisian location), the risk of heart block may approach 5% or a little higher.

In a number of centers, catheter-based cryoablation is used rather than radiofrequency catheter ablation (RFCA) near the compact AV node to minimize the risk of heart block. With cryoablation, heart block is generally reversible with prompt rewarming. However, cryoablation appears to be slightly less effective than radiofrequency as an energy source, especially for deep accessory pathways.

Success rates for curing atrial fibrillation with RFCA are highest (up to 87% in the 5 randomized trials reported through 2006) for paroxysmal atrial fibrillation in the absence of structural heart disease and lowest (50% or less) with persistent atrial fibrillation in the presence of structural heart disease and left atrial enlargement. Repeat procedures are typically needed in 25% or more of patients.

Success rates have historically been based on patient symptoms and periodic electrocardiographic monitoring. Success rates are lower if intensive ambulatory monitoring to detect asymptomatic atrial fibrillation recurrences is used, such as daily monitoring for a month with an auto-triggering event monitor. Some patients require the use of previously ineffective antiarrhythmic drugs to maintain success.

Ventricular tachyarrhythmias

Idiopathic ventricular tachycardia (VT) is curable (success rate approximately 80%), assuming it is readily inducible during electrophysiology studies. The most common location for these VTs is the right ventricular outflow tract. Because these VTs are usually not reentrant in nature, a significant percentage are not inducible. Some cannot be ablated because of their deep septal or epicardial location. Some left ventricular VTs originate near a coronary cusp, which may preclude a successful ablation because of concern regarding coronary artery damage.

Approximately half the cases of VTs associated with structural heart disease can be palliated by catheter ablation. Extensive scarring in these ventricles may limit the efficacy of the relatively small lesions made by RFCA, and multiple VT circuits may also contribute to this moderate success rate. Some form of 3-dimensional mapping is helpful for these complex ablations. In practice, many of these patients have ICDs, and catheter ablation is used as adjunctive therapy for frequent device activations.

Complications of Catheter Ablation

Radiation risk in catheter ablation is low, but it may exceed the risk from common radiologic procedures. The average risk for genetic defects has been computed at 1 case per million births. The average risk for fatal malignancies ranges from 0.3-2.3 deaths per 1000 cases for every 60 minutes of fluoroscopy. Many ablation procedures require less than 60 minutes of fluoroscopy.

Major complications occur in approximately 3% of patients who have ablation procedures, including thromboembolism in less than 1% (higher in some atrial fibrillation ablation series) and death in 0.1-0.2% of all procedures. The incidence of cardiac complications varies, based on the site and type of ablation. Cardiac complications include the following:

• High-grade AV block
• Cardiac tamponade (highest in atrial fibrillation ablation, up to 6%)
• Coronary artery spasm/thrombosis
• Pericarditis
• Valve trauma

Vascular complications, which occur in approximately 2-4% of procedures, include the following:

• Retroperitoneal bleeding
• Hematoma
• Vascular Injury
• Transient ischemic attack/stroke
• Hypotension
• Thromboembolism or air embolism

Pulmonary complications include the following:

• Pulmonary hypertension with and without hemoptysis (secondary to pulmonary vein stenosis)
• Pneumothorax

Miscellaneous complications include the following:

• Left atrial-esophageal fistula
• Acute pyloric spasm/gastric hypomotility
• Phrenic nerve paralysis
• Radiation or electrically induced skin damage
• Infection at access site
• Inappropriate sinus tachycardia
• Proarrhythmia

Long-Term Monitoring

Some physicians empirically treat patients with 4-12 weeks of aspirin therapy to possibly reduce the risk of thromboembolic sequelae.

Echocardiography is not routinely performed unless a complication may have occurred (eg, pericardial effusion). Postprocedure electrophysiologic testing is not performed routinely unless recurrent tachyarrhythmias are suspected.