Disclosure
Background: 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: 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1960s, but surgical treatment for refractory tachyarrhythmias was the mainstay of nonpharmacologic therapy until it was superseded by catheter ablation.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. RF energy produces small, homogeneous, necrotic lesions approximately 5-7 mm in diameter and 3-5 mm in depth. The relative safety of this energy source contributed to the widespread adoption of catheter ablation as a therapeutic modality. Frequency: A study of catheter ablation in elderly patients documented more than 16,000 radiofrequency ablations in Medicare fee-for-service beneficiaries in 1998. Fee-for-service, as opposed to managed care, covered 80% of the Medicare population in 1998. Patient Education: For excellent patient education resources, visit eMedicine's Heart Center. Also, see eMedicine's patient education articles Atrial Flutter, Atrial Fibrillation, and Supraventricular 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 supraventricular tachycardia (SVT), namely atrioventricular nodal reentrant tachycardia (AVNRT) and orthodromic reciprocating tachycardia (ORT). The most commonly performed ablation procedures based on rhythm diagnosis are described in this section. Generic SVT Paroxysmal SVT has 3 main mechanisms. The most common type of generic SVT is AVNRT, accounting for more than half of all cases (see Images 1-3). In the common form of 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 by ablating in this region is quite low (1-2%), and targeting the slow pathway is safer than targeting the fast pathway, which is located closer to the compact AV node. The second most common type of generic SVT is ORT, a reentrant rhythm using the AV node as the anterograde limb and an accessory AV connection (ie, the accessory pathway) as the retrograde limb (see Images 4-5). This tachycardia mechanism accounts for approximately 30% of paroxysmal SVTs. Typically, this rhythm disturbance can be cured by targeting the accessory pathway as it crosses the mitral or tricuspid valve annulus. The least common type of SVT (10% of cases) is a unifocal atrial tachycardia, which can arise from either atrium. These tachycardias are 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 Wolff-Parkinson-White (WPW) syndrome is the most common of the preexcitation syndromes. In this syndrome, rapid antegrade conduction over an accessory AV connection (accessory pathway or bypass tract are older terms) may lead to ventricular fibrillation and sudden death in some patients. Typically, a transition from ORT to atrial fibrillation can be the cause of rapid preexcited tachycardia. RFCA of the accessory pathway cures WPW 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 6-7). 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. Atrial fibrillation The simplest catheter ablation procedure performed in patients with atrial fibrillation is RFCA of the AV junction. This procedure is indicated for patients with high ventricular rates not amenable to drug therapy. RFCA of the AV junction results in excellent rate control, relieves palpitations, and improves functional capacity; however, it requires permanent pacemaker implantation to manage the resulting AV block and requires warfarin to prevent stroke because the atrial fibrillation itself is not affected. RF AV nodal modification is a method of slowing AV nodal conduction without causing heart block. This type of ablation is not commonly performed because it is less therapeutic than AV junction ablation and may result in late heart block. Catheter ablation of atrial tissue to cure atrial fibrillation is still evolving. The procedure is technically demanding, more risky, and less successful than the other ablation procedures described. Nevertheless, the observations of Haissaguerre et al and others that pulmonary vein foci can trigger atrial fibrillation have created an exciting approach to ablating 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 8). 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. The surgical Maze procedure (multiple incisions in both atria) is still the only technique that can cure atrial fibrillation in virtually all patients. Patients with paroxysmal atrial fibrillation and hearts that are not too structurally abnormal have the best success rates with left atrial catheter ablation. Still, no consensus exists on patient selection criteria, the optimal left atrial ablation technique, and what constitutes a clinically successful procedure. 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. In patients with VT due to structural heart disease, catheter ablation is used as adjunctive therapy to the implantable cardioverter-defibrillator (ICD), eg, in patients with frequent ICD discharges.
The current used in RFCA is a sinusoidal high-frequency (eg, 500 kilohertz [kHz]) form of electrical current that causes small lesions within the heart. The primary mechanism of tissue destruction is by thermal injury (ie, desiccation necrosis). The delivery of RF energy causes resistive heating of a narrow rim of tissue in direct contact with the electrode at the tip of the catheter. Deeper tissues are heated by conduction of heat from this perielectrode region. Lesion size is determined by the balance between conduction of heat through the tissue and convective heat loss to the blood pool. The temperature at the electrode-endocardial interface must be approximately 50°C or higher to cause tissue necrosis. As the temperature approaches the boiling point of 100°C, the delivery of current is impeded by coagulum (eg, denatured proteins) on the tip of the catheter. This coagulum may predispose the patient to thromboembolic complications. Current ablation systems allow for temperature-controlled energy delivery and rapidly curtail energy delivery for an impedance rise. Newer technical modifications, such as a larger distal electrode and saline-cooling of this electrode, have helped to minimize impedance rises and to allow creation of larger and deeper lesions. The acute lesion of RFCA consists of a central zone of coagulation necrosis surrounded by a border zone of hemorrhage and inflammation. The presence of this border region may explain the recurrence of tachyarrhythmias days to weeks after the procedure because this region may contain viable arrhythmogenic tissue. |
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Class I indications for catheter ablation
Other indications
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.
Preprocedural considerations The preprocedural evaluation always includes a thorough history and physical examination, review of ECG findings of the tachycardia (12 leads if available), and ECGs performed in sinus rhythm. At minimum, preprocedure laboratory work 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 special situations include exercise testing (especially for exercise-induced tachyarrhythmias), cardiac catheterization, and radionuclide scintigraphy. The patient should report for the procedure after overnight fasting. 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 also discontinued for at least a few doses prior to the procedure. Reproductive-aged women should not be exposed to fluoroscopy if any possibility exists that they are pregnant. Intraprocedural considerations The procedure is typically performed under conscious sedation with intravenous tranquilizers and narcotics. General anesthesia is used in children and selected adults. 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 catheter ablation. 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, ie, (1) transseptal catheterization via the interatrial septum or (2) retrograde catheterization across the aortic valve. Anticoagulation with intravenous heparin is employed to reduce the risk of periprocedural thromboembolism. Postprocedural considerations Many physicians empirically treat patients with 4-12 weeks of aspirin therapy to prevent thromboembolic sequelae. For ablations along the tricuspid or mitral valve annulus, empiric endocarditis prophylaxis prior to interventions with the potential to cause bacteremia is occasionally used for a limited time (6-12 wk) postablation. No data support this practice. Echocardiography is not routinely performed unless a complication may have occurred (eg, pericardial effusion). Postprocedure electrophysiologic testing is not routinely used unless recurrent tachyarrhythmias are suspected.
Radiation risk 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% and death in less than 0.3%. The list below, although not all-inclusive, includes reported complications, most of which are rare or uncommon. Complications
Supraventricular tachyarrhythmias The common forms of SVT (eg, AVNRT, SVT associated with WPW syndrome) are 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. Catheter-based cryoablation is an alternative to RFCA near the compact AV node because the risk of heart block is minimal. This is because heart block is reversible with prompt rewarming. Cryoablation appears to be somewhat less effective than radiofrequency as an energy source, especially for deep accessory pathways. Success rates for curing atrial fibrillation with RFCA are highest (70-80%) 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 is usually based on patient symptoms. Success rates are lower if intensive ambulatory monitoring to detect asymptomatic atrial fibrillation recurrences is used. Some patients require the use of previously ineffective antiarrhythmic drugs to maintain success. Ventricular tachyarrhythmias Idiopathic 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.
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 appear to require some degree of substrate ablation (eg, linear transmural lesions in the left atrium). Techniques are still evolving to address this challenge. Three-dimensional electroanatomic mapping systems, overlayed 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 completeness of ablation lines. Intracardiac echocardiography may be helpful to ensure adequate endocardial contact and to avoid complications from thrombus development. Alternative energy sources are also being investigated in the ablation of atrial fibrillation (eg, cryoablation, ultrasound, laser, microwaves). Research is also focused on developing better methods and tools for catheter ablation of VT, and even VF, 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.
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