Catheter Ablation of Arrhythmias

Procedural Technique and the Electrophysiology Study

The electrophysiology study (EPS) is an indispensable prelude to catheter ablation allowing for the identification of tachycardia mechanism and the most appropriate site for energy delivery. The EPS involves placing electrode catheters in various chambers of the heart for recording, stimulation, mapping, and ablation. Femoral veins and arteries are the most common sites of vascular access used for EPS. Less commonly used sites include the antecubital fossa, subclavian, and jugular veins. Adequate local anesthesia is administered before vascular puncture. A 0.035-inch short J guidewire is placed via the percutaneous Seldinger technique into the femoral vein or artery just below the inguinal ligament. Additional wires, up to three in a single vein, are placed 5 to 10 mm caudal to the first wire. Double- and triple-headed vascular access devices that can accommodate two or three catheters through a single access sheath are commercially available and are used in some laboratories. Hemostatic sheaths (size 6 Fr to 11 Fr) are placed over the guidewires, up to three in a single vein. If more than three catheters are required, another vein is used, typically the contralateral femoral vein.

For His bundle recordings, the femoral approach allows superior catheter stability. However, catheterization of the coronary sinus (CS) is more readily accomplished via the internal jugular, left subclavian, or left antecubital veins. The lateral antecubital veins that drain into the cephalic vein are avoided because of the right angle at which they enter the axillary vein, making catheter manipulation more difficult. These nonfemoral sites are usually reserved for patients with inaccessible femoral access or difficult CS cannulation despite attempting via the femoral approach with a steerable catheter, but in some laboratories, CS cannulation is routinely performed via the left subclavian vein or right internal jugular vein. Although accessing the CS through a femoral vein is generally more difficult, in experienced hands it can be readily accomplished either via a direct approach, often facilitated by bending the catheter in the hepatic vein to achieve a greater posterior angulation, using a steerable catheter, or more indirectly by forming a catheter loop within the right atrium.

The catheters used in an EPS are either steerable or nonsteerable woven Dacron or synthetic catheters containing electrodes that can be manipulated under fluoroscopy in the cardiac chambers. Nonsteerable catheters are available in a variety of electrode configurations and preformed curves, but the most common catheters have either 4 or 10 electrodes for pacing and sensing. Steerable catheters have internal tension wires that allow the tip to be deflected up to 180 degrees or more by manipulating a control at the handle. Diagnostic steerable catheters may contain up to 20 electrodes. Catheters used for radiofrequency ablation are all steerable and generally consist of a 3.5- to 10-mm electrode used for ablation and three proximal recording electrodes. A diagnostic EPS typically requires at least three catheter locations: in the high right atrium near the sinus node, at His bundle area across the superior tricuspid valve, and at the right ventricular apex. Depending on the type of study, additional catheters may be placed in the right ventricular outflow tract, CS, anterolateral right atrium, interatrial septum, left atrium, pulmonary veins, and left ventricle.

Accessing the left ventricular cavity is accomplished either via the mitral valve from transseptal left atrial catheterization or via retrograde aortic approach through the arterial system, typically the femoral artery. Although left ventricular catheterization is not a routine part of a diagnostic EPS, it may have importance in patients with ventricular tachycardia and accessory pathway–mediated tachycardia (atrioventricular reciprocating tachycardia). Detailed catheter mapping of the left ventricular endocardium may also have benefit in defining the myocardial substrate in patients with ventricular arrhythmias, depressed ventricular function, history of myocardial infarction, and congestive heart failure.

Cardiac Mapping Techniques

Understanding the mechanism of arrhythmia is vital, before targeting with ablation, to maximize the success of the procedure. Understanding the arrhythmia mechanism can often be achieved with EPS but can be further refined with the use of cardiac mapping techniques. Cardiac mapping allows the identification of temporal and special relationships of electrical potentials during atrial and ventricular arrhythmias. The focus of cardiac mapping is generally to identify the origin of a focal arrhythmia or a site of critical conduction for a reentrant arrhythmia. The most common approaches to mapping are simple activation mapping, pace mapping, and electroanatomic activation mapping.

Electrograms acquired during mapping techniques are either bipolar or unipolar. Bipolar electrograms reveal the local electrical activity of the heart between two designated electrodes on the catheter. This typically is over an interelectrode spacing ranging from 1 to 10 mm. Unipolar electrograms reveal the local electrical activity at a single catheter point (usually the distal tip) relative to an electrode placed at a distance from the heart. The advantages of using unipolar mapping are that it gives a more precise measure of local activation and it provides information about the direction of impulse propagation. The advantages of using bipolar mapping include superior signal-to-noise ratio and less contribution from distant electrical activity (“far-field” activity). Frequently, accurate mapping involves using both bipolar and unipolar electrograms at different points of the study.

Simple activation mapping is achieved by moving a single roving mapping catheter to various points of interest during an arrhythmia, spontaneous or induced, while measuring local activation times relative to a fiducial marker at a second site, such as the onset of the P wave or QRS complex or a stable intracardiac electrogram. Depending on the arrhythmia mechanism, the area of earliest activation is often a reasonable target for ablation of the arrhythmia.

Pace mapping is a mapping technique that can be used when the patient is not in the arrhythmia. It is often used in conjunction with activation mapping as a second confirmatory test to determine the accuracy of the selected site for ablation, but it can be used as a stand-alone mapping technique if the documented clinical arrhythmia cannot be induced during EPS or is not hemodynamically tolerated. Pace mapping entails pacing the suspected target area for ablation at a rate similar to the clinical arrhythmia, and comparing the 12-lead electrocardiogram (ECG) to the ECG of the arrhythmia. A good pace map will have an exact match in 12 out of 12 leads; however, this is often difficult to achieve. When only minor differences in the ECG configuration and amplitude are noted, the spatial resolution of pace mapping can be as good as 5 mm. However, if only assessing for major differences in the 12-lead ECGs, special resolution may be as poor as 15 mm. In addition to the 12-lead ECG, pace mapping also compares the intracardiac activation sequence seen on the electrophysiology catheters with the sequence observed during the arrhythmia if present or inducible.

Electroanatomic activation mapping relies on the use of specialized three-dimensional electroanatomic systems to assist with intracardiac mapping. The use of these systems can greatly reduce the amount of fluoroscopy time required for a procedure. Most commonly used are the CARTO 3 system (Biosense Webster) and the EnSite NavX system (St. Jude Medical) although the technology of cardiac mapping continues to evolve and newer mapping systems are currently in development. The CARTO 3 mapping system allows three-dimensional electroanatomic mapping using a low-intensity magnetic field and current-based visualization data to provide localization of multiple catheter tips and curves. The system can visualize up to five catheters with and without magnetic sensors but requires the use of compatible Biosense Webster catheters with magnetic sensors for electroanatomic mapping. Biosense Webster mapping catheters have miniature magnetic field sensors at the tip of the catheters, which can determine the location and orientation of the catheter within the magnetic field. With this information, one can create a three-dimensional reconstruction of cardiac chamber geometry. Isochromes of electrical activity can then be overlaid onto this geometry to help define reentrant circuits as well as localize the origin of focal arrhythmias. Additionally, voltage mapping can be performed in sinus rhythm to better delineate the underlying myocardial substrate in any chamber in question. The system has been shown to be highly accurate and reproducible in vitro and in vivo.

The EnSite NavX mapping system also allows for three-dimensional localization of an intracardiac catheter through the use of three pairs of skin patches that send three independent, alternating, low-power currents through a patient's chest, each with a slightly different frequency. These currents can then be used to calculate different levels of impedances in all three planes corresponding to specific anatomic locations within the chest. Mapping catheter electrodes can then be used to measure different levels of impedance corresponding to specific locations in the heart as they are manipulated within the cardiac chambers. Using these calculated impedance coordinates, the system allows for real-time visualization of the position and motion of up to 64 different electrodes on the ablation catheter as well as other standard intracardiac catheters. Like CARTO 3, EnSite NavX can create three-dimensional geography of cardiac chambers and superimpose activation isochromes while mapping an arrhythmia or voltage measurements taken during sinus rhythm. However, unlike the CARTO 3 system, the EnSite NavX system does not require proprietary catheters for localization and mapping. EnSite also offers a noncontact mapping option using a 64-pole balloon catheter that generates mathematically derived electrograms and places them on a map of a cardiac chamber defined by a second, roving contact catheter.

Catheter Ablation

Catheter-based ablation techniques have been so successful in treating a variety of arrhythmias that they have virtually replaced surgical approaches. In catheter-based ablation, energy is delivered to a precise area of the heart. This is most often performed on the endocardial surface of the heart, although epicardial ablation is possible percutaneously via the subxiphoid approach and is performed routinely at many centers. After mapping techniques are used to identify the area or areas to be targeted for ablation, an ablation catheter is positioned in the desired location and connected to an energy source. Radiofrequency (RF) energy is the modality most commonly used clinically, having replaced direct current (DC) ablation because of superior safety and efficacy. Freezing the target area of the heart through a catheter-based cryoablation system is another method that can be used and is most commonly used in pediatric patients and in ablations performed adjacent to the intrinsic conduction system although cryoablation for atrial fibrillation using a cryoballoon catheter is becoming more common.

RF current is typically delivered in a unipolar configuration from the distal tip of the ablation catheter to a cutaneous grounding patch. The energy is generated as an alternating current with a frequency of 300 to 750 kHz. These frequencies produce effective heating with negligible muscle stimulation. During RF ablation, the electrical energy is converted to thermal energy by resistive heating. Most of the heating is concentrated at the tip of the catheter secondary to the small surface area of the tip relative to the cutaneous patch. The heat that is generated is transferred to the adjacent cardiac tissue primarily by conduction and to a lesser extent by radiation, which decreases by the fourth power of the distance from the catheter tip. At steady state the RF lesion size is proportional to the temperature measured at the tissue-catheter interface, as well as proportional to the RF power amplitude.

RF ablation results in thermal injury with coagulation necrosis when tissue heating exceeds approximately 50° C for at least 10 seconds. As heat is produced at the catheter-myocardial interface, the impedance drops. A drop of 5 to 10 Ω is a sign of conductive heating to the adjacent tissue. The time to electrophysiologic effect after onset of RF current delivery is often shorter than one would anticipate for a pure thermal mechanism based on the documented rate of tissue temperature rise contiguous to the electrode. This raises the possibility that there is a contribution of a direct electrical effect in addition to the thermal effect of RF.

RF lesion size and power delivery are limited by tissue heating at the catheter-tissue interface on the endocardium. Newer ablation systems use external or internal saline irrigation to cool the catheter tip, which allows greater power delivery and the creation of deeper and larger lesions. The formation of coagulum may also be decreased with these systems. The catheters typically infuse saline either in a closed system running to the catheter tip or in an open system in which saline flows out small holes at the catheter tip at 17 to 30 mL/min during ablation, similar to irrigated surgical RF ablation devices. Externally irrigated ablation catheters have become the most common catheter used for ablation procedures for the treatment of ventricular tachycardia and atrial fibrillation.

Cryoablation has been used in the surgical treatment of arrhythmias for more than 20 years. Near-transmural lesions can be produced intraoperatively at temperatures of −60° C in the presence of cold cardioplegia. The blood pool presents catheter-based cryoablation systems with a major impediment in achieving adequate temperature. However, a catheter-based closed coolant system has been developed and is in clinical use. The major advantage is the ability to induce a nonpermanent change in tissue conduction, referred to as “ice mapping,” followed by a permanent lesion if the ice mapping reveals a desirable location. This method has been used surgically with temperatures of approximately 0° C producing transient loss of electrical function and −60° C producing irreversible damage. A percutaneous approach is complicated by the warming effect of the circulating blood pool, and mean temperatures of −27° C were needed to achieve transient altering of electrical function. However, the temperature required for irreversible damage was similar to that needed in surgery (−58° C). Cryoablation catheters are most commonly used in the pediatric population and in ablations performed near the intrinsic conduction system where the risk of developing complete heart block is elevated. More recently, a cryoablation balloon has been developed which is capable of delivering a circumferential cryoablation lesion at the antrum of the pulmonary veins. This technology has been shown to reduce procedure times for pulmonary vein isolation with similar efficacy and adverse events compared with standard RF ablation.

Specific Arrhythmias