Catheter Ablation

Anatomy and Physiology

The venous system approached mostly are right and left femoral veins. In the case of difficult coronary sinus cannulation, the internal jugular or subclavian vein approach may be used. Diagnostic catheters are inserted to allow pacing, stimulation, and signal recording at the high right atrium, right ventricle (apex or RVOT), His bundle, and the coronary sinus. The left side can be accessed through an antegrade transeptal puncture from the right atrium into the left atrium or a retrograde aortic approach.[12]

Indications

There are three main indications for catheter ablation. ([1][13]

• Definitive treatment of symptomatic supraventricular tachycardia (SVT) due to atrioventricular re-entrant tachycardia (AVRT), atrioventricular nodal re-entrant tachycardia (AVNRT), unifocal atrial tachycardia, or atrial flutter;
• AF with lifestyle-impairing symptoms and failure of antiarrhythmic agents.
• Symptomatic idiopathic ventricular tachycardia (VT).

However, for VT in the setting of structural heart disease, catheter ablation is generally reserved for failure of drug-therapy or as adjunctive therapy in the setting of frequent implantable cardioverter-defibrillator (ICD) discharges

Contraindications

There are no absolute contraindications for catheter ablation, relative contraindications may include; [14]

The contraindications are limited to vascular access contraindication as DVT for femoral vein access and PAD and aortic dissection in case of retrograde aortic approach.

Also, the presence of intracardiac thrombi, to prevent the risk of embolization.

Bleeding diatheses and coagulopathy are the main contraindication for catheter ablation. Usually, the electrophysiologist can interfere with the patient's safety with the INR up to 3.

Equipment

Catheters

A variety of catheters is available with at least two ring electrodes that can be used for bipolar stimulation and recording. The catheter may be made of woven Dacron or newer synthetic materials such as polyurethane. Woven Dacron catheters are preferred because of their greater durability and physical properties. These catheters have a variable number of electrodes, electrode spacing, and curves to provide a range of options for different purposes. Although they have superior torque characteristics, their greatest advantage is that they are stiff enough to maintain a shape and yet they soften at body temperature so that they are not too stiff for forming loops and bends in the vascular system to adopt a variety of uses.[15]

Synthetic catheters cannot be manipulated or change shapes within the body, so they are less desirable. The synthetic catheters are cheaper and offer smaller sizes (2 to 3 French). Currently, most electrode catheters are size 3 to size 8 French. The smaller sizes are used in children. In adult patients, sizes 5 to 7 French catheters are routinely used. Other diagnostic catheters have a deflectable tip. These are used to reach and record from specific sites (e.g., coronary sinus, crista terminal is, tricuspid valve). In most instances, the standard woven Dacron catheters suffice, and they are significantly cheaper. Mapping catheters fall into two general categories:

1. Deflectable catheters to facilitate positioning for mapping and delivering ablative energy.
2. Catheters with multiple poles (8 to 64) that allow simultaneous acquisition of activation points. 

Some ablation catheters have a cooled tip, one through which saline is infused to allow for enhanced tissue heating without superficial charring or internal cooling.

Ablation catheters deliver RF energy through tips that are typically 3.5 to 5 mm in length but may be as long as 10 mm. Catheters delivering microwave, laser, cryothermal, or pulsed-ultrasound energy to destroy tissue are currently under active investigation.

In the second category, standard catheters with up to 24 poles that can be deflected to map large and/or specific areas of the atrium (e.g., coronary sinus, tricuspid annulus). Shaped catheters as “halo” record from around the tricuspid ring or a lasso catheter on a deflectable shaft to record from 10 to 20 electrodes in the pulmonary vein ostia, and basket catheters which have up to 64 poles or prongs that spring open and which are used to acquire simultaneous data from within a given cardiac chamber.

A catheter is available from which has five flexible splines with four electrodes on each spline allowing one to acquire 20 sites of activation. The 2-mm interelectrode distance allows for high-density mapping. The floppiness of the splines sometimes makes for variable contact in misinterpretation of data.

More recently a new catheter was has been developed a 64-pole roving catheter. This mapping (mini basket) catheter has an 8 F bidirectional deflectable shaft and a basket electrode array (usual mapping diameter 18 mm) with eight splines, each spline containing eight small (0.4 mm2), low-impedance electrodes (total 64 electrodes). The interelectrode spacing along the spline is 2.5 mm (center-to-center). Mapping can be performed with the basket in variable degrees of deployment (diameter ranging 3 mm to 22 mm). The location of each of the 64 electrodes is identified by a combination of a magnetic sensor in the distal region of the catheter and impedance sensing on each of the 64 basket electrodes. The location of each basket electrode is obtained whether the basket is fully or only partially deployed.

Junction Box

The junction box is a rectangular box that receives the intracardiac signals from the catheters and provides the interface to the physiologic recorder. Multiple switches within the junction box are designated to a recording and stimulation channel which can be selected through the recording apparatus. To minimize noise on the channels, the junction box is mounted close to the patient foot.

Recording Apparatus

The physiologic recorder records, displays, and stores intracardiac and surface recordings. It consists of filters, amplifiers, display screens, and recording software. From the junction box, the physiologic signals are introduced into the recorder. These signals are typically low in amplitude and are amplified before displaying and recording. The recording system amplifies and filters each input channel separately, with most current systems supporting up to 64 or more channels. The amplifiers can automatically or manually adjust gain control. The amplifiers should be mounted as close to the patient table as possible, to reduce the cable length of the intracardiac connections and surface ECGs, which minimizes the signal noise. The amplifier is then connected to the main physiologic recorder through a channel, which, ideally, should run separately from electric power cables. Filters are used to eliminate unnecessary signals that distort electrograms (EGMs).

 High pass filters eliminate signals below a given frequency while low pass filters eliminate signals above a given frequency. Most intracardiac electrograms are clearly identified when the signal is filtered between a high pass of 40 Hz and a low pass of 500 Hz.

Stimulator

A programmable stimulator is necessary to obtain electrophysiologic data beyond measurements of conduction intervals. Stimulators are capable of various modes of pacing, including rapid pacing, delivery of single or multiple extra stimuli following a paced drivetrain, and delivery of timed, extra stimuli following sensed beats. Stimulators are capable of delivering variable currents, from 0.1 mA to 10 mA. With the satisfactory positioning of catheters, current thresholds under 2 mA (with 2 ms pulse width) can usually be achieved in both the atrium and ventricle. Higher outputs are seen in the diseased myocardium, within the coronary sinus, and with the use of anti-arrhythmic medications. The output is usually set at twice the diastolic threshold. Pacing at higher outputs is discouraged because may led to the far-field capture and altered the QRS and EGM configuration and may mislead diagnosis, the recommendation is either to start at low amplitude until capture, or start at high output and go low until failure of capture then increase a step.

Cardioverter/Defibrillator

A primary and backup cardioverter/defibrillator should be available throughout all EP studies, defibrillators deliver energy in a biphasic waveform which offers enhanced defibrillation success. Defibrillation pads are attached to the patient and electrically grounded.

Personnel

Following personnel are required for an electrophysiology study/ablation.

• Cardiac electrophysiologist
• Cardiac electrophysiology laboratory technician.
• Nursing staff for administration of drugs.
• Radiographer.
• An anesthesiologist, in rare complex cases

Preparation

Counseling of patients and explanation of the procedure is the main step of preparation.

Complete blood count, renal function, and liver functions should be checked, especially in patients who are on antiarrhythmic drugs.

Coagulation profile is checked, especially for patients on anticoagulation.

The patient is draped and the site of vascular access is cleaned with antiseptic agents similar to other cardiac catheterization procedures.

Technique or Treatment

The electrophysiologic study consists of a systematic analysis of dysrhythmias by recording and measuring a variety of electrophysiologic events with the patient in the basal state and by evaluating the patient's response to programmed electrical stimulation. To perform and interpret the study correctly, we have to understand certain concepts and methods, including the different types of electrogram recordings, measurement of atrioventricular (A-V) conduction intervals, activation mapping, and response to programmed electrical stimulation. Knowledge of the significance of the various responses, particularly to aggressive stimulation protocols, is mandatory before employing such responses to make clinical judgments.

Catheter Positioning

This depends on the type of ablation procedure.

• AVNRT/AVRT

1. HRA catheter: quadripolar fixed curve catheter in the right atrium near the SAN.
2. His catheter: quadripolar catheter fixed curve at the His region, it passes from the IVC and just passes the tricuspid valve to be straddling both the low RA and the RV, and it should have three recordings (A-H-V) catheters.
3. RV catheter: quadripolar fixed curve catheter at the RV apex, and can move the RVOT.
4. Decapolar fixed or steerable catheter into the coronary sinus.

• Atrial flutter: The same basic catheters, plus the Halo catheter, it’s a duo Decapolar catheter, which occupies the whole RA to determine the activation sequence in case of atrial flutter.
• Atrial fibrillation: It involves the basic catheters; the Lasso catheter is introduced, in addition to the ablation catheter to map the pulmonary veins and test for exit block.
• VT ablation: The catheter used in VT ablation may include, RVA and the C.S catheter, together with the steerable deflectable ablation catheter.

Electrogram Recordings

Electrograms can be recorded as unfiltered or filtered unipolar signals or bipolar signals.

• Unipolar signals provide precise activation and give directional information. Unfiltered shows significant far-field activity is recorded that can make it difficult to see small local unipolar signals without increasing the size of the signal so much it swamps the amplifier. Respiratory drift is another problem. Filtering the unipolar signal gives precise localization and removes far-field activity but often results in a very small signal, particularly in infarcted tissue.
• Bipolar signal approximates the timing recorded by the filtered unipolar signal, particularly with narrow interelectrode distances (less than or equal to 2 mm) and removes far-field signals, but does not give the directional information of the unfiltered unipolar signal. For ablation, particularly for the WPW syndrome and idiopathic PVCs/VT, it is important to record both simultaneously to assure the earliest components of the bipolar signal are derived from the distal pole, which is used for ablation, and not the proximal pole.

Mapping System

Cardiac mapping is the process by which arrhythmias are characterized and localized. Conventional mapping involves acquiring electrogram data from fixed and moving catheters and creating mental activation maps with two-dimensional fluoroscopic images.[16]

Three-dimensional anatomic localization of the catheter assists in mapping and ablation. These technologies involve the acquisition of multiple electrogram locations to provide a high-resolution activation, voltage, or propagation map. In addition to correlating local electrograms to three-dimensional cardiac structures, these newer mapping techniques reduce radiation exposure to the patient and physician. The most widely used electroanatomic mapping system localizes the mapping and ablation catheter through a magnetic field. Three coils located beneath the patient generate ultra-low magnetic fields that temporally and spatially code the area within the patient. With a magnetic field sensor in its tip that is referenced to an externally located patch on the patient, the catheter can be displayed and recorded in three dimensions with intracardiac electrograms.[17] 

Another technology offers electroanatomic mapping by creating electrical fields between opposing pairs of patch electrodes located on the patient’s chest. Six patches are placed on the body to create three orthogonal axes with the heart located centrally. A transthoracic electrical field is created through each pair of opposing patch electrodes, and the mapping catheter delivers this signal for processing.

Magnetic navigational systems are more frequently being utilized for mapping and ablation of various arrhythmias as well as for guidance in the placement of left ventricular leads. This system uses large external magnets that sit closely on each side of the patient allowing for magnetic navigation of percutaneous devices. The catheters or guide-wires have small magnetic tips that respond to changes in magnetic field vectors that are programmed by the physician remotely. Advantages of this approach include a softer catheter tip which likely reduces the trauma that can occur with stiffer ablation catheters as well as decreasing physician exposure to radiation. Other robotic navigation systems are also in development.

Measurements of the Basic Intervals [18]

1. P-A interval: 25 ms to 55 ms, measured from the earliest P wave activity on the surface ECG to the earliest a, recorded on the HBE, representing the conduction between the SAN and the AVN to the His.
2. A-H interval: 55 ms to 125 ms, measured from the earliest reproducible A wave on the HBE to the H onset of the His deflection on HBE., representing the conduction from the Low RA to the AVN and then to the His.
3. H-V interval: 35 ms to 55 ms, measured from the most reproducible His deflection on the HBE to the earliest V on the surface ECG, representing the time of conduction from the His to the ventricular myocardium.

Radiofrequency Ablation

• Radiofrequency ablation uses alternating current delivered between the catheter tip and grounding source to deliver energy to tissue, resulting in necrosis.[9] Radiofrequency generators deliver current with a frequency between 300 kHz and 750 kHz, with the generation of heat occurring as a result of resistive and conductive heating. Monitoring of time, power, and impedance are necessary to ensure safe and effective ablation lesions.

Complications

Complications associated with catheter ablation depends on the type of arrhythmia and the site of ablation.[19]

• Death, myocardial infarction, or stroke (0.05% to 0.01%) although the stroke risk is higher with curative atrial fibrillation ablation. [20]
• Heart block, which requires a permanent pacemaker (0.5%) and is mainly dependent on the proximity of the ablation lesion to the atrioventricular node. [21] [22]
• Cardiac trauma and perforation leading to tamponade (1% to 2%).[23]
• Thromboembolic complications including systemic and venous embolism (pulmonary embolism) vary according to the procedure.
• Vascular accesses complications, including arteriovenous fistula, aneurysm, and retroperitoneal bleeding are far more common (2% to 4%).
• For atrial fibrillation ablation, there are complications like pulmonary vein stenosis and phrenic nerve damage, but they can be avoided by identifying the site of the phrenic nerve. [24]
• The incidence of thromboembolic events is common, especially with the increased time of ablation in the atrial-esophageal fistula
• Pulmonary vein stenosis and atrio-esophageal fistulae are the rare complications of atrial fibrillation ablation. [25]

Clinical Significance

Catheter ablation is now the mainstay treatment of most arrhythmia, it can offer a better choice for those suffering recurrent arrhythmias, and is a permanent treatment with a greater than 90% success rate of AVRT and AVNRT ablation. The best choice for the symptomatic accessory pathway, atrial flutter and atrial fibrillation, and ultimately the best choice for those suffering drug-refractory VT or PVC-induced cardiomyopathy.

Enhancing Healthcare Team Outcomes

Catheter ablation is now the mainstay treatment of most arrhythmia, it can offer a better choice for those suffering recurrent arrhythmias,  and is a permanent treatment with a greater than 90% success rate of AVRT and AVNRT ablation. While catheter ablation is usually done by a cardiologist, the monitoring and follow-up of the patients are done by the primary care provider, internist, and nurse practitioner. Even though the success of catheter ablation is high for many atrial arrhythmias, the procedure is also associated with a fair number of serious complications that include death, pulmonary vein stenosis, esophageal perforation, heart block requiring a pacemaker, stroke, phrenic nerve injury, and vascular access complications. It is important to educate the patient on the potential complications before the procedure. [26][27]