CardiologyClinical Features

Cardiac Electrophysiology (EP) Developments

Dr Jonathan Lyne, Consultant Cardiologist and Electrophysiologist, Beacon Hospital and Blackrock Clinic

Cardiac Electrophysiology (EP) describes the study of the heart’s electrical system to identify the mechanisms of cardiac arrhythmia. Despite only being developed during the mid-1960s to 1970s, technological advancements have, and continue to, occur rapidly within this field.

Catheters were first used for intra-cardiac recording in the mid- 1960s, with dedicated protocols for ‘programmed’ stimulation or testing, developed in the mid- 1970s. The first microprocessor based stimulator was developed in 1980, with the ability to deliver a pulse of current through a catheter in the heart to study a patient’s conduction system. A year later, the first ablation procedures were performed, using direct current (DC) shocks through an external defibrillator to ablate a patient’s AV node. All of these involved the sole use of X-ray to place catheters and identify the location of cardiac structures. Unfortunately, DC energy creates a large area of injury lacking the precision required to treat many arrhythmia circuits.

The development of radiofrequency (RF) ablation as a method of creating a permanent thermal myocardial injury occurred some years later in the late 1980s.

It has standardised the treatment of a multitude of cardiac arrhythmias without the reliance on subsequent pacemaker implantation from AV node ablation. Cryoablation techniques – ‘freezing’ as a form of permanent thermal myocardial injury – were also developed around this time. Despite advantages of reduced collateral damage, RF ablation currently remains the most commonly used strategy for treating most arrhythmias.

The first 3D mapping system, developed in 1995, has optimised the integration of an intracardiac anatomy with the electrophysiological features of an arrhythmia. The ability to create a real time cardiac chamber geometry comes from magnetic location technology (Carto, Biosense Webster). It identifies the location of sensors within special catheters and calculates the location of the catheter according to current frequencies thereby creating a reconstruction of a heart as the catheter is moved around a chamber. The subsequent geometry can then display the volume of scar within a chamber (by measuring the peak-to-peak amplitude of signals in contact with myocardium from the sensorbased catheter), and/or the timing of arrhythmias to identify its origin or perpetuating circuit.

Advances in mapping systems has enabled real-time catheter visualisation with up to 1mm precision but is typically only compatible with RF therapy.

This increases the accuracy and safety of targeted RF ablation alongside significant reductions in fluoroscopy and subsequent radiation exposure both for patients and operators. The use of cryotherapy still typically requires high volumes of X-ray in comparison due to the inability of those catheters to recreate a cardiac chamber geometry.

Separately, developments in cardiac imaging have also been utilised for additional safety precautions within Cardiac Electrophysiology. Pre-procedural cardiac imaging, and/or intra-cardiac echocardiography during an ablation procedure may be used by some Electrophysiologists. A relatively recent ability to integrate ultrasound images with 3D mapping systems may improve the efficacy and safety of all ablation procedures, especially for patients with difficult/complex anatomy.

Reconstruction of right atrial anatomy and ablation with zero fluoroscopy using 3D mapping for SVT ablation (septal accessory pathway)

Developments in SVT Ablation

SVT is a term that encompasses atrial arrhythmias other than atrial fibrillation (AF); sinus tachycardia, atrial tachycardias (including atrial flutter), AV node re-entry tachycardia (AVNRT), and AV reentry tachycardia (AVRT).

AV nodal reentry tachycardia (AVNRT) is the most common type of SVT. It results from a circuit circulating within the AV node that includes conduction occurring across two pathways in opposite directions within the AV node. Typically this includes two physiologically distinct channels within the AV node known as the fast and slow pathways.

AV rentry tachycardia occurs when a circuit involves conduction across the AV node but also across an accessory (extra) pathway. These accessory pathways may be visible on an ECG an appear as ‘prexcitation’ such as in Wolff Parkinson White syndrome or may not be visible, ‘concealed’. Electrophysiological testing can be performed to identify the presence of an accessory pathway and to test its function.

Atrial tachycardias are commonly focal in origin but occur from one or more sites within atrial tissue. This includes both atria and their connections to other structures such as pulmonary veins within the left atrium and great vessels such as the aorta.

Atrial flutter commonly results from a circuit within the right atrium that circulates around the tricuspid valve annulus (cavotricuspid isthmus dependent flutter). Other atrial flutter circuits are atypical but may conduct around other structures such as surgical atriotomy scars (incisional flutter) or anatomical boundaries such as the fossa ovalis.

Increasing knowledge and experience of Cardiac Electrophysiology studies and ablation procedures led to updated guidelines released by the European Society of Cardiology in 2019. Whilst some changes were made to the recommended medications in some arrhythmias, the promotion for catheter ablation as a form of chronic therapy has increased significantly.

Historically, SVT ablation procedures were performed without 3D mapping systems; catheters were placed under fluoroscopy guidance and testing manoeuvres using pacing were used to identify the type of SVT. Despite being considered ‘low risk ablation procedures’, patients with SVTs that require ablation near the conduction system of the heart are at risk of permanent pacemaker implantation as a complication. As such, the use of ‘advanced’ 3D mapping systems has meant that mapping of these arrhythmias has increased in popularity due to improved safety and efficacy.

Of late, technological advancements in these systems has resulted in the reduction, or obliteration, of the need for X-ray to perform diagnostic Cardiac Electrophysiology and ablation procedures. The ability to recreate a patient’s cardiac anatomy in 3D using these mapping systems, without X-ray, has become the focus of many Cardiac Electrophysiology labs – pertinent in the current era of a dramatic rise in the knowledge of the effect of radiation on electrophysiologists, staff, and patients alike, with long-term health effects. Even in patients with complex congenital anomalies, the use of 3D mapping for ablation of these SVT arrhythmias is further enhanced by the ability to integrate preprocedural imaging modalities such as CT, MRI, or intra-cardiac echocardiography with the chamber geometry for improved understanding of the mechanism of arrhythmias.

Developments in AF ablation

The demand for treatment of AF is increasing rapidly due to an ageing population and is known to increase the risk of all-cause mortality, heart failure, and thromboembolism. Whilst treatment with anticoagulants and rate control medication is well recognised, there is still a large gap in access to ablation for patients, with only 1-2% of patients with AF estimated to undergo ablation, despite the inclusion in numerous guidelines.

AF ablation techniques vary between paroxysmal and persistent AF patients, but all aim to treat the ‘triggers’ of AF. These triggers are mainly from the pulmonary veins from the left atrium. Electrical isolation of these veins (PVI) is the cornerstone of AF ablation for all patients as first line therapy.

Radiofrequency ablation has been demonstrated as the most costeffective method of treating AF, for both paroxysmal and persistent AF patients. Several approaches have been developed for RF ablation of AF, but the end point for persistent AF ablation patients is still under debate, with long-term success rates for persistent patients being around 70% after one ablation procedure. Recent advances and developments around ablation strategies has vastly improved the success rates of maintaining a normal rhythm in many AF patients although inter-operator variation in ablation strategy varies widely.

As redo procedures have, until recently, demonstrated a disappointing level of chronic pulmonary vein isolation, technological advancements have focused on improving RF lesion durability. The last decade has seen an introduction of contact force-sensing catheters. These ablation catheters provide an indication of the amount of contact that the ablation catheter has with the myocardium. This improved knowledge of tissue contact results in fewer RF ablations and improved lesion formation thereby improving both acute, and chronic success for AF ablation patients.

Of late, there has been vast interest in high power, short duration ablation. Historically, pulmonary vein isolation for AF was performed with relatively low powers (30- 40 Watts) and moderate time durations for each RF lesion, giving lesions that were more affected by conductive heating effects rather than resistive heating. The latest technology and research suggests that high power (90 Watts), short duration RF applications cause a larger zone of resistive heating. This provides shallower but broader lesions, which may increase the lesion durability, lower procedural times, reduce the opportunity for collateral damage to structures such as the oesophagus, and minimise the dwell time of catheters within the arterial system. Where ablation for persistent AF patients requires additional ablation lesions in areas of thicker myocardial tissue, or for other concurrent arrhythmias, the same RF ablation catheters can still be utilised to complete all targets of ablations. Precise temperature feedback from the tip of the ablation catheter has been suggested to improve the safety profile of ablation and has the ability to automatically adjust saline irrigation rates and power to ensure that the tip does not overheat or have any char formation.

Current RF therapy techniques require technical expertise on the part of the physician and have historically been considered more time consuming than balloon counterparts. As such, some Electrophysiologists are known to use Cryoballoon treatments for AF.

Cryotherapy is often utilised when only pulmonary vein isolation is required for a patient, normally those with a ‘normal’ heart and paroxysmal AF. Whilst there is a fast-learning curve associated with this therapy, it cannot be used to treat any other arrhythmias that often occur alongside AF, such as atrial tachycardia or atrial flutter. Similarly, the restrictive sizing of available balloons may result in suboptimal ablation in patients whose pulmonary veins are angulated or abnormally sized.

A new ultra-low cryoablation system has just been released which has the capacity to be circular, linear, or curved in shape. It is hoped therefore that this can be used outside of isolation of the pulmonary veins for successful treatment of more AF patients than current Cryoballoon technology. Ultra-low cryoablation cools the tissue to -190°C compared to -70- -80°C with traditional cryotherapy and thus should create more transmural lesions. Research into this new technology is ongoing.

Laser balloon therapy is another available form of ablative therapy, but is used infrequently within EP centres, possibly due to the more established Cryo and RF therapy techniques currently in existence. Like other Cryoballoon technologies, it is typically used only in patients with paroxysmal AF. It too has a relatively short learning curve and recent developments in the balloon have improved procedural and fluoroscopy times. Additional use of the integration of imaging techniques such as intra-cardiac echocardiography can be especially important when performing Cryoballoon or laser balloon ablation where 3D reconstruction of a patient’s anatomy doesn’t occur during the procedure.

Due to the popularity and speed of balloon therapy techniques for AF, RF balloon therapy technology, with associated integration into 3D mapping systems, is currently on early market release in some European centres. It is hoped that it will provide the learning curve of balloon therapy but with advantages of having the concurrent ability to recreate 3D geometry rapidly and the capacity to safely re-ablate specific target areas within the pulmonary veins in cases of difficult or abnormal vein anatomy.

An alternative form of irreversible myocardial injury, by a technique called electroporation, is receiving a large amount of interest worldwide as the first non-thermal ablation technique for treatment of cardiac arrhythmias. PFA therapy causes cell death via a series of rapid, high voltage electrical pulses which damages targeted cell membranes of tissue in close proximity to the catheter. Having been used for the treatment of tumours in oncology, many companies are currently investigating the optimal waveform ‘recipe’ for safety and long-term success for use within AF ablation. Due to its tissue specificity, it is proposed that it will provide a form of ablation therapy that will limit damage to other noncardiac structures that are at risk from RF or Cryo therapies as well as enabling extremely rapid AF ablation procedural times.

Patients with persistent AF are thought to have other substrates outside of the pulmonary veins, explaining the reduced success rates for these patients if ablation is limited to PVI only. As such, additional triggers within the left and right atrium are sought for these patients. RF ablation is typically the first form of ablation therapy offered for these patients. In patients where at least one RF ablation has failed to maintain sinus rhythm, a hybrid approach has been developed in the last few years. Here, minimally invasive surgery via a pericardioscopic approach from the upper abdomen, enables a cardiac surgeon to use RF energy epicardially to create block ablation lesions in both atria. This is pertinent in areas such as the posterior wall, which is renowned for having epicardial connections, where surgical ablation can assist in overall lesion integrity with concomitant, or subsequent endocardial ablation to validate the surgical ablation lines and, if necessary, add additional lines.

In patients where a surgical option is not feasible, ongoing technological advancements with RF ablation strategies for persistent AF are ongoing. An example of this is Vein of Marshall ethanol infusion as an advocate for endocardial RF ablation of this bundle, thought to initiate or perpetuate AF in persistent patients. The highly challenging and limited success for RF endocardial ablation of the mitral isthmus has limited AF ablation techniques in this region to date and ethanol ablation thus far has identified limited complications but great success rates of both acute and chronic success for persistent AF patients.

Developments in VT ablation

Historically, ablation of VT was only considered after failure of antiarrhythmic drugs and/or an increased frequency in ICD shock therapy. Research into the mechanisms of non-ischemic and ischemic cardiomyopathies have improved strategies for ablating VT over the last two decades and can often prevent some disease advancement.

Pre-procedural imaging is common via MRI or CT to provide anatomical and functional information prior to the EP study. They can be used with delayed enhancement to identify and localise areas of scar as potential targets during the ablation procedure. Importing these scans into the mapping software provides real-time assessment of areas of scar, slow conduction, and border zones with healthy tissue that may cause re-entry VT circuits and be the target of ablation.

In some patients with papillary muscle VT, or VTs in areas of difficult anatomy for catheter manipulation, 3D mapping alongside intracardiac echocardiography enables realtime imaging of the heart during the EP study and ablation. This can help assist with ensuring catheter contact with the tissue during ablation, as well as early detection of complications. The ICE catheter can be used to trace surface contours of the imported ultrasound images, onto prior CT or MRI scans within the 3D mapping system.

As well as improving safety, this can reduce procedure and fluoroscopy times as well as improving success rates. Recent technological advancements in 3D mapping systems allow simultaneous mapping of the volume of scar alongside the ability to localise multiple VTs at the same time, with safety. This is important for patients with multiple ectopics, multiple VTs or substrates, and for whom extremely long procedural times are not well tolerated.

In terms of improvements in catheter technology, use of multipolar catheters with small electrode spacings identifies abnormal electrograms in small areas of myocardium that are missed when creating 3D maps using catheters with widely spaced electrodes. The surface area covered with these catheters, alongside increased electrode count and elongated splines enables extremely fast mapping of chamber geometry, even in hemodynamically unstable VT.

For patients with stable VT, mapping in the arrhythmia does provide the best success rates for ablation procedures. For those with hemodynamically unstable VT, such as patients with ischemia, 3D mapping is often performed in sinus rhythm, with labelling and targeting of abnormal electrograms. However, in patients with high scar volumes, it may be unachievable to target all areas of late conduction when it is unknown which zone of late activation causes or maintains the VT circuit(s). Targeting all areas results in an unnecessarily high volume of ablation and doesn’t necessarily improve success rates.

Use of the scar map to demonstrate areas of diseased tissue, alongside newly described techniques such as ‘DeEP’ and/ or ‘ILAM’ mapping, can help identify more specific ‘delayed conduction’ regions of diseased myocardium that are likely to be critical to the VT circuit. These strategies use various pacing protocols to improve specificity of target ablation areas within a ventricle within areas or channels of noticeable conduction delay. To date this has vastly improved the strategy and success rates for ischemic VTs where the volume of scar is often substantial.

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