New Insights into Cardiac Ablation

Abstract

Catheter ablation has emerged as a significant treatment approach to treat symptoms and regulate heart rate or rhythm. This review highlights the evolution of catheter ablation, focusing on its applications in atrial fibrillation (AF). Radiofrequency (RF) catheter ablation has become a primary treatment option due to its high success rate and minimal adverse effects. Using three-dimensional (3D) reconstruction and mapping systems enhances precision in targeting arrhythmogenic substrates.

1. Introduction

Catheter ablation involves using an electrode catheter to eliminate specific small areas of heart tissue or the conduction system, which are crucial in causing or perpetuating cardiac arrhythmias. In addition to antiarrhythmic medications, catheter ablation provides an effective and potentially curative treatment for patients with complex congenital heart disease, focusing on the source of underlying arrhythmias. The use of 3D reconstruction from MRI or computed tomography is beneficial for understanding heart anatomy, pinpointing the target chamber, and assisting in access planning [1].

Due to its high success rate and minimal adverse effects, radiofrequency (RF) catheter ablation has emerged as a primary treatment option for numerous arrhythmias. In this procedure, one or more electrode catheters are introduced percutaneously through the vasculature to contact cardiac tissues. A diagnostic study is conducted to understand the mechanism of the arrhythmia, and then an ablation catheter is positioned adjacent to the arrhythmogenic substrate. Radiofrequency energy, reaching up to 50 W, is delivered in the form of a continuous, unmodulated sinusoidal waveform, typically for 60 seconds. The application of energy is well tolerated by a mildly sedated patient and results in the creation of a small, well-defined lesion (5 mm). This process involves the destruction of tissue critical for arrhythmogenesis, such as an accessory pathway, and its subsequent replacement with scar tissue, effectively eliminating the arrhythmia [2].

RF catheter ablation has become a primary treatment choice for various arrhythmias due to its high success rate and minimal adverse effects. Electrode catheters are inserted percutaneously through blood vessels to reach the heart tissues. A diagnostic study is performed to understand the arrhythmia mechanism, followed by positioning an ablation catheter near the area causing the arrhythmia. Radiofrequency energy, typically delivered at up to 50 W in a continuous, unmodulated sinusoidal waveform for around 60 seconds, is used. This energy application is well tolerated by patients under mild sedation and leads to the creation of a small, well-defined lesion (5 mm). This process involves the destruction of tissue responsible for causing arrhythmias, such as an accessory pathway, and its replacement with scar tissue, effectively eliminating arrhythmia [2].

2. AF ablation

The development of AF from sporadic and brief occurrences to more sustained forms is a typical progression seen in many affected individuals. However, the specific patterns of this progression can vary significantly among individuals.

While some patients may experience a rapid progression, others may have rare and brief episodes for an extended period. Nevertheless, there is a general trend toward AF progression in all patients. It remains unclear whether AF itself contributes to this progression, prompting the question of whether early AF ablation could be a viable strategy to slow down or halt AF progression [3].

Catheter ablation has become an essential strategy for controlling the rhythm in AF and is the most performed cardiac ablation procedure globally. According to current guidelines, this procedure is recommended for symptomatic patients with paroxysmal or persistent AF who do not respond well to or cannot tolerate antiarrhythmic medications. In some cases, it may also be considered as an initial treatment for asymptomatic patients. Although data from large studies suggest that AF ablation may reduce mortality, heart failure, and stroke risk, evidence from randomized controlled trials is not conclusive. Pulmonary vein isolation, achieved through either point-by-point radiofrequency or cryoballoon techniques, remains primary method in AF ablation. The primary objectives in managing patients with AF include symptom improvement, heart rate or rhythm control, and reducing the risk of stroke [4].

Achieving electrical isolation through circumferential ablation of the pulmonary veins constitutes a fundamental aspect in most AF ablation procedures. This method entails creating a series of radiofrequency lesions, implemented point by point, encircling the two left and two right PVs. This can be achieved either through a singular circumferential lesion surrounding each set of PVs or by incorporating lesions between ipsilateral PVs, resulting in a figure-eight lesion set. The confirmation of PV isolation is commonly conducted using a circular multipolar electrode catheter. The primary tools used for this procedure are irrigated radiofrequency ablation catheters, often used with an electroanatomic mapping system [5].

2.1 The SARA trial

Although several studies have demonstrated superiority of catheter ablation over anti-arrhythmic drugs (AAD) in patients with paroxysmal AF, the Study of Ablation Versus Antiarrhythmic Drugs in Persistent Atrial Fibrillation (SARA) study is the first multicenter, randomized controlled trial conceived to specifically evaluate patients with persistent AF. In this study, catheter ablation was demonstrated to be superior to rhythm control at mid-term follow-up, with a significant QoL improvement. Of note, the ablation approach was PV isolation-only in most cases, with a very low proportion of additional substrate modification. This adds evidence to the hypothesis that limited ablation may be effective in a well-selected population with persistent AF.

This study included 146 patients with persistent drug-refractory AF. The primary endpoint, sustained episodes of AF (>24 hours) after a three-month blanking period, showed that the catheter ablation group had a significantly higher proportion of patients free of sustained AF episodes at 12 months compared to the AAD group (70.4 vs. 43.7%). Additionally, the catheter ablation group demonstrated a higher probability of remaining free of any AF recurrence (>30 seconds) and a lower need for electrical cardioversion. Patients with early recurrences were at a higher risk of reaching the primary endpoint. Quality of life scores did not significantly differ between groups. Complication rates were low in both the catheter ablation group (6.1%) and the AAD group (4.7%). The SARA study concludes that catheter ablation is superior to AAD for rhythm control in patients with persistent AF [6].

3. Mapping

Cardiac mapping involves the precise maneuvering of a mapping or ablation catheter within the targeted region, searching for the site where radiofrequency or cryothermal ablation can effectively treat the arrhythmia.

Techniques for mapping, such as ultra-high-density mapping, isochronal late activation mapping, and ripple mapping, have improved the identification of arrhythmogenic substrates. These advancements not only offer the potential for procedural success but also contribute to minimizing the duration of ablation procedures and reducing the risk of hemodynamic compromise. Sophisticated mapping and ablation techniques, such as epicardial and intramyocardial approaches, enable operators to accurately pinpoint arrhythmogenic substrates [7].

The utilization of a 3D electroanatomic mapping system helps to achieve multiple objectives in catheter ablation procedures. Firstly, it facilitates the localization of catheters without the need for radiographs. Secondly, it calculates and displays 3D representations of electrical activation sequences (activation maps) and local voltage distributions (voltage maps). Thirdly, it visually presents the anatomy of a heart chamber in three dimensions through sequential catheter localization [8].

The superimposition of 3D electroanatomic maps onto the reconstructed surfaces of every cardiac structure enhances the visualization and understanding of the procedure. This operates on a point-by-point basis, requiring a stable arrhythmia and thorough exploration of all relevant sites to generate a comprehensive 3D reconstruction of the activation sequence associated with any arrhythmia. Incomplete mapping may lead to inconclusive maps and unsuccessful ablation attempts [8].

The mapping system consists of an acquisition module linked to a computer capable of concurrently analyzing diverse signals: 32 bipolar electrograms from the basket catheter, 16 bipolar/unipolar electrogram signals, a 12-lead ECG, and a pressure signal. It generates real-time color-coded activation maps, with the electrograms and activation maps presented on a computer monitor. The recorded signals are storable for later offline analysis. Activation marks are automatically generated using algorithms based on peak or slope and manual editing of activation times is performed, as necessary (Figure 1) [9].

Figure 1.

Example of three-dimensional maps acquired using the RHYTHMIA™ system. Left atrial re-entry on the anterior and septal aspect of the mitral annulus; arrows display the direction of rotation around a small area of scar (grey) [1].

Challenges may arise in cases involving a chronically volume-overloaded or scarred myocardium, potentially hindering successful catheter ablation even with precise 3D mapping. The implementation of irrigated tip catheter with increased lesion depth has positively impacted the capacity to produce transmural lesions, addressing certain limitations. However, issues persist in scenarios with restricted catheter-tissue contact, such as in extensively dilated atrial chambers, or when variations in blood flow impact lesion formation [8].

4. Mapping systems

Achieving seamless, transmural lesions in a beating heart is challenging and demands reliable 3D navigation to mitigate potential complications such as pulmonary vein stenosis, perforation, or nerve injuries. Electroanatomical-mapping systems (EAMs) have been devised to aid catheter ablation procedures with reduced radiation exposure compared to fluoroscopy. These systems track intracardiac electrodes in 3D maps, enhancing navigation during ablation. However, conventional mapping systems struggle to detect localized AF drivers due to their sequential spatiotemporal characteristics, intermittent firing, and spatial variability. Consequently, advanced mapping tools have been developed to visualize and comprehend AF-maintaining drivers more effectively [10].

The widely utilized 3D mapping systems include CARTO by Biosense Webster and EnSite Precision by Abbott. Boston Scientific’s Rhythmia system, recently introduced, has significantly contributed to the advancement of high-density mapping for complex arrhythmias [11].

4.1 The CARTO system

The CARTO system, initially introduced in 1996, employs magnetic fields for electroanatomical navigation and geometry reconstruction. This system utilizes ultralow magnetic fields ranging from 0.05 to 0.2 gauss (5 × 10⁻⁵ to 5 × 10⁻⁶ Tesla) for triangulation. Three magnetic generators placed beneath the operating table allow continuous measurement of the catheter’s distance, facilitating precise localization of its tip in space.

Recent advancements in the CARTO system include the capability to visualize non-dedicated catheters (non-magnetic) through impedance-based navigation, utilizing six skin patches like the NavX system. However, these non-dedicated catheters cannot be utilized for geometry construction or local information tagging on maps. Additionally, the CARTO system offers intracardiac echocardiography-based 3D reconstruction and integration of rotational angiography or fluoroscopy views.

The developments in the CARTO system involve automated annotation of radiofrequency application locations with adequate contact force and the integration of fluoroscopic views/movies [10].

4.2 The EnSite NavX system

The EnSite system comprises three pairs of skin patches along with a system reference patch, utilizing impedance-based localization and tracking methods. These patches, when placed on the patient’s skin, establish electrical fields along three perpendicular axes. By emitting low-power currents of 350 mA at 8 kHz frequency through the three pairs of patches, a 3D electrical field centered around the heart is formed.

For anatomical reference, the EnSite system employs either the system reference patch on the patient’s body or an intracardiac electrode, enhancing compensation for cardiac and respiratory motion artifacts. The latest version, the EnSite Precision cardiac mapping system, combines the benefits of hybrid impedance and magnetic field technologies, offering significantly improved precision and accuracy compared to earlier versions.

The Precision system necessitates an additional magnetic field source, termed the EnSite Precision field frame, and two extra sensors on the patient. This frame, affixed beneath the patient’s table, generates a weak magnetic field like the CARTO system’s location pad [11].

4.3 The RHYTHMIA system

The RHYTHMIA system introduces cutting-edge 3D mapping technology characterized by exceptionally high resolution, employing both magnetic and impedance-based localization methods. It utilizes a unique multipolar basket-like catheter equipped with small non-circular electrodes to optimize the recording of electrocardiograms and anatomical reconstruction.

This specialized catheter design enhances the system’s capability to reject “far-field” signals and capture “near-field” local potentials, even at minimal amplitudes. Consequently, mapping is expedited, definitions are precise, and the signal-to-noise ratio is notably high, resulting in fewer artifacts. Additionally, the system incorporates an intelligent algorithm that analyzes wave propagation, automatically identifying areas of blockage with remarkable reliability [10].

5. Advantages of RFA

The advantages of using radiofrequency lesions in the treatment of certain arrhythmias stem from the small size of these lesions. This approach has proven phenomenally successful in addressing arrhythmias with a focal origin or those dependent on a narrow isthmus for maintenance. The effectiveness of ablation is particularly in cases where arrhythmias have anatomically based or directed substrates, such as accessory pathways and atrioventricular nodal re-entry tachycardia (AVNRT).

Accessory pathways, anomalous connections between the atria and ventricles, are commonly found along the mitral or tricuspid valve annulus. The key to successful ablation in such cases lies in precise lesion placement. Fluoroscopically guided by an electrode catheter in the coronary sinus, the ablation catheter’s position is directed along the mitral annulus. The bipolar electrogram, specifically the relative amplitude of atrial and ventricular components, recorded by the ablation catheter, aids in positioning relative to the annulus. Identifying the earliest atrial or ventricular activation during pathway conduction assists in locating the pathway along the annulus. In cases of AVNRT, where the target for catheter ablation is in the posteroseptum, the procedure becomes more predictable. Ablation can be guided solely by anatomical landmarks relative to His bundle and coronary sinus catheter positions using fluoroscopy. Alternatively, an approach involving both anatomic and electrogram considerations may be employed. This combination enhances the precision and effectiveness of the ablation procedure in treating specific arrhythmias [11, 12].

6. Contraindications

• Unstable angina

• Bloodstream infection

• Acute worsening of congestive heart failure not due to the arrhythmia

• Significant tendency for excessive bleeding

• Acute blood clot in the veins of the lower extremities if femoral vein cannulation is desired

• Presence of a mass or blood clot within the heart

7. Timeline of innovations in treating cardiac rhythm disorders

The evolution of strategies for managing heart rhythm disorders has seen significant advancements over time. From surgical antiarrhythmic techniques to the introduction of implantable cardioverter-defibrillators, the landscape of treatment options has expanded. Direct current catheter ablation emerged as a milestone, offering a minimally invasive approach to correct arrhythmias. RFA further revolutionized the field, providing precise and effective treatment with improved outcomes. More recently, pulsed-field ablation has emerged as a promising innovation, offering a non-thermal alternative with potential advantages in safety and efficacy. These progressive developments highlight the ongoing efforts to refine and enhance the management of heart rhythm disorders (Figure 2).

Figure 2.

Evolution of strategies for managing heart rhythm disorders.

8. Special populations

Patient populations at the highest risk but greatest benefit from ablative therapies are those with advanced heart failure. In the setting of patients with progressive cardiac dysfunction from ischemic, non-ischemic, valvular, or inherited disorders, the burden of arrhythmias and the detrimental effect on myocyte performance can be profound. Our center has developed a model to identify high-risk patients in a multi-disciplinary fashion through a combination of specialties: transplant cardiology, cardiothoracic anesthesia, cardiac surgery, and electrophysiology. The utility of each of these providers is to identify high-risk patients, provide high-quality care in the operative theater, and consider the use of left ventricular assist devices for the maintenance of hemodynamic stability. We have previously published this using extracorporeal membrane oxygenation and percutaneous axillary mechanical circulatory support in high-risk ventricular tachycardia ablations [13]. Furthermore, the role of a multi-disciplinary approach in high-risk patients has been further advanced due to recent data from large trials such as CASTLE-HTx demonstrating increased freedom from transplant, durable ventricular assist device, or death in this population [14]. Based on center and surgical comfort, application of early ECMO, percutaneous support, or a combination may allow for appropriate anesthetic dose, stimulation attempts, and longer duration of ablation to achieve a more sustained outcome – without sacrificing cerebral and end-organ perfusion.

9. Advancements in technology

9.1 High-power, short-duration (HPSD) ablation

HPSD ablation is becoming a new option for treating AF instead of using traditional settings with lower power and longer duration. There is concern about the safety of using higher power RF energy, especially on the posterior wall of the heart. Recent animal studies suggest that using 50 watts of energy for 5 seconds works better than lower power, longer duration ablations and has fewer complications.In a small study with humans using contact force-sensing catheters, using 50 watts of energy for about 11 seconds at each spot led to great results without any issues.

The very high-power short-duration (vHPSD) mode uses 90 watts for up to 4 seconds and automatically adjusts the power if it gets too hot. On the other hand, the conventional power temperature-controlled mode uses less than 50 watts and adjusts the water flow and power based on temperature feedback to keep it at the right temperature. To monitor the temperature in real-time, there are six sensors in the catheter’s tip that measure the temperature where it touches the tissue, and three microelectrodes provide detailed electrical signals to map the endocardial surface better and find any conduction gaps. The 3D electroanatomical mapping system helps with this and can reduce the need for using fluoroscopy during the procedure [15].

According to the Multicenter Q-FFICIENCY Trial, temperature-controlled paroxysmal AF ablation using the innovative QDM catheter in very high-power short-duration (vHPSD) mode (90 W, ≤4 seconds), either alone or in combination with conventional power temperature-controlled mode (25–50 W), has proven to be highly efficient and effective in treating atrial fibrillation (AF). This approach maintains safety standards without compromising on efficacy (Figure 3) [17].

Figure 3.

Comparison between HPSD approach and conventional ablation therapy. Lesion dimensions of radiofrequency ablation and heating distribution in myocardial tissue are demonstrated. HPSD approach and conventional ablation therapy could create comparable lesion size. HPSD approach results in an ablation lesion that is heated directly from the catheter during the resistive phase (red part), while, in conventional ablation therapy, myocardial tissue is largely heated because of conductive heating (golden yellow part) [16].

9.2 Pulsed field ablation (PFA)

Pulsed field ablation (PFA) stands as a nonthermal innovative technology for treating cardiac arrhythmias, particularly AF. Instead, it employs high-amplitude pulsed electrical fields to induce tissue ablation via irreversible electroporation, a process that boosts cell membrane permeability, ultimately leading to cell death. Unlike conventional thermal ablation methods, PFA achieves tissue ablation through microsecond-duration high-amplitude electrical pulses, targeting the myocardium by electroporating the sarcolemmal membrane.

A lesion is typically created with 3–4 PFA deliveries, each completed within one heartbeat. The high-voltage electric fields generate irreversible electroporation without significant heating, which allows the produced heat to dissipate through conduction and convection, making it a nonthermal ablation method. This mechanism targets cell membranes while sparing the extracellular matrix. Success with PFA depends on electrode proximity to the target tissue rather than physical contact. Different regions within the heart have varying sensitivities to electroporation, with the ventricular endocardium being more susceptible than the epicardium. PFA is tissue specific – the myocardium is highly vulnerable to injury, surrounding structures such as the esophagus, phrenic nerves, pulmonary veins, and coronary arteries which are relatively resistant. This specificity broadens its therapeutic potential and could improve safety during AF ablation procedures. The rapid vein isolation achievable in seconds suggests that PFA could significantly shorten procedure durations to less than an hour, marking a notable advancement in AF ablation techniques (Figure 4) [19].

Figure 4.

Pulsed field ablation for pulmonary vein isolation in atrial fibrillation [18].

In large animal studies, biphasic PFA multipolar endocardial catheters were found to safely create lasting linear and circular atrial ablations. Reddy et al. conducted three multicenter PFA studies (IMPULSE, PEFCAT, and PEFCAT II) focusing on patients with paroxysmal atrial fibrillation undergoing pulmonary vein isolation. Among 121 patients, PFA alone achieved acute pulmonary vein isolation in all cases. Follow-up remapping after approximately three months showed durable pulmonary vein isolation in most patients, particularly when using the optimized biphasic energy PFA waveform. Pulmonary vein remapping, performed in 110 patients at 93.0 ± 30.1 days, demonstrated durable PVI in 84.8% of pulmonary veins (64.5% of patients) and 96.0% of pulmonary veins (84.1% of patients) treated with the optimized biphasic energy PFA waveform. Furthermore, the one-year estimates for freedom from any atrial arrhythmia were promising, with the entire study population showing a rate of 78.5±3.8% and the optimized biphasic energy energy waveform cohort showing a slightly higher rate of 84.5±5.4% [20].

10. Conclusion

Radiofrequency catheter ablation techniques have dramatically impacted the treatment of cardiac arrhythmias. Initially, surgical approaches were employed, but more recently, catheter ablation has emerged as a crucial and increasingly prominent method for managing these conditions. In the case of most supraventricular arrhythmias, the efficacy of medical treatment with antiarrhythmic drugs is often poor. Moreover, these drugs are associated with issues such as poor or irregular effectiveness, bothersome and occasionally severe side effects, the potential for proarrhythmia, excessive costs, and inconvenience. Advancements in catheter ablation technology, including HPSD ablation and PFA, have significantly influenced the management of cardiac arrhythmias, particularly AF. These innovative techniques offer promising alternatives to traditional approaches, providing more effective and safer options for patients. Future goals for optimizing patient outcomes are utilizing innovative methods and technology to provide ablative options to a larger group of patients.

Highlights

• Catheter ablation is commonly used and considered a standard technology for treating most atrial and ventricular arrhythmias.

• In the case of atrial fibrillation, catheter ablation is increasingly recommended for patients who are symptomatic and unresponsive to drug therapy.

• Pulsed field ablation has emerged as a promising innovation, offering a nonthermal alternative with potential advantages in safety and efficacy.

• Utilizing temporary mechanical circulatory support may provide increased success rates in high-risk patients with severe heart failure or heart failure with reduced ejection fraction.