The symposium’s final session was an overview of new technology, giving a teaser of upcoming evolutions to catheter ablation and AF management including new energies; individualised, patient-tailored modelling of ablation and disease therapy; truly high-density mapping; thermal modelling; and the latest innovations for PV isolation.
Electroporation-assisted PVI
Electroporation is a technology that Dr Fred Wittkampf, from Utrecht, the Netherlands, has been developing for 8 years. This is a method of creating holes in cell membranes; mainly electrically active cells in particular are targeted.
“It has been suggested that we are using a high-energy shock, but I can assure you that is an alternative fact,” joked Dr Wittkampf. “The energy we are using to isolate the PVs is only 14 joules per electrode, equivalent to approximately 0.5 seconds of RF.”
The non-thermal lesions are half-dome shaped, unlike RF lesions that are narrower at the surface of the tissue.
As with RF, tissue contact is essential. The electrode interface impedance is affected by tissue contact and can be measured in real time using a small bipolar current between neighbouring electrode pairs. Mechanical pressure sensors are not required.
During the circular electroporation impulse, the current density decreases linearly with distance from the electrodes as compared to an exponential decay with a point source as with conventional RF ablation. This difference explains the much deeper lesions that can be created with circular electroporation ablation. With sufficient circular contact, pulmonary vein can be isolated with one shot.”
Electroporation can create stunning and transient block without recurrence within 30 minutes or an hour. The disadvantage, he pointed out, “is that you don’t know when to stop ablation. The advantage however, is that you don’t have to wait 30 minutes.”
There have been no incidents of pulmonary vein stenosis or other complications in animal studies during the development of this technology. Technical development, testing and approval has been completed; the protocol for a first human feasibility study, scheduled for the end of 2017, has been submitted to the Institutional Review Board, and a safety and efficacy study will take place thereafter.
Computational Modelling to Guide Interventional Management of Atrial Fibrillation and Atrial Tachycardia
At Johns Hopkins in Baltimore, US, Prof Natalia Trayanova and colleagues are trying to develop a non-invasive predictor of the targets for ablation of flutter or fibrillation. To achieve this, the patient undergoes pre-ablation MRI.
Image processing is done to outline the areas of fibrosis and structural remodelling, which lets the team create a computational model that helps predict the targets for eliminating the arrhythmia. The algorithm for creation of the model is based on the evidence of how electrophysiological remodelling affects persistent AF.
Prof Trayanova’s team is using this modelling technique predominantly for substrate modification in patients with severe fibrotic disease, particularly repeat patients who have fibrosis or structural remodelling from previous ablations. The goal is to create 3D, patient-specific models.
For example, in an atrial model, the team generates the model segmenting the geometry of the atria, delineating the fibrotic and nonfibrotic tissues. Patient-specific fibre orientations are incorporated, and the geometrical model is populated with all the different cell types.
“I can visualise everything from the dynamic of the proteins all the way to the whole organ,” Prof Trayanova explained.
“We can see when the fibrosis does not extend to the endocardia surface, and the other way around. The algorithm places points for pacing – at least 40 locations – and then we deliver stimuli from there to see whether arrhythmia has developed and what it looks like.” Prof Trayanova then gave an overview of her team’s research. She presented case studies in which the model was able to accurately and non-invasively predict targets for ablation, including in patients who had undergone previous failed ablation.
She also discussed some of the challenges, including finding the best way to terminate a re-entrant driver with ablation. Over the course of research, the team has worked out the technical difficulties, and is now focused on improving the precision of target prediction.
High-density Mapping: Too Much of a Good Thing?
There may be some confusion in the definitions of mapping density and mapping resolution in the field of AF treatment. Dr Elad Anter of Beth Israel Deaconess Medical Center in Boston, US, explained the terms. Mapping density relies on the number of times the operator footprint goes over a surface.
Mapping resolution is influenced by electrode size and interelectrode spacing, so mapping with small, closely spaced electrode catheters can improve mapping resolution within areas of low voltage. High-density mapping allows the operator to better identify the entire circuit in a small area that otherwise would be difficult to record distinct electrograms within.
An interpolation limited to 5 mm or below will probably supply sufficient mapping densities, he said. However, for mapping resolution, notably with respect to electrode size and intra-electrode spacing, the point at which you get meaningful information needs to be studied separately.
Dr Anter questioned how much density and resolution we need in practice. Moving from 100 points to 500 points and 800 points provides a lot of information. Now, new mapping technologies are providing 10,000 points. Is this necessary? In many difficult cases, increased resolution enables us to determine where the circuit is, explained Dr Anter, but mapping resolution alone is often not sufficient for understanding the arrhythmia.
Dr Anter discussed a new algorithm that incorporates calculation of vectors and velocities in addition to activation mapping, in order to understand the global propagation of the arrhythmia. This algorithm is called CoherenceMap. The software, made by Biosense Webster, can improve mapping of scar-related AT, he said, noting that its novelty relies on its ability to identify areas of non-continuity.
The Concept of Thermal Modelling
The development of contact force has allowed a better understanding about what type of lesions are being created. However, there are still unknown variables with this new technology, for example angulation of the catheter, the catheter’s interface with the tissue, and temperature.
“Temperature is the result of everything else you do,” said Dr Tom De Potter, from Aalst, Belgium. “If you think about lesion formation as a thermodynamic process, you have certain input variables: energy, contact, interaction of the catheter to the tissue. Then there are modifying variables: tissue characteristics, tissue thickness, blocked flow. A temperature is the result of all this, ultimately leading to the necrosis you want to generate.”
Thermal modelling aims to understand this process, determine the temperature evolution within the tissue, measure tissue temperature and impedance, and estimate catheter-to-tissue pose and temperature field. Software simulates the temperature field based on different models of reality, models of human tissue, and on observed parameters such as contact force and catheter position.
Not only does this give a simulation model of what may be happening in the real world, it tries to make the most accurate prediction of the actual ablation forming under the catheter tip. By using the measured temperature as an input variable for the model, it has the potential to simulate on-going lesion formation and to better guide physicians in optimal energy delivery.
Dr De Potter described in depth his experience with the use of thermal modelling system, integration with existing 3-D visualisation technology, its validation of ablation targets, and the research and literature supporting its use.
“We’ve shown that it’s possible for software to make a thermodynamic model of RF lesion formation that correlates really nicely with animal data,” said Dr De Potter. “A closed-loop system can solve the unknown variable of tip-tissue interaction by using the measured temperature at the tip/tissue interface. Initial clinical experience has improved the algorithm and led to very insightful new research in biophysics. Ultimately, if we can accurately model lesion formation, we should be able to optimise energy titration in individual positions.”
Latest Innovations for PVI Tools
Prof Vivek Reddy of Mount Sinai Hospital in New York, US, described and recapped the newest technologies available for PVI, in four categories: balloons, advances in point-ablation technology, novel global ablation systems, and irreversible electroporation.
A multicentre, single-arm, first-in-human feasibility study recently showed the RF balloon catheter could deliver directionally tailored energy using multiple electrodes for efficient acute PVI in patients with paroxysmal AF. Results showed the RF balloon catheter was able to achieve electrical isolation of all pulmonary veins with a high rate of firstpass isolation and low evidence of latent pulmonary vein re-conduction. The procedural performance with the device was favourable, with 100 % of the treated pulmonary veins electrically isolated without the need for a focal ablation catheter.
“Existing balloon catheters are limited in a number of ways, the most significant limitation being a single ablative element that delivers identical amounts of energy along the full pulmonary vein ostium circumference,” said Prof Reddy. “This can lead to over-ablation of thin tissue, under-ablation of thick tissue, and unnecessary complications. The investigational RF balloon is designed to both optimise safety and efficacy and reduce procedure time.”
The emerging technologies discussed in the symposium show great promise for advancing efficacy of PVI, speed of lesion creation, more accurate target predictions and more. Despite the excitement generated by these innovations, Prof Reddy reminded the audience that, with technological improvements come learning curves and potential for complications.
“My biggest concern is oesophageal damage, and my very strong recommendation for that, based on evidence and experience is mechanical oesophageal deviation,” said Prof Reddy. “In addition, how much ablation is too much? We all want 100 % success with one procedure, and may believe we should ablate more potential targets. But I think we must start thinking about the cost to left atrial function. At some point, we’re going to have to ask: If you get a 5 % or 10 % increase in success, how much atrial function can you lose to justify that? And does it make sense to subject everybody to a very extensive strategy if only a subset need the strategy?”
Weighing the costs and benefits in terms of LA function will be a necessary focus of research and practice as technology continues to develop and evolve in this era of patient-specific ‘precision medicine’.