How Abbott dialed in the waveform for its Volt PFA system

The Abbott Volt pulse field ablation (PFA) system wasn’t the first such system approved for treating atrial fibrillation (AFib), but that allowed the device developer to implement some design and engineering lessons from the first wave of PFA devices before securing its CE mark.

“When I look at first-generation devices, we have taken a deliberately very different development path, especially for the waveform, the recipe of the energy,” Dr. Christopher Piorkowski, the chief medical officer of Abbott’s electrophysiology division, said in an interview with Medical Design & Outsourcing.

“All the other devices out there have developed a couple of waveforms, tested them and went into human clinical use,” he continued. “Our approach was fundamentally different. … Volt has a very different development path and very different expected performance and actually observed performance in the field compared to first-generation devices, just by the way we designed the waveforms on the catheter.”

This is the first of a series of posts where we’ll look at the Abbott Volt PFA system’s design and development, focusing first on the waveform. Piorkowski has previously detailed the Abbott Volt’s unique use of a balloon-in-catheter design for PFA.

The biological effects of PFA

PFA generates an energy field to kill heart muscle cells and isolate erratic signals without causing damage to nearby nerve cells. PFA’s biological effects aren’t yet fully known, and researchers are studying how PFA forms lesions and in what sizes, shapes, depths and durability. PFA safety is also in the spotlight, with potential side effects including hemolysis, bubble formation, coronary spasm and skeletal muscle recruitment.

“All these are biological effects of the energy source and these effects should go into the design and the selection of the recipe of the waveform,” Piorkowski said. “That is what we have done in our waveform and our technology development. Our engineers are at the forefront of understanding the variety of these biological effects in the PFA world.”

Hemolysis, for example, is “something we are not worried about at all” at Abbott, he said. “We have scientific publications from the CE mark trial [showing] we have no clinical relevant hemolysis, and that is attributable to the waveform design.”

“Another example is skeletal muscle recruitment,” he continued. “When you look into the first-generation devices, there’s a lot of skeletal muscle recruitment, and the only way the physician can handle these procedures is by using general anesthesia and very deep sedation to keep the patient calm and keep the catheter stable at an intended ablation target site, because if there’s a lot of skeletal muscle recruitment, the catheter flies away and the lesion is not good. There are potentially adverse events, and that is not something that is desirable. We took out this aspect of significant skeletal muscle recruitment, and we have seen in our clinical studies that it works well. It works as intended. And now going forward, we will produce more scientific evidence on that beneficial aspect of our Volt catheter.”

The dangers of heat in nonthermal PFA

PFA is considered nonthermal ablation because it kills heart muscle cells through electroporation (opening holes in the cell walls) rather than an application of heat with radiofrequency (RF) ablation or freezing temperatures with cryoablation.

But PFA devices can have thermal effects. Johnson & Johnson MedTech recently recalled its Varipulse PFA catheters after an unusually high rate of strokes in patients, and warned after an investigation that those catheters can generate dangerous heat if not used as instructed.

“It is not that RF and PFA are too strictly separated worlds of ablation,” Piorkowski said. “Both are electrical therapies, and there’s significant overlap between these two modalities. It is very easy to create PFA waveforms that have a heavy thermal footprint. And if you see applications where physicians are mandated to wait an extended time between individual PFA applications, you always have the suspicion there’s a thermal footprint at play and that the waiting time is also used to allow cooling down of the tissue.”

He said that’s not the case with the Abbott Volt waveform, and that the system was subject to more regulatory scrutiny on that front than first-generation PFA devices.

“When we started to enter the clinical trials and when we were in discussions with the various regulatory bodies, a lot of learning has happened on the side of the regulatory bodies, and they became aware of of these potential thermal footprints and introduced new testing — which wasn’t there for the first- generation devices — but they introduced it for us where we really needed to document that we have no thermal footprint,” he said.

One test required the development team to deliver 70 pulses at high repetition in the same spot from from the same electrode without a temperature increase in the tissue, he said. “That was essentially a proof point that there is not a thermal footprint in our waveform design and this is really a nonthermal therapy.”

How Abbott dialed in the waveform for the Volt PFA system

Piorkowski declined to divulge details of Abbott’s proprietary PFA waveform, but discussed the technology and its development in general terms.

“A waveform consists of multiple pulses and pauses between the pulses,” he said. “We have to be very humble. Today we understand very little about the cellular effects of these different waveforms in terms of stunning, durable lesions, permanent cell death, etc. And therefore our approach was a very different one. Since we we did not know these cellular mechanisms but we wanted to create a waveform that serves our needs, we did a large empiric testing. So basically, for every biological effect — for lesions, but also for bubbles, for hemolysis, etc. — we developed an individual testing environment in pre-animal models and in animals and then ran hundreds of waveforms through these testing stations and studied the outcome in terms of biological effects. And now we almost have a library of waveforms where we very clearly understand what these waveforms do on a biological level.”

“I personally attended a couple of these experiments,” he continued, “and it was deeply humbling to see if just the pause between trains is changed by two or three milliseconds, suddenly the waveform has a very different behavior — makes way more arrhythmias, way more hemolysis — and it’s only a minimal change and no one can explain why this is happening. Therefore, we took an empiric approach, and I think that was successful.”

Researchers tweaked the waveform parameters — the duration of the pulses, the pauses between the pulses, the number of pulses, polarity of the pulses — and found that hemolysis is “critically influenced by the waveform” (as well as other factors of device design, which we’ll cover later in this series).

Waveform is also a factor in the formation of char and microbubbles during PFA.

“Something that was very surprising to me as well [was] how much the microbubble performance changes depending on the waveform,” Piorkowski said.

Watch Medical Design & Outsourcing for more about microbubbles from Piorkowski and Dr. Brad Sutton, the chief medical officer of Boston Scientific’s Atrial Solutions Business, which includes PFA systems.

Read more from this interview: Why Abbott went with a balloon-in-basket design for its Volt PFA catheter