Medtronic’s Sphere-360 pulsed-field ablation catheter design has some new tricks

The investigational Medtronic Sphere-360 pulsed-field ablation and mapping catheter uses nitinol for a feature that’s unique within Medtronic’s PFA device portfolio.

Medtronic recently offered a first look at its Sphere-360 pulsed field ablation (PFA) and mapping catheter, an investigational device with some design features that are new for the world’s largest device manufacturer.

In an interview with Medical Design & Outsourcing, Tim Laske, VP of research and business development for Medtronic Cardiac Ablation Solutions, discussed the Sphere-360’s design and features.

The large-lattice, over-the-wire PFA Sphere-360 catheter — which Medtronic purchased through its acquisition of Affera — is investigational in the EU but not in the U.S.

Affera founder Doron Harlev and his team designed the Sphere-360 catheter for pulmonary vein isolation (PVI) to treat atrial fibrillation (AFib). Before its acquisition by Medtronic, Affera developed Sphere-360 under the name SpherePVI.

Sphere-360’s nitinol lattice collapses down for catheter delivery through a standard 8.5-French introducer.

“We feel that’s an advantage,” Laske said. “A smaller introduction size [may] result in fewer complications at the introduction site, and it’s a size and form factor that physicians are generally more familiar with.”

The superelastic nitinol catheter expands inside the pulmonary vein for full contact with the vessel wall to kill cardiac myocytes to block irregular heart signals that cause AFib. Sphere-360 is a single-shot PFA catheter, which means it can ablate all of the target heart tissue simultaneously, freeing physicians from repeatedly rotating the device inside a patient’s heart for ablation.

Medtronic’s PulseSelect is also a single-shot catheter, but it does require rotation. Sphere-9 is a smaller, focal catheter.

Sphere-360’s adjustability sets the new PFA catheter apart from the Sphere-9, which Medtronic also acquired from Affera, and Medtronic’s FDA-approved PulseSelect.

Previously: Why Affera’s cardiac ablation technology is worth $1B to Medtronic

Sphere-360 can take the form of a sphere, a linear configuration, a pancake shape for a maximum diameter of 34 mm, or anything in between.

“It’s infinitely variable between those,” Laske said. “One notion that you’ll be hearing more about in the future instead of pulmonary vein isolation is the notion of pulmonary vein ablation. …The goal of pulmonary vein isolation is to prevent triggers within the pulmonary veins from entering the atrium and creating chaos and atrial fibrillation. Early work was done to simply ablate those triggers. Because that becomes challenging and impractical, the step was made to isolate the pulmonary veins and create this barricade so that these triggers could not escape.”

“With some of the newer technologies, there’s an opportunity to do pulmonary vein ablation where you are entering the vein and you’re ablating these triggers as well as creating an antral isolation,” he continued. “Why that can be done with pulsed-field ablation is it appears that it’s not only tissue-selective for myocytes, but it has a very low or no risk of creating pulmonary venous stenosis. … The No. 1 complication that physicians worry about are esophageal fistulas and pulsed-field ablation appears to be safer in that regard. That’s been our experience preclinically — and that’s the hope clinically as well — is that the worry about esophageal fistulas could be a thing of the past, but that remains to be proven.”

Previously: What’s so special about pulsed-field ablation? Medtronic EVP Sean Salmon explains

Nitinol’s dual functions for Medtronic PFA catheters

Sphere-360 doesn’t just use nitinol for the nickel-titanium alloy’s superelasticity properties for catheter deployment and withdrawal. Like Sphere-9, Sphere-360 also uses the conductive properties of nitinol to create the energy field that ablates heart tissue.

“It’s one global electrode,” Laske said. “I won’t get into any details on how that’s designed, but we’re delivering from the entire globe or the sphere and it has six bipolar pairs of electrodes on it.”

Those six electrodes are for mapping, electrogram recording, cardiac stimulation and measuring impedence for tissue contact. The electrodes are electrically isolated from the pulsed-field delivery to act as independent electrodes on the overall superstructure, which contains radiopaque markers for fluoroscopy visualization.

The original form factor of the nitinol used to make the Sphere-360 and Sphere-9 lattice tips is confidential, Laske said.

“What I can say is it requires laser cutting and electropolishing, as is common in use of nitinol,” he said.

Like Sphere-9 and PulseSelect, Sphere-360 also indicates tissue contact for deeper lesions, using those six electrodes for impedance measurements. Medtronic’s PFA portfolio does not measure contact force, with Laske saying the company’s catheter testing on PulseSelect did not find force had an impact on lesion creation.

The Sphere-9 and Sphere-360 catheters’ nitinol lattices are both compliant, meaning they flex when forced against heart tissue, adjusting to the pulmonary vein’s shape for better contact and more effective ablation.

But Sphere-360 is designed to go beyond compliance with conformance “to maintain good, contiguous tissue contact,” Laske said.

PFA system manufacturers have been reluctant to divulge the precise details of their recipes for durable ablation therapy. Laske ballparked it as “on the order of 2,000 volts, base to peak, [and] peak to peak you may be in the 3,000 to 4,000 range.”

Sphere-360’s clinical results

More details about the Medtronic Sphere-360 PFA catheter are available from the first-in-human trial funded by Medtronic’s Affera. Dr. Vivek Reddy (a consultant for Medtronic and Affera) presented interim findings in April as late-breaking data at the European Heart Rhythm Association (EHRA) 2024 Annual Meeting.

The system uses Affera’s HexaPulse generator to deliver “a proprietary monopolar biphasic PF waveform between surface dispersive pads and the six (equal area) sections of the electrode array by independently and sequentially energizing these elements that encompass the entire lattice framework of the single-shot PFA catheter,” Reddy said in his paper. “The waveform is comprised of a train of microsecond pulses ranging from 1.3–2.0 kV that are delivered without cardiac synchronization over 5–6 seconds.”

“The pulse waveform and system hardware/software were optimized during the trial, resulting in three patient cohorts being evaluated: PULSE1, PULSE2, and PULSE3 (this evolution was not pre-specified, but occurred because of suboptimal durability with initial waveforms),” Reddy continued. “Briefly, for these three pulse waveforms: PULSE1 (5.2 second lesion) had a target of two applications per vein, and PULSE2–3 (5.9 second lesion) had a target of four applications per vein. In order to improve lesion durability, propriety changes were also made to the waveform parameters and vectoring.”

There were no primary safety events during the trial, and the PULSE3 waveform achieved 100% freedom from atrial arrhythmia recurrence after a year, Reddy said.

“But any enthusiasm must be tempered by the relatively small number of patients that completed follow-up,” he wrote. “Larger prospective multicenter studies of this waveform with this large-lattice PFA catheter should be conducted — perhaps with greater attention focused on an AF burden endpoint.”

Read more from our interview with Medtronic’s Tim Laske: Medtronic’s Affera Sphere-9 dual-energy ablation and mapping catheter uses nitinol in a new way and How Medtronic designed the PulseSelect pulsed-field ablation system for AFib