Patrice Wendling

June 29, 2016

NICE, FRANCE — Preliminary data suggest cardiac pacing is feasible with implantable cardioverter-defibrillator (ICD) leads placed outside the heart and under the rib cage[1].

Leads were temporarily placed in the extravascular substernal space in 26 patients and consistent ventricular pacing captured in 18.

Among the eight patients with failed capture, three had atrial capture likely related to suboptimal lead placement, one had intermittent capture, and four had no capture, including three cases with high or variable impedance likely due to air introduction around the lead or an excessive amount of saline.

"These data do suggest this is a potentially feasible mechanism of pacing the heart," principal investigator Dr Darius Sholevar (Our Lady of Lourdes Medical Center, Camden, NJ) said at the European Heart Rhythm Association (EHRA) EUROPACE-CARDIOSTIM 2016 meeting.

The substernal leads not only avoid the need for vascular access but may overcome two key limitations of extrathoracic subcutaneous lead placement: high defibrillation thresholds and an inability to deliver antitachycardia pacing, he said.

Medtronic is currently developing the EV-ICD system and presented four abstracts at the meeting, including one suggesting it requires lower energy (defibrillation thresholds) than subcutaneous devices and comparable energy to conventional ICDs.

Sholevar told heartwire from Medscape the approach is likely to be more cost-effective than the alternative approach being worked on of a subcutaneous ICD communicating with a separately implanted leadless RV pacemaker.

"An ideal scenario will be the ability of this technology to provide both low-energy defibrillation and on-demand bradycardia pacing for all patients who receive a device," he said. "Even if this ideal is not possible, the approach offers the possibility of low-energy extravascular defibrillation and a high likelihood of antitachycardia pacing, which could double the usable lifespan of an extravascular ICD compared with current subcutaneous models while offering patients a good chance at painless termination of ventricular arrhythmias."

Device Placement

The Substernal Pacing Acute Clinical Evaluation (SPACE) trial assessed the feasibility of pacing through a 7 French decapolar electrophysiology catheter temporarily placed via percutaneous subxiphoid access into mediastinal tissue under the sternum at or adjacent to the midline. Using fluoroscopy and a special tunneling tool and introducer, the middle pair of electrodes was centered over the maximum anterior right ventricular excursion or the midportion of the RV. The procedure time was limited to 20 minutes.

The mean time to catheter placement was 11.7 minutes. Average pacing thresholds at pulse widths 10 ms, 3 ms, and 1 ms were 7.3 mA, 9.0 mA, and 11.8 mA, respectively.

When asked whether there is a "sweet spot" anatomically that provides the best chance for capture, Sholevar said, "We tested a minimum of seven different pacing vectors in all patients, and in some cases we tested more, and we did find that the widely spaced bipoles going from the mid-RV to the lower portion of the RV was optimal."

Bipolar pacing providing the best capture in 10 patients, unipolar in seven patients, and there was no difference in one.

In 16 patients with pace capture in the bipolar configuration (electrode spacing 19 mm and 10 ms PW), the mean threshold was 5.5 V, but the median was as low as 3.0 V, he added.

The secondary objective of sensing was seen in all patients at an average threshold of 2.98 V to 4.11 mV in the unipolar configuration.

There was low-level chest-wall stimulation in two patients (8%) during pacing, usually in the left parasternal location. "Because these patients were not under conscious sedation, it is difficult to know what the clinical implication of that would be," Sholevar said.

There were two adverse events; one patient had incisional pain that resolved without significant intervention and the very first patient developed an asymptomatic pericardial effusion that was noted at the time of open-heart surgery and drained with no long-term sequelae.

Limitations

Sholevar said study limitations were that the patients did not have indications for an ICD but were undergoing midline sternotomy surgery (CABG in 16 patients, valve replacement in seven, CABG plus valve replacement in two) or subcutaneous ICD implant (one patient).

They also used an off-the-shelf EP catheter (Marinr, Medtronic) with electrodes not optimized for extracardiac pacing, and collection of catheter placement and pacing data was limited to 20 minutes per protocol.

During a discussion of the results, a concern was raised about the position dependence of the pacing efficacy when patients move about and are upright.

Sholevar said pacing and defibrillation efficacy are both likely to improve with leads specifically designed for the substernal space as well as more research on optimal implant location. And "in some of the animal modeling that's been done, this does not seem to be tremendously position-dependent, although you're absolutely right this is very early work. There's going to need to be a lot of further down-the-road discussion before this gets into clinical use."

Medtronic sponsored the study. Sholevar reported no relevant financial relationships.

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