Substernal-Lead ICD May Be Next 'Inflection Point' in ICD Evolution

May 15, 2019

SAN FRANCISCO — An implantable cardioverter defibrillator (ICD) with a lead system that inserts just below the sternum performed promisingly in a recent early experience that sets the stage for a more definitive trial, researchers say.

The investigational EV ICD (Medtronic) system includes a battery generator similar in size and design to a standard transvenous-lead device, but is much smaller than the subcutaneous-lead EMBLEM S-ICD (Boston Scientific), Ian G. Crozier, MBChB, MD, Christchurch Hospital, New Zealand, told | Medscape Cardiology.

And although the substernal-lead device can't provide long-term pacing, it can deliver antitachycardia pacing (ATP) as needed in lieu of shocks and provide postshock backup pacing, "which is a big step up from the subcutaneous device," Crozier said.

Because the lead system never enters the cardiovascular system, hopes are that removing it because of device infection will be far easier than the challenging procedure of transvenous-lead extraction.

In initial experience with the EV ICD in 20 patients with a class I or IIa indication for an ICD, seen at four centers in Australia and New Zealand, the device terminated induced ventricular fibrillation (VF) with shocks of 30 J or less in 17 and greater than 30 J in the rest. The median successful shock energy was 15 J.

It delivered one inappropriate shock during the 3-month follow-up, as well as one successful treatment for symptomatic ventricular tachycardia (VT), said Crozier while presenting the experience here at the Heart Rhythm Society 2019 Scientific Sessions.

Experience with the system has advanced enough that "we can move forward with reasonable safety" to a much larger, worldwide study to support its approval process, he said in an interview.

The substernal-lead system could potentially be only 2 years away from market approval, Crozier said, when it would possibly join the ranks of the transvenous- and subcutaneous-lead devices.

"I think this is the next-generator defibrillator, because it avoids the risks of transvenous leads and is an improvement on the subcutaneous device," he said. "So, provided there isn't a contraindication to this device, it could be the first choice."

The transvenous ICD-lead systems and later the subcutaneous-lead system represented two important milestones in evolution toward a practical, effective implantable defibrillator, observed Sanjeev Saksena, MBBS, MD, RWJ Barnabas Health, Warren, New Jersey, as invited discussant after Crozier's formal presentation.

The substernal-lead ICD may be the next step "on the path to a wholly extracardiac ICD system," said Saksena, an early pioneer of transvenous-lead systems.

"It may be another inflection point in the development of the implantable cardioverter defibrillator."

The current substernal-lead ICD still needs further development, but "I think it's promising," Saksena told | Medscape Cardiology.

"I think this will go through the same evolutionary pathway as other implantable defibrillator systems: we start with a prototype system, the technology gets refined, we get new ideas on how to overcome the obstacles. We'll have those incremental steps to get to a really good system."

With such a limited first experience, whether the current substernal-lead system proves safe and effective enough to be an alternative to transvenous- and subcutaneous-lead systems remains an open question, observed Bruce L. Wilkoff, MD, Cleveland Clinic, in an interview.

"If you exchange one morbidity, let's say transvenous problems for substernal problems, it may be six of one and half a dozen of the other. But if you actually have no further problems from the substernal, then it's a great advance," said Wilkoff, who is a leading authority on transvenous-lead extraction.

If the current substernal-lead system proves successful, he said, it's likely to peacefully coexist with the transvenous- and subcutaneous-lead designs. In fact, the new device isn't going to work for many patients. "For instance, the substernal leads can't go into people who have had prior open-heart surgery. That's a large percentage of the patients."

Also, it will take a much longer follow-up to determine whether its associated infection risk is any lower than with transvenous systems "because there will be infections, eventually," Wilkoff said. When used, however, the substernal system "would eliminate the need for transvenous-lead extraction."

Crozier said "there's no reason to think that the infection rate would be any higher than a transvenous device." It's likely to have an infection risk similar to the S-ICD, and "perhaps less because it's a much smaller can."

And, he said, "our expectation is, if it did get an infection, then the system would be easy to explant — like a subcutaneous system, as opposed to a transvenous system, where if it's been in for a long time and it gets infected, it's a hell of a job to get it out."

Two patients in the current series required removal of leads after 2 months, Crozier said, "and they were removed very easily."

As Crozier described, the EV ICD generator is implanted in a pocket on the left side of the chest, close to the mid-axillary line, similar to placement of an S-ICD. The passive fixation lead with an 8 cm defibrillation coil is inserted via a subxiphoid incision into the substernal space, that is between the sternum and pericardium over the right ventricle. The procedure can be done in a standard cath lab.

Of 21 patients implanted with the device, one was unable to undergo electrical testing because of discovery of mediastinal fibrosis. The remaining patients, including 17 men, were as young as 22 years, but averaged 54 years.

Their mean body-mass index (BMI) was 31 kg/m2; the maximum allowable was 40 kg/m2. Crozier said he expects ultimately there will be no BMI restriction as the experience so far suggests it doesn't make a difference to function or safety.

At defibrillation threshold testing, the lowest successful energy was 15 J in 11 patients, 20 J in four, 30 J in two, and above 30 J in the remainder.

Aside from the one inappropriate shock during the 3-month follow-up, there were two cases of swelling or poor healing, one superficial wound infection, and one patient had inspiratory pain, he said.

Contraindications, besides previous surgery involving a sternotomy, "which obliterates the retrosternal space," Crozier said, include need for permanent pacing.

Saksena pointed to other potential considerations should the substernal-lead system enter practice. As discussant, he noted that "pericardial inflammation and effusions are more common than you would expect in older patients."

And if there is an infection, he said, clinicians should be aware that mediastinitis and osteomyelitis are new potential complications.

Speaking with | Medscape Cardiology, Saksena added that "pulmonary diseases that infringe on this area are an issue. There are many, and pulmonary therapies, which are increasingly mechanical, cause invasion of this area. How do you do this in adults with pulmonary disease and those that require a ventilator? There are many issues to be solved."

Crozier discloses receiving honoraria or speaking or consulting fees and receiving research grants from Medtronic. Saksena had no relevant disclosures. Wilkoff discloses receiving honoraria or speaking or consulting fees from Abbott Vascular and Philips.

Heart Rhythm Society (HRS) 2019 Scientific Sessions: Abstract S-LBCT04. Presented May 11, 2019.

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