The Substernal SPACE: The Final Frontier?

Despite advances in implantable cardioverter-defibrillator (ICD) technology, most systems continue to be limited by the requirement to place leads inside the heart. The totally subcutaneous implantable defibrillator (S-ICD) avoids placement of leads in the heart, but has its own limitations, which include the inability to deliver chronic bradycardia pacing or antitachycardia pacing on demand, and the relatively high energy requirements resulting in a device that is significantly larger than a standard ICD.

The only currently available extravascular ICD system is implanted by tunneling a lead under the skin along the left lateral border of the sternum. A novel idea that is currently under development is placement of a pacing and defibrillation lead under the sternum. Placement of a substernal lead could allow for a lead position that is close enough to capture and defibrillate the heart with energies that are lower than needed with a subcutaneous lead. There are case reports that have described placement of a conventional subcutaneous defibrillator lead under the sternum. This has been done with a commercially available S-ICD to manage high defibrillation thresholds. It was also performed in a patient who was on hemodialysis and needed an ICD, but had occluded central veins and a large dialysis graft that crossed the subcutaneous space overlying the sternum where the S-ICD lead is usually placed.¹  

The first clinical studies to systematically evaluate the feasibility of pacing and defibrillation from the substernal location were published in 2017.^2-3 In the ASD study, 16 patients had a defibrillator lead placed temporarily under the sternum using a percutaneous subxiphoid approach. Defibrillation using a single 35-Joule shock was successful in 93% of patients.³ These data suggested that substernal defibrillation can be achieved with the shock energy available in current transvenous defibrillators. A more recent study of this concept, the Substernal Pacing Acute Clinical Evaluation (SPACE) study, was published just last month.⁴ In the SPACE study, a commercially available electrophysiology catheter was placed in the substernal space and pacing parameters were evaluated. The catheter was placed successfully in 26 patients with a mean placement time of 12 minutes. Successful capture was obtained at 69% of patients using at least one vector.

Data from the ASD and SPACE trials suggest that pacing and defibrillation from the substernal space with a permanently implanted device may be achievable. The follow-up study to ASD and SPACE, the ASD2 study, investigated temporary percutaneous placement of a novel substernal lead to deliver both defibrillation shocks and pacing. This study is scheduled to be presented at the Heart Rhythm Society’s 39th Annual Scientific Sessions this month. 

An ICD with a lead placed in the substernal space using a minimally invasive approach offers several potential advantages compared to both the transvenous and subcutaneous ICDs. This is a new space for electrophysiologists. The risks of placing the lead, the consequences of infection in this space, and the ease of extraction are all unknown. If this concept turns out to have significant advantages to patients, the challenge will be to develop delivery tools and leads that will allow this to be done safely and reliably by cardiac electrophysiologists. The substernal space may be the final frontier for cardiac electrophysiologists. 

References

1. Boyle TA, Cohen J, Carrillo R. Substernal implantation of a subcutaneous implantable cardioverter-defibrillator in a patient with preexisting Hemodialysis Reliable Outflow graft. HeartRhythm Case Rep. 2016;2(5):412-414. 
2. Brouwer TF, Smeding L, Berger WR, et al. Assessment of the Extravascular Implantable Defibrillator: feasibility of substernal ventricular pacing. J Cardiovasc Electrophysiol. 2017;28:674-676.
3. Chan J, Lelakowski J, Murgatroyd F, et al. Novel extravascular defibrillation configuration with a coil in the substernal space: the ASD clinical study. JACC Clin Electrophysiol. 2017;3:905-910.
4. Sholevar DP, Tung S, Kuriachan V, et al. Feasibility of extravascular pacing with a novel substernal electrode configuration: The Substernal Pacing Acute Clinical Evaluation study. Heart Rhythm. 2018;15(4):536-542.