Extravascular ICD Overview

The extravascular ICD (EV-ICD) is the newest defibrillator modality. The Medtronic Aurora generator sits in a left mid-axillary pocket, and the Epsila lead is tunneled and positioned in the substernal space — between the sternum and the pericardium — through a subxiphoid puncture.

The design intent: capture the venous-system-sparing advantages of an S-ICD while restoring the ATP and post-shock pacing capabilities that S-ICD lacks.

How the system works

• Generator location. Left mid-axillary line, between serratus anterior and latissimus dorsi (intermuscular pocket). Generator dimensions are larger than a typical transvenous device because of the longer shock vector requirement, but smaller than the EMBLEM S-ICD.
• Lead anatomy. The Epsila EV lead has a defibrillation coil along its substernal segment, plus two pace/sense electrodes that allow ATP delivery and post-shock pacing. The serpentine (“epsilon”) substernal portion creates a longer effective coil length than a straight lead in the same vertical span, improving defibrillation efficacy.
• Shock vector. Energy is delivered between the substernal coil and the can. The substernal vantage point places the coil immediately anterior to the right ventricle and septum, dramatically closer to the heart than the parasternal S-ICD coil.
• Defibrillation energy. The Aurora system delivers up to 40 J; programmed first shock is typically 35 J. This is meaningfully lower than the 80 J S-ICD can but higher than a modern transvenous ICD because of the larger air-tissue interface.
• Sensing. Bipolar sensing between the two pace electrodes on the lead. Programmable vectors and gain settings; morphology-based discrimination is the main SVT/VT differentiator since there’s no atrial channel.
• ATP. Burst pacing delivered from the substernal lead electrodes. Capture is less reliable than transvenous (pacing across non-cardiac tissue) — verify capture margin programmatically and at follow-up. Typical threshold 5–10 mA with a wider safety margin than transvenous.
• Post-shock pacing. Transient bradycardia support delivered after a shock to bridge asystolic pauses. Programmed window typically 30 seconds. Not a chronic pacing solution.

Comparison: EV-ICD vs S-ICD vs Transvenous ICD

Types and variants

Single platform currently on the US market:

• Generator: Medtronic Aurora EV-ICD
• Lead: Epsila EV substernal lead

No dual-chamber variant — atrial activity is not directly sensed. All discrimination is morphology-based.

The Aurora platform is MRI-conditional under specific scanning protocols.

Indications and candidate selection

Strong EV-ICD candidates:

• ICD indication present (primary or secondary prevention).
• Patient wants to avoid transvenous leads for the long-term lead-reliability and extraction concerns.
• Monomorphic VT history where ATP capability is desired — a critical distinction from S-ICD.
• Venous access concerns (dialysis, prior occluded vessels, congenital absence) where transvenous is not ideal.
• Prior CIED pocket infection where avoiding the venous system entirely matters.

Not appropriate:

• Patient needs chronic bradycardia pacing — EV-ICD doesn’t replace a pacemaker.
• CRT candidate — EV-ICD doesn’t deliver CRT.
• Substernal anatomy not amenable — prior cardiac surgery with adhesions, severe pectus excavatum, prior sternotomy.
• Active substernal infection or hardware.
• Patient with established VF history requiring high-energy defibrillation — better served by transvenous in most cases.
• High body habitus or thick chest wall that would compromise sensing or capture margins.

Implant technique

The implant is a hybrid procedure — EP physician with thoracic-surgery backup is the most common pairing.

Anesthesia and prep

• General anesthesia.
• Single-lumen ETT acceptable.
• Patient supine with left arm abducted; right arm at side or abducted per pocket location preference.
• Chest, abdomen, axilla prepped and draped.
• Antibiotics per pocket-infection prophylaxis protocol.
• TEE optional but useful in selected cases.

Pocket creation

• Left mid-axillary line, between serratus anterior and latissimus dorsi.
• Pocket should accommodate the generator at the level of the 5th–6th intercostal space.
• Intermuscular plane preferred over subcutaneous for lower migration risk and better cosmesis.
• Hemostasis carefully — bleeding into this plane is hard to manage post-closure.

Substernal access

• Subxiphoid puncture. A small skin incision just inferior to the xiphoid. Blunt dissection down to the diaphragmatic-substernal junction.
• Substernal tunneling. A dedicated Medtronic tunneling tool is advanced superiorly under fluoroscopy, hugging the posterior table of the sternum, anterior to the pericardium. The tool has a blunt tip and curved trajectory to ride along the sternum.
• Critical avoidance. The pericardium is millimeters posterior. Pleural reflections are lateral. Sternal perforator vessels lie anterior. Slow, deliberate advance with constant tactile and fluoroscopic feedback.
• Endpoint. Tool tip at the level of the sternal notch (T2–T3 vertebral level).
• Lead passage. The Epsila EV lead is back-loaded through the tunnel and the tool withdrawn, leaving the lead in position.

Lead positioning

• Confirm coil position spanning the cardiac silhouette in AP fluoroscopy (coil overlays the heart from base to apex).
• Confirm substernal location in lateral fluoroscopy (coil should be just posterior to the sternum, anterior to the pericardium).
• Confirm electrical performance:
  • R-wave amplitude ≥ 3 mV (ideally ≥ 5 mV)
  • Pace impedance 400–1500 Ω
  • Shock impedance 40–80 Ω expected
  • Pace threshold for ATP capture: typically 5–10 mA at 1.0 ms pulse width

Tunneling the lead to the pocket

• Subcutaneous tunnel from the subxiphoid incision laterally to the left mid-axillary pocket.
• Keep the tunnel superficial to the latissimus to avoid neurovascular structures.
• Connect the lead to the generator; confirm setscrews and seal.

Defibrillation testing (DFT)

• DFT is routine at EV-ICD implant — unlike modern TV-ICD where it has become optional.
• VF is induced (typically with a 50 Hz burst or DC shock).
• Programmed first shock of 35 J should terminate VF with appropriate safety margin (≥10 J under maximum output).
• If first shock fails: repositioning, vector adjustment, or proceeding to maximum output.
• Document successful DFT in the implant note.

Closure

• Pocket closed in layers; deep absorbable sutures plus skin closure of choice.
• Substernal incision closed in layers; pay attention to deep tissue hemostasis.
• Chest X-ray and PA/lateral fluoroscopic confirmation of final lead position before patient leaves the lab.

Programming considerations

• Detection zones:
  • VT zone: typically 170–200 bpm with ATP attempts before shock.
  • VF zone: ≥200–220 bpm with shock-priority therapy.
• Sensing vector: programmable; reassess at each visit. The substernal vantage gives different morphology than transvenous, so templates need updating after position settles.
• SVT discrimination: morphology-based only (no atrial channel). Templates must be stable; recalibrate periodically.
• ATP burst pacing: 8-pulse burst at ~88% of detected VT cycle length is typical; verify ATP capture at chronic checks.
• Post-shock pacing: 30-second window at 50 bpm; not a chronic pacing solution.
• MRI mode: enable per manufacturer protocol when MRI is scheduled.

Generator change and extraction

• Generator longevity: 7–9 years typical, longer with infrequent therapy.
• Generator change is straightforward — pocket reopening, swap, reconnect to the existing lead.
• Lead revision/extraction is uncommonly required but, when needed, requires careful tunneling re-entry. Long-term substernal extraction data are limited; plan conservatively.
• End-of-system management: most patients will need a system replacement at some point; the field has not converged on standard practice for old leads (leave in place vs extract).

What to know in the lab

• Hybrid mentality. Have thoracic surgery aware and in-house, even if not scrubbed; have a chest tube setup and ultrasound bedside.
• Pleural breach. Mid-procedure ultrasound or fluoro looks for new effusion or pneumothorax. Pleural entry without major sequela can sometimes be managed conservatively; pneumothorax may need a chest tube.
• Pericardial entry. The most serious access complication. Recognize early (sudden hemodynamic change, contrast extravasation on fluoro). Have pericardiocentesis ready.
• Sternal vessel injury. Internal mammary or perforating vessels can be injured during tunneling. Bleeding may be brisk and require chest exploration.

Common issues and troubleshooting

• Inappropriate shocks — morphology template drift, skeletal muscle artifact during exercise, T-wave oversensing. Workup: review event EGM, reprogram vector and gain, retemplate morphology.
• ATP non-capture — confirm pacing threshold from substernal position. If marginal, increase output or accept that ATP success rate will be lower; counsel patient on shock probability.
• Persistent chest discomfort — typically resolves over 6–8 weeks. Persistent pain beyond 3 months warrants imaging to rule out pleural effusion, pericardial irritation, or lead migration.
• Sensing oversensing — far-field skeletal muscle, diaphragm. Vector reselection; gain tuning; rarely lead revision.
• Sensing undersensing — sometimes seen with body habitus changes or weight loss. Lead position can shift; check on imaging.
• Sternal wound infection — uncommon; treat aggressively; may require lead removal.
• Generator pocket complications — hematoma, infection, migration; standard ICD management.

Manufacturer reference

• Medtronic Aurora EV-ICD with Epsila EV Lead