14.1 Subcutaneous ICD (S-ICD), Leadless Pacemakers & Left Atrial Appendage Closure

14.1 Subcutaneous ICD (S-ICD), Leadless Pacemakers & Left Atrial Appendage Closure

The evolution of cardiac electrophysiology has introduced innovative transcatheter and extravascular technologies engineered to mitigate the primary failure modes of conventional transvenous systems—namely vascular occlusion, tricuspid valve disruption, lead fracture, and systemic endovascular infections. For the Registered Cardiac Electrophysiology Specialist (RCES), mastering the biophysics, anatomic landmarks, deployment protocols, and clinical safety thresholds of Subcutaneous Implantable Cardioverter-Defibrillators (S-ICDs), Leadless Pacemakers, and Left Atrial Appendage Closure (LAAC) devices is essential for both procedural success and board certification.

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Subcutaneous Implantable Cardioverter-Defibrillator (S-ICD)

The subcutaneous ICD (Boston Scientific EMBLEM S-ICD system) provides complete extravascular defibrillation. Because no hardware enters the heart or vascular tree, the system eliminates vascular injury, venous thrombosis, hemothorax, and lead-related endocarditis.

                    [Distal Sensing Ring]
                              |
                              |   Defibrillation Coil (8 cm)
                              |
                    [Proximal Sensing Ring]
                              |
                              |   Subcutaneous Parasternal Lead
                              |
    [S-ICD Can] --------------+
    (5th-6th Intercostal Space,
     Mid-to-Posterior Axillary Line)

Clinical Indications & Contraindications

• Primary Indications:
  • Patients at exceptionally high risk for bacteremia or systemic device infection: end-stage renal disease (ESRD) on chronic hemodialysis, immunosuppressed post-transplant patients, and individuals with prior transvenous CIED systemic infections.
  • Young, active patients (<40 years of age) with inherited channelopathies (e.g., Long QT syndrome, Brugada syndrome, catecholaminergic polymorphic VT) or hypertrophic cardiomyopathy who face decades of cumulative transvenous lead stress, insulation degradation, and lead fracture.
  • Compromised or occluded central venous anatomy: bilateral subclavian vein occlusions, superior vena cava (SVC) syndrome, or indwelling central venous catheters/fistulas.
  • Congenital heart defects lacking transvenous access to the right ventricle (e.g., single ventricle physiology, tricuspid atresia, mechanical tricuspid valves).
• Strict Contraindications:
  • Documented indication for chronic anti-bradycardia pacing (sinus node dysfunction, high-grade AV block).
  • Need for cardiac resynchronization therapy (CRT).
  • Recurrent, hemodynamically tolerated monomorphic ventricular tachycardia (VT) that could otherwise be terminated painlessly with anti-tachycardia pacing (ATP). The S-ICD cannot deliver transvenous ATP.
  • Failure to pass pre-implantation surface ECG screening in at least one sensing vector across all postural evaluations.

Pacing Limitation Alert: The S-ICD cannot provide ongoing bradycardia pacing. Its only pacing capability is emergency transthoracic post-shock VOO pacing at 50 bpm for up to 30 seconds following a shock, delivered between the distal coil and the pulse generator can.

Pre-Implantation Surface ECG Vector Screening

Unlike transvenous ICD leads that record localized, high-amplitude near-field endocardial electrograms (where R-waves exceed 5–20 mV and T-waves are minimal), the S-ICD records far-field subcutaneous electrograms spanning the entire thorax. Subcutaneous signals are lower in amplitude (0.5–2.0 mV), making the device vulnerable to T-wave oversensing (TWOS). If a large T-wave is recognized as a separate cardiac depolarization, the device double-counts the heart rate (e.g., an intrinsic rate of 85 bpm is detected as 170 bpm, or 130 bpm is counted as 260 bpm), triggering inappropriate high-voltage shocks.

Pre-implant screening evaluates three sensing vectors using either an automated ECG screening tool (AST) or a manual color-coded template:

1. Primary Vector: Senses from the proximal sensing ring to the distal sensing ring along the left parasternal margin (parallel to the sternum).
2. Secondary Vector: Senses from the distal sensing ring to the pulse generator can (spanning the left anterolateral thorax across the cardiac mass).
3. Alternate Vector: Senses from the proximal sensing ring to the pulse generator can (spanning the left inframammary region).

Screening Vector Topography:
Primary:   [Proximal Ring] <------------------------> [Distal Ring]
Secondary: [Distal Ring]   <------------------------> [S-ICD Can]
Alternate: [Proximal Ring] <------------------------> [S-ICD Can]

Mandatory Postural Screening Protocol

Screening must be performed in both the supine and standing (or upright sitting) positions. In addition, exercise or treadmill testing may be performed in patients with channelopathies or exercise-induced bundle branch blocks. Postural shifts alter diaphragmatic height, heart orientation, and chest wall geometry, which significantly alters subcutaneous QRS and T-wave amplitudes.

• Passing Criteria: To qualify for S-ICD implantation, at least one sensing vector must satisfy the screening template criteria in all evaluated postures. Having two or three passing vectors is clinically preferred, as it provides programmed sensing flexibility should post-implant morphological changes occur.

Implantation Technique & Defibrillation Testing (DFT)

• Surgical Landmarks & Pocket Creation: Modern implantation utilizes a 2-incision technique. The pulse generator is positioned in the left axillary space along the 5th–6th intercostal space at the mid-to-posterior axillary line. The preferred location is an intermuscular pocket created between the serratus anterior and latissimus dorsi muscles. Compared to a superficial subcutaneous pocket, intermuscular placement lowers defibrillation thresholds, minimizes erosion risk, and provides superior cosmetic concealment.
• Parasternal Tunneling: A second incision is made at the xiphoid region. The lead is tunneled subcutaneously from the lateral pocket to the xiphoid, and then vertically superior along the left parasternal edge (1–2 cm left of the sternal midline) up to the level of the manubrium. A suture sleeve secures the distal assembly to the deep fascia.
• Defibrillation Threshold (DFT) Testing: DFT testing under deep sedation or general anesthesia is considered mandatory during S-ICD implantation. The EMBLEM S-ICD delivers an 80-Joule biphasic shock at maximum output. A 10-Joule safety margin is required: the device must reliably detect and terminate induced ventricular fibrillation (VF, typically induced with a 50-Hz burst or alternating current) with a shock of ≤65–70 Joules (standardly tested at 65 J). If DFT testing fails at 65 J, interventions include reversing the shock polarity (can-to-coil vs. coil-to-can), moving the can more posteriorly within the intermuscular space, or re-tunneling the lead closer to the sternum.

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Leadless Pacemaker Systems

Leadless pacemakers represent self-contained capsules housing the battery, electronics, and pacing/sensing electrodes within a single miniaturized biocompatible shell implanted directly into the cardiac myocardium via femoral venous catheter delivery.

System Architectures: Micra vs. Aveir

• Medtronic Micra (Micra VR, Micra AV, Micra AV2):
  • Volume ~0.8 cc; length 25.9 mm.
  • Micra VR: Single-chamber VVI/VVIR pacing with rate-response driven by a 3-axis accelerometer.
  • Micra AV / AV2: Accelerometer-based mechanical atrial sensing delivering VDD pacing. The internal 3-axis accelerometer tracks intracardiac mechanical signals: A1 (mitral/tricuspid valve closure, start of systole), A2 (aortic/pulmonic valve closure, end of systole), A3 (passive ventricular filling / early diastole), and A4 (atrial contraction / kick). The device algorithms filter out A1–A3 and isolate the mechanical A4 signal to trigger synchronized ventricular pacing, achieving atrioventricular synchrony without an atrial lead.
  • Fixation: Four flexible, self-expanding nitinol tines at the distal tip.
• Abbott Aveir (Aveir VR, Aveir DR):
  • Aveir VR: Single-chamber ventricular leadless pacemaker.
  • Aveir DR: Dual-chamber leadless pacing system featuring two distinct leadless capsules: an atrial unit (Aveir AR) implanted in the right atrial appendage or low RA wall, and a ventricular unit (Aveir VR) implanted in the RV septum.
  • Communication: The units communicate beat-to-beat using Conductive Intracardiac Communication (i2i). Subthreshold, high-frequency electrical pulses travel through the intracardiac blood pool and myocardium, synchronizing atrial sensing/pacing with ventricular sensing/pacing without consuming excessive battery power or requiring radiofrequency telemetry.
  • Fixation: Active-fixation distal screw-in helix rotated into the myocardium under fluoroscopic guidance.

Leadless Pacemaker Architectures:

[Medtronic Micra AV]           [Abbott Aveir DR]
  Nitinol Tine Fixation          Active Screw-In Helix Fixation
  Accelerometer Sensing (A1-A4)  Conductive Intracardiac Communication (i2i)
  VDD Single Capsule             True Dual-Chamber (Atrial + Ventricular Capsules)

Implantation Workflow, Target Anatomy & Safety Rules

1. Venous Access & Sheath Introduction: Ultrasound-guided cannulation of the right common femoral vein (strictly above the femoral bifurcation and below the inguinal ligament) prevents retroperitoneal bleeding or arteriovenous fistula formation. Pre-closure is established using two percutaneous suture-mediated devices (e.g., Perclose ProGlide) or a subcutaneous figure-of-eight purse-string suture. A large-bore steerable delivery introducer sheath (23–27 Fr inner diameter, 27–29 Fr outer diameter) is advanced over a stiff 0.035-inch guidewire through the inferior vena cava into the right atrium.
2. Tricuspid Crossing & Anatomical Targeting: The steerable delivery catheter is deflected across the tricuspid valve into the right ventricle under right anterior oblique (RAO) and left anterior oblique (LAO) fluoroscopy.
   • Target Site: The mid-to-low interventricular septum or the right ventricular outflow tract (RVOT) septum.
   • STRICT PROHIBITION: Delivery to the right ventricular free wall or RV apex is strictly contraindicated. The RV free wall is exceptionally thin (1–3 mm), and advancing large-bore delivery systems or torquing fixation mechanisms against it carries a high risk of catastrophic cardiac perforation, hemopericardium, and rapid tamponade.
3. Fixation Testing (The Tug Test / Pull Test):
   • For the Micra, the device is pressed against the septum, the protective sleeve is retracted, and the four nitinol tines deploy into the myocardial trabeculae. Before releasing the safety tether, a tug test is performed: under LAO fluoroscopy, controlled mechanical traction is applied to the tether cord. The operator must verify visible inward tenting of the myocardial wall while confirming that at least 2 out of the 4 nitinol tines remain securely engaged in tissue.
   • For the Aveir, the device is rotated clockwise under continuous fluoroscopic observation (tracking marker rotation counts and helical turn depth) until the active helix is completely seated in myocardial tissue. Controlled backward tension confirms mechanical anchoring.
4. Electrical Parameter Verification:
   • Pacing Capture Threshold: Typically $\le 1.0\text{ V}$ at a 0.24–0.4 ms pulse width.
   • R-Wave Sensing Amplitude: $\ge 5.0–6.0\text{ mV}$.
   • Pacing Impedance: 500 to 1,200 $\Omega$.
   • Current of Injury (COI): Significant ST-segment elevation on the unipolar electrogram recorded from the device tip confirms intimate myocardial contact and membrane disruption, predicting acute and chronic electrical stability.
5. Release and Hemostasis: Once mechanical and electrical parameters are confirmed, the tether cord is cut and flushed or unscrewed, releasing the device. The large-bore femoral sheath is removed, and vascular closure sutures are deployed and tied.

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Left Atrial Appendage Closure (LAAC)

In patients with non-valvular atrial fibrillation (AFib), more than 90% of cardioembolic thrombi originate within the left atrial appendage (LAA). The LAA is a low-flow, trabeculated embryological remnant that promotes blood stasis during disorganized atrial fibrillatory contractions. Left Atrial Appendage Closure (LAAC) provides mechanical exclusion of the appendage from the systemic circulation, preventing thrombus formation and embolization.

                 Left Atrial Cavity
                         |
                         v
        +----------------------------------+
        |           LAA Ostium             |
        |  [Sealing Disc / Fabric Cover]   |
        +----------------------------------+
        |      [Anchor Hooks / Tines]      |
        |        Nitinol Mesh Lobe         |
        +----------------------------------+
                    Apex of LAA

Clinical Indications (HRS / ACC / AHA Guidelines)

• Patients with non-valvular atrial fibrillation who have an elevated thromboembolic stroke risk: $\text{CHA}_2\text{DS}_2\text{-VASc}$ score $\ge 2$ in men, or $\ge 3$ in women.
• AND who possess a formal clinical rationale for non-pharmacological stroke prevention due to high bleeding risk or contraindications to long-term oral anticoagulation:
  • Elevated bleeding risk score: HAS-BLED score $\ge 3$.
  • History of major, life-threatening gastrointestinal, retroperitoneal, or intracranial bleeding.
  • Severe recurrent epistaxis, diffuse vascular malformations (e.g., angiodysplasia), or bleeding diatheses.
  • Severe compliance barriers, documented frailty, extreme fall risk with traumatic head injuries, or lifestyle/occupational risks (e.g., commercial pilot, high-impact trades) precluding lifelong anticoagulation.

Procedural Imaging & Transseptal Puncture Geometry

LAAC is performed under general anesthesia with continuous Transesophageal Echocardiography (TEE) or deep sedation with Intracardiac Echocardiography (ICE).

• Critical TEE Imaging Angles: The LAA is a complex 3-dimensional structure that requires multiplane evaluation at four standardized angles: 0°, 45°, 90°, and 135°.
• Transseptal Puncture Trajectory: Transseptal puncture must be performed in the inferior and posterior region of the fossa ovalis under bicaval and short-axis TEE or ICE guidance.
  • Biophysical Rationale: Targeting the inferior-posterior fossa provides a straight, coaxial trajectory directed toward the long axis of the LAA. An anterior or superior transseptal puncture angles the access sheath toward the Coumadin ridge (ridge between the LAA and left superior pulmonary vein) or roof, resulting in sharp sheath buckling, non-coaxial device orientation, incomplete sealing, and increased perforation risk.

Device Sizing & The PASS Criteria (Watchman FLX)

• Sizing: Baseline angiograms (RAO 20–30°, Cranial 20–30°) and multiplane TEE measurements at 0°, 45°, 90°, and 135° quantify the maximum ostium diameter (landing zone) and available depth. Sizing guidelines mandate selecting a device that is 10% to 20% oversized relative to the largest measured landing zone diameter to guarantee radial anchoring force.
• The PASS Release Criteria: Before releasing the Watchman FLX device from its delivery cable, all four components of the PASS criteria must be strictly fulfilled and documented across all four TEE angles (0°, 45°, 90°, 135°):

1. P - Position: The device must be seated at the true anatomical ostium of the LAA. The threaded insert of the device should align with or sit just distal to the limbus of the left superior pulmonary vein (Coumadin ridge) superiorly and the circumflex coronary artery inferiorly.
2. A - Anchor: Mechanical stability is confirmed via the "tug test". Under live fluoroscopy and echocardiography, the delivery cable is pulled back with firm, steady manual traction. The operator must observe the cardiac silhouette and LAA wall move with the device without device displacement or shift.
3. S - Size (Compression): The device must achieve between 10% and 30% radial compression of its nominal unconstrained diameter (measured at all four angles). For example, a 27-mm device must measure between 18.9 mm and 24.3 mm in situ. Compression <10% indicates inadequate radial force and risks device embolization; compression >30% risks structural distortion, pericardial effusion, or tissue erosion.
4. S - Seal: Complete mechanical exclusion of the appendage. Color Doppler echocardiography must confirm no peri-device leak $>3–5\text{ mm}$ around the perimeter. Leaks $\le 3\text{ mm}$ are considered acceptable and typically resolve as endothelialization occurs.

Post-Implant Antithrombotic Regimens

Endothelialization of the LAAC polyester or PTFE fabric covering requires 45 days to 6 months:

• Standard Watchman FLX Protocol:
  • Days 0–45: Oral anticoagulation (Direct Oral Anticoagulant [DOAC] or Warfarin with target INR 2.0–3.0) PLUS Aspirin (81 mg daily).
  • Day 45 Follow-Up TEE: Evaluates for device-related thrombus (DRT) and residual peri-device leak. If the TEE confirms adequate seal (no leak $>5\text{ mm}$) and absence of thrombus, oral anticoagulation is discontinued.
  • Months 1.5–6: Dual Antiplatelet Therapy (DAPT) with Clopidogrel (75 mg daily) PLUS Aspirin (81 mg daily).
  • Month 6 and Lifelong: Aspirin monotherapy (81 mg daily) indefinitely.
• Abbott Amplatzer Amulet Protocol: Dual-seal design (distal anchoring lobe and proximal sealing disc). Standard post-procedure regimen often employs DAPT (Aspirin + Clopidogrel) from day of implant through 6 months, avoiding systemic oral anticoagulation entirely.

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Insertable Cardiac Monitors (ILR)

CCI task D5 names the ILR (insertable/implantable loop recorder) alongside the PPM, ICD, S-ICD, and CRT, so it is a listed implant the specialist is expected to support.

An ILR is a leadless, subcutaneous, diagnostic-only monitor — it records but never paces, senses for therapy, or shocks. Contemporary devices are injectable, roughly the size of a paperclip, with a battery life of about 3 to 4.5 years.

Pre-implant R-wave mapping with the manufacturer's tool identifies the position giving the largest, cleanest signal, because ILR false positives are dominated by undersensing (missed R waves counted as pauses) and oversensing (myopotentials or T waves counted as tachycardia). As with any device-generated report, the counter is a hypothesis and the stored electrogram is the evidence — reviewing the electrogram before reporting a detected "pause" or "AF episode" is a core specialist responsibility.

An ILR is explanted once the diagnostic question is answered or the battery is depleted, through a small incision under local anesthesia.