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Pacemakers and Intracardiac Devices

Cardiology Residency / DM Level Notes

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1. CLASSIFICATION OF CARDIAC IMPLANTABLE ELECTRICAL DEVICES (CIEDs)

Source: Braunwald's Heart Disease, Chapter 69 (Mina K. Chung & James P. Daubert)

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2. ANATOMY OF A PACING SYSTEM

Pulse Generator (PG)

• Contains battery (lithium iodide; ~5-12 years lifespan) + circuitry
• "End of life" (EOL) signaled by pacing rate or magnet response change (typically rate-responsive or VOO/DOO switch)
• Programmable via telemetry (radiofrequency wand or Bluetooth)

Lead System

• 1-3 leads depending on device type
• Lead body: conducting wires + insulating material (polyurethane or silicone)
• Fixation mechanisms:
  • Active fixation: extendable/retractable screw (helical tip) - can be placed anywhere, used in atrium, septum, His bundle, LV epicardium
  • Passive fixation: tines/fins (rely on trabecular entrapment) - most common in RV apex
• Lead polarity:
  • Unipolar: distal tip (cathode) to generator can (anode) - larger pacing spike, more susceptible to EMI
  • Bipolar: distal tip (cathode) to proximal ring electrode (anode) - smaller spike, less EMI susceptibility

Electrode-Tissue Interface

• Cathodal stimulation used for pacing (distal electrode = cathode)
• Strength-duration curve: relates pulse voltage/current to pulse duration for capture
  • Rheobase: minimum voltage at infinitely long pulse duration (~1.5-2.0 msec)
  • Chronaxie: pulse duration at which threshold = 2x rheobase - this is the most energy-efficient pacing point
  • Energy formula: J = V²t(1/R), so optimizing near chronaxie balances safety and battery longevity
• Pacing threshold increases transiently after implantation (acute injury from active fixation), peaks at ~1-2 weeks, then falls ("maturation")
• Output typically programmed at 2x threshold voltage to maintain adequate safety margin

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3. NASPE/BPEG PACEMAKER CODE (NBD CODE)

• O = None; A = Atrium; V = Ventricle; D = Dual (A+V)
• T = Triggered; I = Inhibited; D = Dual (T+I)
• R = Rate responsive

DDD Timing Cycles (Key)

• AV delay (AVD): interval from atrial event to ventricular pacing (analog of PR interval)
• Lower Rate Interval (LRI): max interval between paced beats = 60,000/LRL (msec)
• Upper Rate Limit (URL): maximum tracking rate to prevent 1:1 atrial tracking at fast sinus rates
• PVARP (Post-Ventricular Atrial Refractory Period): atrial channel blanked after V event - prevents sensing retrograde P waves as pacemaker-mediated tachycardia (PMT)
• TARP = AVD + PVARP: Total Atrial Refractory Period - sets Wenckebach upper rate behavior
• Rate-responsive AV delay: shortens AVD at faster rates to mimic physiology
• Safety pacing: if crosstalk (atrial pulse sensed on ventricular channel), device paces ventricle ~100 ms after atrial pacing to prevent inhibition

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4. SENSING AND CAPTURE PHYSIOLOGY

Capture

• Successful depolarization of myocardium by pacing stimulus
• Loss of capture on ECG: pacing spike not followed by P or QRS
• Common causes: lead dislodgment, exit block (elevated threshold due to fibrosis, electrolyte abnormalities, drugs like flecainide/amiodarone), lead fracture, output programmed too low, battery depletion

Sensing

• Device detects intrinsic cardiac activity via EGM (intracardiac electrogram)
• Oversensing: device "sees" signals it should not (T waves, far-field R waves, myopotentials, EMI) - results in inappropriate inhibition of pacing (dangerous in pacing-dependent patients) or inappropriate ICD shocks
• Undersensing: device fails to detect intrinsic events - results in asynchronous pacing (competitive rhythm risk), loss of rate-responsive behavior
• Sensitivity setting is a voltage threshold: lower number = more sensitive (detects smaller signals)

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5. RATE-RESPONSIVE PACING

• Addresses chronotropic incompetence (heart rate fails to rise appropriately with exertion)
• Sensors used:
  • Accelerometer (piezoelectric): detects body movement/vibration - most common
  • Minute ventilation: detects thoracic impedance changes with breathing
  • QT interval sensor: QT shortens with sympathetic stimulation
  • Dual-sensor devices combine above for more physiologic response
• DDDR or VVIR modes; sensor gain/slope programmable

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6. HEMODYNAMIC ASPECTS OF PACING

• AV synchrony: atrial kick contributes ~20-30% of cardiac output; loss (ventricular pacing without atrial synchrony) reduces CO, raises atrial pressure -> "pacemaker syndrome" (retrograde VA conduction, neck vein pulsations, dizziness, dyspnea)
• Pacing site matters:
  • RV apical pacing: causes dyssynchronous contraction (LBBB pattern) -> adverse remodeling over time, can cause pacing-induced cardiomyopathy
  • RV outflow tract (RVOT) / septal pacing: more physiologic
  • His bundle pacing (HBP): activates the native conduction system -> narrow QRS, most physiologic; technically challenging, higher thresholds
  • Left bundle branch area pacing (LBBAP): emerging technique, more reliable capture than HBP, captures LBB or LBB fascicles deep in septum

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7. INDICATIONS FOR PERMANENT PACEMAKERS

Class I (Symptomatic Bradycardia / High-Degree Block)

• Symptomatic sinus bradycardia (including drug-induced when no alternative exists)
• Symptomatic chronotropic incompetence
• Symptomatic sinus pauses
• Tachycardia-bradycardia syndrome with symptomatic bradycardia

• Third-degree (complete) AV block - regardless of symptoms if rate <40 bpm or escape rhythm unreliable
• Second-degree Mobitz II AV block - block below His, high risk of progression to complete block
• Advanced second-degree block (2:1 or higher) with symptoms
• Symptomatic second-degree Mobitz I (Wenckebach) - less common, usually AV nodal
• Complete AV block after acute MI persisting >5-7 days

• Alternating LBBB and RBBB
• RBBB + LAFB or LPFB with syncope when EP study shows HV ≥100 ms or infra-His block

• Permanent AV block after AV node ablation

• Symptomatic cardioinhibitory carotid sinus hypersensitivity (asystole >3 sec with symptoms)

• Recurrent reflex syncope with documented pauses (DDI or DDD with hysteresis)

Guidelines: 2018 ACC/AHA/HRS Guideline on Bradycardia and Cardiac Conduction Delay (Kusumoto FM et al., Circulation 2019)

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8. PACEMAKER TROUBLESHOOTING

Failure to Pace (No Output)

• Lead fracture, connector pin problem, oversensing (inhibition), battery depletion, "safety switch" to ERI mode

Loss of Capture (Output but No Response)

• Lead dislodgment, elevated threshold (drug effect: flecainide, amiodarone; metabolic: hyperkalemia, hypothyroidism; fibrosis), output too low, insulation break

Oversensing

• Causes: T-wave oversensing, far-field R-wave sensing (atrial channel sensing ventricular events), myopotentials (pectoral muscle in unipolar), EMI
• Consequence: inappropriate pacing inhibition (if pacer dependent = syncope); inappropriate ICD shock
• Fix: decrease sensitivity (raise threshold), switch to bipolar, reprogram refractory periods

Undersensing

• Causes: inadequate EGM amplitude (scar, dislodgment), inappropriate sensitivity setting, intrinsic signal timing within blanking period
• Consequence: competitive pacing, loss of inhibition, potential R-on-T phenomenon

Pacemaker-Mediated Tachycardia (PMT)

• Mechanism: retrograde VA conduction after ventricular pacing -> retrograde P wave sensed outside PVARP -> triggers another V pace -> loop
• Rate: upper tracking rate
• Termination: magnet (asynchronous pacing), lengthen PVARP, enable PMT termination algorithm (most modern devices)

Pacemaker Syndrome

• Retrograde VA conduction with VVI pacing -> no AV synchrony -> low CO, cannon A waves, dyspnea, dizziness
• Treatment: upgrade to dual-chamber system

Runaway Pacemaker

• Rare; device malfunction causing very rapid pacing (>200 bpm); battery end-of-life or component failure
• Emergency: magnet application, device re-programming or emergent explant

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9. IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS (ICDs)

Device Types

1. Transvenous ICD (TV-ICD): lead in RV (sensing/pacing + shocking coil), ± SVC coil; generator in left pectoral pocket
   • Single-chamber (VR): RV only
   • Dual-chamber (DR): RA + RV leads - better discrimination of SVT vs VT
   • CRT-D: RA + RV + LV (CS) leads
2. Subcutaneous ICD (S-ICD): no intravascular lead; generator at left axilla; subcutaneous coil + sensing electrode along sternum
   • Advantages: avoids transvenous lead complications; for patients with no pacing/CRT need
   • Disadvantages: no antibradycardia pacing, no ATP, larger device, inappropriate shocks from T-wave oversensing
3. Extravascular ICD (EV-ICD, e.g., EMPOWER): lead tunneled substernally (no intravascular), generator left pectoral; can provide ATP
4. Wearable cardioverter-defibrillator (WCD): external vest; temporary protection while deciding on ICD or bridge during waiting period (e.g., newly diagnosed LV dysfunction)

Shock Waveform

• Biphasic waveform: 500-1400 V; reverses polarity mid-shock - reduces defibrillation energy requirement by ~30% vs monophasic
• Defibrillation threshold (DFT): tested at implant (or not, as per contemporary practice); safety margin typically ≥10 J below maximum device output

ICD Therapies

1. Antitachycardia Pacing (ATP): sequence of rapid pacing pulses (burst or ramp) to terminate VT without a shock - effective in up to 90% of VTs <250 bpm; painless; first-line therapy for VT
2. Low-energy cardioversion: synchronized shock at lower energy; for organized VT
3. High-energy shock: defibrillation; for VF or failed cardioversion
4. Antibradycardia pacing: backup pacing (post-shock asystole, drug-induced bradycardia)

Detection Zones

• VF zone (typically >200-220 bpm): shock therapy
• VT zone (typically 150-200 bpm): ATP first, then shock if ATP fails
• VT monitor zone (optional): detection + recording only, no therapy
• Detection enhancements (SVT-VT discrimination):
  • Onset: sudden onset favors VT; gradual onset favors SVT
  • Stability: irregular RR intervals favor AF
  • Morphology: EGM morphology compared to template
  • AV relationship (dual-chamber): VA dissociation confirms VT

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10. INDICATIONS FOR ICD IMPLANTATION

Secondary Prevention (post VF/VT arrest - strongest evidence)

• Survivors of cardiac arrest from VF or hemodynamically unstable VT not due to reversible causes
• Sustained VT with structural heart disease
• Meta-analysis: ICD vs amiodarone -> 50% reduction in arrhythmic death, 25% reduction in all-cause mortality

Primary Prevention

• MADIT-I / MADIT-II / SCD-HeFT trials established benefit
• SCD-HeFT: EF ≤35%, NYHA II-III, optimal medical therapy -> amiodarone vs ICD vs placebo; ICD reduced all-cause mortality 23% vs placebo
• MADIT-II: post-MI, EF ≤30% -> ICD reduced mortality 31%
• Guideline: ICD for LVEF ≤35%, NYHA II-III, ischemic cardiomyopathy, on optimal medical therapy ≥3 months, expected survival >1 year with good functional status

• DEFINITE, SCD-HeFT: benefit also shown in NICM with EF ≤35%, NYHA II-III
• DANISH trial (2016): ICD vs control in NICM - no significant reduction in all-cause mortality (HR 0.87, p=0.28), though significant reduction in SCD; contemporary HF therapy may have diluted benefit
• Still indicated by guidelines for NICM, EF ≤35%, NYHA II-III after ≥3 months optimal therapy

• HCM: ICD for aborted cardiac arrest, spontaneous sustained VT, ≥2 major SCD risk factors (FHx SCD, NSVT, syncope, LV wall thickness ≥30 mm, abnormal BP response to exercise)
• Arrhythmogenic right ventricular cardiomyopathy (ARVC): ICD for aborted arrest, sustained VT; consider for high-risk features
• Brugada syndrome: ICD for aborted VF/VT; quinidine as adjunct; high-risk features (spontaneous type 1 pattern + syncope)
• Long QT syndrome: ICD if aborted arrest or VT on beta-blockers; beta-blockers first line
• Catecholaminergic polymorphic VT (CPVT): nadolol/flecainide first; ICD if VT/VF on therapy
• Sarcoidosis with cardiac involvement: ICD often indicated with EF <35% or significant conduction disease
• Chagas disease: ICD for VT/VF, high-risk features

• EF ≤35% after 3 months optimal medical therapy (regardless of etiology in most guidelines)
• Note: Waiting period is critical - LVEF must be reassessed after 3-6 months of GDMT; early ICD implantation post-MI (within 40 days) has NOT shown benefit (DINAMIT trial)

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11. CARDIAC RESYNCHRONIZATION THERAPY (CRT)

Mechanism

• Patients with LBBB (or wide QRS) have dyssynchronous LV contraction (late lateral wall activation) -> reduced LV efficiency
• CRT paces RV + LV simultaneously (biventricular pacing) to restore synchrony -> improved stroke volume, reduced MR, reverse remodeling over time

Indications (Class I)

• LVEF ≤35% + LBBB morphology + QRS ≥150 ms + NYHA II-IV + on GDMT ≥3 months
• NYHA II on GDMT: CRT-D (ICD + CRT) - MADIT-CRT, RAFT trials
• NYHA III-IV: CRT-P or CRT-D - CARE-HF, COMPANION trials

Key Trials

Key teaching point: CRT is harmful in narrow QRS (<130 ms). QRS morphology matters: LBBB gets greatest benefit; RBBB/non-specific IVCD benefit is uncertain.

LV Lead Placement

• LV lead placed via coronary sinus into a lateral or posterolateral branch
• Problem in ~5-10%: cannot reach ideal vein; alternatives include surgical epicardial lead, LV endocardial pacing
• Adequate biventricular pacing: must achieve >98% BiV pacing for optimal benefit

AV Node Ablation + CRT

• When AF + rapid ventricular rate cannot be controlled medically and CRT device is implanted, AV node ablation ensures 100% biventricular pacing
• Rate control alone + CRT: underperforms ablation + CRT in patients needing full resynchronization

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12. LEADLESS AND NOVEL DEVICES

Leadless Pacemaker (Micra, Aveir)

• Capsule implanted directly into RV via femoral venous approach; no subcutaneous pocket, no transvenous lead
• Single-chamber: Micra VR (VVIR) - for AF + bradycardia
• Dual-chamber: Micra AV (VDDR) - uses mechanical sensing of atrial contraction to trigger ventricular pacing; Aveir DR - two separate communicating capsules
• Complications: cardiac perforation/tamponade (~1.5%), device embolization, vascular access issues; retrieval is possible but challenging
• Advantages: no pocket (no pocket hematoma/infection), no lead (no lead failure)

Subcutaneous ICD (S-ICD) - Boston Scientific EMBLEM

• No intravascular component; entirely subcutaneous
• Sensing via 3 sensing vectors from subcutaneous electrode
• Pre-implant screening ECG mandatory (SENSE screening) to ensure adequate T:R ratio to avoid T-wave oversensing
• Cannot deliver ATP or chronic pacing - not suitable for pacing-dependent patients, VT requiring ATP, or patients needing CRT
• Particularly preferred in: young patients (preserve vasculature), prior TV-ICD lead infections, no pacing need

Extravascular ICD (EV-ICD)

• Lead tunneled under sternum; generator left pectorally
• Combines benefits of S-ICD (no intravascular) with ability to deliver ATP
• Emerging technology (EFFORTLESS EV-ICD registry)

Conduction System Pacing (CSP)

• His bundle pacing (HBP): electrode placed at His bundle; corrects LBBB by capturing native conduction; narrow QRS
  • Challenges: higher threshold, risk of lead dislodgment, sensing issues
• Left bundle branch area pacing (LBBAP) / Left bundle branch pacing (LBBP): electrode screwed deep into interventricular septum to capture LBB directly
  • More stable, lower thresholds than HBP, corrects LBBB
  • Increasingly used as alternative to biventricular CRT (CSP-CRT or His-optimized CRT)
  • LBBP-RESYNC trial: ongoing evaluation vs BVP-CRT

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13. COMPLICATIONS OF CIEDs

Procedural

Long-term

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14. ELECTROMAGNETIC INTERFERENCE (EMI)

• Over a pacemaker: applies asynchronous (VOO/DOO) pacing at magnet rate (~85 bpm typically; manufacturer-specific) - prevents oversensing inhibition
• Over an ICD: suspends tachyarrhythmia detection (no shock); does NOT change pacing mode

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15. ICD TROUBLESHOOTING

Appropriate Shocks

• Delivery of shock for true VT/VF - desired
• If recurrent: optimize anti-arrhythmic drugs (amiodarone, sotalol, mexiletine), catheter ablation for scar-related VT, reassess reversible causes

Inappropriate Shocks (IS)

• Definition: ICD shock delivered for non-VT/VF rhythm
• Most common cause: AF with rapid ventricular rate (most frequent overall), T-wave oversensing, SVT, lead fracture/noise
• T-wave oversensing: more common with S-ICD; manage by adjusting sensing vectors, programming sensitivity
• Lead noise (fracture): large-amplitude, non-physiologic signals; seen as very short RR intervals on EGM; reprogramming or lead replacement
• Consequence of IS: significant psychological distress, possible proarrhythmia (shock on vulnerable period), increased mortality in some analyses
• Management of IS: identify and eliminate cause; antiarrhythmics for AF; shock reduction algorithms; S-ICD screening

VT Storm

• 3 separate episodes of VT requiring ICD intervention within 24 hours

• Management:
  1. Correct reversible causes (ischemia, electrolytes, drugs)
  2. Amiodarone IV (150 mg bolus, then infusion)
  3. Beta-blockers IV (metoprolol, propranolol)
  4. Deep sedation (reduces sympathetic tone)
  5. Catheter ablation (urgent if storm persists)
  6. Temporary overdrive pacing if bradycardia-dependent VT
  7. Mechanical circulatory support (IABP, Impella) for cardiogenic shock

Failure to Shock / Detect VF

• VF within detection programming (confirm detection turned ON)
• Oversensing causing double-counting (VF detection criteria reached falsely); undersensing of VF
• Elevated DFT (drug effect: amiodarone increases DFT; hypothyroidism)
• Lead fracture, connector issue

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16. REMOTE MONITORING

• All modern CIEDs transmit data wirelessly to a base station (home monitor) -> manufacturer server -> clinician portal
• Allows detection of: arrhythmias, lead problems (impedance change), battery status, fluid status (intrathoracic impedance as proxy for pulmonary congestion)
• IN-TIME, CONNECT, TRUST trials: remote monitoring reduces mortality and hospitalization
• Standard of care: remote monitoring + in-office device checks every 6-12 months

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17. SPECIAL SITUATIONS

CIED in Pregnancy

• Pacing is safe; lead implantation during pregnancy if urgently needed (use echocardiographic guidance to reduce radiation)
• ICDs can deliver shocks safely to fetus without harm at standard energies

Perioperative CIED Management

• If pacing-dependent: reprogram to asynchronous mode pre-op; have external pacing/defibrillation available
• ICD detection: suspend via magnet or reprogramming; ensure magnet available throughout case
• Postoperatively: restore original settings; check device interrogation

End of Life / CIED Deactivation

• Ethically permissible to deactivate ICD at patient/family request (avoiding unwanted shocks at end of life)
• Pacemaker deactivation more complex (patient may be pacemaker-dependent)
• Requires advance care planning; palliative care team involvement

CIED Extraction (Transvenous Lead Removal - TLE)

• Indications: infection (Class I), lead malfunction affecting clinical management, lead recalled for safety
• Techniques: simple traction, locking stylets, laser sheaths (Spectranetics), mechanical dilator sheaths, femoral workstation
• Risks: vascular/cardiac tear (~1% major complications at high-volume centers), hemothorax, tamponade; requires cardiac surgical backup

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18. KEY EXAM/CLINICAL PEARLS

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Sources

• Braunwald's Heart Disease, Chapter 69 - Mina K. Chung & James P. Daubert (CIEDs)
• Braunwald's Heart Disease, Chapter 68 (Bradyarrhythmias and Conduction Disorders)
• Fuster and Hurst's The Heart, 15th Edition, Chapter 37 (Ventricular Arrhythmias/SCD) and Chapter 36 (AF)
• Referenced guidelines: ACC/AHA/HRS 2018 Bradycardia Guideline; AHA/ACC/HRS 2017 VT/SCD Guideline; ESC 2021 Pacing GuidelinesThese are complete, residency/DM-level notes built from base to top. Here's a quick orientation to the structure:

1. Device classification (what each CIED is)
2. Hardware anatomy (PG, leads, electrodes)
3. NBD pacing code (understand any programmed mode)
4. Sensing & capture physics (strength-duration curve, chronaxie, threshold maturation)
5. Rate-responsive pacing (sensors, chronotropic incompetence)
6. Hemodynamics of pacing (AV synchrony, pacing site effects, His/LBBAP)
7. Pacemaker indications (SND, AV block, bifascicular, hypersensitive carotid - Class I/II)
8. Pacemaker troubleshooting (failure to pace, loss of capture, over/undersensing, PMT, runaway pacemaker)
9. ICD types (TV-ICD, S-ICD, EV-ICD, WCD) + shock physics
10. ICD therapy zones (ATP, cardioversion, defibrillation, detection zones)
11. ICD indications (secondary prevention meta-analysis; primary prevention trials: MADIT, SCD-HeFT, DANISH, with key LVEF/QRS thresholds)
12. CRT (mechanism, indications, key trials with data: CARE-HF, COMPANION, MADIT-CRT, RAFT, ECHO-CRT)
13. Leadless, S-ICD, CSP/HBP/LBBP (emerging tech)
14. Complications (procedural + long-term including infection management)
15. EMI (MRI, cautery, magnet behavior)
16. ICD troubleshooting (inappropriate shocks, VT storm algorithm)
17. Remote monitoring, special situations (pregnancy, perioperative, end-of-life, TLE)
18. High-yield exam pearls (table format)