Cardiac Pacemakers — Indications and Types

• Sinus Node Dysfunction (SND): The sinoatrial (SA) node normally generates spontaneous action potentials at 60-100 bpm through automaticity mediated by If (funny current) and L-type calcium channels. SND results from either inadequate impulse formation (sinus bradycardia, sinus pauses, sinus arrest) or inadequate impulse conduction from the SA node (sinoatrial exit block). Intrinsic causes include fibrosis, amyloidosis, and inflammatory infiltration, while extrinsic causes involve autonomic dysfunction or medication effects. The result is symptomatic bradycardia insufficient to maintain adequate cardiac output for metabolic demands.

• Atrioventricular (AV) Block: Normal AV nodal conduction allows coordinated atrial-to-ventricular transmission with appropriate delay for optimal ventricular filling. AV block occurs at three levels: (1) First-degree AV block (prolonged PR interval, all impulses conduct), (2) Second-degree AV block (intermittent conduction failure—Mobitz I involves progressive PR prolongation within the AV node, while Mobitz II involves sudden conduction failure distal to the AV node), and (3) Third-degree (complete) AV block (complete dissociation between atrial and ventricular rhythms). High-degree AV block (≥2 consecutive P waves fail to conduct) necessitates pacing because escape rhythms are unreliable, slow, and unstable.

• Cardiac Resynchronization Therapy (CRT) Mechanism: In heart failure with left bundle branch block (LBBB), ventricular dyssynchrony results from delayed left ventricular (LV) activation. This creates mechanical inefficiency with delayed LV contraction relative to right ventricular (RV) contraction, increasing wall stress and reducing ejection fraction. Biventricular pacing (pacing both RV and LV simultaneously via coronary sinus lead placement) restores synchronous ventricular contraction, improving dP/dt, reducing mitral regurgitation, decreasing LV end-diastolic volume, and ultimately improving ejection fraction by 10-15% in responders.

• Rate-Responsive Pacing: Fixed-rate pacing fails to meet physiologic demands during activity. Modern pacemakers incorporate sensors (accelerometer, minute ventilation, or impedance-based) that detect activity or metabolic demand and increase pacing rate accordingly, maintaining adequate cardiac output across varying physiologic states.

Indications for Permanent Pacemaker Implantation

Sinus Node Dysfunction

• Intrinsic SND: fibrosis (age-related most common), amyloidosis, sarcoidosis, myocarditis, Chagas disease, infiltrative cardiomyopathies
• Extrinsic SND: beta-blockers, calcium channel blockers, digoxin, amiodarone, clonidine, lithium; hypothyroidism; sleep apnea; increased vagal tone
• Documented symptomatic bradycardia (<40 bpm) with symptoms OR asymptomatic bradycardia <40 bpm if alternate cause excluded

Atrioventricular Block

• Permanent third-degree AV block (any location)
• High-degree AV block with symptoms or ventricular rate <40 bpm at rest
• Symptomatic second-degree AV block (Mobitz II or advanced second-degree)
• First-degree or second-degree AV block with prolonged PR/block if undergoing node-ablating procedures (e.g., AV nodal ablation for atrial fibrillation) or if secondary to infiltrative disease
• Post-surgical AV block not expected to resolve
• Acquired AV block from myocardial infarction (especially inferior MI with involvement of AV nodal artery)
• Congenital AV block with symptoms or QRS duration >120 ms

Cardiac Resynchronization Therapy (CRT)

• LVEF ≤35% (or ≤40% in certain trials)
• NYHA Class II-IV heart failure symptoms
• QRS duration ≥120 ms, ideally ≥150 ms (LBBB pattern preferred)
• Sinus rhythm (CRT-P) or with indication for ICD (CRT-D)
• Optimal medical therapy including ACE inhibitors/ARBs, beta-blockers, aldosterone antagonists

Other Indications

• Carotid sinus hypersensitivity with recurrent syncope
• Vasovagal syncope (specialized dual-chamber pacing with special algorithms)
• Hypertrophic cardiomyopathy with symptomatic LVOT obstruction refractory to medical therapy
• Obstructive sleep apnea-related bradycardia (selected cases)
• Post-transplantation bradycardia
• Tachycardia-bradycardia syndrome

Risk Factors for Requiring Pacemaker

• Age >65 years
• Male gender (SND more common in males)
• Chronic kidney disease
• Diabetes mellitus
• Systemic hypertension
• Infiltrative or inflammatory cardiac diseases
• Prior cardiac surgery
• Chronic medication use (antiarrhythmics, beta-blockers)

Symptoms from Bradycardia/Conduction Block

• Syncope or presyncope: sudden loss of consciousness or near-syncope from critically reduced cardiac output; classic presentation is sudden onset without prodrome (distinguishing from vasovagal syncope)
• Dyspnea: exertional dyspnea from inadequate cardiac output response to activity; dyspnea at rest in advanced bradycardia
• Fatigue: persistent fatigue, exercise intolerance, reduced functional capacity from chronotropic incompetence
• Palpitations: sensation of irregular heartbeat or awareness of slow heart rate; may report skipped beats (especially with Mobitz II block or pauses)
• Dizziness and lightheadedness: positional dizziness, worse with exertion
• Chest discomfort: atypical chest pain, pressure sensation
• Confusion or altered mental status: in elderly patients with severe bradycardia reducing cerebral perfusion

Physical Examination Findings

• Bradycardia: resting heart rate <40-50 bpm or irregular rhythm with long pauses
• Hypotension: reduced systolic and diastolic BP from decreased cardiac output
• Cannon a waves: prominent jugular venous pulsations from atrial contraction against closed tricuspid valve during complete heart block (pathognomonic for complete AV block)
• Variable S1 intensity: in complete heart block, S1 varies because atrial contraction occurs at different points in the cardiac cycle relative to ventricular systole
• Peripheral hypoperfusion: cool extremities, delayed capillary refill
• Hepatomegaly: from hepatic congestion in cardiogenic shock from severe bradycardia
• Signs of heart failure: pulmonary rales, peripheral edema, elevated JVP (if secondary to cardiomyopathy)

Electrocardiogram (ECG) — Primary Diagnostic Tool

• Sinus Node Dysfunction:
• Sinus bradycardia <60 bpm (normal variant vs. pathologic requires correlation with symptoms)
• Sinus pause ≥3 seconds (diagnostic of pathologic SND)
• Sinoatrial exit block (P waves absent for 1-2 cycle lengths, pause equals 2× normal PP interval)
• Atrial fibrillation with slow ventricular response (tachy-brady syndrome)
• Bradycardia-tachycardia pattern

• First-Degree AV Block:
• PR interval >200 ms (>5 small squares)
• All P waves conduct
• Not itself an indication for pacing unless symptomatic

• Second-Degree AV Block — Mobitz Type I (Wenckebach):
• Progressive PR prolongation until a P wave fails to conduct
• PR interval resets after blocked beat
• Usually occurs within AV node (narrow QRS complex)
• Generally benign; pacing indicated only if symptomatic

• Second-Degree AV Block — Mobitz Type II:
• Sudden dropped beat without preceding PR prolongation
• Fixed PR interval in conducted beats
• Usually occurs distal to AV node (wide QRS complex, BBB pattern)
• Requires pacing (high risk for progression to complete block)

• High-Degree AV Block:
• Two or more consecutive P waves fail to conduct
• Not necessarily complete dissociation (some AV conduction preserved)
• Requires pacing (symptomatic or not, if rhythm will not resolve)

• Third-Degree (Complete) AV Block:
• Complete dissociation between P waves and QRS complexes
• Atrial rate independent of ventricular escape rate
• Escape rhythm may be narrow (junctional, 40-60 bpm) or wide (ventricular, 20-40 bpm)
• Wide escape complex indicates infranodal block and mandates pacing

Holter Monitor or Event Monitor

• Captures arrhythmias in SND when standard ECG is normal
• Correlates symptoms with rhythm abnormalities
• Documents sinus pauses, frequency, and duration of bradycardia
• Essential for establishing symptom-rhythm correlation

Electrophysiology Study (EPS)

• Sinus Node Recovery Time (SNRT): measures time for normal sinus rhythm to resume after atrial pacing; SNRT >1500 ms is abnormal and suggests SND
• Sinoatrial Conduction Time (SACT): assesses conduction from SA node; prolonged SACT suggests SA node dysfunction
• AV Node Properties: measures AV nodal conduction time (AH interval), AV nodal refractory periods, and ability to conduct rapid atrial rates
• Indicated when: diagnosis unclear, syncope of unknown origin, need to distinguish SND from other causes, or to establish prognosis
• Less commonly used now given good clinical correlation with symptoms and ECG findings

Stress Testing

• Used selectively in SND to document chronotropic incompetence (inability to increase heart rate appropriately with exercise)
• Normal: heart rate increases to 60% of age-predicted maximum with moderate exercise
• Chronotropic incompetence: blunted heart rate response to exercise (diagnostic for rate-responsive pacemaker indication)

Echocardiography

• Assesses LVEF, ventricular dimensions, wall motion abnormalities
• Essential for CRT candidates (LVEF ≤35%)
• Documents QRS duration and LBBB morphology (best responders to CRT)
• Identifies mechanical dyssynchrony if QRS <120 ms (tissue Doppler imaging or strain imaging)
• Rules out structural heart disease requiring pacing (infiltrative diseases, cardiomyopathy)

Cardiac MRI

• Useful in infiltrative diseases (amyloidosis, sarcoidosis) causing SND
• Detects fibrosis patterns in restrictive cardiomyopathy

Diagnostic Criteria for Symptomatic Bradycardia Requiring Pacing

• Symptom-rhythm correlation on monitoring (syncope, presyncope, dyspnea, or fatigue with documented bradycardia)
• Heart rate <40 bpm or recurrent pauses ≥3 seconds
• No alternative explanation for symptoms (volume depletion, anemia, infection, medication)
• Failure of symptoms to respond to trial of medication withdrawal or adjustment

Pacemaker Types and Selection

1. Single-Chamber Ventricular Pacing (VVI/VVIR)

• Electrode in right ventricle only
• Senses ventricular activity; paces ventricle
• Least expensive option
• Indications: Permanent atrial fibrillation with bradycardia, limited life expectancy, severe renal disease making device follow-up difficult
• Disadvantages: Loss of AV synchrony reduces cardiac output 10-25%, increases pulmonary congestion, promotes atrial fibrillation
• Contraindication: Sinus node dysfunction (requires dual-chamber pacing to preserve AV synchrony)

2. Dual-Chamber Pacing (DDD/DDDR)

• Electrodes in right atrium and right ventricle
• Senses and paces both chambers
• Most common type for SND and AV block
• Advantages: Maintains AV synchrony, rate-responsive variants adjust to activity, reduces atrial fibrillation incidence vs. VVI
• Indications: Sinus node dysfunction, AV block with intact sinus function, syncope with carotid hypersensitivity
• Rate-responsive variant (DDDR): Includes sensor for automatic rate adjustment during exercise
• Programming:
• Lower rate limit: typically 60 bpm at rest, increases with activity in rate-responsive mode
• Upper rate limit: typically 120-140 bpm to prevent tracking of rapid atrial rates
• AV delay: typically 150-200 ms to optimize ventricular filling
• Sensor-driven rate: programmed based on sensor type (accelerometer most common)

3. Cardiac Resynchronization Therapy (CRT) — Biventricular Pacing

• Three electrodes: right atrium, right ventricle, and left ventricle (via coronary sinus)
• Paces both ventricles nearly simultaneously to restore synchrony
• CRT-P (Pacemaker): Biventricular pacing only
• CRT-D (Defibrillator): Biventricular pacing + ICD capability
• Indications:
• LVEF ≤35% (or ≤40% in specific trials)
• NYHA Class II-IV heart failure
• QRS ≥120 ms (ideally ≥150 ms for ischemic cardiomyopathy)
• LBBB pattern (best predictor of response)
• Sinus rhythm preferred (though works in some AF patients with rate control)
• Expected Benefits:
• LVEF improvement by 10-15% in responders
• Reduction in hospitalizations for heart failure
• Reduction in mortality in systolic dysfunction
• Class I recommendation in major guidelines for appropriate candidates
• Programming:
• Simultaneous RV-LV pacing (VV delay ~0-40 ms) or LV-first pacing (LV precedes RV by 5-20 ms)
• Lower rate limit typically 60 bpm
• Rate-responsive capability
• Optimization may include echocardiographic guidance (AV delay, VV delay)

4. Leadless Pacemakers (Emerging Technology)

• Self-contained device implanted directly in right ventricular cavity via femoral vein
• No transvenous lead required
• Advantages: No lead complications (infection, fracture, dislodgment), smaller implantation site, potential for multiple devices
• Disadvantages: Single-chamber only (no atrial pacing), higher cost, removal difficult, limited battery life
• Current FDA approval: Limited to VVI pacing (Micra system approved 2016)
• Dual-chamber leadless systems in development

Implantation Procedure

• Local anesthesia + sedation (general anesthesia optional)
• Subclavian or cephalic vein access (subclavian preferred; cephalic easier but

Periprocedural (first 24–48 hours)

• Pneumothorax: subclavian puncture lacerates the pleural apex; presents with pleuritic pain, hypoxia, absent breath sounds on the implant side. Routine post-implant chest radiograph screens for it. Tension physiology (hypotension, tracheal deviation) is an emergency requiring immediate needle decompression.
• Cardiac perforation/tamponade: stiff lead tip through the thin RV free wall or atrial appendage; hypotension, pulsus paradoxus, distended neck veins, and new pericardial effusion on echo. Emergency — pericardiocentesis and surgical backup.
• Pocket hematoma: most often from periprocedural anticoagulant bridging. The BRUISE CONTROL data support continuing warfarin over heparin bridging. Hematomas should not be aspirated blindly, as this seeds infection.
• Lead dislodgement: highest risk in the first month; manifests as failure to capture, failure to sense, or loss of pacing spikes in the expected chamber.

Infection

• Pocket infection and CIED-related endocarditis: Staphylococcus aureus and coagulase-negative staphylococci predominate. Erythema, fluctuance, or lead erosion through skin; fever with staphylococcal bacteremia in a device patient implies infection until proven otherwise. Obtain blood cultures and transesophageal echocardiography for lead vegetations. Per the AHA/HRS consensus on CIED infection, complete device and lead extraction is required — antibiotics alone fail. Empiric vancomycin is dosed to a 24-hour AUC/MIC of 400–600 (2020 IDSA/ASHP consensus), not a trough target.

Late device–patient complications

• Pacemaker syndrome: VVI pacing with retrograde VA conduction causes atrial contraction against closed AV valves — fatigue, cannon a waves, hypotension with pacing. Treatment is upgrade to dual-chamber pacing.
• Pacemaker-mediated tachycardia: retrograde P wave sensed by the atrial lead re-triggers ventricular pacing in an endless loop at the upper rate limit. Magnet application (asynchronous mode) breaks it; definitive fix is lengthening PVARP.
• RV pacing–induced cardiomyopathy: a high RV pacing burden creates an iatrogenic LBBB-like dyssynchrony and falling LVEF; consider upgrade to CRT or conduction system pacing.
• Lead fracture/insulation failure: subclavian crush between clavicle and first rib causes oversensing of noise → inappropriate inhibition of pacing (asystole in a pacemaker-dependent patient) or inappropriate ICD shocks. Both are emergencies; a magnet restores asynchronous pacing or suspends ICD tachytherapy.
• Other: phrenic nerve/diaphragmatic stimulation from an LV coronary sinus lead (hiccups synchronous with pacing), subclavian venous stenosis, and Twiddler syndrome (patient rotates the generator, coiling and retracting leads).

• Cannon a waves plus variable S1: the examiner's shorthand for complete AV block. Pair it with AV dissociation on ECG and the answer is pacing, not atropine, if the escape is wide and slow.
• Level of block drives urgency, not the degree alone: Mobitz I (Wenckebach) is intranodal, narrow-QRS, and atropine-responsive — pace only if symptomatic. Mobitz II is infranodal, wide-QRS, atropine-*unresponsive* (atropine can worsen it by speeding the atrial rate), and warrants permanent pacing per the 2018 ACC/AHA/HRS bradycardia guideline.
• Inferior MI vs anterior MI block: inferior MI causes AV nodal ischemia (right coronary/AV nodal artery) — usually transient, responds to atropine, rarely needs a permanent device. Anterior MI with new block means septal necrosis of the His-Purkinje system — ominous, often permanent pacing.
• Single best next step for unstable bradycardia (ACLS/AHA): atropine, then transcutaneous pacing and/or a chronotropic infusion (dopamine or epinephrine) as a bridge to transvenous pacing — permanent implantation is never the acute answer.
• Magnet effects differ by device: over a pacemaker, a magnet forces asynchronous pacing (VOO/DOO), useful for oversensing or pacemaker-mediated tachycardia. Over an ICD, a magnet suspends shock therapy only and does not change pacing. Confusing the two is the classic distractor.
• CRT responder profile: LVEF ≤35%, sinus rhythm, LBBB with the widest QRS, on guideline-directed medical therapy. Remember that current 2022 AHA/ACC/HFSA HFrEF therapy is four classes — ARNI (or ACEI/ARB), beta blocker, MRA, and SGLT2 inhibitor — before device consideration.
• Failure to capture = spike with no complex: think lead dislodgement, but always check hyperkalemia, acute ischemia, and class I antiarrhythmics, which raise the myocardial capture threshold. Failure to sense produces spikes marching into T waves (R-on-T risk).
• Paced or LBBB rhythm does not equal STEMI: a new LBBB is not a stand-alone STEMI criterion — apply Sgarbossa (modified Sgarbossa in ventricular-paced rhythms) rather than reflexively activating the cath lab.