Heart Block (AV Block)

Topic Overview

Summary

Atrioventricular (AV) block represents impaired electrical conduction between the atria and ventricles, ranging from mild PR interval prolongation (first-degree) to complete absence of AV conduction (third-degree or complete heart block). The clinical spectrum varies from asymptomatic ECG findings to life-threatening bradycardia causing syncope, heart failure, and sudden cardiac death.

Classification is based on ECG morphology and conduction pattern: first-degree block (prolonged PR interval > 200 ms with 1:1 conduction), second-degree block (intermittent failure of AV conduction—subdivided into Mobitz type I [Wenckebach] and Mobitz type II), and third-degree (complete) block with complete AV dissociation. Mobitz type II and complete heart block carry high risk of progression to asystole and require urgent pacing evaluation. [1,2]

The site of block—nodal (AV node) versus infranodal (His bundle or bundle branches)—determines prognosis and escape rhythm reliability. AV nodal blocks typically have narrow QRS escape rhythms and respond to atropine; infranodal blocks present with wide QRS escape, poor haemodynamic reserve, and require pacing. [3,4]

Aetiology includes acute myocardial infarction (especially inferior MI), degenerative conduction system disease (Lenègre's and Lev's disease), drugs (beta-blockers, calcium channel blockers, digoxin), electrolyte disturbances (hyperkalaemia), infiltrative diseases (sarcoidosis, amyloidosis), and infectious causes (Lyme disease, endocarditis). [5,6]

Emergency management follows the Resuscitation Council UK Advanced Life Support (ALS) bradycardia algorithm: assess for adverse features (shock, syncope, myocardial ischaemia, heart failure), administer atropine 500 mcg IV (up to 3 mg), initiate transcutaneous pacing if atropine fails, and arrange urgent transvenous pacing. Permanent pacemaker implantation is indicated for symptomatic bradycardia, Mobitz type II, complete heart block, and post-MI persistent conduction abnormalities. [7,8]

Key Facts

• First-degree block: PR interval > 200 ms; 1:1 AV conduction; usually benign; no treatment required
• Mobitz type I (Wenckebach): Progressive PR prolongation → dropped QRS; grouped beating pattern; AV nodal level; benign in most cases
• Mobitz type II: Fixed PR interval → sudden dropped QRS; infranodal; high risk progression to complete block; requires pacing
• Third-degree (complete) block: Complete AV dissociation; P waves and QRS complexes independent; escape rhythm sustains circulation; always requires pacing
• Emergency treatment: Atropine 500 mcg IV (effective in nodal blocks); transcutaneous pacing; transvenous pacing; isoprenaline/adrenaline infusion
• Permanent pacing indications: Symptomatic bradycardia, Mobitz type II (even if asymptomatic), complete heart block, post-MI persistent block
• Mortality: Untreated complete heart block carries 50% 1-year mortality; permanent pacing improves survival to near-normal [9,10]

Clinical Pearls

"Mobitz I wanders, Mobitz II drops" — Mobitz type I shows progressive PR prolongation before dropped beat; Mobitz type II has fixed PR then sudden drop

Atropine fails in infranodal block — Atropine increases AV nodal conduction but has no effect on His-Purkinje disease; complete heart block often unresponsive

Broad QRS escape = unstable ventricle — Escape rhythm less than 4lt;4lt;40 bpm with wide QRS indicates infranodal block; unreliable, requires immediate pacing

Inferior MI block is transient; anterior MI block is permanent — Inferior MI causes AV nodal ischaemia (reversible); anterior MI damages His-Purkinje (extensive infarction, poor prognosis)

2:1 AV block is ambiguous — Cannot distinguish Mobitz I from II; treat as high-risk if wide QRS or infranodal features

Why This Matters Clinically

Heart block represents a critical diagnosis in emergency and acute medicine. High-grade AV block (Mobitz type II, complete block) can progress unpredictably to ventricular standstill (asystole), making early recognition and pacing essential. Stokes-Adams attacks—sudden syncope from transient asystole—carry risk of injury and sudden death. [11,12]

In acute coronary syndromes, AV block localizes the culprit vessel and predicts prognosis: inferior MI with transient AV nodal block (RCA occlusion) has good prognosis with reperfusion; anterior MI with infranodal block (proximal LAD occlusion) indicates extensive myocardial damage and high mortality despite pacing. [13,14]

Permanent pacemaker implantation transforms prognosis: untreated complete heart block has 50% 1-year mortality versus near-normal survival with pacing. Modern devices (dual-chamber, rate-responsive, conduction system pacing) maintain physiological AV synchrony and improve quality of life. [9,15]

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Visual Summary

Visual assets to be added:

• ECG examples: first-degree, Mobitz I, Mobitz II, 2:1 block, complete heart block
• Comparison diagram: Mobitz I vs Mobitz II vs complete block
• ALS bradycardia algorithm flowchart
• Transcutaneous pacing pad placement
• Permanent pacemaker indications decision tree
• Anatomical diagram: AV conduction system (AV node → His bundle → bundle branches)
• Post-MI heart block: inferior vs anterior MI prognosis

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Epidemiology

Incidence and Prevalence

First-degree AV block is common in the general population, affecting 1-2% of apparently healthy adults, with prevalence increasing with age. It is often an incidental finding on routine ECG and rarely progresses to higher-grade block in the absence of structural heart disease. [16]

Second-degree AV block is less common. Mobitz type I (Wenckebach) occurs in healthy young adults (especially athletes due to high vagal tone) and during sleep. Prevalence in the general population is estimated at 0.003-0.02%. Mobitz type II is rare and almost always indicates structural conduction system disease. [17]

Third-degree (complete) AV block has an estimated prevalence of 0.02-0.04% in the general population. Incidence increases sharply with age due to degenerative conduction disease (Lenègre's disease, Lev's disease). Annual incidence in adults > 65 years is approximately 200 per 100,000. [18]

Post-myocardial infarction: AV block complicates 6-14% of acute MI. Inferior MI (RCA occlusion) causes transient AV nodal ischaemia in 10-15% of cases; anterior MI (proximal LAD occlusion) causes permanent infranodal block in 2-5%, indicating extensive septal infarction. [13,14]

Age and Sex

• Age: Incidence of high-grade AV block increases exponentially with age. Degenerative conduction system disease (fibrosis, calcification) is the leading cause in elderly patients
• Sex: Slight male predominance in degenerative conduction disease; no significant sex difference in most aetiologies

Risk Factors

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Pathophysiology

Normal AV Conduction

The cardiac conduction system propagates electrical impulses from the sinoatrial (SA) node through the atria, AV node, His bundle, bundle branches, and Purkinje fibres to ventricular myocardium. Normal AV conduction time (PR interval) is 120-200 ms. [3]

AV nodal conduction: The AV node imposes physiological delay (80-120 ms), allowing atrial systole to complete ventricular filling before ventricular contraction. AV nodal conduction is modulated by autonomic tone (accelerated by sympathetic, slowed by parasympathetic input).

His-Purkinje conduction: Rapid conduction (less than 4lt;40 ms) through the His bundle, left and right bundle branches, and Purkinje network ensures synchronous ventricular activation.

Levels of Block

Electrophysiological significance: His bundle electrography can distinguish AV nodal from infranodal block. Prolonged AH interval indicates AV nodal disease; prolonged HV interval or block below His indicates infranodal disease (worse prognosis). [4]

First-Degree AV Block

• Pathophysiology: Delayed conduction through the AV node (most common), intra-Hisian, or infra-Hisian (rare)
• ECG: PR interval > 200 ms; every P wave conducts to QRS (1:1 conduction)
• Clinical significance: Usually benign; may progress to higher-grade block if associated with bundle branch block (bifascicular block) or very prolonged PR (> 300 ms)

Second-Degree AV Block — Mobitz Type I (Wenckebach)

• Pathophysiology: Progressive fatigue of AV nodal conduction. Each successive impulse conducts more slowly until one fails completely, resetting the cycle
• ECG: Progressive PR interval prolongation → dropped QRS → grouped beating pattern (e.g., 3:2, 4:3 conduction ratio). PR interval is longest before dropped beat and shortest after dropped beat
• Mechanism: AV nodal refractoriness increases with each conducted beat. The increment in PR prolongation decreases with each beat (largest increment between 1st and 2nd beat)
• Clinical significance: Usually benign, especially in young/athletic individuals or during sleep (high vagal tone). Rarely progresses to complete block [17]

Second-Degree AV Block — Mobitz Type II

• Pathophysiology: Sudden failure of infranodal conduction (His bundle or bundle branches) without prior PR prolongation. Represents phase 4 depolarization failure in His-Purkinje cells
• ECG: Fixed PR interval → sudden dropped QRS. Often associated with bundle branch block (wide QRS). May have 2:1, 3:1, or variable conduction ratio
• Clinical significance: High risk of progression to complete heart block and asystole. Requires pacing even if asymptomatic [2,8]

2:1 AV Block

• Pathophysiology: Every second P wave conducts (2:1 ratio). Cannot classify as Mobitz I or II because only one conducted PR interval is visible
• Diagnostic approach: If QRS is narrow, likely AV nodal (Mobitz I pattern); if QRS is wide, likely infranodal (Mobitz II pattern). Electrophysiology study can definitively localize block level [3]

Third-Degree (Complete) Heart Block

• Pathophysiology: Complete failure of AV conduction. No atrial impulses reach the ventricles. Ventricular activation depends entirely on escape pacemaker below the block
• ECG: Complete AV dissociation—P waves and QRS complexes are independent (PP and RR intervals regular but unrelated). Atrial rate > ventricular rate
• Escape rhythms:
  • "Junctional escape (AV nodal or proximal His block): Rate 40-60 bpm, narrow QRS, relatively stable"
  • "Ventricular escape (infranodal block): Rate 20-40 bpm, wide QRS, unstable and unreliable"
• Clinical significance: Always pathological. Risk of ventricular standstill if escape rhythm fails. Requires permanent pacing [9,10]

Mechanisms by Aetiology

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Clinical Presentation

Symptoms

Symptoms depend on degree of block, ventricular rate, and haemodynamic reserve.

Asymptomatic:

• First-degree AV block (incidental ECG finding)
• Mobitz type I in young/athletic patients
• Well-compensated complete block with adequate escape rate (> 40 bpm)

Symptomatic bradycardia:

• Dizziness, lightheadedness, presyncope: Transient cerebral hypoperfusion
• Syncope (Stokes-Adams attacks): Sudden loss of consciousness due to ventricular standstill or severe bradycardia (less than 3lt;30 bpm). Episode lasts seconds to minutes; rapid recovery without postictal confusion (unlike seizure). High risk of traumatic injury [11,12]
• Fatigue, exercise intolerance: Inadequate chronotropic response
• Dyspnoea: Heart failure due to inadequate cardiac output
• Chest pain: May occur with severe bradycardia or underlying ischaemia
• Sudden cardiac death: Asystole if escape rhythm fails

History suggestive of aetiology:

• Recent MI, angina (ischaemic block)
• Medications: beta-blockers, calcium channel blockers, digoxin, amiodarone
• Tick bite, erythema migrans rash (Lyme disease)
• Systemic symptoms: weight loss, night sweats (sarcoidosis, infiltrative disease)
• Family history of early pacemaker, sudden death (genetic conduction disease)

Signs

Cardiovascular examination:

• Bradycardia: Heart rate 30-60 bpm (complete block) or intermittently irregular (second-degree block)
• Cannon A waves in jugular venous pulse (JVP): Occur when atria contract against closed tricuspid valve (AV dissociation in complete block) [10]
• Variable intensity of first heart sound (S1): PR interval varies (Mobitz I) or AV dissociation (complete block) causes variable ventricular filling
• Hypotension: Systolic BP less than 9lt;9lt;90 mmHg if severe bradycardia or poor escape rhythm
• Heart failure signs: Elevated JVP, pulmonary crackles, peripheral oedema (bradycardia-induced cardiomyopathy)

Stokes-Adams attack (witnessed):

• Sudden pallor, loss of consciousness
• Rapid recovery (seconds to 1-2 minutes) without confusion
• Flushing on recovery (reactive hyperaemia)
• No tonic-clonic movements (unless prolonged cerebral hypoperfusion causes secondary anoxic seizure)

Red Flags

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Clinical Examination

General Inspection

• Level of consciousness: Alert vs drowsy vs unconscious (severe bradycardia, Stokes-Adams attack)
• Pallor, cyanosis: Poor perfusion
• Distress: Dyspnoea, chest pain

Vital Signs

• Heart rate: Bradycardia (30-60 bpm in complete block); irregular in second-degree block
• Blood pressure: May be maintained or low (systolic less than 9lt;9lt;90 mmHg indicates haemodynamic compromise)
• Respiratory rate: Tachypnoea if heart failure
• Oxygen saturation: May be reduced if pulmonary oedema

Cardiovascular Examination

Jugular venous pulse (JVP):

• Cannon A waves: Giant A waves occurring irregularly (when right atrium contracts against closed tricuspid valve during ventricular systole). Pathognomonic of AV dissociation (complete heart block, ventricular tachycardia)
• Elevated JVP: Heart failure

Palpation:

• Pulse: Slow, regular (complete block with stable escape) or irregularly irregular (Mobitz I, atrial fibrillation with slow ventricular response—differential diagnosis)
• Apex beat: May be displaced if cardiomyopathy

Auscultation:

• Variable S1 intensity: PR interval varies (Mobitz I) or AV dissociation (complete block) causes variable LV filling and force of mitral valve closure
• S3 or S4 gallop: Heart failure
• Murmurs: Aortic stenosis (may cause syncope; co-existent with Lev's disease)

Peripheral perfusion:

• Cool peripheries, prolonged capillary refill time (poor cardiac output)

Respiratory and Systemic Examination

• Pulmonary crackles: Left ventricular failure, pulmonary oedema
• Peripheral oedema: Right ventricular failure
• Hepatomegaly: Right heart failure
• Erythema migrans rash: Lyme disease
• Lymphadenopathy, hepatosplenomegaly: Sarcoidosis, lymphoma
• Neurological signs: Myotonic dystrophy (myotonia, frontal balding, ptosis, dysarthria)

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Investigations

ECG — Diagnostic

ECG is the definitive diagnostic investigation for heart block.

First-Degree AV Block

• PR interval > 200 ms (5 small squares at 25 mm/s)
• Every P wave conducts to QRS (1:1 conduction)
• QRS morphology: Narrow (AV nodal delay) or wide (if co-existent bundle branch block)

Second-Degree AV Block — Mobitz Type I (Wenckebach)

• Progressive PR prolongation → dropped QRS → cycle repeats
• Grouped beating pattern (e.g., groups of 3, 4, or 5 QRS complexes)
• PR interval longest before dropped beat, shortest after dropped beat
• Increment in PR prolongation decreases with each successive beat (Wenckebach periodicity)
• Usually narrow QRS

Second-Degree AV Block — Mobitz Type II

• Fixed PR interval (constant for conducted beats)
• Sudden dropped QRS without prior PR prolongation
• Conduction ratio may be 3:2, 4:3, or variable
• Often associated with wide QRS (bundle branch block)

2:1 AV Block

• Two P waves for every QRS (2:1 conduction)
• Cannot distinguish Mobitz I from II (only one conducted PR visible)
• If QRS narrow → likely AV nodal (Mobitz I pattern)
• If QRS wide → likely infranodal (Mobitz II pattern)

Third-Degree (Complete) Heart Block

• Complete AV dissociation: P waves and QRS complexes are independent
• Atrial rate > ventricular rate (e.g., atrial 70-90 bpm, ventricular 30-50 bpm)
• Regular PP intervals and regular RR intervals, but no relationship between them
• Escape rhythm QRS morphology:
  • "Narrow QRS (40-60 bpm): Junctional escape (AV nodal or proximal His block)"
  • "Wide QRS (20-40 bpm): Ventricular escape (infranodal block)—UNSTABLE"

Blood Tests

Echocardiography (Transthoracic)

• Structural heart disease: LV systolic dysfunction (ischaemic cardiomyopathy), valvular heart disease (aortic stenosis, endocarditis), infiltrative cardiomyopathy (amyloidosis—sparkling myocardium, restrictive filling)
• Septal abscess: Endocarditis complication causing AV block
• LV ejection fraction: Bradycardia-induced cardiomyopathy (may improve post-pacing)

Ambulatory ECG Monitoring

• 24-hour Holter monitor: Intermittent AV block, paroxysmal complete block, nocturnal Wenckebach (normal variant in athletes)
• Event recorder (external or implantable loop recorder): Infrequent symptoms; captures rhythm during syncope

Electrophysiology Study (EPS)

• Indications: Unclear diagnosis (2:1 block, suspected intermittent block), symptomatic but no documented high-grade block, assessment of His-Purkinje conduction in bundle branch block
• Findings:
  • "Prolonged AH interval (> 125 ms): AV nodal disease"
  • "Prolonged HV interval (> 55 ms): His-Purkinje disease (indication for pacing)"
  • "Block below His bundle: Infranodal disease [4]"

Exercise Testing

• Indication: Assess chronotropic competence; provoke AV block
• Findings: Worsening AV block with exercise suggests infranodal disease (abnormal—should improve with sympathetic stimulation)

Cardiac MRI

• Indications: Suspected infiltrative disease (sarcoidosis, amyloidosis), myocarditis
• Findings: Late gadolinium enhancement (LGE) in septum/conduction system; oedema (myocarditis); amyloid pattern (global subendocardial LGE)

Other Investigations (Aetiology-Specific)

• Chest X-ray: Sarcoidosis (bilateral hilar lymphadenopathy), heart failure (pulmonary oedema, cardiomegaly)
• CT chest: Sarcoidosis staging
• Endomyocardial biopsy: Myocarditis, infiltrative disease (rarely required)

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Classification & Staging

Degree of AV Block

Anatomical Classification

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Management

Emergency Management: ALS Bradycardia Algorithm

Follow Resuscitation Council UK Advanced Life Support (ALS) Guidelines. [7]

Step 1: Assess Patient and Identify Adverse Features

Adverse features indicate haemodynamic compromise requiring urgent treatment:

• Shock: Hypotension (systolic BP less than 9lt;9lt;90 mmHg), cool peripheries, altered mental status
• Syncope: Stokes-Adams attack
• Myocardial ischaemia: Chest pain, ECG ischaemia, elevated troponin
• Heart failure: Pulmonary oedema, elevated JVP, dyspnoea

Step 2: If Adverse Features Present → Urgent Treatment

1. Atropine

• Dose: 500 mcg IV bolus
• Repeat: Every 3-5 minutes up to maximum 3 mg (6 doses)
• Mechanism: Blocks vagal (parasympathetic) input to AV node, increasing AV nodal conduction
• Effectiveness: Works for AV nodal blocks (first-degree, Mobitz I); ineffective for infranodal blocks (Mobitz II, complete heart block with wide QRS)
• Side effects: Dry mouth, blurred vision, urinary retention; rarely precipitates tachyarrhythmia

2. Transcutaneous (External) Pacing

• Indication: Atropine ineffective or contraindicated; high-risk blocks (Mobitz II, complete block)
• Technique:
  • Apply pacing pads (anterior-posterior or anterior-lateral)
  • Set rate 60-90 bpm
  • Increase output (mA) until electrical capture (pacer spike followed by wide QRS)
  • Confirm mechanical capture (palpable pulse, improving BP)
• Analgesia: External pacing is painful; give sedation/analgesia (e.g., IV morphine, midazolam) if patient conscious
• Limitations: Temporary measure; arrange transvenous pacing urgently

3. Inotropic/Chronotropic Infusions

• Indications: Atropine/pacing ineffective or unavailable
• Agents:
  • "Adrenaline: 2-10 mcg/min IV infusion"
  • "Isoprenaline: 1-10 mcg/min IV infusion (beta-agonist; increases heart rate)"
  • "Dopamine: 2-10 mcg/kg/min IV infusion"
• Use with caution: May worsen ischaemia; temporary only

4. Transvenous Temporary Pacing

• Indications: Sustained bradycardia requiring pacing; failure of transcutaneous pacing
• Technique: Pacing wire inserted via internal jugular, subclavian, or femoral vein; positioned in right ventricle under fluoroscopy or ECG guidance
• Complications: Pneumothorax, bleeding, lead displacement, arrhythmias, perforation
• Temporary measure: Bridge to permanent pacemaker or resolution of reversible cause

Step 3: If NO Adverse Features → Monitor and Observe

Risk of Asystole: Even if haemodynamically stable, certain features predict high risk of progression to asystole and warrant urgent pacing:

• Recent asystole
• Mobitz type II AV block
• Complete heart block (especially wide QRS escape less than 4lt;4lt;40 bpm)
• Ventricular pause > 3 seconds

Monitoring:

• Continuous cardiac telemetry
• Frequent vital signs (BP, HR, SpO₂)
• Prepare for transcutaneous pacing (pads on standby)
• Urgent cardiology referral

Reversible Causes — Treat Underlying Aetiology

Permanent Pacemaker Implantation — Indications

Indications follow 2021 ESC Guidelines on Cardiac Pacing and Cardiac Resynchronization Therapy. [8]

Class I Indications (Definite; Always Indicated)

• Symptomatic bradycardia due to AV block (syncope, presyncope, heart failure)
• Third-degree (complete) AV block (even if asymptomatic)
• Mobitz type II second-degree AV block (even if asymptomatic)
• High-grade AV block (≥2 consecutive non-conducted P waves)
• Post-MI persistent AV block (≥48 hours post-MI, unlikely to resolve)
• Asymptomatic complete AV block with ventricular rate less than 4lt;4lt;40 bpm or pauses > 3 seconds while awake

Class IIa Indications (Reasonable; Probably Indicated)

• Asymptomatic Mobitz type II with narrow QRS (some advocate pacing due to progression risk)
• First-degree AV block with PR > 300 ms and symptoms (pacemaker syndrome—loss of AV synchrony)
• 2:1 AV block with wide QRS (likely infranodal)

Class IIb Indications (May Be Considered)

• Asymptomatic first-degree AV block with PR > 300 ms and LV dysfunction

NOT Indicated

• Asymptomatic first-degree AV block with PR less than 3lt;3lt;300 ms
• Asymptomatic Mobitz type I (unless very slow ventricular rate or long pauses)
• AV block expected to resolve (e.g., drug-induced, Lyme disease with antibiotic treatment)

Pacemaker Types

Post-Pacemaker Care

• Wound care: Avoid vigorous arm movement for 4-6 weeks (lead displacement risk)
• Device check: 6 weeks post-implant, then annually or remotely
• Antibiotic prophylaxis: NOT routinely recommended for dental procedures (endocarditis risk low with modern devices)
• MRI compatibility: Modern pacemakers are MRI-conditional (check device specifications)
• Driving restrictions (UK DVLA):
  • "Group 1 (car/motorcycle): 1 week off driving post-pacemaker implant"
  • "Group 2 (lorry/bus): 6 weeks off driving"

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Management Algorithm

AV BLOCK DETECTED ON ECG
         |
         ├─→ First-degree (PR > 200 ms, 1:1 conduction)
         |         |
         |         ├─→ Asymptomatic → Monitor; no treatment
         |         └─→ Symptomatic (PR > 300 ms, heart failure) → Consider pacing
         |
         ├─→ Second-degree Mobitz I (progressive PR prolongation → dropped QRS)
         |         |
         |         ├─→ Young/athletic, asymptomatic → Monitor; no treatment
         |         └─→ Symptomatic → Pacemaker
         |
         ├─→ Second-degree Mobitz II (fixed PR → dropped QRS)
         |         |
         |         └─→ PERMANENT PACEMAKER (even if asymptomatic)
         |
         ├─→ 2:1 AV block
         |         |
         |         ├─→ Narrow QRS → Likely Mobitz I; monitor or pace if symptomatic
         |         └─→ Wide QRS → Treat as Mobitz II; PERMANENT PACEMAKER
         |
         └─→ Third-degree (complete heart block)
                   |
                   ├─→ ADVERSE FEATURES? (shock, syncope, ischaemia, HF)
                   |         |
                   |         ├─→ YES → URGENT TREATMENT:
                   |         |         1. Atropine 500 mcg IV (up to 3 mg)
                   |         |         2. Transcutaneous pacing
                   |         |         3. Transvenous pacing
                   |         |         4. Inotropes (adrenaline/isoprenaline)
                   |         |
                   |         └─→ NO → Monitor, urgent cardiology referral
                   |
                   └─→ PERMANENT PACEMAKER (always indicated)

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Complications

Complications of Heart Block

Complications of Temporary Pacing

Complications of Permanent Pacing

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Prognosis & Outcomes

First-Degree and Mobitz Type I (Wenckebach)

• Prognosis: Excellent in otherwise healthy individuals
• Progression: Rarely progresses to high-grade block unless associated with structural heart disease or bundle branch block
• Management: Observation; no treatment required in most cases

Mobitz Type II

• Natural history: High risk of progression to complete heart block (up to 35% within 1 year if untreated)
• Sudden death risk: Significant risk of asystole
• Prognosis with pacing: Excellent; near-normal life expectancy [2,8]

Third-Degree (Complete) Heart Block

• Untreated: 50% 1-year mortality due to sudden cardiac death, heart failure, or syncope with traumatic injury [9]
• Pacemaker treatment: Improves survival to near-normal; quality of life significantly improved [10]
• Escape rhythm prognosis:
  • "Junctional escape (narrow QRS, 40-60 bpm): Relatively stable; better prognosis"
  • "Ventricular escape (wide QRS, less than 4lt;4lt;40 bpm): Unreliable; high risk of asystole; urgent pacing required"

Post-Myocardial Infarction AV Block

Pacemaker Outcomes

• Symptom resolution: Syncope, presyncope, and fatigue resolve in > 90% of patients post-pacing
• Heart failure improvement: Bradycardia-induced cardiomyopathy may improve (LVEF increase of 10-20%) within 3-6 months of pacing
• Longevity: Modern pacemakers last 7-12 years before battery replacement required
• Complications: Low (1-5% major complications at implant; less than 1lt;1lt;1% annual complication rate thereafter)

Long-Term Follow-Up

• Device checks: 6 weeks post-implant, then annually or via remote monitoring
• Lead surveillance: Annual interrogation for lead integrity, sensing, and pacing thresholds
• Battery monitoring: Elective replacement indicator (ERI) typically 3-6 months before end of battery life

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Evidence & Guidelines

Key Guidelines

1. Glikson M, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520. PMID: 34455427

   • Comprehensive guidelines for pacemaker indications, device selection, implantation technique, and follow-up

2. Resuscitation Council UK. Adult Advanced Life Support Guidelines. 2021. resus.org.uk

   • Emergency management of bradycardia including atropine dosing, transcutaneous pacing, and transvenous pacing indications

3. Kusumoto FM, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019;140(8):e382-e482. PMID: 30586772

   • Detailed US guidelines for bradycardia management and pacing indications

Key Evidence

4. Narula OS. Longitudinal dissociation in the His bundle. Bundle branch block due to asynchronous conduction within the His bundle in man. Circulation. 1977;56(6):996-1006. PMID: 923072

   • Landmark electrophysiology study defining intra-Hisian block

5. Gregoratos G, et al. ACC/AHA Guidelines for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices. Circulation. 1998;97(13):1325-1335. PMID: 9570205

   • Historical guideline establishing pacemaker indication framework

6. Wan D, et al. Lyme carditis and high-degree atrioventricular block. Am J Med. 2018;131(9):1103.e1-1103.e5. PMID: 29727588

   • Lyme disease as reversible cause of AV block; antibiotic therapy effective

7. Soar J, et al. European Resuscitation Council Guidelines 2021: Adult Advanced Life Support. Resuscitation. 2021;161:115-151. PMID: 33773835

   • Evidence-based emergency management of bradycardia

8. Brignole M, et al. 2013 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2013;34(29):2281-2329. PMID: 23801822

   • Previous ESC guidelines; basis for 2021 update

9. Edhag O, Swahn A. Prognosis of patients paced for chronic atrioventricular block. Acta Med Scand. 1976;200(6):457-463. PMID: 793533

   • Classic study demonstrating survival benefit of pacing in complete heart block

10. Shaw DB, et al. Survival in second degree atrioventricular block. Br Heart J. 1985;53(6):587-593. PMID: 4005079

    • Natural history of second-degree AV block; mortality data

11. Brignole M, et al. New classification of haemodynamics of vasovagal syncope: beyond the VASIS classification. Europace. 2000;2(1):66-76. PMID: 11225595

    • Classification of syncope mechanisms including cardiogenic (Stokes-Adams)

12. Alboni P, et al. Clinical spectrum of neurally mediated reflex syncopes. Europace. 2004;6(1):55-62. PMID: 14697726

    • Differentiation of syncope aetiologies

13. Berger PB, et al. Complete atrioventricular block complicating inferior wall acute myocardial infarction treated with reperfusion therapy. Am J Cardiol. 1993;72(15):1154-1160. PMID: 8237799

    • Inferior MI heart block characteristics and prognosis

14. Hindman MC, et al. The clinical significance of bundle branch block complicating acute myocardial infarction. Circulation. 1978;58(4):679-688. PMID: 688577

    • Anterior MI heart block prognosis and mortality

15. Sharma PS, et al. Permanent His bundle pacing is feasible, safe, and superior to right ventricular pacing in routine clinical practice. Heart Rhythm. 2015;12(2):305-312. PMID: 25179489

    • Conduction system pacing (His bundle pacing) outcomes

16. Cheng S, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA. 2009;301(24):2571-2577. PMID: 19549974

    • Epidemiology and outcomes of first-degree AV block

17. Barold SS, Hayes DL. Second-degree atrioventricular block: a reappraisal. Mayo Clin Proc. 2001;76(1):44-57. PMID: 11155413

    • Classification and management of second-degree AV block

18. Priori SG, et al. 2015 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Eur Heart J. 2015;36(41):2793-2867. PMID: 26320108

    • Sudden cardiac death risk stratification

19. Regueiro A, et al. Impact of New-Onset Left Bundle Branch Block and Periprocedural Permanent Pacemaker Implantation on Clinical Outcomes in Patients Undergoing Transcatheter Aortic Valve Replacement. Circ Cardiovasc Interv. 2016;9(5):e003635. PMID: 27142264

    • AV block complicating TAVI procedure

20. Schott JJ, et al. Cardiac conduction defects associate with mutations in SCN5A. Nat Genet. 1999;23(1):20-21. PMID: 10471492

    • Genetic basis of progressive cardiac conduction defect

21. Auffret V, et al. Analysis of Conduction Abnormalities and Permanent Pacemaker Implantation After Transcatheter Aortic Valve Replacement. J Cardiothorac Vasc Anesth. 2020;34(4):966-974. PMID: 31558391

    • Comprehensive analysis of post-TAVI conduction abnormalities and pacemaker requirements

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Patient & Family Information

What Is Heart Block?

Heart block (also called AV block or atrioventricular block) is a problem with your heart's electrical system. Normally, electrical signals travel from the top chambers of your heart (atria) to the bottom chambers (ventricles), making your heart beat regularly. In heart block, these signals are delayed or blocked.

Types of Heart Block

First-degree heart block:

• The electrical signals are slightly delayed but all get through
• Your heart beats normally
• You usually have no symptoms
• No treatment is needed in most cases

Second-degree heart block:

• Some electrical signals don't get through
• Your heart may beat irregularly or slowly
• You may feel dizzy, tired, or faint
• Treatment depends on the type—some people need a pacemaker

Third-degree (complete) heart block:

• No electrical signals get through from top to bottom chambers
• Your heart beats very slowly (30-50 beats per minute instead of 60-100)
• You may feel very dizzy, faint, or short of breath
• This is serious and always needs treatment with a pacemaker

Symptoms

Many people with mild heart block have no symptoms. More severe heart block can cause:

• Dizziness or lightheadedness
• Fainting (blackouts)
• Feeling very tired or weak
• Shortness of breath
• Chest discomfort

Seek urgent medical help if you experience fainting or blackouts.

Causes

Heart block can be caused by:

• Heart disease (heart attacks, heart muscle problems)
• Ageing (wear and tear of the heart's electrical system)
• Medications (some blood pressure and heart rhythm drugs)
• Infections (Lyme disease from tick bites, heart infections)
• Heart surgery or procedures

Tests

• ECG (electrocardiogram): The main test—shows the electrical activity of your heart
• Blood tests: Check for causes like infection or medication levels
• Echocardiogram (heart ultrasound): Checks the structure of your heart
• 24-hour ECG monitor: Records your heart rhythm over 24 hours to detect intermittent problems

Treatment

Mild heart block (first-degree, some second-degree):

• Often no treatment needed
• Monitor with regular check-ups
• Review medications that might be slowing your heart

Severe heart block (some second-degree, all third-degree):

• Pacemaker: A small device (about the size of a matchbox) implanted under the skin below your collarbone. It sends electrical signals to keep your heart beating regularly
• Emergency treatment: If you're very unwell, temporary pacing wires or medications may be used until a pacemaker is fitted

Living with a Pacemaker

What to expect:

• Minor surgery to implant the device (usually under local anaesthetic)
• Small scar below collarbone
• Device lasts 7-12 years before battery needs replacing
• Most people return to normal activities within a few weeks

Precautions:

• Avoid vigorous arm movements on the pacemaker side for 4-6 weeks after implantation
• Carry a pacemaker ID card
• Tell healthcare staff you have a pacemaker before any procedures
• Modern pacemakers are safe with most electrical devices (mobile phones, microwaves, airport security)
• Inform DVLA (UK): You can drive again 1 week after pacemaker insertion (car/motorcycle)

Follow-up:

• Device check 6 weeks after implantation, then yearly (or remote monitoring)
• Battery lasts 7-12 years; you'll be alerted well before replacement is needed

Prognosis

• Mild heart block: Excellent outlook; usually doesn't progress
• Severe heart block with pacemaker: Excellent outlook; symptoms resolve, and life expectancy is normal
• Untreated severe heart block: High risk of fainting, injury, and sudden cardiac arrest—pacemaker is life-saving

When to Seek Help

Call 999 or go to A&E if you experience:

• Fainting or blackouts
• Severe dizziness or feeling faint
• Chest pain or severe shortness of breath
• Very slow heart rate (less than 4lt;40 bpm) with symptoms

Resources and Support

• British Heart Foundation: bhf.org.uk — Information on heart block and pacemakers
• Arrhythmia Alliance: heartrhythmalliance.org — Patient support for heart rhythm problems
• NHS: nhs.uk/conditions/heart-block — Trusted NHS information

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Examination Focus (MRCP PACES, Clinical Finals)

Viva Questions and Model Answers

Q1: How would you distinguish Mobitz type I from Mobitz type II on ECG?

Model answer:

"Mobitz type I (Wenckebach phenomenon) shows progressive PR interval prolongation before a dropped QRS complex, creating a grouped beating pattern. The PR interval is longest just before the dropped beat and shortest just after. The RR interval shortens with each beat before the dropped beat.

Mobitz type II shows a fixed PR interval for all conducted beats, then sudden failure of AV conduction (dropped QRS) without prior PR prolongation. It is often associated with bundle branch block (wide QRS) and indicates infranodal disease. Mobitz type II has a high risk of progression to complete heart block and requires permanent pacing even if asymptomatic."

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Q2: Why does atropine work in AV nodal block but not in infranodal block?

Model answer:

"Atropine is an anticholinergic agent that blocks vagal (parasympathetic) input to the AV node. The AV node has significant vagal innervation, so blocking this parasympathetic tone increases AV nodal conduction velocity and reduces the refractory period, improving AV conduction in AV nodal blocks (first-degree, Mobitz type I).

However, the His bundle and bundle branches (infranodal structures) have minimal vagal innervation. Infranodal block is due to structural disease (fibrosis, ischaemia, degeneration) rather than autonomic tone. Therefore, atropine has no effect on infranodal blocks such as Mobitz type II or complete heart block with wide QRS escape."

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Q3: Explain the prognosis of heart block complicating inferior versus anterior myocardial infarction.

Model answer:

"In inferior MI (typically RCA occlusion), the AV node receives its blood supply from the AV nodal artery, a branch of the RCA. Ischaemia causes reversible AV nodal dysfunction, leading to transient AV block (often first-degree or Mobitz type I, occasionally complete block). The escape rhythm is usually junctional (narrow QRS, 40-60 bpm), which is relatively stable. AV block typically resolves within 48-72 hours following reperfusion. Prognosis is good if the MI is otherwise uncomplicated.

In anterior MI (proximal LAD occlusion), the septum—including the His bundle and bundle branches—is infarcted. This causes infranodal block with a wide QRS ventricular escape rhythm (less than 4lt;40 bpm), which is unreliable. The presence of AV block in anterior MI indicates massive septal infarction with high mortality (50-80%). AV block is usually permanent and requires permanent pacing, though overall prognosis remains poor due to extensive myocardial damage."

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Q4: What are the indications for permanent pacemaker implantation in AV block?

Model answer:

"According to the 2021 ESC Guidelines, Class I indications (definite, always indicated) for permanent pacing in AV block include:

• Symptomatic bradycardia due to AV block (syncope, presyncope, heart failure)
• Third-degree (complete) AV block, even if asymptomatic
• Mobitz type II second-degree AV block, even if asymptomatic
• High-grade AV block (two or more consecutive non-conducted P waves)
• Post-MI persistent AV block (lasting > 48 hours, unlikely to resolve)
• Asymptomatic complete AV block with ventricular rate less than 4lt;4lt;40 bpm or pauses > 3 seconds while awake

Class IIa indications (reasonable, probably indicated) include asymptomatic first-degree AV block with very prolonged PR (> 300 ms) causing symptoms from loss of AV synchrony (pacemaker syndrome).

Pacing is NOT indicated for asymptomatic first-degree block or asymptomatic Mobitz type I unless there are very long pauses or very slow ventricular rates."

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Q5: Describe the ALS bradycardia algorithm for managing severe symptomatic heart block.

Model answer:

"The ALS bradycardia algorithm begins with assessment for adverse features: shock, syncope, myocardial ischaemia, or heart failure.

If adverse features are present, proceed with urgent treatment:

1. Atropine 500 mcg IV, repeated every 3-5 minutes up to a maximum of 3 mg (6 doses). Atropine is effective for AV nodal blocks but ineffective for infranodal blocks.
2. Transcutaneous (external) pacing: Apply pacing pads, set rate to 60-90 bpm, increase output until electrical and mechanical capture achieved. Provide analgesia/sedation as external pacing is painful.
3. Transvenous temporary pacing: Insert pacing wire via internal jugular or subclavian vein into the right ventricle. This is the definitive temporary measure.
4. Inotropic/chronotropic infusions (adrenaline 2-10 mcg/min, isoprenaline, dopamine) if pacing unavailable or ineffective.

If no adverse features but risk of asystole (recent asystole, Mobitz type II, complete block with wide QRS, pauses > 3 seconds), prepare for pacing and monitor closely with urgent cardiology referral."

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Q6: What are the complications of permanent pacemaker implantation?

Model answer:

"Complications can be divided into early (periprocedural) and late:

Early complications:

• Pneumothorax (1-2%): From subclavian or internal jugular venous access
• Bleeding or haematoma at the pacemaker pocket (2-5%)
• Lead displacement (1-5%, especially in first 6 weeks)
• Ventricular arrhythmias during lead insertion
• Cardiac perforation or tamponade (less than 1lt;1%)

Late complications:

• Infection (1-2%): Pocket infection or lead endocarditis; requires device extraction
• Lead fracture (0.5-1% per year): Detected on device interrogation
• Pacemaker syndrome (5-10% with VVI pacing): Loss of AV synchrony causing fatigue, dyspnoea, presyncope; managed by upgrading to dual-chamber (DDD) pacing
• RV pacing-induced cardiomyopathy (10-20% with chronic RV pacing): Can consider conduction system pacing (His bundle or left bundle branch area pacing) to avoid this
• Battery depletion: Requires generator replacement after 7-12 years

Overall, permanent pacing is safe with low complication rates and transforms prognosis in high-grade AV block."

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Clinical Scenario (OSCE/PACES Station)

Scenario: A 78-year-old man presents to A&E with recurrent episodes of dizziness and one episode of syncope. His pulse is 38 bpm and regular. ECG shows complete heart block with a wide QRS escape rhythm.

Tasks:

1. Take a focused history
2. Interpret the ECG
3. Outline immediate management

Model approach:

History:

• Details of syncope: Sudden onset? Duration? Recovery? Any warning? Any injury?
• Previous episodes of dizziness or syncope?
• Chest pain, dyspnoea, palpitations?
• Past medical history: Ischaemic heart disease, previous MI, hypertension, diabetes
• Medications: Beta-blockers, calcium channel blockers, digoxin, amiodarone
• Recent infections, tick bites (Lyme disease)

ECG interpretation: "The ECG shows complete heart block (third-degree AV block). There is complete AV dissociation with regular P waves and regular QRS complexes occurring independently. The atrial rate is approximately 80 bpm and the ventricular rate is 38 bpm. The escape rhythm has a wide QRS morphology, indicating an infranodal (ventricular) escape pacemaker. This is an unstable rhythm with high risk of asystole."

Immediate management:

1. Assess for adverse features: This patient has syncope and severe bradycardia (38 bpm), which are adverse features
2. Atropine 500 mcg IV: Unlikely to be effective (infranodal block) but should be tried as first-line
3. Transcutaneous pacing: Apply pacing pads, initiate pacing at 60-70 bpm, confirm capture. Provide analgesia/sedation
4. Urgent cardiology referral: Arrange transvenous temporary pacing as a bridge to permanent pacemaker
5. Monitoring: Continuous telemetry, frequent vital signs, defibrillator on standby
6. Investigations: Bloods (U&Es, troponin, digoxin level if on digoxin), echocardiography
7. Definitive treatment: Permanent pacemaker implantation (Class I indication)

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Additional Clinical Scenarios

Scenario 2: Young Patient with AV Block

Case: A 28-year-old woman presents with fatigue and documented first-degree AV block (PR 240 ms) on pre-employment ECG. She is asymptomatic.

Approach:

• History: Enquire about systemic symptoms (fever, rash, joint pain), family history of early pacemaker or sudden death, travel history (Lyme disease endemic areas), medications
• Examination: Look for signs of systemic disease (sarcoidosis, myotonic dystrophy—ptosis, myotonia, frontal balding)
• Investigations:
  • "Echocardiography: Exclude structural heart disease, cardiomyopathy"
  • Lyme serology if exposure risk
  • Consider genetic testing if family history (SCN5A mutations)
  • "Exercise ECG: Assess for worsening AV block (abnormal)"
• Management: If asymptomatic with isolated first-degree block, no treatment required. Annual follow-up with ECG. Advise to report symptoms (syncope, presyncope). If exercise-induced worsening or very prolonged PR (> 300 ms) with symptoms, refer to electrophysiology

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Scenario 3: Post-TAVI AV Block

Case: An 82-year-old man undergoes transcatheter aortic valve implantation (TAVI) for severe aortic stenosis. Post-procedure, he develops complete heart block.

Background: AV block complicates 10-30% of TAVI procedures due to mechanical trauma to the AV node and His bundle during valve deployment. Risk factors include pre-existing RBBB, large prosthesis size, and deep implantation. [19]

Management:

• Immediate: Temporary pacing if symptomatic or rate less than 4lt;4lt;40 bpm
• Observation: Monitor for 48-72 hours; some recover spontaneously
• Permanent pacing indication: Persistent complete heart block or high-grade AV block beyond 48 hours post-TAVI
• Device choice: Dual-chamber pacemaker (DDD) to maintain AV synchrony

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Scenario 4: Drug-Induced AV Block

Case: A 70-year-old man with hypertension and atrial fibrillation is on bisoprolol 10 mg and diltiazem 240 mg. He presents with dizziness and is found to have Mobitz type I AV block.

Pathophysiology: Beta-blockers and non-dihydropyridine calcium channel blockers (diltiazem, verapamil) both slow AV nodal conduction. Combination therapy increases risk of AV block.

Management:

• Stop or reduce offending drugs: Discontinue diltiazem (non-dihydropyridine CCB); reduce bisoprolol dose or switch to alternative rate control (e.g., digoxin—though also slows AV conduction)
• Monitor: Repeat ECG after 48-72 hours to assess resolution
• If persistent or high-grade block: Consider permanent pacing if symptomatic or Mobitz II/complete block develops
• Alternative rate control strategies: Consider catheter ablation of AV node + permanent pacemaker in refractory atrial fibrillation ("ablate and pace" strategy)

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Scenario 5: Congenital Complete Heart Block

Case: A neonate is found to have complete heart block on antenatal ultrasound. Mother has systemic lupus erythematosus (SLE).

Pathophysiology: Maternal anti-Ro/SSA and anti-La/SSB antibodies cross the placenta and damage fetal AV node, causing irreversible complete heart block. Risk is 1-2% in mothers with these antibodies.

Management:

• Antenatal monitoring: Fetal echocardiography to assess heart rate, ventricular function, and signs of heart failure (hydrops fetalis)
• Delivery planning: Early delivery if fetal distress; pacing may be required in neonatal period
• Postnatal: Permanent pacemaker if ventricular rate less than 5lt;5lt;55 bpm, structural heart disease, or heart failure
• Maternal counselling: High recurrence risk (15-20%) in subsequent pregnancies

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Special Populations

Athletes

Presentation: Highly trained athletes often have high vagal tone, leading to:

• Sinus bradycardia (30-40 bpm at rest)
• First-degree AV block (PR > 200 ms)
• Mobitz type I (Wenckebach) during sleep or rest

Distinguishing physiological from pathological:

• Physiological: Resolves with exercise (increased sympathetic tone); narrow QRS; no symptoms; structurally normal heart
• Pathological: Worsens or persists with exercise; wide QRS; symptoms; structural heart disease

Management: No treatment if physiological. If symptomatic or high-grade block, investigate as for non-athletes.

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Pregnancy

AV block in pregnancy:

• Pre-existing AV block: Usually well-tolerated if paced
• New AV block in pregnancy: Rare; consider acute myocarditis (viral, peripartum cardiomyopathy), Lyme disease, or drug effects

Pacemaker in pregnancy:

• Safe; can proceed with pregnancy and vaginal delivery
• Avoid lithotomy position (compresses IVC, reduces venous return)
• Pacing threshold may change during pregnancy (monitor device)

Emergency pacing in pregnancy:

• Transcutaneous and transvenous pacing safe in pregnancy if required
• Shield fetus during fluoroscopy; use echo guidance if possible

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Elderly and Frailty

Considerations:

• Degenerative conduction disease (Lenègre's, Lev's disease) is common
• Polypharmacy increases risk of drug-induced AV block
• Comorbidities (dementia, limited mobility) may influence pacing decision

Pacing in frail elderly:

• Permanent pacing still improves quality of life and prevents syncope (fall prevention)
• Consider single-chamber (VVI) pacing if AF or limited life expectancy
• Discuss goals of care; pacing may not be appropriate in palliative patients

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Differential Diagnoses

Conditions Mimicking AV Block

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Advanced Topics

Electrophysiology Study (EPS) Findings

Indications for EPS in AV block:

• Uncertain diagnosis (e.g., 2:1 block—cannot distinguish Mobitz I vs II)
• Symptoms suggestive of intermittent high-grade block but no documentation
• Risk stratification in asymptomatic bifascicular block

Key measurements:

• AH interval (atrial to His bundle): Normal 55-125 ms. Prolonged in AV nodal disease
• HV interval (His to ventricle): Normal 35-55 ms. Prolonged (> 55 ms) indicates His-Purkinje disease (high risk; pacing indicated)
• Block below His: Definitive indication for pacing

Provocative testing:

• Atrial pacing at increasing rates to provoke AV block
• If 2:1 block occurs at rates less than 1lt;1lt;130 bpm, suggests infranodal disease

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Conduction System Pacing

Traditional right ventricular (RV) pacing causes ventricular dyssynchrony (electrical activation spreads slowly from RV pacing site, mimicking LBBB). Chronic RV pacing can cause LV dysfunction (pacing-induced cardiomyopathy) in 10-20% of patients. [15]

Conduction system pacing alternatives:

Indications: Patients requiring high-burden ventricular pacing (e.g., complete heart block, especially if reduced LVEF or expected long-term pacing).

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Cardiac Resynchronization Therapy (CRT)

Indication in AV block: Patients with:

• AV block requiring pacing
• LV systolic dysfunction (LVEF ≤35%)
• LBBB or anticipated high RV pacing burden (> 40%)

Device: Biventricular pacemaker (CRT-P) or CRT with defibrillator (CRT-D). Paces both RV and LV to maintain ventricular synchrony.

Outcome: Improves LV function, reduces heart failure hospitalisations, and improves survival compared to RV pacing alone in heart failure patients.

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Rare and Infiltrative Causes

Cardiac Sarcoidosis

Pathophysiology: Granulomatous infiltration of the myocardium, particularly the basal septum (conduction system).

Presentation: AV block (often high-grade), ventricular arrhythmias, heart failure, sudden cardiac death.

Diagnosis:

• Cardiac MRI: Late gadolinium enhancement in septum; oedema on T2-weighted imaging
• PET scan: Increased FDG uptake in active inflammation
• Endomyocardial biopsy: Non-caseating granulomas (low sensitivity—patchy disease)
• Systemic evaluation: Chest CT (hilar lymphadenopathy), serum ACE, lymphocyte count

Management:

• Immunosuppression: Corticosteroids (prednisolone 0.5-1 mg/kg) for active inflammation; may prevent progression
• Permanent pacemaker: High-grade AV block or symptomatic bradycardia
• ICD: High risk of sudden death; consider if ventricular arrhythmias or LVEF less than 3lt;3lt;35%

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Cardiac Amyloidosis

Pathophysiology: Amyloid protein deposition in myocardium and conduction system.

Presentation: Restrictive cardiomyopathy, AV block, atrial arrhythmias, heart failure.

Diagnosis:

• Echocardiography: Thickened LV walls, "sparkling" myocardium, restrictive filling, biatrial enlargement
• Cardiac MRI: Diffuse global subendocardial late gadolinium enhancement
• Biopsy: Congo red staining (apple-green birefringence); immunohistochemistry for AL vs ATTR
• Nuclear imaging: Technetium-99m bone scans (DPD, PYP, HMDP)—high uptake in ATTR cardiac amyloidosis

Management:

• Permanent pacemaker: Symptomatic AV block
• ICD: Risk of sudden death if advanced disease
• Disease-specific therapy:
  • "AL amyloidosis: Chemotherapy (bortezomib, melphalan)"
  • "ATTR amyloidosis: Tafamidis (transthyretin stabilizer), patisiran (gene silencing)"

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Lyme Disease (Lyme Carditis)

Pathophysiology: Borrelia burgdorferi spirochaete directly invades cardiac tissue, causing myocarditis and inflammation of the AV node.

Epidemiology: Occurs in 1-10% of Lyme disease cases. Endemic areas: Northeastern US, Europe.

Presentation: AV block (often fluctuating—can progress from first-degree to complete block within hours), myopericarditis. History of tick bite, erythema migrans rash (bull's-eye rash).

Diagnosis:

• Serology: Enzyme-linked immunosorbent assay (ELISA) for Borrelia IgM/IgG; confirm with Western blot
• ECG: Variable AV block (may fluctuate hour-to-hour)

Management:

• Antibiotics: Ceftriaxone 2 g IV daily for 14-21 days (or doxycycline 100 mg PO BD if mild)
• Temporary pacing: If complete heart block or symptomatic bradycardia
• Permanent pacing: Rarely required; AV block usually resolves with antibiotic therapy within 1-6 weeks [6]
• Monitoring: Continuous telemetry; may need temporary pacing during antibiotic treatment

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Drug Toxicity Syndromes

Digoxin Toxicity

Mechanism: Digoxin inhibits Na+/K+-ATPase, increasing intracellular calcium. Enhances vagal tone, slowing AV nodal conduction.

AV block in digoxin toxicity: First-degree, Mobitz I, or complete heart block. Often associated with other arrhythmias (atrial tachycardia with AV block, ventricular ectopy).

Other features: Nausea, vomiting, confusion, yellow-green vision (xanthopsia).

Diagnosis: Digoxin level > 2.5 mcg/L (therapeutic 0.8-2.0 mcg/L). Hypokalaemia exacerbates toxicity.

Management:

• Stop digoxin
• Correct hypokalaemia: Potassium replacement (careful—avoid hyperkalaemia which also causes AV block)
• Atropine: May improve AV nodal conduction
• Digoxin-specific antibody fragments (DigiFab): Indication: life-threatening arrhythmias, severe bradycardia unresponsive to atropine, hyperkalaemia > 5.5 mmol/L with digoxin toxicity. Dose: Number of vials = (serum digoxin [ng/mL] × weight [kg]) / 100
• Temporary pacing: If severe symptomatic bradycardia

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Beta-Blocker and Calcium Channel Blocker Toxicity

Combined toxicity: Profound AV block, bradycardia, hypotension, cardiogenic shock.

Management:

• Stop drugs
• IV fluids: For hypotension
• Atropine: May help AV nodal block (limited efficacy)
• Calcium gluconate: 10-20 mL of 10% solution IV over 10 minutes (for calcium channel blocker toxicity; counteracts calcium channel blockade)
• Glucagon: 5-10 mg IV bolus, then infusion 1-10 mg/hr (bypasses beta-receptors, increases cAMP and cardiac contractility/chronotropy)
• High-dose insulin euglycaemic therapy (HIET): Insulin 1 unit/kg IV bolus, then 1-10 units/kg/hr infusion with 50% dextrose to maintain euglycaemia. Mechanism: Insulin has positive inotropic and chronotropic effects independent of beta-receptors
• Intralipid (lipid emulsion therapy): 20% lipid emulsion 1.5 mL/kg IV bolus, then infusion 0.25 mL/kg/min. Indicated for severe refractory cardiovascular collapse (beta-blocker or CCB overdose)
• Transcutaneous/transvenous pacing: If severe bradycardia unresponsive to pharmacotherapy
• Extracorporeal membrane oxygenation (ECMO): Last resort for refractory cardiogenic shock

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Genetic and Familial AV Block

Progressive Cardiac Conduction Defect (PCCD)

Genetics: Autosomal dominant. Mutations in SCN5A (encodes cardiac sodium channel Nav1.5). Loss-of-function mutations reduce sodium current, slowing conduction. [20]

Phenotype: Progressive conduction disease starting in 3rd-4th decade. Sequence: First-degree AV block → bundle branch block → high-grade AV block → complete heart block.

Associated conditions: Sick sinus syndrome, atrial fibrillation, ventricular arrhythmias (Brugada syndrome phenotype in some SCN5A mutations).

Management:

• Genetic counselling: Family screening (ECG, genetic testing)
• Pacemaker: Indicated when symptomatic or high-grade block develops
• ICD: Consider if concurrent Brugada phenotype or family history of sudden death

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Myotonic Dystrophy Type 1

Genetics: Autosomal dominant. CTG repeat expansion in DMPK gene.

Cardiac involvement: Progressive conduction disease (AV block, bundle branch block), atrial arrhythmias, dilated cardiomyopathy, sudden death.

Clinical features: Myotonia (delayed muscle relaxation), muscle weakness, frontal balding, ptosis, cataracts, endocrine abnormalities (diabetes, hypogonadism).

Cardiac screening: Annual ECG, echocardiography. Holter monitoring if palpitations or syncope.

Management:

• Pacemaker: High-grade AV block or symptomatic bradycardia
• ICD: Consider if ventricular arrhythmias or LVEF less than 3lt;3lt;35%
• Anaesthetic risk: Sensitivity to sedatives and anaesthetics; risk of malignant arrhythmias perioperatively

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Pacemaker Coding and Programming

NBG Pacing Code

Pacemakers are described using a 5-letter code:

Common modes:

• VVI: Ventricular pacing, ventricular sensing, inhibited by sensed event. Used in permanent AF + AV block
• DDD: Dual-chamber pacing and sensing, dual response. Maintains AV synchrony
• VVIR/DDDR: Rate-responsive (R) versions; adjust rate based on activity sensor

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Pacemaker Programming Basics

Key parameters:

• Lower rate limit (LRL): Minimum paced heart rate (e.g., 60 bpm)
• Upper rate limit (URL): Maximum paced heart rate (e.g., 130 bpm)
• AV delay: Time between atrial pacing/sensing and ventricular pacing (mimics PR interval; typically 150-200 ms)
• PVARP (post-ventricular atrial refractory period): Time after ventricular event during which atrial sensing is inhibited (prevents pacemaker-mediated tachycardia)

Mode switching: DDD pacemakers can automatically switch to VVI mode if atrial tachyarrhythmia (e.g., AF) detected, preventing rapid ventricular pacing.

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Emerging Therapies and Research

Leadless Pacemakers

Technology: Self-contained pacemaker capsule (no leads) implanted directly into RV via femoral vein. Examples: Micra (Medtronic), Nanostim (St Jude—withdrawn).

Advantages: No lead complications (fracture, infection); no pocket infection; suitable for patients with limited venous access.

Limitations: Single-chamber (VVI) only; battery non-replaceable (entire device extracted when depleted); no dual-chamber or CRT capability (though dual-chamber leadless systems in development).

Indications: Permanent AF with AV block requiring VVI pacing; high infection risk; venous occlusion.

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Biological Pacing (Gene Therapy)

Concept: Viral vector delivery of genes to create a biological pacemaker by converting ventricular myocytes into pacemaker-like cells.

Approaches:

• Overexpression of HCN channels (hyperpolarization-activated cyclic nucleotide-gated channels) to create pacemaker current (If)
• Suppression of inward rectifier potassium current (IK1) to promote automaticity
• Conversion of fibroblasts to pacemaker cells using transcription factors (TBX18)

Status: Preclinical and early clinical trials. Proof-of-concept demonstrated in animal models. Not yet in routine clinical use.

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Quality Improvement and Audit

Key Metrics for AV Block Management

Emergency department:

• Time from presentation to ECG diagnosis in bradycardia: Target less than 1lt;1lt;10 minutes
• Time from diagnosis to transcutaneous pacing in unstable complete heart block: Target less than 3lt;3lt;30 minutes
• Appropriate use of atropine in AV block (should not be overused in infranodal block)

Cardiology service:

• Time from referral to permanent pacemaker implantation: Target less than 4lt;4lt;48 hours for urgent cases
• Complication rate (pneumothorax, lead displacement, infection): Benchmark less than 5lt;5lt;5%
• Device infection rate: Benchmark less than 1lt;1lt;1% per year
• Post-pacemaker follow-up: 100% of patients should have 6-week check and annual review

Outcomes:

• Syncope recurrence post-pacemaker: Should be near 0%
• Patient-reported quality of life improvement post-pacing
• All-cause mortality in paced vs non-paced complete heart block (audit pacing appropriateness)

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Summary: Key Take-Home Points

1. Heart block classification: First-degree (PR > 200 ms), Mobitz I (progressive PR prolongation), Mobitz II (fixed PR → dropped QRS), complete (AV dissociation)
2. "Mobitz I wanders, Mobitz II drops": Mobitz I is benign; Mobitz II requires pacing
3. Atropine works for AV nodal blocks only: Ineffective for infranodal (His-Purkinje) disease
4. Complete heart block is an emergency: 50% 1-year mortality untreated; permanent pacing is life-saving
5. Post-MI block prognosis differs: Inferior MI (transient, good prognosis); anterior MI (permanent, poor prognosis)
6. Pacemaker indications: Symptomatic bradycardia, Mobitz II, complete block, post-MI persistent block
7. ALS bradycardia algorithm: Atropine → transcutaneous pacing → transvenous pacing → permanent pacing
8. Broad QRS escape = high risk: Ventricular escape less than 4lt;4lt;40 bpm is unstable; needs immediate pacing
9. Reversible causes matter: Drugs (beta-blockers, CCBs, digoxin), hyperkalaemia, Lyme disease—treat underlying cause
10. Infiltrative diseases: Sarcoidosis, amyloidosis—require immunosuppression or disease-specific therapy plus pacing
11. Conduction system pacing: His bundle or left bundle branch area pacing avoids RV pacing-induced cardiomyopathy
12. Young patients with AV block: Investigate for infiltrative disease, Lyme disease, genetic causes (SCN5A mutations, myotonic dystrophy)

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