Overview
Heart Block (AV Block)
Topic Overview
Summary
Heart block (atrioventricular block) is impaired conduction from atria to ventricles. It ranges from first-degree (prolonged PR, benign) to complete heart block (no AV conduction, potentially fatal). Mobitz II and complete heart block are high-risk and usually require pacing. Causes include ischaemic heart disease, drugs (beta-blockers, digoxin), and conduction system fibrosis. Emergency management follows the ALS bradycardia algorithm: atropine, transcutaneous pacing, then transvenous pacing.
Key Facts
- First-degree: Prolonged PR (over 200 ms); benign
- Second-degree Mobitz I (Wenckebach): Progressive PR prolongation → dropped beat; usually benign
- Second-degree Mobitz II: Fixed PR → sudden dropped beat; high risk; needs pacing
- Third-degree (complete): No AV conduction; escape rhythm; always needs pacing
- Emergency treatment: Atropine (may not work in Mobitz II/complete); transcutaneous pacing; transvenous pacing
- Permanent pacing: Indicated for symptomatic bradycardia, Mobitz II, complete heart block
Clinical Pearls
"Mobitz I — benign; Mobitz II — bad" — Mobitz II has high risk of progressing to complete heart block
Atropine is unlikely to work in complete heart block (acts on AV node, not infra-nodal block)
Broad QRS escape in complete heart block = unstable; needs immediate pacing
Why This Matters Clinically
High-grade heart block causes syncope, heart failure, and sudden death. Recognising the ECG patterns and initiating pacing promptly is life-saving.
Visual Summary
Visual assets to be added:
- ECG examples of first, second, and third-degree block
- ALS bradycardia algorithm
- Transcutaneous pacing technique
- Heart block classification diagram
Epidemiology
Incidence
- First-degree block: Common (1-2% of population)
- Complete heart block: 0.02-0.04% of population
- Risk increases with age
Demographics
- Elderly (degenerative conduction disease)
- Patients with ischaemic heart disease
Causes
| Category | Examples |
|---|---|
| Ischaemic | Acute MI (especially inferior MI), chronic IHD |
| Degenerative | Lenegre's disease, Lev's disease |
| Drugs | Beta-blockers, digoxin, calcium channel blockers, amiodarone |
| Infective | Lyme disease, endocarditis, myocarditis |
| Structural | Cardiac surgery, congenital |
| Electrolyte | Hyperkalaemia |
| Infiltrative | Sarcoidosis, amyloidosis |
Pathophysiology
AV Conduction System
- SA node → atria → AV node → Bundle of His → bundle branches → Purkinje fibres → ventricles
Levels of Block
| Level | Location | QRS |
|---|---|---|
| Supra-Hisian | AV node | Narrow (AV nodal escape) |
| Infra-Hisian | His bundle or below | Wide (ventricular escape) |
First-Degree Block
- Slow conduction through AV node
- All impulses conducted
- PR over 200 ms
Second-Degree Block — Mobitz I (Wenckebach)
- Progressive AV nodal fatigue
- PR prolongs until beat dropped
- Grouped beating pattern
Second-Degree Block — Mobitz II
- Infra-nodal disease
- Sudden dropped beat without prior PR prolongation
- High risk of complete block
Third-Degree (Complete) Block
- No atrial impulses conducted
- Escape rhythm (junctional or ventricular) maintains circulation
- Atria and ventricles beat independently (AV dissociation)
Clinical Presentation
Symptoms
Signs
Red Flags
| Finding | Significance |
|---|---|
| Syncope | Stokes-Adams attack |
| Heart failure | Bradycardia-induced |
| Mobitz II or complete block | Needs pacing |
| Broad complex escape | Unstable; urgent pacing |
| Post-MI | May need temporary pacing |
Often asymptomatic (first-degree, Mobitz I)
Common presentation.
Dizziness, lightheadedness
Common presentation.
Syncope, near-syncope
Common presentation.
Fatigue
Common presentation.
Dyspnoea (heart failure)
Common presentation.
Sudden cardiac death (complete heart block)
Common presentation.
Clinical Examination
Vital Signs
- Heart rate (often 30-50 in complete block)
- Blood pressure (may be maintained or low)
Cardiovascular
- Irregular pulse (second-degree) or regular slow pulse (complete)
- Cannon A waves (JVP)
- Variable S1
- Signs of heart failure
Investigations
ECG — Essential
| Type | ECG Features |
|---|---|
| First-degree | PR over 200 ms; all P waves conducted |
| Mobitz I (Wenckebach) | Progressive PR prolongation → dropped QRS; grouped beating |
| Mobitz II | Fixed PR interval; sudden dropped QRS; often wide QRS |
| Third-degree | No relationship between P waves and QRS; AV dissociation |
Blood Tests
| Test | Purpose |
|---|---|
| U&E | Hyperkalaemia |
| Digoxin level | Toxicity |
| Troponin | Acute MI |
| TFTs | Hypothyroidism |
Other
- Echocardiography (structural heart disease)
- Holter monitor (if intermittent block suspected)
- Electrophysiology study (if diagnosis unclear)
Classification & Staging
Degree of Block
| Type | Description | Risk |
|---|---|---|
| First-degree | Prolonged PR; all conducted | Low |
| Second-degree Mobitz I | Progressive PR prolongation | Low |
| Second-degree Mobitz II | Sudden dropped beat | High |
| Third-degree (complete) | No AV conduction | High |
2:1 Block
- Cannot distinguish Mobitz I from II
- Treat as high risk if wide QRS
Management
ALS Bradycardia Algorithm
1. Assess for Adverse Features:
- Shock
- Syncope
- Myocardial ischaemia
- Heart failure
2. If Adverse Features Present:
- Atropine 500 mcg IV (repeat up to 3 mg)
- Transcutaneous pacing (if atropine ineffective)
- Transvenous pacing (for sustained pacing)
- Inotropes (dopamine, isoprenaline, adrenaline infusion)
3. If No Adverse Features:
- Monitor
- Seek expert help
- Prepare for pacing if risk of asystole
Risk of Asystole (Need Pacing Even if Stable)
- Recent asystole
- Mobitz II AV block
- Complete heart block with broad QRS
- Ventricular pause over 3 seconds
Permanent Pacemaker Indications
- Symptomatic bradycardia due to AV block
- Mobitz II (even if asymptomatic)
- Complete heart block
- Post-MI persistent block
Reversible Causes
- Stop offending drugs (beta-blocker, digoxin)
- Correct hyperkalaemia
- Treat Lyme disease, myocarditis
Complications
Of Heart Block
- Syncope (Stokes-Adams attacks)
- Heart failure
- Sudden cardiac death (complete block without escape)
- Bradycardia-induced cardiomyopathy
Of Pacing
- Lead displacement
- Infection
- Pneumothorax (transvenous)
- Pacing failure
Prognosis & Outcomes
First-Degree and Mobitz I
- Excellent prognosis
- Usually benign
Mobitz II and Complete Heart Block
- High risk without pacing
- Excellent prognosis with permanent pacemaker
Post-MI Block
- Inferior MI: Usually transient (AV nodal, resolves)
- Anterior MI: Often permanent (extensive conduction damage)
Evidence & Guidelines
Key Guidelines
- Resuscitation Council UK ALS Guidelines (2021)
- ESC Guidelines on Cardiac Pacing (2021)
Key Evidence
- Transcutaneous pacing is effective as bridge to transvenous pacing
- Permanent pacing improves symptoms and survival in high-grade block
Patient & Family Information
What is Heart Block?
Heart block is when the electrical signals that control your heartbeat are slowed or blocked. This can make your heart beat too slowly.
Types
- First-degree: Mild; usually no treatment needed
- Second-degree: May need monitoring or a pacemaker
- Third-degree (complete): Always needs a pacemaker
Symptoms
- Dizziness, fainting
- Tiredness
- Shortness of breath
Treatment
- Medication review
- Pacemaker (a small device implanted under the skin to control your heartbeat)
Resources
References
Primary Guidelines
- Resuscitation Council UK. Adult Advanced Life Support Guidelines. 2021. resus.org.uk
- Glikson M, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520. PMID: 34455427
Key Studies
- Epstein AE, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities. Circulation. 2008;117(21):e350-e408. PMID: 18483207