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AV Block

Clinical Subject Page

Atrioventricular Block (AV Block)

ICD-10

I44.30

Specialty

Cardiology

Onset

Acute & Chronic

Reviewed

June 2026

On This Page

Overview

Atrioventricular (AV) block is a heart conduction disorder in which the electrical signals from the atria (upper chambers) to the ventricles (lower chambers) are delayed or completely blocked. This can cause a slow heart rate (bradycardia) and reduce blood flow to the body.

Etiology & Risk Factors

    • Cardiac Causes

      • Ischemic heart disease (especially acute myocardial infarction)
      • Congenital heart disease and congenital complete AV block
      • Post-cardiac procedures (e.g., valve surgery, catheter ablation, TAVI)
      • Myocarditis
      • Cardiomyopathies (e.g., amyloidosis, sarcoidosis)
      • Degenerative fibrosis of the conduction system (Lenègre-Lev syndrome)

      Noncardiac Causes

      • Increased vagal tone (e.g., athletes, vomiting, suctioning, intubation)
      • Electrolyte disorders (especially hyperkalemia)
      • Acid-base disorders
      • Cardiotoxic drugs
        • Beta-blockers
        • Calcium channel blockers
        • Digoxin
      • Infections
        • Lyme carditis
        • Bacterial endocarditis
        • Acute rheumatic fever
      • Endocrine disorders
        • Thyroid disease
        • Adrenal disease
      • Neuromuscular disorders
        • Myotonic dystrophy

      Risk Factors

      • Coronary artery disease or acute MI
      • Previous cardiac surgery or catheter ablation
      • Structural heart disease
      • Cardiomyopathy
      • Myocarditis
      • Electrolyte abnormalities (especially hyperkalemia)
      • Use of beta-blockers, calcium channel blockers, or digoxin
      • Lyme disease
      • High vagal tone (e.g., trained athletes)

Pathophysiology

        • AV block occurs when the electrical impulse from the atria to the ventricles is interrupted or delayed anywhere in the AV conduction system, including:
          • AV node
          • Bundle of His
          • Right and left bundle branches
        • AV nodal block
          • The impulse is delayed at the AV node.
          • Ventricular conduction remains normal → narrow QRS complex.
        • Infranodal block
          • The block occurs at or below the Bundle of His.
          • Ventricular depolarization is impaired → wide QRS complex.
        • More distal (lower) blocks
          • Result from more extensive damage to the conduction system.
          • Have a higher risk of progressing to complete (third-degree) heart block.

        Simple Flow

        Damage to the AV conduction system → Delayed or blocked impulse from atria to ventricles → Slow ventricular rate (bradycardia) → ↓ Cardiac output → Dizziness, syncope, or heart failure

Clinical Presentation

  1. Many Patients

    • Asymptomatic (especially first-degree and Mobitz type I AV block)

    Common Symptoms (due to bradycardia)

    • Fatigue
    • Exercise intolerance
    • Shortness of breath (dyspnea)
    • Dizziness
    • Syncope (fainting)
    • Palpitations (especially with irregular rhythms such as Mobitz I)

    Other Symptoms

    • Feeling of pulsations in the neck or chest (due to loss of AV synchrony)
    • Heart failure symptoms (fatigue, dyspnea, exercise intolerance)

    Severe Presentation

    • Stokes-Adams attacks (sudden brief loss of consciousness due to ventricular asystole)
    • Cardiac arrest (especially in advanced AV block)

History Taking

  • “Have you felt dizzy or lightheaded?”
  • “Have you ever fainted or nearly fainted?”
  • “Do you feel unusually tired?”
  • “Do you get short of breath during activity?”
  • “Do you notice your heart beating slowly or skipping beats?”
  • “Do you have chest pain?”
  • “Have your symptoms started suddenly or gradually?”
  • “Have you had a recent heart attack or heart surgery?”
  • “Are you taking medications such as beta-blockers, calcium channel blockers, or digoxin?”
  • “Have you been diagnosed with Lyme disease or thyroid disease?”

Degrees of AV Block

Atrioventricular (AV) Block · Classification & Management

AV BLOCK = DELAYED OR FAILED CONDUCTION FROM ATRIA TO VENTRICLES Spectrum ranges from benign PR prolongation to complete dissociation between atria and ventricles. Severity dictates urgency — Mobitz II and complete (3rd-degree) block carry risk of sudden progression to asystole.
Classification of AV Block
1
1st Degree
PR interval prolonged >200 ms. Every P wave is followed by a QRS — conduction is delayed, not dropped.
Usually benign
2
2nd Degree — Mobitz I
Progressive PR prolongation until a P wave is not conducted (dropped QRS), then cycle resets. Classic Wenckebach pattern.
Usually benign
3
2nd Degree — Mobitz II
Constant PR interval with intermittent, unpredictable dropped QRS complexes. Often fixed ratio (e.g. 2:1, 3:1).
Higher risk
4
High-Grade Block
Two or more consecutive P waves fail to conduct, but some AV conduction still occurs. Sits between Mobitz II and complete block.
Pre-complete block
5
3rd Degree (Complete)
Complete dissociation — no relationship between P waves and QRS. Ventricles driven by an escape rhythm (junctional or ventricular).
Medical emergency
Degree ECG Findings Typical Cause / Level of Block Management
1st Degree PR > 200 ms fixed, all P waves conduct Often AV node delay. Increased vagal tone, AV nodal disease, some drugs (beta-blockers, CCBs, digoxin). No treatment usually required. Monitor; review/adjust contributing drugs if symptomatic.
2nd Degree — Mobitz I Progressive PR lengthening → dropped beat → resets Usually AV node level. Increased vagal tone, inferior MI, drugs. Usually benign and asymptomatic. Observe; treat reversible cause. Pacing rarely needed.
2nd Degree — Mobitz II Fixed PR, sudden non-conducted P waves, often fixed ratio Infranodal (His-Purkinje/bundle branch) disease. Anterior MI, fibrosis, structural heart disease. Higher risk of progression to complete block. Admit, cardiology referral, permanent pacemaker usually indicated even if asymptomatic.
High-Grade Block ≥2 consecutive P waves not conducted, some conduction persists Usually infranodal. Similar substrate to Mobitz II. Treat as high risk — same urgency as Mobitz II/complete block. Pacing usually indicated.
3rd Degree (Complete) No P-QRS relationship, independent atrial & ventricular rates AV node or infranodal. Can be congenital, degenerative (Lenegre/Lev), MI, post-cardiac surgery, infiltrative disease. Medical emergency if unstable or symptomatic. Atropine/isoprenaline or transcutaneous pacing as bridge; permanent pacemaker definitive treatment.
Causes to Consider
Ischaemia — inferior MI (AV node, often Mobitz I/transient) vs anterior MI (infranodal, often Mobitz II/complete)
Drugs — beta-blockers, calcium channel blockers, digoxin, amiodarone
Degenerative disease — fibrosis of the conduction system (most common cause of chronic complete block in older adults)
Increased vagal tone — athletes, sleep, vasovagal episodes (typically 1st degree/Mobitz I)
Infective/inflammatory — Lyme disease, endocarditis (aortic root abscess), myocarditis, sarcoidosis
Post-surgical/procedural — cardiac surgery, TAVI, ablation near the conduction system
Electrolyte disturbance — hyperkalaemia
Congenital — congenital complete heart block (e.g. maternal anti-Ro/La antibodies)
Acute & Pacing Considerations
Assess for instability — hypotension, syncope, heart failure, chest pain, rate-related symptoms
Atropine 0.5 mg IV — first-line if symptomatic bradycardia; repeat to max 3 mg. Less effective in infranodal block.
Transcutaneous pacing — bridge therapy if atropine fails or block is infranodal; sedate/analgese as it is uncomfortable
Isoprenaline/adrenaline infusion — alternative bridge while awaiting pacing
Transvenous pacing wire — for unstable patients or high-grade/complete block awaiting permanent pacing
Permanent pacemaker indicated — symptomatic Mobitz I, any Mobitz II, high-grade block, complete heart block (even if asymptomatic)
Identify & treat reversible cause — stop/hold offending drugs, correct electrolytes, treat ischaemia/infection

Investigations

    • 1. 12-Lead ECG – First-line (Gold Standard)

      • Confirms the type and degree of AV block.

      2. Continuous ECG Monitoring

      • Detects intermittent or high-grade AV block.
      • Used in symptomatic or hospitalized patients.

      3. Laboratory Tests

      • Electrolytes (especially potassium)
      • Troponin (if myocardial infarction is suspected)
      • Renal function tests
      • Liver function tests
      • TSH
      • Blood gas (serum pH)
      • Lyme disease antibody testing (if suspected)
      • Drug levels (e.g., digoxin), when indicated

      4. Cardiac Imaging

      • Transthoracic echocardiography (TTE) to evaluate structural heart disease.
      • Consider cardiac MRI, CT, or TEE if further evaluation is needed.

Diagnosis

Diagnostic Approach

  • 12-lead ECG is the gold standard to diagnose and classify the degree of AV block.
  • Assess whether the patient is stable or unstable (signs of unstable bradycardia).
  • Identify the underlying cause (e.g., myocardial infarction, electrolyte abnormalities, medication toxicity).

ECG Findings

  • First-degree AV block: PR interval >200 ms.
  • Mobitz I (Wenckebach): Progressive PR prolongation → dropped QRS.
  • Mobitz II: Constant PR interval with intermittent dropped QRS complexes.
  • Third-degree AV block: AV dissociation (no relationship between P waves and QRS complexes).
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Management

1. Initial Management

  • Obtain a 12-lead ECG immediately.
  • Assess for unstable bradycardia.
  • Apply transcutaneous pacing pads if needed.

2. Unstable Patients

  • IV atropine
  • Transcutaneous pacing
  • Transvenous pacing if required
  • IV chronotropic medication (e.g., epinephrine)
  • Stabilize first, then investigate the cause.

3. Stable Patients

  • Treat the underlying cause:
    • Correct electrolyte abnormalities.
    • Treat acute myocardial infarction.
    • Stop or adjust AV nodal-blocking drugs (e.g., beta-blockers, calcium channel blockers, digoxin).
  • Perform TTE if structural heart disease is suspected.

4. Low-Risk AV Block

(First-degree, Mobitz I, and some 2:1 AV blocks)

  • Usually observation and follow-up if asymptomatic.
  • Monitor with periodic ECGs.
  • Pacemaker only for selected symptomatic patients.

5. High-Risk AV Block

(Mobitz II, high-grade AV block, third-degree AV block)

  • Continuous cardiac monitoring
  • Temporary pacing if necessary.
  • Permanent pacemaker for most patients with irreversible AV block.

Complications

      • Progression to complete (third-degree) heart block
      • Symptomatic bradycardia
      • Syncope (Stokes–Adams attacks)
      • Heart failure (due to reduced cardiac output)
      • Cardiac arrest
      • Sudden cardiac death (especially with untreated high-grade AV block)

Prognosis

  • First-degree AV block and Mobitz I generally have a good prognosis and often only require observation if asymptomatic.
  • Mobitz II and third-degree AV block have a higher risk of progression and usually require permanent pacemaker implantation.
  • Prognosis improves significantly when reversible causes are treated and high-grade AV block is managed with a permanent pacemaker.

Key Points / Clinical Pearls

  • AV block = delayed or blocked electrical conduction from the atria to the ventricles.
  • Three types: First-degree, Second-degree (Mobitz I & II), and Third-degree (complete heart block).
  • Gold-standard diagnosis: 12-lead ECG.
  • First-degree: PR interval >200 ms.
  • Mobitz I: Progressive PR prolongation → dropped QRS.
  • Mobitz II: Constant PR interval → dropped QRS (high risk).
  • Third-degree: Complete AV dissociation.
  • First-degree and Mobitz I: Usually observation if asymptomatic.
  • Mobitz II and third-degree: Usually require a permanent pacemaker.
  • Always identify and treat reversible causes (e.g., myocardial infarction, electrolyte abnormalities, medication toxicity).