Atrioventricular (heart) block !
Heart block (atrioventricular block) is a delay or interruption of the electrical signal from the upper to the lower chambers, which can slow the pulse.
Updated 2026-10-29
Structures involved
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- Atrioventricular (AV) node
- Bundle of His
- Bundle branches
- Sinoatrial (SA) node
- Right coronary artery
- Posterior descending artery (PDA)
- Interventricular septum
- Right atrium
In histology
In the clinical exam
In physiology
- The ventricular action potential
- The SA node pacemaker potential
- The cardiac conduction system
- ECG basics: waves, intervals and leads
- How arrhythmias arise
- Central venous pressure and the jugular pulse
What it is
Delay or interruption of conduction from the atria to the ventricles through the atrioventricular node or the His-Purkinje system, graded as first, second or third degree.
Who gets it
Higher-degree block becomes more common with age, mostly from degeneration of the conduction system. In Colombia and Latin America Chagas cardiomyopathy is an important cause of conduction disease and pacemaker need in younger adults.
Causes
- Age-related fibrosis of the conduction system
- Ischaemia, especially inferior myocardial infarction (right coronary artery)
- Drugs: beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, some antiarrhythmics
- Chagas cardiomyopathy and other infiltrative or inflammatory diseases
- High vagal tone in athletes (first degree and Mobitz I)
- After cardiac surgery or valve procedures; congenital block
Risk factors
- Older age
- Coronary artery disease
- Chagas disease
- AV-nodal blocking drugs
- Hyperkalaemia
- Aortic valve disease and its treatment
Pathophysiology
Block in the AV node usually causes gradual, narrow-QRS conduction failure and often responds to atropine. Block below the node (His bundle or bundle branches) is less predictable, gives wide escape rhythms and may suddenly progress to complete block with slow ventricular rates and low output.
Classification
- First degree: PR interval prolonged (over 200 ms), every P conducted
- Second degree Mobitz I (Wenckebach): PR lengthens progressively until a P is not conducted
- Second degree Mobitz II: constant PR with sudden dropped beats; usually infranodal
- Third degree (complete): P waves and QRS complexes independent (AV dissociation)
Symptoms and signs
- No symptoms in first degree and many Mobitz I cases
- Fatigue and exercise intolerance
- Dizziness, presyncope or syncope (Stokes-Adams attacks)
- Breathlessness or heart failure from slow rates
- Bradycardia; cannon a waves in the jugular pulse in complete block
Diagnosis
- Clinical
- Bradycardia and symptoms prompt review of drugs, ischaemia, electrolytes and Chagas exposure.
- Tests
- 12-lead ECG classifies the degree; Holter or event monitoring captures intermittent block. Potassium, thyroid function, troponin and Chagas serology as indicated; electrophysiology study in selected cases.
- Imaging
- Echocardiography looks for structural heart disease, ventricular function and valve disease.
Differential diagnosis
- Sinus bradycardia or sinus node dysfunction
- Blocked atrial premature beats
- Atrial fibrillation with slow ventricular response
- Vasovagal syncope
- Hyperkalaemia or drug toxicity
Goal of treatment
Maintain an adequate heart rate and cardiac output, remove reversible causes and prevent syncope and sudden death.
Treatment
- Conservative
- Observation for first degree and asymptomatic Mobitz I block; review of AV-nodal blocking drugs.
- Medical
- Correction of reversible causes: stopping offending drugs, treating hyperkalaemia or ischaemia.
- Acute
- For unstable bradycardia: anticholinergic drugs (atropine), then chronotropic infusions or transcutaneous or transvenous temporary pacing.
- Procedure
- Permanent pacemaker for symptomatic block, Mobitz II and complete block not due to a reversible cause.
- Follow-up
- Device follow-up and periodic ECG monitoring.
Drugs for this condition
Complications
- Syncope and falls with injury
- Heart failure
- Ventricular arrhythmias and sudden cardiac death
- Pacemaker complications: infection, lead problems
Prognosis
First degree and Mobitz I block usually have a good outlook. Mobitz II and complete block carry a risk of syncope and sudden death, which pacing largely removes.
Test yourself
Think of the answer, then tap the question to see it.
How does Mobitz I differ from Mobitz II on the ECG?
Mobitz I: PR lengthens progressively before a dropped beat. Mobitz II: PR stays constant and beats drop suddenly.
Why is Mobitz II more dangerous than Mobitz I?
It is usually below the AV node and can progress suddenly to complete heart block.
Which ECG finding defines third-degree heart block?
AV dissociation: P waves and QRS complexes occur independently, with a slow escape rhythm.
Which coronary artery usually supplies the AV node?
The right coronary artery (in right-dominant hearts), so inferior infarcts can cause AV block.
What is the definitive treatment of symptomatic complete heart block?
A permanent pacemaker, after excluding reversible causes.
Related conditions
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Atrioventricular Block (StatPearls (NCBI Bookshelf), 2024).
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