What is AV Block?
It is a disorder of the heart’s electrical conduction system in which impulses from the atria are delayed or fail to reach the ventricles. The AV node normally acts as a gateway regulating conduction; disruption leads to varying degrees of heart block. It is commonly classified into first-degree, second-degree (Mobitz I and Mobitz II), and third-degree (complete) block.
Types of AV Block
1. First-Degree
All atrial impulses are conducted, but with a delay. It is usually benign and often asymptomatic.
2. Second-Degree
- Mobitz I (Wenckebach): Progressive prolongation of PR interval until a beat is dropped. Often transient and less serious.
- Mobitz II: Sudden dropped beats without prior PR prolongation. More dangerous and may progress to complete heart block.
3. Third-Degree (Complete)
No atrial impulses reach the ventricles. The ventricles rely on an escape rhythm, which is slow and unreliable. This is a medical emergency.
Causes
There are manifold causes of block that are following.
Cardiac Causes
- Ischemic heart disease (e.g., Myocardial Infarction)
- Degenerative fibrosis of the conduction system
- Cardiomyopathies
- Congenital heart defects
Non-Cardiac Causes
- Electrolyte imbalances (e.g., hyperkalemia)
- Infections (e.g., Lyme Disease)
- Increased vagal tone (common in athletes)
Drug-Induced Causes
- Beta-blockers
- Calcium channel blockers (e.g., Verapamil)
- Digoxin
- Antiarrhythmic drugs
Signs and Symptoms of AV Block
The symptoms of block depend on the severity of the block:
Mild (First-degree, some Mobitz I)
- Often asymptomatic
- Mild fatigue
- Lightheadedness
Moderate to Severe (Mobitz II, Third-degree)
- Dizziness or syncope (fainting)
- Shortness of breath
- Chest pain
- Bradycardia (slow pulse)
- Hypotension
- Confusion (especially in elderly)
Diagnosis
A physician may ask for the following investigations according to the patient’s symptoms.
- Electrocardiogram (ECG): Key tool showing PR interval changes and dropped beats
- Holter monitoring: Detects intermittent blocks
- Blood tests: Identify electrolyte imbalance or drug toxicity
- Echocardiography: Evaluates structural heart disease
Treatment of AV Block
1. First-Degree AV Block
- Usually no treatment required
- Monitor and avoid causative drugs
2. Second-Degree AV Block
Mobitz I AV Block
- Often no treatment needed
- If symptomatic → Atropine may be used
Mobitz II AV Block
- Requires urgent intervention
- 0.6mg IV atropine is injected if fails then temporary pacing is recommended
- Permanent pacemaker is usually indicated for long term treatment
3. Third-Degree AV Block
- Medical emergency
- Injection atropine (3mg intravenous) or injection isoprenaline (2mg in 500ml 5% dextrose water, infused at 10-60 ml/hr) till temporary pacemaker is placed.
- Definitive treatment: Permanent pacemaker for lifelong management
General Management Principles
- Treat underlying cause (e.g., ischemia, infection)
- Stop or adjust causative medications
- Correct electrolyte imbalances
- Continuous cardiac monitoring in severe cases
Severe Complications
- Heart failure
- Sudden cardiac arrest
Prognosis of AV Block
- First-degree and Mobitz I blocks generally have a good prognosis
- Mobitz II and third-degree blocks carry significant risk without treatment
- Pacemaker therapy significantly improves survival and quality of life
Conclusion
It is a spectrum of conduction abnormalities ranging from benign delay to life-threatening complete block. Early recognition through ECG and timely management—especially pacemaker insertion in advanced cases—are crucial in preventing complications and improving outcomes.