U01.07.024 AV block

Atrioventricular (AV) block is a conduction abnormality where the electrical signal from the atria to the ventricles is delayed or completely blocked. It is classified into three degrees based on severity.

Types of AV Block

First-Degree AV Block

Definition: Prolonged PR interval (> 200 msec).
Clinical Features:
- Benign and asymptomatic.
- No treatment is required.

Second-Degree AV Block

The second-degree AV block is further divided into Mobitz Type I (Wenckebach) and Mobitz Type II.

Mobitz Type I (Wenckebach)

Mechanism: Progressive lengthening of the PR interval until a beat is “dropped” (a P wave not followed by a QRS complex).
Clinical Features:
- Usually asymptomatic.
- Variable RR interval with a regularly irregular pattern.

Mobitz Type II

Mechanism: Dropped beats without a preceding change in PR interval (unlike Type I).
Clinical Features:
- May progress to third-degree AV block.
- Often requires a pacemaker.

Third-Degree (Complete) AV Block

Mechanism: The atria and ventricles beat independently (P waves and QRS complexes are not rhythmically associated).
Clinical Features:
- Atrial rate > Ventricular rate.
- Usually requires a pacemaker.
- Can be caused by Lyme disease.

Summary Table of AV Blocks

Type	Mechanism	ECG Findings	Clinical Significance	Treatment
First-Degree	Delayed AV conduction	PR interval > 200 msec	Asymptomatic, benign	None required
Second-Degree (Mobitz I)	Progressive PR lengthening until dropped beat	Regularly irregular RR interval	Usually asymptomatic	None required
Second-Degree (Mobitz II)	Dropped beats without PR length change	Sudden absent QRS complex	May progress to 3rd-degree block	Pacemaker
Third-Degree (Complete)	Atria and ventricles beat independently	P waves and QRS not rhythmically associated	Atrial rate > Ventricular rate	Pacemaker (possible Lyme disease association)

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