Physiology · Cardiovascular System

Heart block

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Definition

Heart block (atrioventricular block, AV block) is a defect in transmission of cardiac impulses from atria to ventricles.
There are three types: first, second and third degree; first and second degree are incomplete heart block, third degree is complete heart block.

Classification of Heart Block

Types of AV (heart) block

Type	Conduction	Key ECG feature
First degree	All atrial impulses reach ventricle, but delayed	Prolonged PR interval
Second degree, Mobitz I (Wenckebach)	Progressive delay till a beat is dropped	PR interval progressively lengthens, then a QRS is dropped
Second degree, Mobitz II	Only some atrial impulses conducted	Fixed-ratio non-conducted P waves, e.g. 2:1 or 3:1
Third degree (complete)	No atrial impulse reaches ventricle	Atrioventricular dissociation; P waves unrelated to QRS

Type

First degree

Conduction: All atrial impulses reach ventricle, but delayed
Key ECG feature: Prolonged PR interval

Second degree, Mobitz I (Wenckebach)

Conduction: Progressive delay till a beat is dropped
Key ECG feature: PR interval progressively lengthens, then a QRS is dropped

Second degree, Mobitz II

Conduction: Only some atrial impulses conducted
Key ECG feature: Fixed-ratio non-conducted P waves, e.g. 2:1 or 3:1

Third degree (complete)

Conduction: No atrial impulse reaches ventricle
Key ECG feature: Atrioventricular dissociation; P waves unrelated to QRS