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Atrioventricular (AV) block: the different types and patient management

By Cristiano Antonino On Sep 27, 2022

Atrioventricular block (sometimes abbreviated BAV) is a defect in the conduction system of the heart, which occurs in the portion between the atrium and ventricle

Under physiological conditions, the sinoatrial node determines the heart rhythm by generating electrical signals that propagate to the ventricle.

In the case of an atrioventricular block, the electrical impulse does not reach the ventricles or is impaired.

Even if there is a complete interruption of conduction from the sinoatrial node, the ventricles continue to contract, albeit at a slower rate, thanks to the natural pacemaker.

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Atrioventricular (AV) Block, DELAYED vs. INTERMITTENT vs. COMPLETE:

First degree AV block: delayed conduction from the atrium to the ventricles in which the PR interval is >200 milliseconds (0.20 seconds) without interruption in atrioventricular conduction.

Second degree AV block: intermittent block of atrial conduction to the ventricles, often in patterns such as 2:1, 3:1, etc.

Mobitz I (Wenckebach).
Mobitz II.

Third degree AV block: complete block of signal from the atria to the ventricles. (The ventricles are “on their own.”)

High-grade AV block: 2 consecutive blocked P waves. (Usually an advanced form of Mobitz II.)

First Degree AV Atrioventricular Block

First degree AV block: delayed conduction from the atrium to the ventricles in which the PR interval is >200 milliseconds (0.20 seconds) without interruption in atrioventricular conduction.

Essentially, it’s not a true block but only a delay: the impulse from the SA node reaches the AV node at a slower-than-normal speed.

This still results in a normal QRS and usually an otherwise normal underlying rhythm. Usually the delay originates at the AV node, but may also be along the bundle of His or Purkinje fibers.

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PR Interval, The conduction between the atria and ventricles is the PR interval:

P → QR. This includes the pacemaker SA node propagating the atrial depolarization (P wave) and then further conduction through the AV node, bundle of His, bundle branches, and transmission terminating in the Purkinje fibers.

A first degree AV block is defined by a delay in the PR interval of > 200 ms.

Causes of atrioventricular block

With each atrial pulsation of electrical conductivity, the SA node becomes increasingly refractory, until a conduction lands on an unfirable SA node which cannot depolarize.

Structural abnormality in the SA node.

Drugs (digoxin, beta blockers, and some calcium channel blockers).

Vagal inhibitory influences (autonomic nervous system).

MI.

Cardiomyopathy and carditis.

Signs and Symptoms

Rarely are there symptoms due to the benign nature of a first degree AV block, so the diagnosis is usually via ECG and measurement of the PR interval (>200 ms).

Pacemaker Syndrome: symptoms, if present, include a frightening and/or uncomfortable awareness of one’s own heartbeat when the atrium closes against a closed mitral valve or if atrial contraction is coincidently right after ventricular systole → incomplete atrial filling.