Atrioventricular (AV) Blocks

DELAYED vs. INTERMITTENT vs. COMPLETE:

1. 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.

2. Second degree AV block: intermittent block of atrial conduction to the ventricles, often in patterns such as 2:1, 3:1, etc.
   
   a. Mobitz I (Wenckebach).
   b. Mobitz II.

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

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

 

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First Degree AV 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.

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

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.

 

Management

Asymptomatic patients require no therapy.

Pacemaker Syndrome patients can have a permanent pacemaker implanted, but this is actually rare.