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 the 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. The delay usually 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

After each depolarization, the SA node enters a refractory period during which incoming impulses cannot trigger a new action potential. If an atrial impulse arrives during this refractory period, the SA node will not depolarize.

A few things can cause this kind of delay:

• Structural abnormality in the SA node.
• Drugs (digoxin, beta blockers, and some calcium channel blockers).
• Vagal inhibitory influences (autonomic nervous system).
• Myocardial infarct (MI).
• Cardiomyopathy and carditis.
• And sometimes, it’s just the AV node itself getting a bit older or damaged.

This delay isn’t caused by any problem with the SA node, which is where the heartbeat starts. The SA node still fires normally; it’s just that the signal slows down a bit on its way to the ventricles.

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.