Atrioventricular (AV) Blocks: REVIEW

Delayed vs. Intermittent vs. Complete:

1. First degree AV block: delayed conduction from 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.)

 

Second Degree AV Block

Second-degree AV block is an INTERMITTENT block of atrial conduction to the ventricle, often in patterns such as 2:1, 3:1, etc., designated as

a. Mobitz I  and
b. Mobitz II

 

MOBITZ II Second Degree AV Block

Second-degree type II (Mobitz II) rhythms are cardiac arrhythmias that have intermittent, non-conducted P waves (without the progressive prolongation of the PR interval, like in Mobitz I).

Whereas Mobitz I second-degree AV block usually results from AV node abnormalities, Mobitz type II second-degree AV block usually means there is a disease in the His-Purkinje system that causes failure to conduct an impulse from the atria to the ventricles. Mobitz I block occurs at the AV node, but Mobitz II block occurs below the AV node and within the bundle of His or both bundle branches.

 

Causes

This type of block originates within the AV Junction.

REVERSIBLE CAUSES:

A. Pathologic

• Myocardial ischemia of the conduction system
• Cardiomyopathy
• Myocarditis, endocarditis
• Hyperkalemia
• Hypervagal tone

B. Iatrogenic

• AV node-blocking medication
• Cardiac surgery
• Catheter ablation
• Catheter aortic valve implantation.

IRREVERSIBLE CAUSES: idiopathic cardiac conduction disease and myocardial fibrosis or sclerosis.

 

Signs and Symptoms

Mobitz II patients who only have an occasional missed beat and who have a normal heart rate of 60-100 are usually asymptomatic, but most are symptomatic from a reduced cardiac output, making them pale and diaphoretic.

Diagnosis of Mobitz II can be made by 

1. Irregular pulse, and
2. Suggestive symptoms:

• Irregular pulse
• Possible bradycardia (may have normal sinus rate)
• Fatigue
• Dyspnea
• Chest pain
• Syncope ("Stokes-Adams attacks")
• Cardiac arrest

3. EKG: persistent, unchanging PR intervals followed by block of one or more P waves that fail to conduct effectively below the SA node, causing a missed QRS.

 

Management

If hyperkalemia is the reversible cause, this should, by definition of "reversible," be reversed.

STABLE PATIENTS

Although stable Mobitz II patients don't require immediate pharmacologic or pacemaker therapy, they should be observed closely because Mobitz II itself is unstable and can progress to complete (third degree) heart block. Symptoms from even stable bradycardia will require an implantable pacemaker.

UNSTABLE PATIENTS

Unstable patients (hypotension, altered mental status, shock, chest pain, and pulmonary edema) require immediate pharmacologic therapy (atropine IF transcutaneous pacing is not immediately available) and a temporary pacemaker to increase heart rate and cardiac output. Once stable, any potentially reversible causes should be treated, followed by a permanent pacemaker, especially in patients without a reversible cause of the arrhythmia.

Atropine and Mobitz Type II Heart Block: While atropine is often the first-line pharmacological agent for treating bradycardia, it is generally ineffective in Mobitz Type II heart blocks. This is because Mobitz II block occurs below the AV node, typically in the His-Purkinje system, where atropine has little effect on conduction. Atropine primarily acts on the AV node, enhancing conduction through it, but it does not improve conduction in lower blocks like those seen in Mobitz II.

When Atropine Might Be Used: However, in unstable patients (e.g., those presenting with hypotension, altered mental status, or shock) and when transcutaneous pacing (TCP) is not immediately available, atropine can be considered as a temporary measure. It may provide some benefit, though its effectiveness is limited in this type of block. Definitive treatment with pacing or pharmacological support remains the priority.

Primary Treatment (TCP and Dopamine): The preferred treatment for symptomatic Mobitz II patients is transcutaneous pacing (TCP). When TCP is delayed, or unavailable, dopamine or epinephrine infusions may be recommended by your protocols to maintain cardiac output and perfusion by supporting heart rate. These medications act as inotropic agents, increasing heart rate and improving perfusion until pacing can be initiated or a permanent pacemaker is considered.

 

High-Grade AV Block

An advanced or "high-grade AV block" is an advanced form of Mobitz Type II second-degree AV block in which there is more than one dropped beat--one or more P waves in a row do not precede a QRS complex. The ratio can be regular or irregular, such as 3:1, 4:1, or others.

 

The Mobitz I vs. Mobitz II Cheatsheet

SIMILARITIES:

• P waves are normal
• Mobitz I and II both involve interruption of conduction from atria to the ventricles
• Both can be asymptomatic; both can be symptomatic. (Most type I are asymptomatic; most type II are symptomatic.)
• Both may have 2:1 AV contraction ratio, but > 2:1 is always Mobitz II.

In 2:1 AV block, every other P wave is conducted (every other P wave is also non-conducted); it isn't possible to determine whether a 2:1 AV block is Mobitz I or II, since both types can result in this pattern.

DIFFERENCES:

MOBITZ I:

• PR interval prolonged > 200 ms progressively.
• RR intervals get shorter
• Disease of AV node

Pattern: progressively lengthening PR intervals until there is a dropped beat due to intrinsic AV node disease. ? Does not require a pacemaker unless cause is irreversible and patient is symptomatic.

MOBITZ II:

• No change in PR interval, even after the dropped beat
• RR intervals are constant
• Disease below AV node, in Bundle of His and/or Purkinje fibers

Pattern: ratio of conducted beats to missed beats, e.g., 2:1, 3:1, etc.; usually due to non-reversible structural damage (often from infarct) to the conducting system.

Requires a pacemaker even when a patient is asymptomatic.