What are the causes of vagally mediated heart block (Vagally Mediated (VM) Heart Block)?

From the Guidelines

Vagally mediated heart block (VM) is primarily caused by heightened parasympathetic tone.

Causes of Vagally Mediated Heart Block

The causes of vagally mediated heart block can be attributed to various factors, including:

• Increased parasympathetic tone, which can occur during sleep or as a result of vasovagal syncope 1
• Vasovagal syncope, which can cause a sudden increase in parasympathetic tone, leading to bradycardia and potentially atrioventricular block 1
• Heightened parasympathetic tone, which can mediate any type of atrioventricular block, including first-degree, second-degree, and third-degree atrioventricular block 1 Some key points to note about vagally mediated heart block include:
• It can be asymptomatic if the periods of atrioventricular block occur at night while sleeping 1
• It can be recognized by the presence of concomitant sinus node slowing (P-P prolongation) during sleep 1
• It is often accompanied by a significant vasodepressor effect in patients with vasovagal syncope 1

From the Research

Causes of Vagally Mediated Heart Block

The causes of vagally mediated heart block (VM heart block) can be attributed to several factors, including:

• Increased parasympathetic tone, which may lead to vagally mediated atrioventricular block (AVB) 2, 3
• Excessive vagal activation, which can cause transient or permanent atrioventricular block cases 3
• Autonomic nervous system imbalance, specifically the sympatho-vagal balance, which plays a role in the genesis of atrial fibrillation (AF) and potentially VM heart block 4
• Slowing of the sinus rate, which is associated with vagally mediated atrioventricular (AV) block 5

Pathophysiology

The pathophysiology of VM heart block involves:

• Paroxysmal AV block, localized within the AV node, associated with slowing of the sinus rate 5
• Increased parasympathetic tone, which can lead to AVB 2, 3
• Autonomic nervous system imbalance, which can contribute to the development of VM heart block 4

Diagnosis and Management

The diagnosis and management of VM heart block involve:

• Differentiation from intrinsic AV block, based on the behavior of the sinus rate 5
• Identification of asymptomatic or symptomatic events, such as syncope or presyncope 5
• Consideration of pacemaker implantation in symptomatic patients, but not in asymptomatic patients 2, 5