Nocturnal Second-Degree Type I AV Block: A Benign Manifestation of Vagal Tone in Sleep

Second-degree type I AV block, also known as Mobitz I or Wenckebach, is typically characterized by a progressive prolongation of the PR interval followed by a dropped beat. While often evaluated for underlying conduction disease, it is not always pathological—especially when it occurs exclusively during sleep.

This article explores the physiological basis for Mobitz I blocks seen at night, the role of autonomic tone, and why this phenomenon is generally benign in healthy individuals.

Understanding Mobitz I AV Block

Mobitz I is caused by gradual fatigue of conduction through the atrioventricular (AV) node, which is rich in calcium channel-dependent tissue and highly responsive to autonomic influences.

Key features:

• Progressive PR interval lengthening

• Followed by a non-conducted P wave (dropped QRS)

• After the dropped beat, the PR interval resets and the cycle repeats

The block typically occurs within the AV node itself, as opposed to Mobitz II, which is usually infranodal.

The Role of Autonomic Tone in Sleep

During non-REM sleep, particularly in the deep stages, the body enters a state of increased parasympathetic (vagal) tone and decreased sympathetic drive. This shift in autonomic balance affects the heart in the following ways:

• Sinus node: Reduced firing rate → bradycardia

• AV node: Slower conduction and longer refractory period → PR interval prolongation

• Increased AV nodal refractoriness makes it more likely that a P wave will arrive during the refractory period, resulting in a dropped beat

This is not due to structural disease, but rather transient, reversible AV nodal suppression due to high vagal tone.

Why Is Nocturnal Mobitz I Benign?

Several factors support its benign nature:

1. Occurs in Healthy Individuals

• Especially common in young adults, athletes, and individuals with high baseline vagal tone

• Often seen on Holter monitors during overnight monitoring

2. Normal AV Node Physiology

• The AV node is intrinsically sensitive to vagal stimulation

• Unlike Mobitz II, Mobitz I does not imply conduction system disease below the AV node, which carries more prognostic weight

3. Resolves with Arousal or Exercise

• Upon waking, sympathetic tone rises, and AV nodal conduction normalizes

• The block is rate-related and not fixed

4. No Associated Symptoms

• Most patients are asymptomatic during episodes

• When symptoms (e.g., fatigue or bradycardia) do occur, they are mild and self-limited  

Clinical Takeaway

Second-degree type I AV block that occurs only during sleep is a physiological vagally mediated phenomenon, not a sign of disease. It reflects the natural balance of autonomic tone during rest, particularly in individuals with robust parasympathetic tone. Recognizing this pattern can prevent unnecessary workup, pacemaker implantation, or concern in otherwise healthy individuals.