Bradyarrhythmias and Atrioventricular Conduction Disorders

Bradyarrhythmias are abnormally slow heart rhythms, arising either from failure of impulse generation in the sinus node or from impaired conduction through the atrioventricular node and His-Purkinje system. Atrioventricular conduction disorders, or heart block, span first-degree delay through complete dissociation of atria and ventricles. This entry summarizes the conduction-system anatomy, the grading of block, and the clinical significance of slow rhythms as a reference topic.

Definition

Bradyarrhythmias are heart rhythms slower than expected for the physiological context, caused by impaired impulse formation in the sinoatrial node or impaired conduction in the atrioventricular node or His-Purkinje system; heart block denotes delayed or failed conduction from atria to ventricles.

Scope

The entry covers sinus node dysfunction and the spectrum of atrioventricular block (first-degree, second-degree types I and II, and third-degree/complete block), the relevant conduction-system anatomy, and the principles of escape rhythms and cardiac pacing as a conceptual remedy for symptomatic bradycardia. It is descriptive and educational and does not provide individualized clinical or device guidance.

Key concepts

• Sinus node dysfunction (sick sinus syndrome)
• First-degree atrioventricular block
• Second-degree block (Mobitz type I/Wenckebach and type II)
• Third-degree (complete) atrioventricular block
• Escape rhythms
• His-Purkinje conduction
• Cardiac pacing as conceptual treatment of symptomatic bradycardia

Mechanisms

Slow rhythms result either from inadequate impulse generation or from blocked conduction. Sinus node dysfunction reflects abnormal automaticity or exit block at the sinoatrial node, often from fibrosis and aging. Atrioventricular block is graded by the site and severity of the conduction defect: first-degree block is uniform PR prolongation; Mobitz type I (Wenckebach) shows progressive PR lengthening before a dropped beat and usually localizes to the atrioventricular node; Mobitz type II shows abrupt non-conducted beats and typically localizes below the node in the His-Purkinje system, carrying a higher risk of progression; third-degree block is complete atrioventricular dissociation, with the ventricles driven by a slower escape rhythm. The reliability and rate of the escape pacemaker determine how dangerous a given block is (Kusumoto et al., 2019).

Clinical relevance

Bradyarrhythmias and conduction disease can cause fatigue, dizziness, syncope, or — in high-grade block with an unreliable escape rhythm — hemodynamic collapse, and they are the principal indication for permanent cardiac pacing. This entry describes the disorders and their evidence framework for educational reference and is not a basis for individual diagnosis or treatment.

Epidemiology

Sinus node dysfunction and atrioventricular conduction disease increase markedly with age, reflecting progressive fibrosis of the conduction system, and together they account for the majority of permanent pacemaker implantations; conduction disease can also be precipitated by ischemia, infiltrative disease, infection, or medication effects.

Evidence & guidelines

The ACC/AHA/HRS bradycardia guideline (Kusumoto et al., 2019) and the ESC pacing guideline (Glikson et al., 2021) provide the contemporary framework for evaluating and grading conduction disorders and for the indications and modes of cardiac pacing.

History

Clinical description of slow pulse and dropped beats preceded the electrocardiogram, but the grading of atrioventricular block — including the Wenckebach periodicity described in the late nineteenth century and Mobitz's later classification — became precise with electrocardiography. The development of the implantable cardiac pacemaker in the late 1950s converted complete heart block from a frequently fatal condition into a treatable one.

Seminal works

• kusumoto-2019
• glikson-2021

Frequently asked questions

What is the difference between Mobitz type I and type II second-degree block?

In Mobitz type I (Wenckebach), the PR interval lengthens progressively until a beat is dropped, and the block is usually within the atrioventricular node; in Mobitz type II, conducted beats have a constant PR interval with sudden non-conducted beats, the defect is typically below the node, and the risk of progressing to complete block is higher.

Why can complete heart block be dangerous?

In third-degree block the atria and ventricles beat independently, and the ventricles rely on a backup (escape) rhythm that may be slow and unreliable, which can cause severe symptoms or, if the escape fails, life-threatening pauses.