Atrioventricular Conduction in the Electrocardiogram

Atrioventricular Conduction in the Electrocardiogram represents the electrical conduction process through the atrioventricular (AV) node and the specialized conduction system that connects the atria and ventricles. It is a critical component of the cardiac cycle, ensuring coordinated contraction and efficient pumping of blood by delaying the electrical impulse traveling from the atria to the ventricles. This conduction process is reflected in specific segments and intervals on the electrocardiogram (ECG), primarily the PR interval and the morphology of the P wave and QRS complex.

Electrophysiological Basis of Atrioventricular Conduction

The atrioventricular conduction system includes the AV node, bundle of His, right and left bundle branches, and Purkinje fibers. After the sinoatrial (SA) node generates an impulse causing atrial depolarization (seen as the P wave), the impulse reaches the AV node. The AV node introduces a physiological delay, allowing the atria to contract and complete ventricular filling before ventricular depolarization begins.

This delay is crucial and is reflected in the PR segment of the ECG, which represents the time between the end of atrial depolarization and the start of ventricular depolarization. The conduction velocity in the AV node is slower than in other cardiac tissues due to smaller diameter fibers and fewer gap junctions, contributing to the delay.

Following the AV node, the impulse travels rapidly through the His-Purkinje system, resulting in a synchronized ventricular depolarization visualized as the QRS complex.

Representation of Atrioventricular Conduction on the Electrocardiogram

P Wave

The P wave corresponds to atrial depolarization initiated by the SA node. The morphology and duration of the P wave reflect atrial conduction but are also indirectly related to AV conduction because a normal P wave must be followed by a normal PR interval for proper AV conduction.

PR Interval

The PR interval extends from the beginning of the P wave to the beginning of the QRS complex. It encompasses atrial depolarization, AV nodal delay, and conduction through the His-Purkinje system up to the initiation of ventricular depolarization. The normal PR interval duration is between 120 and 200 milliseconds.

Changes in the PR interval provide information about the integrity and conduction velocity of the AV node and the AV conduction system:

• Prolonged PR interval (First-degree AV block): Indicates delayed AV conduction.
• Shortened PR interval: May indicate pre-excitation syndromes like Wolff-Parkinson-White syndrome.
• Variable PR interval: Suggests types of second-degree AV block.

PR Segment

The PR segment is the portion of the PR interval between the end of the P wave and the start of the QRS complex. It represents the electrical conduction delay in the AV node and the His bundle. Elevation or depression of the PR segment can be clinically significant, for example, in pericarditis.

QRS Complex

The QRS complex reflects ventricular depolarization. While not a direct measure of AV nodal conduction, the timing of the QRS onset relative to the P wave (i.e., the PR interval) depends on AV conduction. Abnormal conduction distal to the AV node (bundle branch blocks) alters the QRS morphology and duration but not the PR interval unless the AV node is also involved.

Clinical Implications of Atrioventricular Conduction Abnormalities

First-Degree AV Block

Characterized by a prolonged PR interval (>200 ms) with all atrial impulses conducted to the ventricles. It usually indicates slowed conduction through the AV node.

Second-Degree AV Block

Divided into:

• Mobitz Type I (Wenckebach): Progressive prolongation of the PR interval culminating in a dropped QRS complex. It reflects reversible conduction delay within the AV node.
• Mobitz Type II: Sudden failure of AV conduction without prior PR prolongation, often due to His-Purkinje system disease, with a risk of progression to complete heart block.

Third-Degree (Complete) AV Block

Complete dissociation between atrial and ventricular activity; no impulses conducted through the AV node. The ECG shows independent P waves and QRS complexes, with ventricular escape rhythms.

Pre-excitation Syndromes

Conditions like Wolff-Parkinson-White syndrome involve accessory pathways bypassing the AV node, leading to a shortened PR interval and characteristic delta waves on the ECG.

Quantitative Assessment of Atrioventricular Conduction

The PR interval can be measured from the onset of the P wave to the start of the QRS complex on the ECG tracing. Normal values vary slightly with age and heart rate but generally lie within 120–200 milliseconds.

Mathematically, the PR interval duration (t_PR) includes:

Where:

• $\mathrm{t\_\{P\; wave\}}$ is the duration of atrial depolarization,
• $\mathrm{t\_\{AV\; node\; delay\}}$ is the conduction delay in the AV node,
• $\mathrm{t\_\{His-Purkinje\; conduction\}}$ is the rapid conduction time through the His-Purkinje system.

Prolongation of $\mathrm{t\_\{AV\; node\; delay\}}$ is the most common cause of PR interval prolongation.

Summary of Electrocardiographic Features Related to Atrioventricular Conduction

Summary Diagram of Atrioventricular Conduction and ECG Correlation

This diagram illustrates the pathway of electrical conduction from the sinoatrial node through the atrioventricular node and bundle branches, alongside the corresponding features on the ECG where atrioventricular conduction is represented.

Summary

Atrioventricular conduction in the electrocardiogram is represented primarily by the PR interval, which reflects the time taken for the electrical impulse to travel from the atria through the AV node to the ventricles. Variations in the duration and morphology of the PR interval and associated segments provide important diagnostic information about the integrity and function of the AV conduction system. Understanding these features is essential for diagnosing conduction blocks, pre-excitation syndromes, and other cardiac conduction abnormalities.