Atrioventricular (AV) block describes a delay or complete interruption of the electrical signal as it travels from the heart’s upper chambers, the atria, to the lower chambers, the ventricles. This conduction system failure affects the heart’s rhythm. The electrocardiogram (ECG) is the primary tool used to diagnose and categorize these blocks. This article will explain the visual characteristics of the three main degrees of AV block—First, Second, and Third—as they appear on an ECG tracing.
Reading the ECG: Essential Components
The ECG is a recording of the electrical activity of the heart over time, represented by a series of waves and complexes. For analyzing AV conduction, three main components are examined. The P wave, a small, rounded wave, represents the electrical activation of the atria. The QRS complex, a sharp, larger deflection, represents the rapid electrical activation of the ventricles.
The time it takes for the electrical impulse to travel from the atria, through a relay station called the AV node, and into the ventricles is measured by the PR interval. A normal PR interval ranges between 0.12 and 0.20 seconds in duration, or three to five small boxes on standard ECG paper. This measurement is taken from the beginning of the P wave to the start of the QRS complex. The duration of the PR interval defines the different types of AV block.
Identifying First-Degree Block
First-degree AV block is the least severe form and is characterized exclusively by a prolonged PR interval. On the ECG tracing, the distance between the start of the P wave and the start of the following QRS complex is greater than 0.20 seconds. This prolonged time indicates that the electrical impulse is moving slower than normal through the AV node, but it is not completely blocked.
A significant visual feature is that every single P wave is consistently followed by a QRS complex. No beats are dropped, and the rhythm of the QRS complexes is typically regular. The consistently stretched-out PR interval is the sole visual hallmark of this condition.
Distinguishing Second-Degree Blocks
Second-degree AV blocks involve intermittent failure of the electrical impulse to reach the ventricles, resulting in dropped beats. This category is separated into two distinct types, Mobitz Type I and Mobitz Type II. The difference lies in the behavior of the PR interval leading up to the dropped beat.
Mobitz Type I, also known as Wenckebach phenomenon, displays a characteristic cyclical pattern. With each successive beat, the PR interval gets progressively longer. This progressive lengthening continues until an atrial impulse completely fails to conduct, and a P wave appears without a following QRS complex, which is the dropped beat. After the dropped beat, the cycle immediately resets, creating a recognizable pattern of “grouped beats” on the tracing.
Mobitz Type II block shows a constant PR interval for all conducted beats. The PR interval may be normal or prolonged, but it does not change length before a dropped QRS complex occurs. The failure to conduct is sudden and unpredictable, appearing as an abrupt interruption in the rhythm without the preceding gradual lengthening seen in Mobitz Type I. This block is considered more concerning because of its potential to progress to complete heart block. The consistency of the PR interval, followed by a sudden, unexpected interruption, is the key visual differentiator for Mobitz Type II.
Visualizing Third-Degree Block
Third-degree AV block, also called complete heart block, represents a complete failure of the electrical signal to pass from the atria to the ventricles. On the ECG, the defining visual characteristic is complete atrioventricular (AV) dissociation. This means the atria and the ventricles are beating independently of each other, operating under two separate pacemakers.
The P waves, representing the atrial rate, will march out regularly and at a faster rate, typically 60 to 100 beats per minute. Simultaneously, the QRS complexes, representing the ventricular rate, will also be regular but significantly slower, usually between 30 and 45 beats per minute. They are being driven by a slower “escape” pacemaker lower down in the conduction system.
When analyzing the tracing, there is no discernible relationship between the P waves and the QRS complexes. The P waves appear to float across the strip, sometimes landing directly on top of, or hidden within, the QRS complex or the T wave. The PR interval is constantly varying, and the atrial rate is always faster than the ventricular rate. The appearance of the QRS complex itself offers another clue: a narrow QRS complex suggests the escape rhythm originates higher up, while a wide QRS complex suggests a lower origin in the ventricles.