An electrocardiogram (ECG) provides a visual representation of the heart’s electrical activity, serving as a rapid diagnostic tool. This test involves placing small patches, called electrodes, on the skin to detect the tiny electrical impulses generated by the heart. These impulses coordinate the heart’s muscle contractions, ensuring proper blood flow throughout the body. An atrioventricular (AV) block represents a disruption in this electrical signaling pathway between the upper chambers (atria) and lower chambers (ventricles) of the heart. This article explores the specific appearances of different AV blocks on an ECG strip.
Understanding ECG Basics
To interpret an ECG, understanding its fundamental components is helpful. Each heartbeat recorded on an ECG strip consists of several distinct waves and intervals. The P wave, a small upward deflection, signifies the electrical activation (depolarization) of the atria, leading to their contraction. The QRS complex represents the rapid electrical activation of the ventricles, prompting their contraction and the pumping of blood. This complex is a sharp, larger deflection.
After the QRS complex, the T wave indicates the electrical recovery (repolarization) of the ventricles. The PR interval is a key measurement, reflecting the time from the beginning of the P wave to the start of the QRS complex. This interval measures the duration it takes for an electrical impulse to travel from the atria, through the AV node, and into the ventricles. A normal PR interval ranges from 0.12 to 0.20 seconds. Analyzing these elements helps assess the heart’s electrical rhythm and identify abnormalities.
First-Degree AV Block Appearance
First-degree AV block presents a distinct, often benign, pattern on an ECG. Its defining characteristic is a consistently prolonged PR interval, exceeding 0.20 seconds. Despite this delay, every electrical impulse originating from the atria successfully conducts through the AV node and activates the ventricles. On the ECG strip, every P wave is consistently followed by a QRS complex, meaning there are no “dropped” or missing QRS complexes. This condition is frequently asymptomatic and may be an incidental finding on a routine ECG.
Second-Degree AV Block Appearance
Second-degree AV block involves some atrial impulses failing to reach the ventricles, resulting in “dropped” QRS complexes. This category is divided into two types: Mobitz Type I and Mobitz Type II, each with unique ECG characteristics. Differentiating between these types is important due to their differing clinical implications.
Mobitz Type I
Mobitz Type I, also known as Wenckebach phenomenon, is characterized by a progressive lengthening of the PR interval with each consecutive beat. This prolongation continues until an atrial impulse is completely blocked, meaning a P wave appears without a subsequent QRS complex. After this dropped beat, the pattern resets, and the PR interval shortens before gradually lengthening in the next cycle. This creates a distinctive “grouped beating” appearance on the ECG, where QRS complexes seem to cluster together, followed by a pause.
Mobitz Type II
In contrast, Mobitz Type II presents with a constant PR interval for the conducted beats, but suddenly, a P wave is not followed by a QRS complex. There is no progressive lengthening of the PR interval before the dropped beat. The P waves continue to appear at a regular rate, but some fail to conduct to the ventricles. This type often indicates a block lower in the heart’s conduction system, making it more concerning than Mobitz Type I.
Third-Degree AV Block Appearance
Third-degree AV block, also known as complete heart block, is the most severe form of AV block. On an ECG, this condition is characterized by a complete electrical dissociation between the atria and the ventricles. No atrial impulses are conducted through the AV node to the ventricles.
The atria and ventricles beat independently. P waves appear regularly at their own rate, typically faster than the ventricular rate. QRS complexes appear at their own, much slower, regular rate, initiated by an escape pacemaker lower in the heart. Because there is no relationship between atrial and ventricular activity, the PR interval varies widely and randomly across the ECG strip.