Atrioventricular (AV) block is a condition describing a delay or complete interruption of the electrical signal traveling from the heart’s upper chambers (atria) to its lower chambers (ventricles). This disruption alters the heart’s rhythm, ranging from a minor delay to a severe condition. The electrocardiogram (ECG) is the primary tool doctors use to diagnose and classify the severity of these conduction issues. The unique patterns displayed on the ECG strip allow for precise identification of the block’s location and type.
The Components of a Normal Heart Rhythm
Understanding AV block begins with recognizing the appearance of a normal electrical cycle on an ECG. The tracing consists of distinct waves and segments, each representing a specific phase of the heart’s electrical activity. The P wave, the first positive deflection, signifies the depolarization and contraction of the atria.
This is followed by the QRS complex, a sharp, larger wave that represents the rapid depolarization and contraction of the ventricles. The final component, the T wave, indicates the repolarization, or electrical recovery, of the ventricles. The relationship between these components determines the health of the heart’s conduction system.
The time it takes for the electrical impulse to travel from the atria to the ventricles is measured by the PR interval. This interval is calculated from the beginning of the P wave to the start of the QRS complex. In a healthy adult, the PR interval should measure between 120 and 200 milliseconds, or three to five small squares on standard ECG paper.
The PR interval reflects the conduction speed through the atrioventricular node, which acts as the gatekeeper between the upper and lower chambers. Any change in this measurement, particularly a prolongation, is the defining characteristic used to diagnose the various degrees of AV block.
First-Degree Atrioventricular Block
First-degree AV block is characterized by a prolonged delay in the electrical signal’s journey from the atria to the ventricles. On an ECG, this appears as a PR interval that consistently measures longer than 200 milliseconds. This delay means the electrical signal is traveling slower than normal, usually due to an issue within the AV node itself.
Despite the delay, every atrial impulse successfully reaches and activates the ventricles. Consequently, every P wave is followed by a QRS complex, meaning no beats are dropped. The ventricular rhythm remains regular, though the extended pause between atrial and ventricular contraction may be pronounced.
The ECG strip shows a regular rhythm where the QRS complexes occur later than expected following the P waves. This form of AV block is often considered delayed conduction rather than a true block, and it is frequently asymptomatic. The consistent, prolonged PR interval is the sole diagnostic criterion.
Second-Degree Atrioventricular Blocks
Second-degree AV blocks involve an intermittent failure of the atrial signal to reach the ventricles, resulting in a dropped beat. This category separates into two distinct types: Mobitz Type I and Mobitz Type II. They are differentiated solely by the pattern of the PR interval leading up to the dropped beat, which indicates different block locations and implications for patient risk.
Mobitz Type I (Wenckebach)
Mobitz Type I, also known as Wenckebach phenomenon, displays a characteristic pattern of progressive conduction fatigue on the ECG. With each successive beat, the PR interval gradually lengthens, showing an increasing delay in AV nodal conduction. This lengthening continues until the delay becomes so significant that an atrial impulse fails to conduct to the ventricles.
A P wave appears without a QRS complex, resulting in a “dropped” beat. Following the dropped beat, the conduction system recovers, and the cycle immediately resets with a normal PR interval. This restarts the pattern of progressive lengthening. This cyclic pattern of lengthening PR intervals culminating in a dropped beat is the unique signature of Mobitz Type I block.
Mobitz Type II
Mobitz Type II block presents a different and more abrupt pattern on the ECG. The PR interval preceding the conducted QRS complexes remains constant and fixed. This interval may be normal or prolonged, but it does not show the progressive lengthening seen in Mobitz Type I.
The defining characteristic is the sudden failure of a P wave to conduct, resulting in a dropped QRS complex without any preceding change in the PR interval. The block usually occurs below the AV node, within the bundle of His or the bundle branches. This intermittent failure of conduction with a fixed PR interval distinguishes Mobitz Type II and is often associated with a higher risk of progressing to a more severe block.
Third-Degree Atrioventricular Block
Third-degree AV block, also referred to as complete heart block, represents the most severe form of conduction failure. There is a total interruption of the electrical signal traveling from the atria to the ventricles. No atrial impulses are conducted through the AV node, leading to complete atrioventricular dissociation.
The ECG tracing reveals two entirely independent rhythms: one for the atria and one for the ventricles. P waves occur regularly, representing the intrinsic rhythm of the atria, but they bear no relationship to the QRS complexes. P waves may sometimes be hidden within the QRS complex or the T wave.
The ventricles, cut off from the atrial signal, must rely on an escape rhythm generated lower down in the conduction system to maintain a heartbeat. This ventricular escape rhythm is typically slow and regular, often resulting in a heart rate of 40 beats per minute or less. The lack of any fixed PR interval between the P waves and the QRS complexes confirms the diagnosis.