The atrioventricular (AV) node acts as a critical gateway for electrical impulses traveling from the atria to the ventricles of the heart. When there’s a delay or blockage at this gateway, it can lead to various types of heart blocks, each with distinct patterns observable on an electrocardiogram (EKG). This article explores these heart block patterns in detail to aid in their recognition and understanding.
First-Degree Heart Block
A first-degree heart block represents a minor delay in the conduction of electrical impulses at the AV node, similar to a slight traffic slowdown on a highway. On an EKG, the hallmark of a first-degree heart block is a prolonged PR interval—the time it takes for the impulse to travel from the atria to the ventricles—that exceeds 0.2 seconds. Despite this delay, every atrial impulse still reaches the ventricles, so the heart rhythm remains regular.
Second-Degree Heart Block
Second-degree heart blocks are characterized by intermittent failures of electrical conduction from the atria to the ventricles. They are further classified into two types: Mobitz Type I (Wenckebach) and Mobitz Type II.
Mobitz Type I (Wenckebach)
In Mobitz Type I heart block, there is a progressive delay in AV node conduction until an impulse fails to conduct, resulting in a missed ventricular beat. This cycle then repeats.
On the EKG, this appears as progressively lengthening PR intervals with each beat until a QRS complex (which represents ventricular depolarization) is dropped. After the missed beat, the PR interval resets to a shorter duration, and the pattern starts over. Additionally, the R-R intervals (the time between ventricular contractions) typically shorten before the dropped beat.
A useful diagnostic tip is to compare the PR intervals just before and after the missing QRS complex; the PR interval following the dropped beat is shorter.
Mobitz Type II
Mobitz Type II heart block is more serious and can progress to a complete heart block. In this type, the AV node fails to conduct impulses intermittently without prior changes in the PR interval.
This means the PR intervals remain consistent, but some beats are suddenly dropped. On the EKG, there are more P waves (atrial depolarizations) than QRS complexes because some atrial impulses do not reach the ventricles. This irregularity can lead to bradycardia and symptoms such as dizziness or syncope.
2:1 Heart Block
A 2:1 heart block occurs when every alternate atrial impulse fails to conduct to the ventricles, resulting in one conducted beat followed by one blocked beat. This pattern makes it challenging to distinguish between Mobitz Type I and Mobitz Type II because only two beats are compared at a time. However, certain clues can aid differentiation:
- Examination of Other EKG Leads: Observing for patterns indicative of Mobitz Type I or II in different parts of the EKG may provide hints.
- Response to Physical Activity: If the heart rate does not increase appropriately with exercise—a condition known as chronotropic incompetence—it suggests Mobitz Type II.
- Presence of Symptoms: Symptoms like fatigue, lightheadedness, or fainting are more commonly associated with Mobitz Type II due to its potential to cause significant bradycardia.
Third-Degree Heart Block (Complete Heart Block)
In a third-degree heart block, there is a complete dissociation between atrial and ventricular activity. The electrical impulses from the atria do not conduct to the ventricles at all. As a result, the atria and ventricles beat independently.
On the EKG, both the P-P intervals (atrial rate) and R-R intervals (ventricular rate) are regular, but there is no relationship between them—they are not synchronized. The atrial rate is usually faster than the ventricular rate. The ventricles often rely on an escape rhythm originating from a secondary pacemaker within the heart’s conduction system, such as a junctional or idioventricular rhythm, to maintain a heartbeat.
Leave a Reply