The heart serves as the cornerstone of human physiology, tirelessly pumping blood to supply our body with oxygen and vital nutrients.
Heart block is a condition where the heart’s electrical signaling system is disrupted, impacting both the heart’s rhythm and overall health.
This article breaks down the various types, including the popular heart block poem that simplifies the learning process.
What is a Heart Block?
Heart block, medically referred to as atrioventricular (AV) block, is a condition characterized by a disruption or delay in the electrical signals that coordinate the heart’s contractions.
In a healthy heart, electrical impulses travel from the sinoatrial (SA) node, the heart’s natural pacemaker, through the atria and then to the ventricles via the atrioventricular (AV) node.
Heart block occurs when these impulses are partially or completely blocked, leading to an irregular or slow heart rhythm.
Hear Block Types
There are three primary types of heart blocks, classified based on the severity of the electrical signal disruption:
First-Degree Heart Block
In this mildest form, electrical signals experience a slight delay as they pass through the AV node, but all of them eventually reach the ventricles.
First-degree heart block usually does not cause any noticeable symptoms and may not require treatment. It is often discovered incidentally during an electrocardiogram (ECG) for other reasons.
Second-Degree Heart Block
This type is further divided into two subtypes:
- Mobitz Type I (Wenckebach)
- Mobitz Type II
In Mobitz Type I, the electrical signals progressively slow down until one signal fails to reach the ventricles, causing a skipped beat.
Mobitz Type II is more serious, as some of the electrical signals are unable to reach the ventricles without warning.
Both types can lead to symptoms like dizziness, fatigue, or fainting, and may require treatment, depending on the severity and underlying cause.
Third-Degree Heart Block
The most severe form, third-degree heart block, occurs when no electrical signals can pass from the atria to the ventricles. This results in the ventricles beating independently of the atria, at a significantly slower rate.
Complete heart block can lead to severe symptoms like fatigue, dizziness, fainting, and even cardiac arrest. It requires immediate medical attention and often necessitates the implantation of a pacemaker to regulate the heart’s rhythm.
Heart Block Poem
This poem serves as a mnemonic to help remember the different types of atrioventricular (AV) heart blocks and their key characteristics as seen on an electrocardiogram (ECG):
If the R is far from P,
then you have a FIRST-DEGREE.
Longer, longer, longer, drop!
Then you have a WENCKEBACK.
If some P’s don’t get through,
then you have MOBITZ II.
If P’s and Q’s don’t agree,
then you have a THIRD DEGREE.
Let’s break down each line of the heart block poem and explain its meaning:
“If the R is far from P, then you have a FIRST-DEGREE.”
In a first-degree heart block, the PR interval (the time between the onset of the P wave and the start of the QRS complex) on the ECG is consistently prolonged, meaning the time it takes for the electrical signal to travel from the atria to the ventricles is longer than usual.
This line reminds us that a prolonged PR interval is indicative of a first-degree heart block.
“Longer, longer, longer, drop! Then you have a WENCKEBACK.”
This line refers to a second-degree heart block, Mobitz Type I (Wenckebach). In this type of block, the PR interval progressively lengthens until a QRS complex is dropped (i.e., a ventricular beat is skipped).
The poem emphasizes the pattern of increasing PR intervals followed by a dropped QRS complex as the key feature of the Wenckebach heart block.
“If some P’s don’t get through, then you have MOBITZ II.”
This line describes a second-degree heart block, Mobitz Type II. In this type, some of the P waves are not followed by QRS complexes, meaning that the electrical signals from the atria occasionally fail to reach the ventricles.
The poem highlights the presence of P waves without corresponding QRS complexes as the defining characteristic of Mobitz Type II heart block.
“If P’s and Q’s don’t agree, then you have a THIRD DEGREE.”
The final line of the poem pertains to a third-degree heart block, also known as a complete heart block. In this most severe form of heart block, none of the electrical signals from the atria (P waves) are able to reach the ventricles (QRS complexes).
Consequently, the atria and ventricles contract independently of each other, leading to a complete dissociation between P waves and QRS complexes on the ECG.
This line serves as a reminder that a lack of correlation between P waves and QRS complexes signifies a third-degree heart block.
Heart Block Trick
The poem above is an effective way to simplify the process of understanding and memorizing the different types of heart blocks. However, it can be difficult to memorize.
Use this heart block trick as a shorter, simplified version:
- First-degree = Far away P
- Wenckebach = Longer, then Drop
- Second Degree = Drop randomly
- Third Degree = Beat independently
Cardiac Conduction System
The cardiac conduction system is a complex network of specialized cells and pathways responsible for generating and transmitting electrical impulses that coordinate the heart’s contractions.
This system ensures that the heart beats in a regular, organized manner, efficiently pumping blood throughout the body.
Here are the key components of the cardiac conduction system and their roles:
- Sinoatrial (SA) node: Often referred to as the heart’s natural pacemaker, the SA node is a cluster of specialized cells located in the upper right atrium. It generates electrical impulses that initiate the heart’s contractions. These impulses are generated at a regular rate, typically between 60 and 100 times per minute at rest.
- Atrioventricular (AV) node: Situated in the lower right atrium near the tricuspid valve, the AV node serves as a relay station for electrical signals coming from the SA node. It temporarily delays the impulse, allowing the atria to complete their contraction and fully empty blood into the ventricles before the ventricles contract.
- Bundle of His: Also known as the atrioventricular bundle, the Bundle of His is a group of specialized fibers that transmit electrical impulses from the AV node to the ventricles. The Bundle of His is located in the upper part of the interventricular septum, which separates the left and right ventricles.
- Bundle branches: The Bundle of His divides into two branches, the right bundle branch and the left bundle branch. The right bundle branch carries electrical signals to the right ventricle, while the left bundle branch further divides into the left anterior fascicle and the left posterior fascicle, carrying impulses to the left ventricle. These branches run through the interventricular septum and deliver electrical signals to the ventricles’ muscular walls.
- Purkinje fibers: These specialized fibers extend from the bundle branches and spread throughout the ventricular walls. They are responsible for distributing electrical signals to the ventricular muscle cells, ensuring a coordinated and efficient contraction of the ventricles.
Summary: The cardiac conduction system is a sophisticated electrical system that controls the heart’s contractions. Starting from the SA node, electrical impulses travel through the atria, the AV node, the Bundle of His, the bundle branches, and finally, the Purkinje fibers, resulting in a well-coordinated and efficient contraction of the atria and ventricles to pump blood throughout the body.
Understanding heart blocks and their various types is essential for recognizing potential disruptions in the heart’s electrical signaling system.
This article has provided an overview of the different degrees of heart blocks, along with an explanation of the heart block poem, which serves as a helpful mnemonic for remembering their key characteristics.
By grasping the fundamental concepts and distinctions between heart block types, we can better appreciate the importance of maintaining our heart’s health and rhythm.
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
- Kashou AH, Goyal A, Nguyen T, et al. Atrioventricular Block. [Updated 2023 Feb 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
- Park DS, Fishman GI. The cardiac conduction system. Circulation. 2011.