Difference Between 1st 2nd and 3rd Degree Heart Block

The key difference between 1^st 2^nd and 3^rd Degree Heart Block is that in the first-degree heart blocks, all the electric impulses that originate in the SA node are conducted to the ventricles, but there is a delay in the propagation of the electrical activity, which is indicated by a prolongation of the PR interval. Failure of some of the p waves to propagate into the ventricles is the characteristic feature of second-degree heart blocks. None of the P waves that generate in the atria are conducted to the ventricles in the third-degree heart blocks.

Conduction system of the heart is made up of a few major components which include the SA node, AV node, bundle of his, right bundle branch block and left bundle branch block. When there are defects in this conduction system that give rise to heart blocks. There are three main varieties of heart blocks as first, second and third-degree heart blocks.

CONTENTS

1. Overview and Key Difference
2. What is 1st Degree Heart Block
3. What is 2nd Degree Heart Block
4. What is 3rd Degree Heart Block
5. Similarities Between 1st 2nd and 3rd Degree Heart Block
6. Side by Side Comparison – 1st vs 2nd vs 3rd Degree Heart Block in Tabular Form 
7. Summary

What is a 1^st Degree Heart Block?

All the electric impulses originated in the SA node are conducted to the ventricles, but there is a delay in the propagation of the electrical activity which is indicated by a prolongation of the PR interval.

Figure 01: Heart Rate of 1st Degree Heart Block

First-degree heart block is usually a benign condition but can be due to coronary artery disease, acute rheumatic carditis, and digoxin toxicity.

What is a 2^nd Degree Heart Block?

Failure of some of the p waves to propagate into the ventricles is the characteristic feature of second-degree heart blocks. There are three main varieties of 2^nd-degree heart blocks.

• Mobitz type 1

There is a progressive prolongation of the PR interval which ultimately ends up with the failure of a P wave to propagate into the ventricles. This is known as the Wenckebach phenomenon also.

• Mobitz type 2

Figure 02: Heart Rates of 2nd Degree Heart Block

The PR interval remains the same with no fluctuations but an occasional P wave is lost without been conducted into the ventricles.

• The third group is characterized by the presence of a missing P wave for every 2 or 3 conducted P waves.

Mobitz type 2 and the third group are the pathological varieties.

What is 3^rd Degree Heart Block?

None of the P waves generated in the atria are conducted to the ventricles. The ventricular contraction happens by generating intrinsic impulses. Therefore, there is no relationship between the P waves and QRS complexes.

Figure 03: Heart Rate of 3rd Degree Heart Block

These blocks can be due to infarction in which case they are only transient. A chronic block is most likely to be due to the fibrosis of the bundle of his.

What is the Similarity Between 1^st 2^nd and 3^rd Degree Heart Block?

• All the conditions are due to defects in the conduction system of the heart.

What is the Difference Between 1^st 2^nd and 3^rd Degree Heart Block?

All the electric impulses originated in the SA node are conducted to the ventricles in the 1st heart block, but there is a delay in the propagation of the electrical activity which is indicated by a prolongation of the PR interval. While in the 2^nd heart block, failure of some of the p waves to propagate into the ventricles is the characteristic feature of second-degree heart blocks. None of the P waves generated in the atria is conducted to the ventricles in the 3rd degree heart block. This is the main difference between 1^st 2^nd and 3^rd Degree Heart Block.

Summary – 1st 2nd vs 3rd Degree Heart Block

Heart blocks arise secondary to the defects in the conduction system of the heart. In the first-degree heart blocks all the electric impulses that originate in the SA node are conducted to the ventricles, but there is a delay in the propagation of the electrical activity that is indicated by a prolongation of the PR interval.  Failure of some of the p waves to propagate into the ventricles is the characteristic feature of second-degree heart blocks. None of the P waves generated in the atria is conducted to the ventricles in the third-degree heart blocks. This is the difference between 1^st 2^nd and 3^rd Degree Heart Block.

Reference:

1. Hampton, John R. 8th ed., Churchill Livingstone, 2013

Image Courtesy:

1.’First Degree AV Block ECG Unlabeled’By Andrewmeyerson – Own work, (CC BY-SA 3.0) via Commons Wikimedia 
2.’Second degree heart block’By Npatchett – Own work, (CC BY-SA 4.0) via Commons Wikimedia
3.’Rhythm strip showing third degree heart block’By MoodyGroove at English Wikipedia – Own work, (Public Domain) via Commons Wikimedia  

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