Removing the Road Blocks: Understanding Heart Blocks

cardiac conduction pathway

Ever wondered about the differences between a first, second and third degree atrioventricular (AV) heart block? And not to mention that second degree AV blocks can be further differentiated into type 1 or type 2? You know, because it wasn’t confusing enough as it was! This article is going to explain it all to you! But before we start making heart blocks something that is easy to understand, we should start with a bit of normal electrophysiology through the heart. After all, we can’t understand what is abnormal until we truly understand what is normal!

The cardiac conduction system at it’s simplest form consists of the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, left and right bundle branches and purkinje fibres. This entire system works on the principle of hierarchy meaning that there is a clear nurse unit manager (SA node) that runs the show for the majority of the time, an associate nurse unit manager (AV node) that will take over running the show when the nurse unit manager goes on annual leave, and the clinical staff (purkinje fibres) that do not want to run the show but will try their best if they absolutely have to!

Heart conduction system

In a normal functioning conduction system, the SA node firing off causes atrial contraction and results in the P wave. The electrical conduction travels to the AV node where it is slowed down momentarily to allow time for the blood from the contracted atria to enter the ventricles, prior to ventricular contraction. This is observed by the PR interval. The electrical impulse then travels down the bundle of His, left and right bundle branches and to the purkinje fibres where it causes ventricular contraction. This is observed by the QRS complex. 

pqrst wave

Think of the AV node like a boom gate in the middle of a road that controls traffic. When working normally, it will allow each car to pass through from point A to point V…pun intended! However, an issue with the boom gate will result in a road block. The same applies for the AV node not allowing the electrical impulse to pass through normally. The severity of this road block determines the degree of the heart block. It could range from merely causing some delayed traffic through the boom gate (first degree), to some traffic getting through and some traffic getting blocked (second degree), or no traffic getting through at all (third degree).

If there is an issue with your AV node, you have some type of heart block. There are two important things to remember regarding this:

  • The PR interval is normally 0.12 – 0.2 seconds meaning that if the PR interval is longer than this, you have a delay through the AV node and therefore an issue with your AV node
    • As people don’t remember numbers as well as they remember patterns, I always remind people that 1 little ECG box is 0.04 seconds which means that 5 little boxes that make up 1 big ECG box is 0.2 seconds. Therefore, a normal PR interval should always be less than 1 big ECG box!
  • Every P wave should be conducted through the AV node to result in a narrow QRS complex meaning that if there is no follow through with a QRS complex at any point, you have a complete blockage through the AV node and therefore an issue with your AV node

First Degree Heart Block

First degree heart block is simple; it is merely a prolonged PR interval of more than 0.2 seconds or one big ECG box! Every P wave conducts through the AV node and results in a QRS complex, there is just a bit of a longer delay between the two than normal.

Heart block first degree

Many people have a first degree heart block without even knowing it and it generally does not cause any problems…

Second Degree Heart Block

There are two types of second degree heart block; type 1 and type 2.

Second degree heart block type 1  is also known as Mobitz I or more popularly, Wenckebach. It is characterised by a progressive lengthening of the PR interval until finally a QRS complex is not conducted through. It is almost as if the QRS complex is walking away from the P wave but is “winking back” each time until it disappears out of sight. 

Heart block second degree type 1

Second degree heart block type 2 is also known as Mobitz Type II. It does not possess a pattern like Wenckebach and is characterised by intermittently non-conducted P waves through the AV node resulting in intermittently non-conducted QRS complexes. This is like listening to a really long and monotoned story. You may start with the best intentions to tune in, but then all of sudden you find yourself tuning out. You might catch yourself doing this and tune straight back in again, or you might tune out for a little while longer before tuning back in.

Heart block second degree type 2

Mobitz Type II may not seem that bad when it doesn’t conduct one QRS complex through every few minutes, but what if it decides not to conduct through three QRS complexes in a row? Or 5? Or 10? Remember that with no QRS complex, you have no blood pressure! The loss of predictability makes Mobitz Type II a worse type of heart block than Wenckebach/Mobitz Type I.

Third Degree Heart Block

Third degree heart block is often referred to as complete heart block. As the name suggests, the AV node does not conduct any electrical impulses from the atria to the ventricles. The SA node will  continue depolarising without the ability to conduct the electrical signal down to the purkinje fibres. As a result, the purkinje fibres assume that neither the SA or the AV not are functioning and try to help by conducting independently. Therefore, the SA node and purkinje fibres will depolarise at a rate independent of each other. This is the worst type of heart block that may render the patient with a very low blood pressure and increase their risk of sudden cardiac death. 

Heart block third degree

As the SA node normally fires at a rate of 60 – 100 bpm, you will see P waves at a rate of 60 – 100 bpm. However, none of these P waves will be followed by a narrow QRS complex as nothing is being conducted through the AV node. The purkinje fibres will fire at a rate of 20 – 40 bpm, however due to the origin of the impulse starting from below the AV node, the QRS complex will be widened. This is because an impulse starting below the AV node cannot get onto the express lane that is known as the bundle of His, and will therefore have to take the long way all around the ventricles to cause contraction. The longer the time spent transmitting the electrical impulse through the heart, the wider the QRS complex.

I hope that you have found this article helpful in differentiating between the different types of heart block. Remember, that coming first is always better than coming third. Therefore, a first degree heart block is better than a second degree heart block, which in turn is better than a third degree heart block. The same applies for Mobitz Type 1 and Type 2; being number 1 is always better than being number 2! You can also check out the flowchart found in How to Diagnose ANY Cardiac Rhythm Systematically to help you figure out the right rhythm.

If you found the artistic genius of Jorge Muniz as brilliant as I do from his heart block comics above, you can see more of his work at his Medcomic site by clicking here!


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