Cardiac Conduction & Tachyarrhythmias: SA Node, AV Node & Purkinje Fibers Explained!

Added: 2026-05-19

[00:00:00]So, foundation layer in the sense is just to make you understand certain concepts, but uh it's not going to be that extensive like what you've done in batch one and batch two, but still it will be of some use to understand certain things.

[00:00:12]So, first I think because physiology is already over, you must be able to understand the important things that are uh going to help in cardiac conduction.

[00:00:20]Like, for example, we begin with the SA node, then it's going to the impulse is going to go to the AV node.

[00:00:26]Then, probably uh it's going to be the His bundle system. Then, from there it's going to enter the bundle branches, and we have two bundle branches, right? So, we have the right bundle branch, and we have the left bundle branch. And the left bundle branch alone divides into two. So, one is going to the left anterior fascicle, and second is going to be the left posterior fascicle, right? Left anterior fascicle, left posterior fascicle. And finally, you have the Purkinje fibers, which is the fastest.

[00:00:48]The fastest conduction is going to occur through the Purkinje fibers. It's approximately around 4 m per second.

[00:00:55]Right?

[00:00:56]Similarly, the slowest conduction is going to be via the AV node. AV node is going to be the slowest, approximately 0.01 to 0.02 m per second, very, very slow. So, I told you multiple things in the previous sessions also, but that's not important.

[00:01:10]So, why AV node is slow. AV node is calcium dependent. AV node has very little gap junctions, and that's the reason why AV node is the slowest. The laziness and letharginess of AV node is a characteristic property which saves against so many things we'll discuss in the future. So, Purkinje fibers are the fastest, AV node is the slowest.

[00:01:29]And if you look at the diagram, you can understand. It starts from the SA node.

[00:01:33]It uh I mean, these internodal pathways are not very important for you. We have the total backman and wenkyback three internodal pathways within the atrium is there, not really important for you.

[00:01:42]It won't be exam questions. And then, you hit the AV node. AV node is located in an area called as triangle of Koch.

[00:01:49]I think everyone must be aware of that, triangle of Koch. That's where the AV node is located.

[00:01:54]And from there, you get the bundle of His.

[00:01:57]And from the bundle office you get the bundle branches. That is a right bundle branch and the left bundle branch. And the left bundle actually divides into two, the anterior fascicle and the posterior fascicle, even though it's not shown here. And finally you have the Purkinje fibers, right? So this is the whole conduction system of the heart.

[00:02:14]And what are the mechanisms by which you can develop arrhythmogenesis? Especially we're talking about tachyarrhythmia mechanisms.

[00:02:21]So there are three principal mechanisms of tachyarrhythmias. One is going to be the enhanced automaticity.

[00:02:27]So this enhanced automaticity can be enhanced normal automaticity, or it can be abnormal automaticity. So tissues that are usually automatic becomes more automatic. That's called enhanced normal automaticity, like sinus tachycardia. SA node is usually automatic, but if it becomes more automatic, you get sinus tachycardia.

[00:02:49]And