Rapid and regular tachycardias can either be broad or narrow. In this video from our ECG Mastery: Blue Belt course, you'll take a deep dive into narrow complex tachycardias and learn why they are narrow, how to interpret them on an ECG, what the differential diagnosis is, and much, much more.
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[00:00:00] Welcome back. Now that we have a rapid, regular, and broad. Let's focus on rapid, regular, and narrow complex tachycardias. This is a quick recap of what we already know. When we talked about tachycardias, we discriminated it between rapid and regular, and rapid and irregular. We said that rapid and regular could come
[00:00:30] in two manifestations: broad and regular, and narrow and regular. If the rhythm was irregular, especially if the QRS complexes were narrow, we said that in most instances this was atrial fibrillation or AF but we're not going to talk about this for today. Also, we've already covered the broad and regular tachycardia, so we're also not going to talk about those either. So, what we're left with are the narrow and regular
[00:01:00] tachycardias. Let's check them out. I told you in the last module that the QRS complex is narrow. If the impulse travels from the atria to the ventricles, through the AV node, the bundle of His, and the bundle branches. So for example, if a patient has sinus tachycardia, meaning that the sinus node discharges at a rate of 100 per minute or above, when all the other bundles and fascicles and the ventricles are open, the QRS
[00:01:30] complex will be narrow, and the ECG will look something like in the next slides. Let me explain what happens in sinus tachycardia, using this already familiar ladder diagram. So in sinus tachycardia, the sinus node discharges at a rate of 100 beats or above per minute. The sinus node is located, here, so it discharges then the impulse travels down through the atria causing a P wave, through the AV node,
[00:02:00] causing ventricular depolarization, then the next impulse starts, travels through the atria, causes a P wave—this blue box—travels down to the ventricles through the AV node, causes a QRS complex, ventricular depolarization, and so forth. So since depolarization of the atria occurs from top to bottom, you see the arrow is directed downwards towards lead 1 and 2,
[00:02:30] P waves will be positive in these leads. And since atrial depolarization occurs before ventricular depolarization, the P waves are preceding the QRS complexes. Now, you might think, well, yeah, of course, that's always the case. But as you will learn, there are some cases of narrow complex tachycardias where that is not the case. One thing that I like to mention about sinus tachycardia is the fact that it's a symptom of another extra cardiac cause,
[00:03:00] something like exertion, fever, pulmonary embolism, anemia, and so forth. So, the heart rate increases when the underlying cause is present and ends when it's gone again, that's a little different to atrial tachycardia. And I'm going to tell you how in just a bit. In atrial tachycardia, an ectopic area, within the atria, takes over the pacemaker function and then depolarizes the atria upon [00:03:30] which the impulse travels down to the AV node, through the AV node, through the bundles, and into the ventricles. Let's have a look at atrial tachycardia using our well-known ladder diagram. So let's recap. Atrial focus discharges, the impulse travels down to the ventricles and the same thing happens over here, in the ladder diagram. Atrial focus discharges, depolarizes the atria, impulse travels to the ventricles, depolarizes the ventricles, then you see the
[00:04:00] QRS complex, atrial focus discharges, depolarizes the atria, P wave travels down to the ventricles, QRS complex, and so forth. As in sinus tachycardia, the impulse depolarizes the atria before it depolarizes the ventricles. Due to the fact that this atrial focus can be situated anywhere in the atria, the atrial vector, that's the vector that points away from that focus,
[00:04:30] can show into any direction pretty much. So, that's one important take-home message, that in atrial tachycardia the P can be positive in one and two, but it can also be negative. Most of the time, however, the Ps in atrial tachycardia are positive in leads one and two. And how can you tell the difference between those two types of tachycardias, between sinus tachycardia and atrial tachycardia, from a clinical perspective? Well, it's really important to take a clinical history. In sinus tachycardia, the onset
[00:05:00] of the fast heart rate is gradual, and it depends upon the underlying cause of the tachycardia. The cause usually doesn't come and go away suddenly but it comes and goes gradually. So, sinus tachycardia also comes and goes gradually. In contrast, atrial tachycardia look something like, that. It comes suddenly and it also disappears suddenly. There are situations when an atrial impulse travels to the ventricles and then back to the
[00:05:30] atria again, through an additional conduction bundle. Back in the atria, the impulse thing causes another wave of atrial depolarization, travels down to the ventricles again and causes ventricular depolarization. This is called a circus movement or reentrant tachycardia. One of these reentrant tachycardias can occur in patients with Wolff-Parkinson-White syndrome. Let's check it out. We remember that in WPW syndrome or Wolff-Parkinson-White syndrome, the
[00:06:00] atrial impulse can depolarize the ventricles through two different pathways. One, would be the AV node and the other one would be the bundle of Kent. If there was only the AV node, the QRS complex would look completely normal and narrow. But since there's also the bundle of Kent, we can also see a Delta wave. Now let's see what happens when there's a premature atrial beat. As you can see, the premature
[00:06:30] atrial beat reaches the bundle of Kent in its refractory period, so the impulse can no longer travel down to the ventricles, through the bundle of Kent. However, the AV node was already out of its refractory period so the impulse could travel down to the ventricles through the AV node. And in this case, the QRS complex would look completely normal without a Delta wave. What happens next? Well, the bundle of Kent leaves its refractory period and this impulse travels back up
[00:07:00] to the atria and a circus movement tachycardia is established. Now, this type of tachycardia is called AVRT, atrioventricular reentrant tachycardia. The atrial depolarization is already marked in this anatomical representation and on this ladder diagram. Now let's see what happens after that. Well, the impulse travels down through the AV node to the ventricles. What happens then?
[00:07:30] The ventricles are depolarized and we can see a QRS complex. Next, the impulse travels back up into the atria, causing a retrograde depolarization of the atria and causing a negative P wave. Now, why is that P wave negative again? Well, actually, because the vector of atrial depolarization, points from bottom to top. So, away from leads one and two, then the impulse reaches
[00:08:00] the ventricles again, causes a QRS complex, travels back up into the atria, causing a retrograde P, traveling down to the ventricles again, and so forth. Now, you see why it's so important to look if the P waves come before or after the QRS complex and if the P waves are positive or negative in leads one and two. Now let's turn to another situation where there is a circus movement tachycardia.
[00:08:30] The situation of AVNRT or AV nodal reentrant tachycardia. As you can see, here, some people have two separate conduction tract in their AV node. One with slow conduction properties and the other one with fast conduction properties. Usually, the impulse travels down from the atria, to the ventricles, through the fast tract. AV nodal reentrant tachycardia
[00:09:00] can occur, when by chance, the fast tract is intermittently refractory, and the atrial impulse passes the AV node by the slow tract . So, here, travels down, after reaching the bundle of His, the fast tract may now be opened and take the impulse back into the atria again, and so another circus movement is born. Let's see what AVNRT looks like on the ladder diagram.
[00:09:30] So first, depolarization goes down into the ventricles and we can see a QRS complex. Then, the impulse travels back up into the atria, where it causes a retrograde P wave. The P wave is negative in leads one and two. Then the impulse travels back down into the ventricles, again, where it causes depolarization and a QRS complex. Then it travels back up causing
[00:10:00] a retrograde P wave and so forth. Please note, that the negative P wave in a AVNRT comes immediately after the QRS complex, which will be important later on when you learn to differentiate between AVRT and AVNRT. Sometimes, the retrograde P wave is so close to the QRS that it actually gets almost buried in the QRS, and here's a little trick how you can differentiate it from the QRS. All you have to do is to look for a pseudo R prime when you suspect
[00:10:30] AVNRT with a hidden P wave. And that's what the pseudo R prime looks like. If you see something like this, a little knob at the end of the QRS then that could be a pseudo R prime, which would be a sign for the presence of AVNRT. So by now, we can already differentiate between sinus tachycardia, atrial tachycardia, AVNRT, and AVRT. Let's try to differentiate between those two entities and those two entities, first. So, in sinus tachycardia and atrial tachycardia,
[00:11:00] P waves always come before the QRS complex. So, that's what they have in common. In sinus tachycardia, the P waves are always positive in leads one and two. In atrial tachycardia, on the other hand, P waves can be positive or negative in those leads . However, as we've learned most of the time, these Ps are positive in leads one and two. Now, what about the circus movement tachycardias, AVNRT and AVRT. While, here,
[00:11:30] the P waves are always negative and they usually come after the QRS and sometimes inside the QRS, as we've just learned with the pseudo R prime trick. But there's one more trick how you can differentiate between AVNRT and AVRT and I'm going to teach it to you in just a bit. Here, the two tachycardia side by side again. Here's AVRT and here's AVNRT. As you can see up here and as you've already learned in AVRT,
[00:12:00] the circus movement is much larger than in AVNRT. What does that mean? Well, that means that once the impulse reaches the ventricles and once the QRS complex develops, it takes longer for the impulse to reach the atria again in AVRT than in AVNRT, right? So, the distance of the P waves to the QRS complex in AVNRT must be shorter than the distance of the P waves to the QRS complex in AVRT. Let's check it out.
[00:12:30] So, you see that, here, the retrograde P wave is already in the T wave, while here, the retrograde P wave is still in the ST segment. So, it comes earlier than in AVRT, and that is the trick that I was talking about. That's how you can differentiate between AVRT and AVNRT. The distance between QRS and P wave is short in AVNRT and long in AVRT.
[00:13:00] So, in a nutshell, a regular tachycardia, with narrow QRS complexes, can have one of four possible causes. It can be due to sinus tachycardia, where the sinus node fires at a rate of over 100 beats per minute. It can be due to atrial tachycardia or an ectopic atrial focus takes over the pacemaker function. It can be due to reentrant tachycardia in the setting
[00:13:30] of AVNRT, or a reentrant tachycardia in the setting of the Wolff-Parkinson-White syndrome, also called AVRT. And not to forget it could also be atrial flutter, with regular two to one conduction, which means two flutter waves per QRS complex. Here, however, the flutter waves should have given us a clue in the first place. You can see them, here. In the following exercises, you should decide if tachycardia is regular or irregular,
[00:14:00] if the QRS complexes are broad or narrow, you should try to identify P waves and QRS complexes, and finally, you should try to come up with a correct diagnosis. Don't miss out on the videos explaining the solution to each case. You'll learn a lot from them. So, Take care and I'll see you in the next module.