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Studying for a test? Prepare with these 15 lessons on Circulatory system diseases.
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Video transcript
- Atrial flutter is one of the heart arrhythmias that's also known as just flutter. It's one of the supraventricular tachycardias. Now remember that the heart's dominant pacemaker is the SA node and the SA node is going to send the signal to the AV node which will then conduct the signal down to the lower half of the heart which makes the ventricles contract. In case the SA node fails, the heart has several backup pacemakers called automaticity foci. These backup pacers only fire in the event that the signal from the SA node is not efficiently or effectively reaching the AV node and when they do fire, they fire at a rate between 60 and 80 beats per minute or BPM. In atrial flutter, there's a really irritable automaticity focus and I'm drawing that here in purple. This irritable focus is going to fire at a rate between 250 and 300 beats per minute, which is a lot faster than the rest of the foci. The electrical signal travels around in a circular pattern and moves around again, and again, and again, and again, and again. So this causes the atria to contract at a rate between 250 and 300 beats per minute. Every time the signal goes around it's going to hit the AV node and remember the AV node is what conducts the signal down to the lower half of the heart and makes the ventricles contract. So you might think because of that, the ventricles are also going to contract at a rate between 250 and 300 beats per minute but in reality the ventricles usually contract at a slower rate around 150, beats per minute. Now why is that? That's because there's a built in mechanism in the AV node called a refractory period. Other tissues have this too. After the AV node conducts the signal, to the lower half of the heart, there's a window of time also known as the refractory period where the AV node can't fire again even if it gets the signal to do so. So this is sort of like a backup mechanism to prevent the AV node from over-firing which will then prevent the ventricles from contracting too quickly. So let's look at this in EKG. With an atrial flutter EKG you're going to see multiple P waves and regular spaced QRS intervals. By regular I mean that the space from this R to R interval is going to be the same as this R to R interval which is the same as this R to R interval. Now why do we have these multiple P waves? Remember, that there's an irritable automaticity focus that's over-firing, so it's going to fire, fire, fire but it won't conduct signal through the AV node every time it fires because of the refractory period. So for example here, we have the focus firing, causing the atria to contract, it hits AV node, conducts, makes QRS complex and it tries to fire again but because we're in the refractory period it's not going to conduct through the AV node and you're not going to get another QRS complex. Instead you're going to see another P wave when it tries to fire again. In this particular example we have, three P waves, for every QRS. So this is called three to one conduction. In atrial flutter you can also have two to one conduction, where you'd have two P waves for every QRS or even four or five P waves for every QRS but this example is three to one conduction and if you look closely you can appreciate that the lines that the P waves make, make a certain pattern and they kind of resemble the teeth on a saw. So classic A flutter is said to have a saw tooth pattern and the saw teeth are the P waves.