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So let's get started! Every single time the heart contracts and we have a heartbeat there is an electrical waveform that is created on an EKG. Remember from our other lessons that when there is an electrical conduction it causes a mechanical action. So every time the atria contract it produces a P wave and when the ventricles contract it produces a QRS. And when the ventricles relax or depolarize, a T wave is produced. Let's break down each of these waveforms into more detail!
Let’s start with the P wave. Remember the SA node, every single time the SA node sends an electrical impulse across the atria, the atria contract or depolarize. When this action takes place, it creates the P wave on an EKG, the P wave is an upward or positive deflection. The PR interval on an EKG is very important also, the PR interval is measured from the beginning of the P wave to the beginning of the QRS. This PR interval indicates the seconds it takes for the electrical current to reach the ventricles. When SA node starts the electrical impulse and the atria contract, the impulse is then sent to the AV node, remember that the AV node slows down the impulse so the atria can contract all the way. Then the AV node and bundle of His send the impulse to the ventricles. So the time it takes for the atria to initiate a contraction, contract and send the impulse to the ventricles is called a PR interval. This is normally done in 0.12-0.20 seconds. That’s not a lot of time! Anything longer than that, like 0.24-0.28 seconds means that it is taking the electrical current too long to get to the ventricles. That is when people have first degree heart blocks but we will talk about those in another lesson!
So after the atria has finished contracting, it sends the impulse to the ventricles. When the ventricles contract or depolarize, a QRS complex is seen on an EKG. This waveform has negative and positive waveforms or downward and upward waveforms depending on the lead. As you can see the waveform on a QRS complex, especially the R wave is a higher amplitude, that is because of the force of contraction the ventricle has to pump to effectively get the blood out to the body creates a greater voltage and amplitude. The QRS interval is the time it takes for the electrical current to travel through the ventricles, it starts at the beginning of the Q wave to the end of the S wave. It should normally take about 0.06-0.12 seconds to complete. Again that is not that long, the heart should not take that long to contract. The heart naturally beats about 80 times a minute so it needs to do this pretty quickly to keep up. If a QRS interval is long, you will hear the term wide QRS complex and it means that its usually greater than 0.12 seconds and its taking the heart way too long to contract.
And lastly, the good old T wave. It is the last waveform on an EKG. When the electrical conduction has finished and depolarization is complete, the ventricles repolarize so they can rest and fill up with blood and get ready for the next contraction. When potassium is reentering the cell, it causes the ventricle to relax, this is what produces the T wave on an EKG. This is a positive deflection. The QT interval on an EKG is the time it takes for the ventricles to contract and relax, it starts at the beginning of the Q wave and ends at the end of the T wave, it should normally take 0.36-0.44 seconds for this to take place. Your might hear prolonged QT interval also and this means that it takes longer than 0.44 seconds for the ventricle to contract and relax, this is not good. When the ventricles take too long to contract and relax, it can cause deadly arrhythmias. So always keep an eye out for that QT interval. ST segment is from the end of the QRS complex to the beginning of the T wave, it’s a flat line and it’s the time between ventricular depolarization and repolarization.
It is important to understand that the PQRST waveforms can indicate what is going on in the heart and which chamber is contracting and when. Meaning, when you see a P wave you know that is the atria contracting, if there are abnormal P waves, you would know it has to do with the atria. Same with the QRS, it means the ventricles are contracting, so when you see abnormalities in the QRS complex, you will know it has something to do with the ventricles. Of course the T wave is the ventricles relaxing, so if the T wave is abnormal you would know the ventricles are not relaxing like they should. With that being said, it is also important to understand that the atria contract and then the ventricles contract. It just simply has to work that way, therefore you should always see a P wave followed by a QRS. This is how you can always tell if you patient is a sinus rhythm. Do you have a 1 P wave followed by 1 QRS, and of course a T wave. Sometimes there is a U wave which is not clearly understood but it’s thought to be repolarization of the Purkinje fibers. Not everyone has a U wave so you may not see them too often.
All right so you guys. It is so important, I don't think I can stress this enough how important it is for you to understand that pqrst wave form because it helps you identify which chamber is contrasting and at what time.
So again looking at this, we have a P wave, P wave. P waves tell, if you recall every time you see your P wave it means what the Atria is contracting or is depolarizing. And then we identify our QRS, QRS, QRS complex. Every time that we see a QRS, it means what guys? The ventricles are contracting. So you should always always always see a P wave followed by a QRS, P wave followed by a QRS, P wave followed by a QRS. And then of course don't forget about our T waves. T waves are very important, it means that the ventricles are relaxing. So you might ask yourself, how come you don't have a waveform for the Atria to relax? Well that usually happens around here. So because the qrs is so much bigger, you don't see a waveform where the Atria are relaxing. So with this being said, you know that the P wave means the Atria are Contracting. So you should always see a P wave followed by a QRS because the Atria contract and the ventricles contract, atria contracts, ventricles contract, so you should always have a p wave followed by a QRS. And of course if the ventricles contract they have to relax. If not, we have a dead patient. So you always have to see a t wave. Every now and then, I'm not sure if you guys have ever heard of a u wave, every now and then a patient may have a u wave, and this is usually occurs right after the T wave. It's not really understood what they u wave is there for it they believe that it may be repolarization of the purkinje fibers. But not everybody has one. In my career I think I've seen like two patients with a u wave but what's interesting about that is that you may get it confused and think it's a p wave. Cuz there's a T waves pqrst and then a u wave, the you'll see a P wave again followed by a PQRST. So here's our new but I still sometimes when you look at it you're like wait a minute or is that an extra P wave what is it? And then you get everybody involved and you start analyzing it until you realize no, this patient has a U-wave. So again, it doesn't happen too often and you will not see it that often. But just know that every now and then very rarely a u wave decides to come along whenever it feels like it. That just depends on the patient, their electrical conduction, on whether it shows up.
To recap, these key points are your biggest takeaway from this lesson. Remember to keep it simple so you can understand the basics. The P wave is when the atria depolarize, the PR interval is how long it takes for the electrical current to reach the ventricles. The QRS complex is ventricular depolarization and the QRS interval is how long it takes for the electrical current to travel through the ventricles and the T wave is when the ventricles repolarize.
I hope this makes sense to you guys and that you are able to understand the basic waveforms on an EKG. Make sure you check out all of the resources attached to this lesson. Now, go out and be your best self today! And, as always, happy nursing!
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