EKG Interpretation, part 3: Analysis of Heart Rhythm, Heart Rate, P wave, PR interval and QRS complex

by Cathy Parkes July 09, 2020

Full Transcript

Alright. In this video, we are going to interpret an EKG and go through the different steps in doing so. The first step will be to identify the heart rhythm as either regular or irregular. Then we'll talk about how to calculate the heart rate. Then we will analyze the P wave, the PR interval, as well as the QRS complex. All right. Let's talk about how to analyze the heart rhythm. So we want to check to see if the rhythm is regular or irregular, and we want to check both the atrial heart rhythm as well as the ventricular heart rhythm. So this strip that I have here on the left shows how we would go about analyzing the ventricular heart rhythm. So we would measure the distance with calipers between one R wave to the next. And you can see that it's the same distance between all our waves. So we have a regular ventricular rhythm here, as opposed to this graphic on the right. You can see when we measure the distance between the R waves, that they vary. This is a smaller distance here between this R wave and this R wave, than it is between these two R waves. So this would have an irregular ventricular heart rhythm.

To analyze the atrial heart rhythm, we would do the same thing with calipers, but measure between the P waves, to see if the distance between each P wave is equal or if it varies, like it does here in the second strip. So you can see the distance between this P wave and this P wave is not the same as the distance between this P wave and this one. So in this case, we have an irregular ventricular heart rhythm as well as an irregular atrial rhythm.

 Now let's talk about how to calculate the heart rate. If the rhythm is regular, the best way to calculate the heart rate is using the small box method. Again, this is just for regular rhythms. So with the small box method, you would count the number of small boxes between the R waves. So you can see that it's blown up here, and we can count 12 small boxes between the R waves. There will be exactly 12 small boxes between each of these R waves because it's a regular rhythm. So then you would take 1,500 and divide it by the number of small boxes between the R waves, and that will give you the heart rate in beats per minute. So you remember before we talked about there are 1,500 small boxes in a minute. So 1,500 divided by 12 here equals 125 beats per minute. So that is going to be our heart rate.

All right. Now let's talk about the second way of calculating heart rate, which is the big block method, which should only be used for regular rhythms, just like the small box method we just went over. So when it comes to regular rhythms, I personally prefer using the small box method because it is more accurate; however, a lot of people use the big block method. And so I want to explain it here because it is quick, although it is not quite as accurate. So with this method, we would count the number of big blocks between the R waves. So in this case, we have 5 big blocks. Then we would 300 and divide it by 5, which gives us 60 beats per minute. So if you recall in my last video, I shared that there are 300 big boxes in a minute, which is why we take 300 and divide it by the number of big boxes between those R waves.

Alternatively, with this method, you can kind of memorize this number sequence over here to determine the heart rate. So if you go just 1 box, that's 300; 2 boxes, 150; 3 boxes, 100; 4 boxes is 75; 5 boxes is 60; and so on. So kind of the math behind this memory sequence method is that if you just have 1 box between R waves, then you would take 300 divided by 1, which would be 300, and if you had two boxes between R-waves, then you would take 300 divided by 2. So it's basically doing what we did over here but just kind of memorizing the sequence as a quick way to determine the heart rate. So again, this is the big block method, definitely an option for calculating heart rate, but in my opinion, the small box method is better. 

Alright. Now, let's go over the last method for calculating heart rate, which is the six-second strip method. This is the method you will need to use if you are dealing with an irregular heart rhythm. If your heart rhythm is regular, then definitely use the small block method, which is much more accurate. But if your rhythm is irregular, then you're going to need to use this method that I will describe here. So with the six-second strip method, you want to, first of all, make sure you're dealing with a six-second strip. So how do you know that? Well, as I shared before, each big block is 0.2 seconds in duration. So five big blocks is one second in duration, so you need to have 30 big boxes to have a six-second strip. Then, you want to calculate, or not calculate but count, the number of QRS complexes within those six seconds. So in this example, we have five QRS complexes. So we would take 5 times 10 which would give us 50 beats per minute, approximately. I do want to offer this one caveat, though. There are some sources and some instructors who want you to count cycles as opposed to QRS complexes which will, in some cases, give you a different result. So if we were counting cycles-- that means I would start here and this would be one cycle, this would be two cycles, three cycles, four cycles, and a little bit, right? So we would have just a little bit over 40 beats per minute if I counted cycles. But most teachers and most sources have you counting the number of QRS complexes. But you will need to clarify that with your particular instructor to see how they want you to calculate the heart rate with the six-second strip method.

Alright. Now that we've determined the heart rhythm and rate, let's analyze the P waves on our EKG strip. So first of all, we want to make sure there are actually P waves on the EKG strip because there are some dysrhythmias where P waves are missing. Then we want to make sure they are occurring regularly, and we want to make sure they have a consistent appearance across the strip. So P waves should be upright, smooth, and rounded. And like we talked about before, the amplitude, or height, should be up to 2.5 millimeters high. And then we want to make sure there is one P wave for each QRS complex on the strip. So we can see here on the left that we have a P wave followed by a QRS complex. P wave followed by a QRS complex across the board. However, if you look here on the right, we can see that we have an inverted P wave. So there is a P wave with each QRS complex, but it is inverted, which is abnormal. And we'll talk about what causes that abnormal P wave when we get into specific dysrhythmias. But that's definitely something you will want to note when you are analyzing the P waves on your EKG.

 The next step in EKG interpretation is analyzing the PR intervals on your strip. So when we are measuring the PR interval, we want to use calipers to make sure we get an accurate measurement. So like I shared before, the PR interval should be between three and five small boxes wide, okay, which is 0.12 to 0.2 seconds in duration. So you can see that the PR interval here on the left is normal. It is five small boxes in duration as opposed to the interval here on the right, which is prolonged. It is over five small boxes wide, so you'll definitely want to make note of that. You also want to make sure that the intervals are consistent across the strip. There are some dysrhythmias where the PR interval will gradually lengthen, and you'll definitely want to make note of that.

Right. Next, let's analyze our QRS complexes. So like I shared before, the QRS complexes should be under 0.2 seconds in duration or under three small boxes in width, okay? They should be nice and narrow. Also, we want to make sure they have a consistent appearance across the strip, and we want to make sure there is a QRS complex for each P wave. So you can see here on the left, we have a P wave followed by a QRS complex, P wave, QRS complex, and so on. On this side, you can see that our QRS complexes are not normal. They are wide. We have a wide QRS complex, and we do not have any P waves with this dysrhythmia as well. So we will get into what can cause a wide QRS complex and why we would have a missing P wave.

So now that we have reviewed the different steps on how to analyze the EKGs, including identification of the rhythm, the heart rate, analysis of the P wave, PR interval as well as the QRS complex, we are ready to put it all together and identify the key dysrhythmias that you'll need to know. Thanks so much for watching!


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