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

by Cathy Parkes July 09, 2020

In this article, we cover how to interpret an EKG and we go through the different steps in doing so. The EKG Interpretation video series follows along with our EKG Interpretation Flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX.

Analyzing regular versus irregular heart rhythms

When checking to see if a heart rhythm is regular or irregular, we need to check both the atrial heart rhythm as well as the ventricular heart rhythm.

Ventricular heart rhythm

The ventricular heart rhythm is represented by the R wave on an EKG strip. To determine if the ventricular rhythm is regular vs irregular, we would measure the distance between R waves with a caliper. A regular ventricular heart rhythm will have the same distance between R waves while an irregular rhythm will have varying distances between them. 

Atrial heart rhythm

The atrial heart rhythm is represented by the P wave on an EKG. To determine if the atrial rhythm is regular vs irregular, we would measure the distance between P waves with a caliper. A regular atrial heart rhythm will have the same distance between P waves while an irregular rhythm will have varying distances between them. 

How to calculate heart rate

There are three ways to calculate a heart rate using an EKG. If the rhythm is regular, the small box method and big block method can be used. If the rhythm is irregular, the six second strip method can be used. 

Small box method

With the small box method, you count the number of small boxes between R waves, then divide 1,500 by that number, and that will give you the heart rate in beats per minute. The reason we use 1,500 is because there are 1,500 small boxes in one minute on an EKG. You are basically checking how many spaces-between-R-waves there are in a minute, and that's your heart rate! A great example of this process is as follows:

  • We observe/count 12 small boxes between R waves on an EKG
  • 1,500 is the number of small boxes in one minute on an EKG
  • We take 1,500 divided by 12 and get a heart rate of 125 beats per minute.

Big block method

The big block method is very similar to the small box method explained above. In the big block method we count the number of big blocks between R waves as opposed to small boxes. You then take 300, which is the number of big boxes in a minute on an EKG, and divide that by the number of big blocks counted. A great example of this process is as follows:

  • We count 5 big blocks between R waves on an EKG
  • 300 is the number of big boxes in one minute on an EKG
  • We take 300 divided by 5 and get a heart beats per minute of 60

Memory sequence method

You can take the big block method and, instead of counting and dividing, you can just memorize the number sequence to determine the heart rate. This sequence is as follows:

  • 1 box = 300 bpm
  • 2 boxes = 150 bpm
  • 3 boxes = 100 bpm
  • 4 boxes = 75 bpm
  • 5 boxes = 60 bpm

Memorizing the sequence is just a quick way to determine the heart rate, but it's important to understand the big block method first so you know why those numbers of boxes equal those numbers of beats per minute.

Six-second strip method

All of the methods we just covered work well when you are dealing with a regular rhythm. A regular rhythm is the same pattern of waves that repeats, which is why you can zoom in to count boxes and extrapolate that to the rest of the rhythm. But what if the heart rate is not regular? What if the spaces between R waves is different across the strip? When you are dealing with an irregular heart rhythm, you can use the six-second strip method. 

The first step in calculating heart rate with the six-second strip method is to first ensure you are dealing with a 6-second EKG strip. A 6-second strip is made up of 30 big boxes. Each big block is 0.2 seconds in duration, so 5 big blocks is equal 1 second in total duration (.2 x 5 = 1), meaning you would need a total of 30 big boxes to make a 6-second strip. 

Once you know you are dealing with a 6 second strip, you count the number of QRS complexes within those 6 seconds and multiply by 10. For example, if we count 5 QRS complexes within a 6 second strip, we would get 50 beats per minute, approximately. 

Cycles vs. QRS Complexes

Some sources, or some nursing instructors, will ask you to count cycles as opposed to QRS complexes. A cycle is defined by a full heartbeat captured by a EKG. Meaning that the EKG shows the P-wave, QRS complex, and T wave. In some cases, this will give you a different result. In the above example with five QRS complexes, you may only see four cycles plus a little bit extra, which would give you 40+ bpm instead of 50. 

Most teachers and most sources will prefer the QRS complex method, but you should clarify that with your instructor to make sure you choose the right method. 

Analyzing P waves on a EKG strip

First, make sure that there are actually P waves on the EKG strip because there are certain dysrthythmias where P waves are missing. 

Once you’ve determined that a strip includes P waves, you’ll want to make sure the P waves are occurring regularly and have a consistent appearance across the strip. P waves should be upright, smooth and rounded with an amplitude, or height, of up to 2.5 millimeters high. 

Then we want to make sure there is one P wave for each QRS complex on the strip. There are certain dysrhythmias that cause abnormalities, which we will cover in later articles, that can cause the P wave to be inverted. These types of abnormalities are definitely things you’ll want to take note of while analyzing P waves on an EKG strip.

Analyzing PR intervals

The PR interval is the period in milliseconds from the beginning of the P wave to the beginning of the QRS complex. It's helpful to use calipers against the EKG strip to make sure you get an accurate measurement of boxes. 

The normal PR interval should be between 3 to 5 small boxes wide, which is 0.12 to 0.20 seconds in duration. If the PR interval is longer than 5 small boxes, it is considered to be abnormal and should be taken note of. 

PR intervals should also remain consistent across the strip. There are some dysrhythmias where the PR interval will gradually lengthen, which is also something to take note of as an abnormality. 

Analyzing QRS complexes

The QRS complex is the combination of three of the graphical deflections seen on a typical EKG. Think of Q, R, S, from the alphabet — they come immediately one after the other.

QRS complexes should be under 0.20 seconds in duration or under three small boxes in width. They should be narrow and have a consistent appearance across the strip. 

There should also be a QRS complex for each P wave, however there are certain dysrhythmias that can cause a wide QRS complex with no P waves, which we mentioned above. We will cover why this happens in later articles, but any abnormalities should be taken note of when analyzing an EKG.


Full Transcript

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.

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.

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.

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 take 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.

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.

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 count the number of QRS complexes within those six seconds [and multiply by 10].

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.

Now that we've determined the heart rhythm and rate, let's analyze the P waves on our EKG strip.

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. When we are measuring the PR interval, we want to use calipers to make sure we get an accurate measurement.

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.

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.

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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