Electrocardiogram (ECG) - Online Tutor, Practice Problems & Exam Prep

Electrocardiograms (ECGs) are vital for assessing heart function, depicting the electrical activity of the heart through waves and complexes. Key components include the P wave (atria depolarization), QRS complex (ventricles depolarization), and T wave (ventricles repolarization). Abnormalities like bradycardia (slow heart rate), tachycardia (fast heart rate), and fibrillation (uncoordinated contractions) indicate potential pathologies. Understanding these patterns aids in diagnosing heart conditions, emphasizing the importance of recognizing normal versus pathological ECGs for effective cardiovascular health management.

Electrocardiogram

This is an electrocardiogram, and almost certainly you are familiar with this sight and with this sound. Now, we're going to talk about electrocardiograms in a lot more detail. Before we do, I just want to acknowledge we don't normally talk about medical tests in a lot of detail. We're going to talk about electrocardiograms because they tell you so much about the heart. They're so common, and you are very likely to see questions about it.

Before we really dive in, though, I just want to note we have two words here that are very similar. We have electrocardiograph and electrocardiogram. Let's talk about the distinction between these two. Electrocardiograph, that's the instrument. That's the machine that measures the electrical activity in the heart using electrodes on the body. Now the electrocardiogram, what we sometimes call the ECG or EKG, I'll come back to those letters in just one second. That's the recording of the cardiac electrical activity. So this distinction, it's a fine one, and it's probably okay if you get it confused. What you really want to be able to do is to be able to read an electrocardiogram and say what's going on in the heart, but electrocardiograph, technically, that's the machine. The readout from that machine is the electrocardiogram. The way I remember it is I think of a telegraph and a telegram. Now that's that old-timey way of sending messages that you may be familiar with from, like, movies about the old west. The telegram, that's the readout you get. That's the message you get. It's sent by a machine called a telegraph. So that distinction is the same that we're talking about here.

So what we really want to talk about is that readout. What does it look like and how do we read it? That's the electrocardiogram. Now we said we can call this an ECG, and that just comes from electrocardiogram, or sometimes people call it an EKG. That comes from the same word, just if you spell it in German. Now, I speak English, so I'm going to be trying to say ECG, but for historical reasons, sometimes people still call it an EKG. Again, you probably don't need to really worry about that. If you get mixed up and you call it an EKG, just tell your professor you like to speak in German.

These, again, we said these are the recordings of the cardiac electrical activity. What you're really recording, though, what you're measuring is the depolarization and the repolarization of the atria and the ventricles. We are looking at these cardiac contractile cells. Remember, that's the majority of the heart muscle. And when we looked at the action potentials in these cardiac contractile cells, remember the depolarization and the repolarization are separated by that plateau phase when it just sort of stays depolarized for a little while. So we can see those two things happen separately.

Now this means that we're actually going to be looking for four things when we look at this ECG. We're looking for the depolarization and the repolarization of the atria, and the depolarization and the repolarization of the ventricles. Now, spoiler, you can't actually see all four of those, but I want you to keep those four things in mind as we're going through this. So we're going to draw this out here, and we have the voltage on the y-axis and time on the x-axis, and we also have our heart there showing the electrical conduction system for reference.

Alright. So we start out, we go along, and we get this first little blip that we have, sort of colored there in that yellow-orange color, and we've labeled this the P wave. Alright. So if you think, what starts the electrical conduction system of the heart? Well, that's when the sinoatrial node, the SA node, depolarizes, and that starts a wave of depolarization that spreads through the atria. So that's what we're seeing here. We're seeing those contractile cells in the atria depolarizing. Right? So that P wave, we're going to say the SA node fires, and then what you're actually seeing here is that the atria depolarize.

Well, remember the atria depolarize, and then they go into those cells go into that plateau phase. And during that plateau phase, that's when those cells actually contract. So I've highlighted the atria there in red to illustrate now is when those atria are contracting.

Alright. Well, following that, now we get sort of the major feature of one of these electrocardiograms. We get this QRS complex. This QRS complex is going to be really big compared to that p wave, because these are when this is when the ventricles depolarize. And the ventricles, well, there's just so much more mass in the ventricles than there is in the atria. So when all those cells depolarize at basically the same time, it is a massive electrical activity. So we're going to say here the QRS complex. We can see the ventricles depolarize. And I just want to note, you may wonder why does it go down for the Q, way up for the R, and then down again for the S. Well, the Q and the S specifically, remember, we're measuring electrical activity in the heart, but this electrical activity, these action potentials are spreading through the heart like a wave, and the heart is this very complex three-dimensional structure. And we're measuring this from nodes on different parts of the body, from electrodes on different parts of the body is what I meant to say. Excuse me. So sometimes those waves are sort of moving towards those electrodes, sometimes they're moving away from the electrodes, and so that sort of down for the Q and the down for the S, that's sort of just actually an artifact of how the how this electrical activity is measured. What you really want to see is just how big that that R wave is, this massive depolarization, because you have a lot a lot of mass in the heart all depolarizing at the same time in those ventricles.

Now we also want to note we're looking for 4 things here. We were looking for the atria and the ventricles to depolarize. We've seen that. But we also want to see the repolarization. Well, our atria depolarized, but we're not going to be able to see them repolarize because it's happening now. So this is also atrial repolarization. And the atria just are so small in terms of mass compared to the ventricles, that that repolarization of the atria just sort of gets lost in the wash of this QRS complex. It's happening at the same time, but you can't really see it because the ventricles are just so much bigger.

Alright. Well, now the ventricles are contracting. So now remember we're in that plateau phase of these cells. That means that this is when the ventricles are really contracting. So the ventricles are contracting, and then they need to repolarize, and we see that in the T wave. So the T wave, we can see the ventricles repolarize.

Alright. Now as we look at this, there's going to be different intervals and segments that people sometimes measure. An interval is when you're measuring more than one part of this readout at the same time, and that is going to include at least one of these waves. A segment is going to be one of the times when the line is flat between the waves. So just to give you some examples of this, we'll, include one of each of those here. We can look at what's called the PR interval. Now the PR interval starts with the P wave, includes the P wave, and it ends at the beginning of the QRS complex. Now you may say why isn't it called the PQ interval? Well, sometimes that Q wave is very small and you can't see it. So normally when we talk about something like that, we just refer to the R wave because the R wave is so big and so obvious. So the PR interval, we said there, that's when those atria are contracting. But we also need to think, why aren't the ventricles starting to contract right then too? Remember, action potential waves of depolarization that spread really fast. Remember, this is what happens. This is the AV node delay. Remember that depolarization is going to get to that AV node very quickly. It's the AV node is going to get that signal during that first P wave, but the ventricles don't contract for a little while because that AV node puts that 100 millisecond pause on things. It says, let's just wait a second. Let's give a second for the atria to contract, Then we'll spread that action potential to the ventricles.

Right. Now between the QRS complex and the T wave, we can see here this is called the ST segment. And so we've highlighted that on the readout there as well, on that electrocardiogram there. So the ST segment, well, we said this is when those ventricles are contracting. And this specifically, we can say this is definitely that ventricle plateau phase. Right? The plateau phase when the ventricles are depolarized, and they're just sort of staying there depolarized before they repolarize again.

Alright. Now this is a normal ECG, but of course when people look at an ECG, they're looking for things that are wrong. So we're going to practice looking at some things that are wrong with an ECG, and trying to figure out how to read those as well. We don't need to be an expert at it, but, honestly, if you get pretty good understanding what each one of these figures on the ECG represents, well, if it doesn't look right, you should be able to figure out what part of that heart's electrical conduction system is causing a little bit of a problem. Again, we'll look at that more coming up. First, we have an example in practice problems. I'll see you there.

Electrocardiogram (ECG) Example 1

Our example says, on the ECG below, identify the following, and it has four things that we need to do with this ECG. So as I look at this ECG or electrocardiogram, I see those waves and complexes that we've been over, so let's see what we need to do with it. It says, a, correctly label one heart contraction with the letters p, q, r, s, and t. Alright. So look at the ECG, pick out one heart contraction, and think where would you put those letters, p, q, r, s, and t. Alright. Well, when I look at it, the first thing I want to do is I want to identify the different heart contractions, because we can see the heart contracting a few times here. So this pattern that I'm circling here, that's one heart contraction, the depolarization of the atria, the depolarization of the ventricles, and then the repolarization of the ventricles. And I see that happen once, twice, three times, and then this 4th time we see it starting. We don't see the whole thing there because the end of it is cut off. Alright. So now to label the p, q, r, s, and t, I'm gonna pick the first one. Well, start with p. That's the first thing you see. That's that first little bump there. The p wave, remember, that's depolarization of the atria. Next, we see the QRS. So it goes down for the Q, goes way up for the R, and then goes down a little bit. Again for the S. That QRS complex, remember, is depolarization of the ventricles. Now remember, the repolarization of the atria happens at the same time, but you can't see it on an ECG because the depolarization of the ventricles is such a bigger voltage change that it just sort of gets lost in the wash. Alright. That means finally, we got one more thing here. We got one more bump right down here. That's our t wave. Remember, the t wave is the repolarization of the ventricles. Alright. That means that I have done a. I'm gonna cross that out. Next, we wanna put a box around the sections which measure the activity of the ventricles. Alright. So where would you put your boxes on this ECG? Well, I'm gonna put my boxes sort of starting at the beginning of that QRS complex and then ending right after the t wave. Remember, QRS, that's the depolarization of the ventricles, and the t wave, that's the repolarization. So everywhere in that box has to do with the ventricles. So I'm gonna keep going. I'm gonna put this down here a few times. There's another box. Here's another box. And then on the 4th one, well, we can see the activity of the ventricles. We don't see that t wave because it's cut off, but we can still put a box there. Alright. That means I've done b, and I'm gonna move on to c. Circle the sections that measure the activity of the atria. Alright. Where are you gonna put your circles? Well, the atria, we can see on an ECG. We can see when they depolarize as part of that p wave. So I'm gonna circle my p waves. Now technically speaking, the repolarization is happening during that QRS complex, but we can't see it on the ECG, so I'm not gonna circle that part. Alright. So I've circled my 4 p waves. That was the activity of the atria. I'm done with c. I'm gonna move on to d. He says, in this section of readout, how many times can you observe the ventricles repolarizing? Alright. So take a look. How many repolarizations of the ventricles do you see? Well, remember, repolarization of the ventricle, that's the t wave. So really, we're just asking you to count the t waves, and I see 1 t wave, 2 t waves, 3 t waves, and you can't see the 4th t wave in that last contraction there. So I counted to 3. That's gonna be my answer. I see 3 times that the ventricles repolarize. Alright. With that, I've answered the question. We got more practice problems to follow. Give them a try.

When analyzing an ECG, you measure the time between the beginning of the QRS complex and the end of the T wave. Which physiological events are included in this interval?

• A. Atrial depolarization and repolarization.
• B. Ventricular depolarization and repolarization.
• C. Atrial repolarization only.
• D. Ventricular depolarization only.

During an ECG analysis of a patient, the doctor detects an abnormally prolonged PR interval. Which stage of cardiac conduction is most likely responsible for this delay?

• A. Conduction through the SA node.
• B. Atrial repolarization.
• C. Conduction through the AV node.
• D. Ventricular repolarization.

Electrocardiogram: Pathologies

We've been looking at electrocardiograms or ECGs, and we've been seeing how these waves and complexes of the ECG correspond to normal heart function. But of course, normally when someone's looking at an ECG, they're looking to see if there's something wrong with the heart. They're looking for pathologies. So we just want to start by saying that ECGs help health professions diagnose heart pathologies, and by pathologies, of course, we just mean anything that might be wrong with heart function.

Now I just want to be clear here. Almost certainly, you are not expected to be an expert at diagnosing heart pathologies using ECGs in your class. But if you understand what a normal ECG is supposed to look like and you understand what those different waves and complexes represent in the heart, when you see one that's abnormal, it's actually not that hard to usually figure out, at least generally, what might be going wrong in the heart. That's what we want to practice here.

So to start that, we just want to look at this normal ECG in the top right here that we're using as a reference. We can see here it has this normal P wave. That's the atria depolarizing. We have the QRS complex. Remember, that's the ventricles depolarizing, followed by this T wave, and that T wave is the ventricles repolarizing. Well, now we see that this pattern just repeats nice and normally in a nice steady rhythm.

Now, if an ECG doesn't look like that, we call it an arrhythmia, and we can break this word down. 'A' means not, and arrhythmia, well, that refers to the rhythm. It doesn't have a rhythm or doesn't have the right rhythm, at least. So, we're going to say here an arrhythmia or arrhythmias are abnormal heart rhythms. And arrhythmias can vary from, you know, minor and very inconsequential to very serious and potentially fatal.

Now to start this out and start looking at them, we just want to first define what a normal heart rate is. And so we're going to say here in a normal ECG, the resting heart rate is expected to be between 60 and 100 beats per minute. And we call that out because the first thing that sort of the easiest thing that jumps out when you look at an ECG is that heart rate. How fast is this pattern repeating? And one type of arrhythmia is going to be if it's too slow, and one's going to be if it's too fast. So if it's too slow, we call it bradycardia. Bradycardia is if that resting heart rate is too low or too slow, and we're going to say that that is a resting heart rate that is going to be below that 60 beats per minute. Now we can look at this ECG right here, and we can see that in this ECG, we have a simple bradycardia. We can see a P wave. We can see the QRS complex. We can see the T wave. That all looks normal. We can see it here. But then as it repeats, it's just kinda too spread out. Those heartbeats are just spread out, and it's just beating too slowly.

Now a lot of times when you see a bradycardia, it's diagnosed as bradycardia with something else. There it's going to be something else wrong here. We don't see that in this ECG. Here, we see a normal ECG just with a simple bradycardia. Something like that, it can be concerning or it might not be concerning depending on the patient. People with excellent cardiovascular fitness will sometimes have or often have heart rates below that 60 beats per minute. In fact, you know, people, if you're a competitive marathoner or a competitive cyclist, often they have heart rates down in the forties beats per minute.

Now they have physiological changes going on in their heart that puts them into an abnormal range, but it's not unhealthy. They just have this abnormal cardiovascular fitness. Alright. Now in the other direction, we have tachycardia. Tachycardia is going to be if that resting heart rate is too fast. Now that's going to be a resting heart rate of above 100 beats per minute, and we can see here we have another ECG here that is just a simple tachycardia. We can see the normal P wave, that normal QRS complex, and the normal T wave. But that pattern, well, it pattern is normal. It just repeats too fast. That heart is just beating too fast.

So again, this is a simple tachycardia. Often, there will be something that's more complex where you have tachycardia with other things going on, other problems with those waves. Again, this is a normal tachycardia, but unlike bradycardia, there's really no time that someone's happy to have tachycardia. If your resting heart rate is above 100 beats per minute, that pretty much universally says there's something unhealthy going on. There's some pathology there. Alright.

Our next one is going to be fibrillation. Fibrillation is going to be uncoordinated contraction. And when we first started talking about the intrinsic cardiac conduction system, we said that coordinated contraction was one of the most important things that that conduction system does for the heart. You need your atria to contract as one. You need the ventricles to contract as one so that it can push blood through the heart. Well, if it doesn't do that, if all these cells are contracting, but they're just kinda doing it on their own, you get this quivering, fluttering heart that can't push blood.

Now fibrillation can be either atrial or ventricular. Atrial fibrillation is serious, but it's usually not fatal, at least not initially. In fact, plenty of people live a very long time with atrial fibrillation. As long as those ventricles are contracting at least somewhat regularly, you're able to push blood through the body, and that's enough to keep you alive. In fact, it's so common that you'll see ads on TV for drugs that treat atrial fibrillation or as the ads often call it, AFib.

Alright. So that's atrial fibrillation. Now in contrast, ventricular fibrillation, well, ventricular fibrillation is pretty much always fatal, if not immediately treated. And we can see here this is an ECG of a ventricular fibrillation. And you can see there's just completely uncoordinated polarizations and depolarizations just going off all over the place. You can't see any recognizable waves. This is just a heart just fluttering, not pumping blood in any coordinated way.

Now when we say immediately treated, you've probably heard it the way you treat this. This is fibrillation. You treat it with a defibrillator. That's where you've seen in, like, TV shows with a clear. You get the paddles. You put that big, hold electric shock through. Now that electric shock coordinates the heart. It causes all of the heart cells to depolarize at one time, and then, hopefully, they repolarize simultaneously, and the SA node or the AV node can take back over and coordinate heart function again. Alright.

As we move on, I want to note those first three that we talked about, bradycardia, tachycardia, and fibrillation, almost certainly, you should know those vocabulary words. Heart block is the next one we're going to do, and it's less likely that you need to know what a heart block is. But like anything, check with your notes, know what your professor expects of you. We put this here again because just knowing what normal heart function is supposed to look like, it's not that hard to recognize when something goes wrong, and I want to point that out by using a heart block.

So a heart block is going to be a disrupted signal, and this is going to be an issue with that conduction circuit. So to see what I mean, we can look at this ECG down here, and you can see we have a normal P wave. We have a normal QRS complex. We have a normal T wave. That all looks great. And now you expect it to repeat again, so we get a normal P wave and then nothing. Right. So it looks to me like the atria depolarized and then the ventricles didn't follow. You follow it along some more. You get another P wave. Well, now this one follows correctly. QRS followed by the T wave, and then you get another P wave. And then again, nothing. So as I look at this ECG, what it looks like to me is that sometimes those atria are depolarizing, and the ventricles aren't depolarizing afterward. That's quite serious. So if I were to say what's going on here, well, it's something with the conduction circuit, Something in between the time the atria depolarized and the ventricles depolarized, something in there is stalling out. So maybe something with the AV node may be to blame. Okay.

Again, you almost certainly don't need to be an expert at recognizing all sorts of different pathologies. I'd be familiar with those first three vocabulary words that we said here, the bradycardia, the tachycardia, and the fibrillation. And I would also just practice seeing, well if

Electrocardiogram (ECG) Example 2

Here we see that a normal ECG and 2 pathological ECGs are shown below, and we have this table down here that shows our 1, 2, 3 different ECGs or electrocardiograms that we're going to need to analyze. And it says a, for each ECG, identify it as normal or pathological. B, if possible, identify and label at least 1 p wave, QRS complex, and t wave on each diagram. We also note here not all waves or complexes may be present in all ECGs. And then, finally, it says, in simple terms, what part of the heart's conduction system do you think may be failing each pathological ECG? Alright. So as we look at this, I think, for me, the easiest way to do this is just start labeling these complexes and waves, and if I can do it, it's going to be normal. If I have trouble, it's going to be abnormal. So in this first one, I see a p wave, I see a QRS complex, and then I see a t wave. Though that looks relatively normal, this does kinda look like a big space there, but we'll come back to that in a second. Now as I keep going, I have a p wave and then nothing. It looks like I have another p wave and then nothing. I have another p wave, and then we have another QRS complex. Alright. So I was able to label things there, but as you look at this ECG, does that look normal or pathological to you? Well, to me, it looks pathological, so I'm going to put a p there. And it looks pathological because it looks like I have these p waves that are not followed by QRS complexes. That's a problem. So in simple terms, what part of this conduction system do you think may be failing? Well, the p wave is depolarization of the atria. The atria, therefore, are depolarizing here, but the ventricles seem not to be depolarizing regularly. That means that that action potential stalls out somewhere going from the atria to the ventricles. So what does that action potential have to pass through? It has to pass through the AV node and the AV bundle. So because this ventricle isn't contracting regularly, it tells me that that action potential must be stalling out somewhere in here. Now the technical term that we're looking at, this is a heart block, and it's actually quite serious. Alright. As we look at our second one, I can label things again. Well, I'm going to look for my p wave, QRS complex, and t wave, and look, p wave QRS and my t wave. That looks normal. And as I look down, that normal pattern just repeats over and over again. So does that look normal or pathological to you? Well, this just looks like our normal ECG. I'm going to give it an N. Therefore, what's the failing part of the conduction system? Nothing's failing, so I don't really need to write anything in that box. Alright. Our final one here. Let's see what we can label in this ECG down here at the bottom. Well, as I look there, I don't think I can name a p wave. I do see depolarizations here that represent the depolarization of the ventricles. Those seem to be happening. Right? I see a lot of those QRS complexes, but I can't see the p waves and I can't see the t waves. So knowing that, normal or pathological. Well, this looks pathological to me. So because I see that QRS complex, that tells me that the ventricles are contracting. But in between, when I would expect to see that normal P wave, I just get this sort of fluttering, this random depolarization and repolarization. Now, I know that's not from the ventricles, because the ventricles are giving that QRS complex. So what must be happening here? This is atrial fibrillation. Now you can see in sort of more simple terms, you could say there's something wrong with the atria. The atria are not passing that action potential correctly. But what we're seeing here is this sort of random unorganized depolarization on the cells in the atria that we call fibrillation. Okay. Now again, you do not need to be an expert at reading these ECGs and identifying pathologies. But hopefully, you see here that by knowing what you're supposed to see, it's actually not that hard to figure out what is pathological and what might actually be going wrong in the heart when you see it. Alright. With that, we got more practice to follow. Give it a try.

Fibrillation can happen in either the atrial or the ventricles. When fibrillation happens in the ventricle it is usually immediately be life threatening. Which ECG below do you think shows ventricular fibrillation?

• A.
• B.
• C.
• D.

A varsity athlete went for their physical, which included an ECG. The doctors noticed that the student’s heart rate was unusually low, but they told the student not to worry. What term would the doctors include in their chart to describe the student’s heart?

• A. Fibrillation.
• B. Bradycardia.
• C. Heart Block.
• D. Tachycardia.

Here’s what students ask on this topic:

What is an electrocardiogram (ECG) and how does it work?

An electrocardiogram (ECG or EKG) is a medical test that records the electrical activity of the heart over a period of time using electrodes placed on the skin. The ECG machine, known as an electrocardiograph, detects the electrical signals generated by the heart as it contracts and relaxes. These signals are then displayed as waves on a graph, known as an electrocardiogram. The main components of an ECG include the P wave (representing atrial depolarization), the QRS complex (ventricular depolarization), and the T wave (ventricular repolarization). By analyzing these waves, healthcare professionals can assess heart function and identify any abnormalities or pathologies.

What are the key components of an ECG and what do they represent?

The key components of an ECG are the P wave, QRS complex, and T wave. The P wave represents the depolarization of the atria, which is the electrical activity that triggers atrial contraction. The QRS complex represents the depolarization of the ventricles, which is the electrical activity that triggers ventricular contraction. The T wave represents the repolarization of the ventricles, which is the process of the ventricles returning to their resting state. These components are crucial for understanding the electrical activity of the heart and diagnosing potential heart conditions.

What is the difference between bradycardia and tachycardia?

Bradycardia and tachycardia are both types of arrhythmias, which are abnormal heart rhythms. Bradycardia refers to a resting heart rate that is too slow, typically below 60 beats per minute. It can be normal in well-trained athletes but may indicate an underlying issue in others. Tachycardia, on the other hand, refers to a resting heart rate that is too fast, typically above 100 beats per minute. Unlike bradycardia, tachycardia is generally considered unhealthy and indicates a potential pathology. Both conditions can be identified through an ECG by observing the frequency of the heartbeats.

How can an ECG help diagnose heart pathologies?

An ECG helps diagnose heart pathologies by providing a visual representation of the heart's electrical activity. By analyzing the P wave, QRS complex, and T wave, healthcare professionals can identify abnormalities in heart function. For example, bradycardia (slow heart rate) and tachycardia (fast heart rate) can be detected by observing the frequency of the heartbeats. Fibrillation, which is uncoordinated contraction of the heart muscles, can also be identified through irregular and chaotic wave patterns. By comparing the ECG to a normal heart rhythm, professionals can pinpoint specific issues within the heart's conduction system and determine the appropriate treatment.

What is the significance of the P wave, QRS complex, and T wave in an ECG?

The P wave, QRS complex, and T wave are significant components of an ECG that represent different phases of the heart's electrical activity. The P wave indicates atrial depolarization, which is the electrical activity that causes the atria to contract. The QRS complex represents ventricular depolarization, which is the electrical activity that causes the ventricles to contract. The T wave signifies ventricular repolarization, which is the process of the ventricles returning to their resting state. These waves are essential for diagnosing heart conditions, as any abnormalities in their shape, size, or timing can indicate potential pathologies.