Interpreting ECG duration

Find out how to determine whether the duration of a certain part of the ECG is inaccurate.

Franz Wiesbauer, MD MPH
Franz Wiesbauer, MD MPH
13th Apr 2018 • 4m read

Sometimes the duration of certain parts of an ECG can be off the mark. In this video from our ECG Mastery: Yellow Belt course, you'll find out how to dig deeper into your patient's ECG and interpret the duration of the P wave, PR interval, and QRS complex in order to determine whether there is a discrepancy.

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Video Transcript

[00:00:00] As we've learned in the previous sessions, several diagnoses can already be made, just by performing simple measurements on the time or x-axis. In order to understand if the duration of certain parts of the ECG are off, we need to look at the normal values first. In this session, we're going to talk about the duration of the P wave, the duration of the

[00:00:30] PR interval, and the duration of the QRS complex. Let's first look at the P wave duration. Do you remember from the last chapter, P wave duration starts when the P wave leaves the isoelectric line and it ends when it reaches the isoelectric line again. The normal duration of the P wave is below 0.12 seconds. If the atria are enlarged, especially when the left atrium is enlarged, depolarization takes a little longer

[00:01:00] and the P wave will, therefore, also be wider than normal. In these instances, we also have a second peak of the P wave, usually seen in lead 2, and this entity is called P mitrale. Please note that an enlargement of the right atrium will not cause a prolongation of the P wave. The take-home message here is that when the P wave is 0.12 seconds long or above, that usually means that the

[00:01:30] left atrium is enlarged. We also call this type of P, P mitrale. Next, let's talk about the duration of the PR interval. The PR interval starts at the beginning of the P wave and ends at the beginning of the QRS complex. It represents the time the atrial depolarization takes to travel through the AV node, in order to reach the ventricles. So, the PR interval represents AV conduction time. The normal value for

[00:02:00] the PR interval is 0.12 to 0.2 seconds. Let's look at some situations when the PR interval is considered abnormal. First, let's look at the situation when the PR interval is longer than 0.2 seconds. As in this example. As you can see here, the PR interval is 1, 2, 3, 4, 5, 6, 7, almost 8 mm long. We know that 1 mm stands for

[00:02:30] 0.04 seconds and 0.04 times 8 is 0.32 seconds, so that's way over 0.2 seconds. And in this special case, there is a constant prolongation of the PR interval. We see prolongation here, we see a prolongation here, and also here, and this is an example of a first degree AV block. You're going to learn much more about this entity in later

[00:03:00] chapters. Now, let's turn to situations when the PR interval is shorter than 0.12 seconds. When the PR interval is shorter than 0.12 seconds, that means that ventricular depolarization starts earlier than normal. Since the AV node has a certain maximum conduction velocity, that usually indicates that there is an additional, faster pathway, through which the impulse can travel from the atria to the ventricles. So, this time is

[00:03:30] shortened. These special circumstances are called preexcitation syndromes. There are two important forms of preexcitation syndromes that you should remember. First, there's the so-called Lown-Ganong-Levine Syndrome. In this syndrome, the QRS immediately follows the P wave and the PR interval is below 0.12 seconds. Let's look at an example.

[00:04:00] As you can see here, the QRS, this entity here immediately follows the P wave at a distance of somewhat between 0.08 and 0.1 seconds, so that's definitely too short. The impulse is traveling through an accessory pathway from the atria to the ventricles. This accessory pathway conducts the impulse faster than the AV node and that's why the PR interval is shortened. The second entity that

[00:04:30] causes a shortening of the PR interval is the so-called Wolff-Parkinson-White syndrome or WPW syndrome. Here, the QRS or delta wave, immediately follow the P wave and the PR interval, is below 0.12 seconds. Let's look at an example again. Here, we have a strange looking entity. We have the P wave and then immediately the QRS complex starts but we have this strange thing here, and that's what's called a

[00:05:00] delta wave. Delta because it resembles the Greek letter Delta. The regular components of the QRS complex follow after the delta wave. And finally, let's talk about the duration of the QRS complex. Here you can see a normal QRS complex. It starts here and ends here. So, this is the QRS duration. As you can see here, the duration of the QRS complex is about 1, 2,

[00:05:30] 2.5 mm long. So, 2.5 times 0.04 is 0.1 seconds. And the normal duration of the QRS complex is 0.1 seconds or below so we have a normal QRS duration in this case. Now, let's look at another example. You can already see that something must be wrong here. Let's look at the QRS duration. The QRS stars here and ends here. The duration is over 0.12 seconds. Let's count, 1,

[00:06:00] 2, 3, 3.5—3.5 times 0.04 that's 0.14 seconds and that's longer than 0.12 seconds. So, what we have here is a complete bundle branch block. As you probably know, the depolarization is taken down from the AV node to the ventricles through the bundle branches. There's a right bundle branch and the left bundle branch. And when one of these branches is blocked, depolarization of the ventricle

[00:06:30] takes longer than normal, and that's why the QRS complex is broadened in this instance. The following examples may seem familiar to you. But at this time, not only the measurement but also the correct diagnosis is required. Note that there may be more than only one abnormality in a single example.

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