# Step 3: Measuring the PR Interval (Nursing)

by Rhonda Lawes, PhD, RN

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00:01 So let's go on to step three.

00:03 Now, we're going to measure the PR interval.

00:06 Now, the PR interval is the time from the beginning of the atrial depolarization, to the beginning of the ventricular depolarization.

00:14 It includes the P wave, and then that short isoelectric line that follows it.

00:20 So that's what we're looking at the PR interval.

00:24 It's the P wave and the short ISO electric line that follows it.

00:28 Hey, before we go on, see if you can pause and remember the first two steps of interpreting ECGs.

00:43 Good. I want to encourage you to do more of that, as you're studying.

00:47 I promise you, that's going to move the information into your long term memory, which is where you need it when you're taking a test, or taking care of a patient.

00:56 So this is step three. Measuring the PR interval, we know that's from the P wave to the short isoelectric line after it.

01:04 It represents the beginning of atrial depolarization, to the very beginning of ventricular depolarization.

01:13 Okay, so take another look at our beating heart.

01:15 We can see that the PR interval represents the time from the beginning of atrial depolarization, to the beginning of ventricular depolarization.

01:24 So from the beginning of the P wave to the beginning of the QRS complex, including that short isoelectric line, that's the PR interval.

01:33 Remember that during the PR interval, the electrical impulse within the heart is traveling from the atria to the ventricles, and it's got that little delay in the AV node.

01:43 That's why there's no electrical activity and the line is flat.

01:47 So when you're measuring the PR interval, keep in mind that the PR interval does vary with heart rate.

01:54 So as the heart rate gets faster, the PR interval gets shorter.

01:58 As the heart rate gets slower, you've got the opposite effect.

02:01 But a normal PR interval is 0.12 to 0.20 seconds in adults.

02:09 Okay, now, just in case, I want to review that.

02:12 How do I know that that is 0.12? Remember, a tiny box is 0.04 seconds, right.

02:19 And a big box is 0.2 seconds.

02:22 So when you measure the PR interval, right, from the beginning of the P wave to just before the QRS and you count the number of boxes that are underneath there, that's going to give you the length or the time that it takes for that impulse to make it through.

02:38 So we know that the normal PR interval is at least 0.12 So what can we learn when a PR interval is less than 0.12? Well hang on because this is pretty cool.

02:50 This tells us that the impulse originated in an ectopic pacemaker.

02:55 Now, it's probably somewhere close to the AV node or the bundle of His and it traveled down an abnormal conduction pathway.

03:02 We call that an accessory pathway.

03:04 Now, this bypasses the AV node and depolarize the ventricles earlier than usual, right.

03:11 That's why that PR interval is shorter.

03:15 Now, Wolff-Parkinson-White is an example of such a condition that we'll have a PR interval less than 0.12.

03:23 So let's look at the opposite example.

03:26 What if the PR interval is greater than normal or greater than 0.20? Well, this might indicate something like a first degree AVblock.

03:36 That means it's taking longer for that electrical conduction to make it through the heart.

03:41 This is a delay in conduction through the atria.

03:44 Now, this can also cause a delay of conduction to the AV node, or the bundle of His.

03:50 On the other hand, what do you think can make a PR interval longer? Long PR intervals are seen in patients with a first degree AV block.

04:00 As I mentioned before, the purpose of the AV node is to delay the impulse coming from the SA node in the atria to the Purkinje fibers in the ventricles.

04:10 In AV block, this pauses exaggerated.

04:14 And the delay is prolonged beyond what the ventricle usually need to finish filling up.

04:19 This is reflected in the ECG strip as a long PR interval.

The lecture Step 3: Measuring the PR Interval (Nursing) by Rhonda Lawes, PhD, RN is from the course The Basics of ECG Strips (Nursing).

### Included Quiz Questions

1. A longer than normal PR interval
2. A shorter than normal PR interval
3. An absent PR interval
4. A PR interval that comes after the QRS complex
1. 0.12 – 0.20 seconds
2. 0.01 – 0.1 seconds
3. 1 – 2 seconds
4. 0.2 – 0.30 seconds

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