When a nurse is interpreting an EKG, it is important the nurse knows how to measure a PR interval. The PR interval represents atrioventricular (AV) node conduction time. The AV node is part of the electrical conduction system of the heart, and is known as the gatekeeper of the electrical system. The PR interval tends to be longer in older adults, and shortens when the heart rate increases. For example, a patient with tachycardia will have a shorter PR interval compared to a patient with sinus bradycardia. It is very important when interpreting heart rhythms as the nurse that the PR interval is not increasing in length and is baseline for that specific patient. A PR interval that has increased in length could represent a heart block. In this article, I am going to show you a simple way on how to measure the PR interval. At the end of the article, you can test your knowledge on how to measure PR intervals. In addition, I recommend that you check out how to measure the QRS complex and how to count a heart rate on an EKG using the 6 six second rule.
Video Tutorial on How to Measure the PR Interval
In this video, I explain how to measure the PR interval.
Steps on how to Measure the PR Interval
A normal PR interval measures 0.12-0.20 seconds which is 3 to 5 small boxes on the EKG strip. When you measure a PR interval, you start measuring at the BEGINNING of the p-wave until the BEGINNING of the QRS complex. Here are some simple steps on how to measure the PR interval:
Find the P-wave on the EKG strip
Determine where the PR interval is, and to do this you start measuring at the beginning of the p-wave until the beginning of the QRS complex.
Count the SMALL boxes between there measurements.
Remember each box represents 0.04 seconds. So, if you measure 2 boxes the measurement of the PR interval would be 0.08 seconds.
ECG/EKG Study Guide and Workbook for Nursing Students
“ECG/EKG Interpretation Study Guide and Workbook by Nurse Sarah”. This book contain 100 pages of content featuring 26 ECG rhythm break downs, 51 ECG rhythm analysis practice problems, 100 comprehensive ECG practice questions, worksheets, chart summaries, and more. You can get an eBook version here: “Nurse Sarah ECG Book” or a physical copy here: “ECG/EKG Interpretation Study Guide by Nurse Sarah“.
PR Interval Quiz
What is the measurement of the PR interval in figure 1? (See answer key below) What is the measurement of the PR interval in figure 2? (See answer key below) What is the measurement of the PR interval in figure 3 ? (See answer key below) Answer Key: Figure 1: 0.08 seconds…note this is an abnormal PR interval (Rational: There is 1 small box and 2 (0.5 boxes) which when you add 0.02 +0.02+0.04=0.08 seconds) Figure 2: 0.16 seconds (Rational: There are 4 small boxes, which when you add the boxes 0.04+0.04+0.04+0.04=0.16 seconds) Figure 3: 0.28 seconds…note this is an abnormal PR interval (Rational: There are 7 small boxes, which when you add the boxes 0.04+0.04+0.04+0.04+0.04+0.04+0.04=0.28 seconds)
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Here are some simple steps on how to measure the PR interval: 
What is the measurement of the PR interval in figure 1? (See answer key below) 
What is the measurement of the PR interval in figure 2? (See answer key below) 
What is the measurement of the PR interval in figure 3 ? (See answer key below) Answer Key: Figure 1: 0.08 seconds…note this is an abnormal PR interval (Rational: There is 1 small box and 2 (0.5 boxes) which when you add 0.02 +0.02+0.04=0.08 seconds) Figure 2: 0.16 seconds (Rational: There are 4 small boxes, which when you add the boxes 0.04+0.04+0.04+0.04=0.16 seconds) Figure 3: 0.28 seconds…note this is an abnormal PR interval (Rational: There are 7 small boxes, which when you add the boxes 0.04+0.04+0.04+0.04+0.04+0.04+0.04=0.28 seconds)
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