ECG Level 1 Tutorial: Analyze the Waveforms
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Tutorial: Analyze the Waveforms
This module will teach the basic waveforms on ECG and a stepwise approach of how to analyze them. Practice each step and get feedback on your performance.
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Tutorial: Analyze the Waveforms PR Interval
Lessons
42
Times Practiced
1284
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Progress
Accuracy
Efficiency
Accuracy
Efficiency
1 Anatomy Anatomy
2 Waveform Identification Waveform Identification
3 Waveform Identification #2 Waveform Identification #2
4 Determine the heart rate Determine the heart rate
5 Do you have Rhythm? Do you have Rhythm?
6 P waves! Where are you? P waves! Where are you?
7 How many P waves? How many P waves?
8 P wave Size and Shape P wave Size and Shape
Current lesson 9 PR Interval PR Interval
10 P conducted to QRS? P conducted to QRS?
11 QRS Width QRS Width
PR Interval
In this lesson, we will cover how to measure the PR interval. Remember that the PR interval starts at the start of the P wave and ends at the start of the QRS:
the PR interval

There are 2 questions you must ask when you measure the PR interval:
  • how long is it?
  • is it changing?

One little square on the ECG is 0.04 seconds. The normal PR interval is
  • 3-5 little squares
  • = 0.12-0.20 seconds

The PR interval options are:
  • normal
  • too short
  • too long, constant
  • too long, changing
  • not applicable (in other words, does not exist)

Not applicable??
  • if you have no P wave, you can have no PR interval
  • if you have no QRS, you can have no PR interval
  • if the P wave is ridiculously far away (or even after the QRS), then you have no PR interval

Please note: the QRS complex does not always contain a Q, an R, and an S wave. Sometimes there is only a single deflection (which, if upward would be called an R and if downward would be called a Q). Regardless of the configuration of waveforms within the QRS complex, we still measure to the start of ventricular depolarization, even if it starts with an R wave instead of starting with a Q wave.

What it means when the PR interval is abnormal:
  • PR interval is long:
    • there is slow conduction through the AV node (called first degree heart block)
  • PR interval is short:
    • origin is the AV node (called "junctional" rhythm)
    • accessory pathway is present
In the Rhythms Diagnostic Criteria tutorial, we will cover in more detail about first degree heart block and junctional rhythm and we have super wicked animated videos to try to help explain all this stuff. Accessory pathways are rare and will be described in a more advanced tutorial later.

Examples: for each ECG, indicate the length of the PR interval and if it is changing:
long PR interval
PR interval is 6 little squares = 0.24 sec = long, and is constant

What about this one?
changing PR interval
The PR interval is changing. It starts normal then is long. Then you can see a QRS is missing. Zoomed in, we are showing blue lines to indicate the 2 different PR intervals:
changing PR interval

Here is a difficult one:
unknown PR interval
The PR interval in this example is different every time. Therefore, we cannot describe it. The PR interval in this example is "not applicable".

Here is another example where the PR interval is not applicable:
no PR interval from 3rd degree heart block


Practice PR interval analysis now.