1. Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program8
PR Intervals
Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

2. PR Intervals
Another important step of assessing the ECG is analyzing the PR intervals
Ask the students “What are the 1st, 2nd, 3rd and 4th steps of analyzing an ECG tracing?”
Answer: Assess the rate, regularity, P waves and QRS complexes. Q

3. PR Interval
Distance from the beginning of the P wave to the beginning of the Q wave
Morphology is a P wave and a flat line
Instructional points:
A normal PR interval indicates the impulse originated from SA node (or close to it) and traveled through the atria and AV node in a regular and unobstructed course.
Remind the student we measure the R wave if the Q wave is absent. I

4. PR Interval
Denotes depolarization of the heart from the SA node through the atria, AV node and His-Purkinje system

5. PR Intervals
Their presence or absence as well as duration can help us identify if and how well the impulses are traveling through the heart’s conductive pathway
Important to know because delays or interruption of conduction through AV node and/or junction can lead to serious or even life-threatening conditions

6. Measuring PR Intervals
If the PR intervals are identifiable, measure from where the P wave begins to where the Q wave begins
To make it easier, find a P wave that begins on a vertical line and then measure to where the Q or R wave begins

7. PR Intervals
Are considered abnormal if they are shorter, longer, progressively longer, vary or absent

8. Shorter P’R Intervals
Shorter P’R intervals occur when the impulse originates in the atria close to the AV junction or in the AV junction

9. Shorter P’R Intervals
Can occur when an impulse arises from a supraventricular site but travels through abnormal accessory pathways to the ventricles
Leads to premature ventricular depolarization called preexcitation

10. Longer P’R Intervals
Can occur when there is a delay in impulse conduction through the AV node

11. Varying P’R Intervals
Can occur when the pacemaker site moves from beat to beat causing the P’ waves to appear different and the P’R intervals to vary

12. Varying P’R Intervals
In one form of AV heart block, the PR intervals are progressively longer until a QRS complex is dropped and then the cycle repeats

13. Absent PR Intervals Occurs with:
atrial rates that exceed 250 BPM and produce flutter waves or a chaotic baseline and absence of P waves preceding the QRS complexes
ventricular dysrhythmias

14. Absent PR Intervals
Also seen in the most severe form of AV heart block

15. More P Waves and Constant PR Intervals
In on form of AV heart block, some sinus beats are blocked in the AV node and do not reach the ventricles
PR intervals associated with P waves that are conducted through to the ventricles are constant

16. Practice Makes Perfect
In this tracing, determine the type of PR interval
Answer: PRI seconds in duration. I

17. Practice Makes Perfect
In this tracing, determine the type of PR interval
Answer: PRI has a longer than normal duration at 0.30 seconds. I

18. Practice Makes Perfect
In this tracing, determine the type of PR interval
Answer: PRI seconds in duration I

19. Practice Makes Perfect
In this tracing, determine the type of PR interval
Answer: PRI seconds in duration. I

20. Practice Makes Perfect
Determine the type of PR interval
Answer: PRI is immeasurable (as the P waves are not associated with the QRS complexes) I

21. Summary
Another important step of analyzing an ECG rhythm is examining the PR intervals
PR interval is the distance from the beginning of the P wave to the beginning of the Q wave
Duration of the PR interval is normally 0.12 to 0.20 seconds
PR intervals are considered abnormal if they are shorter, longer, absent or vary

22. Summary
Shorter P’R intervals occur when the impulse originates in the atria close to the AV junction or in the AV junction itself
AV heart block is the most common cause of longer PR intervals
When the pacemaker site moves from beat to beat it causes the P’ waves to appear different and the P’R intervals to vary

23. Summary
In one for of AV heart block, the PR intervals are progressively longer until a QRS complex is dropped and then the cycle repeats
There is an absence of PR intervals in atrial rates that exceed 250 BPM producing flutter waves or a chaotic looking baseline and with ventricular dysrhythmias
In the most severe form of AV heart block the PR intervals are not measurable

24. Summary
In one form of AV heart block, some of the sinus beats are blocked in the AV node and do not reach the ventricles. The PR intervals associated with the P waves that are conducted through to the ventricles are constant in duration