Module title = Tutorial: Origins of Activity

Lesson title = PR Interval Rules

This is lesson 3 of 8 in this module

Rule: A normal or long PR interval rules out the AV node

Let's explore why this is true. Remember that there is a normal delay of conduction through the AV node (yellow arrow). It is this delay that creates the normal PR interval. If conduction is extra slow through the AV node, then the PR interval will be long.
AV node direction of conduction

If the origin is inside the AV node (a junctional pacemaker), then the regular delayed conduction through the AV node is lost. This is because the AV node sends a signal both up to the atria (yellow arrows) and down to the ventricles (blue arrow) at the same time:
AV node signal escapes top and bottom at same time

With a junctional pacemaker, 3 situations can be created, depending on the exact timing of the yellow vs. blue arrows:

P wave before QRS, but with a short PR interval
P wave and QRS at same time: PR interval will not exist
P wave AFTER QRS: PR interval will not exist

In all 3 situations, a normal or long PR interval never occurs; therefore a normal or long PR interval rules out a junctional origin. You can review the details of a junctional rhythm to see why the PR interval is short in lesson 10 in the Diagnostic Criteria Module.

Rule: A normal or long PR interval rules out the ventricles

If the origin is the ventricle, the ventricle will depolarize first. Therefore, we will not see a P wave before the QRS and the PR interval will not exist. Boom: ventricles ruled out!

Rarely, P waves can be conducted up the AV node in reverse direction from the ventricles, but of course, these P waves would be after the QRS so the PR interval will not exist in this situation.

Rule: P waves with QRS complexes and NO consistent PR interval = 2 pacemakers

Let's think about this one for a moment. If the P waves are getting conducted to the ventricles, then you will have a consistent PR interval. Thus, you only have 1 pacemaker. If there is no consistent PR intervals (in other words, there is an absent PR interval), then this means that the P waves are NOT getting conducted to the ventricles. Therefore you need a pacemaker for the P waves and a second pacemaker for the QRS complexes. The pacemaker for the P waves will be the SA node the vast majority of the time (but could also be the atria). The pacemaker for the ventricles will be the AV node (if the QRS is narrow) or the ventricles themselves (if the QRS is wide).

In this setting of no PR interval, the rate of P waves is almost always faster than the rate of the ventricles, since the "lower down" you go in the heart, the slower the pacemakers get ... and the AV node and ventricles are lower than the SA node is.

Heads up!!
Sometimes with a ventricular origin, there will still be P waves generated from the SA node (you would need both the SA node and the ventricles generating electrical impulses at the same time so there would be 2 different origins of activity at the same time). The P waves will have a different rate compared to the QRS complexes. As a result of this, by random chance, a P wave might occur before a QRS and falsely appear to have the occasional normal PR interval. Don't be fooled however, by a random pairing of the P wave and QRS. You are smarter than that!

The (Rare) Exception to these 2 Rules:
As with all rules, there are exceptions. A condition called Wolff-Parkinson-White (WPW) has a short PR interval and can originate from the SA node or atria. However, if you are just learning ECG's, leave this uncommon diagnosis for later learning when you are more advanced. Learn the rules first, and the exceptions later.

Summary:

a normal or long PR interval rules out the AV node
a normal or long PR interval rules out the ventricles
rare exception: WPW can have a short PR interval and originate from the SA node or atria

Lesson 3 of 8

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Module title = Tutorial: Origins of Activity

Lesson title = PR Interval Rules