Q 1: What is the normal duration of  PR interval? 

Q 2: What does PR interval represent?

Q 3:  What does prolonged PR interval mean?

Q 4: What does shortened PR interval mean?

Answer

Answers

Q 1: What is the normal duration of  PR interval?

Normal duration: 0.12-2.0 seconds (3-5 horizontal boxes). This is measured from the onset of the P wave to the onset of the QRS complex regardless if the initial wave is a Q or R wave.

 
Q 2: What does PR interval represent?

Represents: atria to ventricular conduction time (through His bundle)

Q 3:  What does prolonged PR interval mean?

If the PR interval is greater than 0.2 sec, then an AV block is present. There are several types of AV blocks:

• 1st degree AV Block: PR>0.20 sec.
• 2nd degree AV Block: 2 types:

• Type I (Mobitz I or Wenckeback): increasing PR interval until a QRS complex is dropped. It is usually benign.
• Type 2 (Mobitz II): QRS dropped without any progressive increase in PR interval (i.e., PR interval is constant but still >0.20 sec). Here's an example.

• 3rd degree AV Block: atria and ventricles are electrically dissociated. Therefore, P waves and QRS complexes will occur independent of each other. As always, use the QRS complexes to determine heart rate. Here's an example.

Q 4: What does shortened PR interval mean?

A PR interval that is <0.12 sec (when associated with a prolonged QRS) should prompt evaluation for Wolff-Parkinson-White Syndrome (WPW).
<0.12 sec when associated with prolonged QRS should prompt evaluation for Wolff-Parkinson syndrome (WPW).