The PR interval is the time (in seconds) from the beginning of the P wave (onset of atrial depolarization) to the beginning of the QRS complex (onset of ventricular depolarization). The normal PR interval duration range is from 0.12 sec - 0.20 sec, measured from the initial deflection of the P-wave to the initial deflection of the QRS complex. The PR interval is longer with high vagal tone. A prolonged PR interval can correspond to impaired AV node conduction.
Although electrical activity begins at the P wave, actually atrial contraction begins later at about the middle of the P-wave and continues during the PR segment. A Wiggers diagram can be used to illustrate that the left atrial pressure (indicating left atrial contraction) beginning to rise at about half way through the P-wave and continuing into the PR segment. In terms of electrical activity, the PR segment is a time lag to allow atrial systole to occur, filling the ventricles before ventricular systole. Most of the delay occurs in the AV node. The AV node is slow conducting, causing a delay in conduction of about 0.1 seconds.
The animation below shows conduction in the heart during the PR interval. Note the relation between the EKG and the electrical impulses (red dots) traveling through the heart is followed later by atrial contraction (blinking stars and blue & red arrows showing pumping of blood ). Also note that the animation had been slowed about 10 times the normal heart rate to show the details.
- Last Updated: 01/03/00.
- Copyright © 1997-9 Molson Medical Informatics Project. All Rights Reserved.
The heart rate calculated using the RR intervals is the venticular rate. In sinus rythme, the venticular rate corresponds to the atrial rate. Tthe atrial rate can be determined from the PP interval using either of the two methods above.