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The EKG Waveform
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The PR Segment
PR segment is the portion on the EKG wave from the end of the P wave to the beginning of the QRS complex, lasting about 0.1 seconds. The PR segment corresponds to the time between the end of atrial depolarization to the onset of ventricular depolarization. The PR segment is an isoelectric segment, that is, no wave or deflection is recorded. During the PR segment, the impulse travels from the AV node through the conducting tissue (bundle branches, and Purkinje fibers) towards the ventricles. (Note a wave will be recorded only after the impulses exit the conducting systems and activates the ventricular muscle to give the QRS complex). Most of the delay in the PR segment occurs in the AV node. 

Although the PR segment is isoelectric, the atrial are actually contracting, filling the ventricles before ventricular systole. A Wiggers diagram can be used to illustrate the increase in atrial pressure during the PR segment. 

The animation below shows conduction  in the heart during the PR segment. Note the relation between the EKG and the electrical activity (red dots).  The animation had been slowed about 10 times normal heart rate to show the details. 

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Last Updated: 01/03/00.
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