thaler chapter 1

1   The Basics

52

− 90°

− 90°

180°

0°

A

0°

180°

+90°

lead III

B

+90°

lead III

The PR Interval The PR interval represents the time from the start of atrial depolarization to the start of ventricular depolarization. It includes the delay in conduction that occurs at the AV node. The PR interval normally lasts from 0.12 to 0.2 seconds (3 to 5 mm on the EKG paper). Rotation of the heart within the chest cavity redirects the perceived direction of current flow. Lead III is normally oriented perpendicularly to atrial depolarization. With the apex of the heart turned leftward, lead III will view atrial depolarization as receding and will record a wave that is largely negative.