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1
The Basics
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).
lead III
lead III
0°
+90°
−
90°
180°
−
90°
B
A
0°
180°
+90°
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.