ECG Tutorial   

Introduction
Anatomy
Heart Rate
PR Interval
QRS Interval
QT Interval
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To determine the QRS Interval.

The QRS interval represents the time it takes for depolarization of the ventricles. Normal depolarization requires normal function of the right and left bundle branches. The QRS duration may vary with the size of the heart and is longer in the base-apex lead. A block in either the right or left bundle branch delays depolarization of the ventricle supplied by the blocked bundle, resulting in a prolonged QRS duration (rare in large animals).