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Lecture 6

BIOC63H3 Lecture Notes - Lecture 6: Pr Interval, Qrs Complex, Qt Interval

Biological Sciences
Course Code
Ivana Stehlik

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The Electrocardiogram (ECG) (Figure 1)
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The P Wave (Figure 2)
Electrical impulses originating in the SA node produce various waves on the ECG as they spread
throughout the heart. The first wave in the cardiac cycle is known as the P wave, which
represents depolarization of the right and left atria prior to their contraction. The beginning of
the P wave is recognized as the first abrupt or gradual deviation from the baseline and its end is
the point at which the waveform returns to the baseline. It is usually smooth and round.
The PR Segment (Figure 3)
The PR segment is part of the PR interval and is that area noted from the end of the P wave to the
beginning of the QRS complex. It represents atrial depolarization and conduction through the
AV node. There is a delay in conduction through the AV node, allowing time for the contents of
the atria to empty into the ventricles before ventricular contraction begins. The impulse then
spreads to the bundle of His, bundle branches, and purkinje fibers. The PR segment is normally
an isoelectric (flat) line.
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