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The Basics
P Waves, QRS Complexes, T Waves,
and Some Straight Lines
Let’s follow one cycle of cardiac contraction (systole) and relaxation
(diastole), focusing on the electrical events that produce the basic
waves and lines of the standard EKG.
Atrial Depolarization
The sinus node fires spontaneously (an event not visible on the EKG),
and a wave of depolarization begins to spread outward into the atrial
myocardium, much as if a pebble were dropped into a calm lake.
Depolarization of the atrial myocardial cells results in atrial contraction.
During atrial depolarization and contraction, electrodes placed
on the surface of the body record a small burst of electrical activity
lasting a fraction of a second. This is the
P wave
. It is a recording of
the spread of depolarization through the atrial myocardium from start
to finish.
Each cycle of normal cardiac contraction and relaxation begins when
the sinus node depolarizes spontaneously. The wave of depolarization
then propagates through both atria, causing them to contract.