thaler chapter 1

1   The Basics

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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.

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.

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.