The electrocardiogram or ECG, records the electrical activity of the heart. It is used to determine if the heart has an abnormal rhythm. If the heart muscle is abnormally thick, or if the cardiac muscle is receiving less than optimal oxygenated blood. The normal pattern of electrical activity that is seen on an ECG is called a sinus rhythm. Each heartbeat produces a regular, identifiable pattern; the P wave, the QRS complex, and the T wave. These segments identify the electrical activity of specific parts of the heart.

The SA node is known as the pacemaker of the heart and begins the normal heartbeat by producing an electrical signal. This signal spreads through the atria, or upper chambers of the heart, causing them to contract. Electrical activity in the atria is seen on the ECG as the P wave. The signal then reaches the AV node, which produces an electrical signal that continues through the bundle of His, the bundle branches, and the Purkinje fibers. This causes the ventricles, the lower chambers of the heart, to contract. Electrical activity in the ventricles is seen as the QRS and the T wave. For each beat, the cycle begins again at the SA node.

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