Electrocardiography (EKG, ECG) tracings show a characteristic pattern of electrical impulses that are generated by the heart. The different parts of an EKG tracing of a heartbeat are called the P wave, the QRS complex, the ST segment, and the T wave.

See a picture of EKG components and intervals.

• The P wave is a record of the movement of electrical activity through the upper heart chambers (the atria) and is recorded when they contract.
• The QRS complex is a record of the movement of electrical impulses through the lower heart chambers (the ventricles) and is recorded when they contract.
• The ST segment appears as a straight, level line between the QRS complex and the T wave. Elevated or lowered ST segments may mean the heart muscle is damaged or not receiving enough blood.
• The T wave corresponds to the period when the lower heart chambers are relaxing and preparing for their next muscle contraction.