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Electrocardiogram (ECG or EKG) is a non-invasive test to gauge the heart's condition. It records and shows the electrical signal generated by a heart when it pumps. The electrical signal synchronizes all the pumping acivities during a heart beat. ECG is useful for detecting certain types of conditions and because of its low cost, it is the most common test for screening heart diseases.
A heart consists of two pumps (right and left) and each pump has two chambers (upper and lower). The upper chamber is call atrium and the lower chamber is called ventricle. The right pump circulates blood from other parts of the body to the lung and the left pump circulates blood from the lung to the rest of the body.
Each ECG cycle has 5 waves - PQRST. The P wave corresponds to the atrial depolalrization and the pumping of blood from the atrium to the ventricle. The QRS complex corresponds to the depolarization of blood and the pumping of blood out from the ventricle to the body and lung. The T wave corresponds to the repolarization of the ventricle and the recovery of the ventricle for next cycle.
The following diagram is from Electrocardiogram and shows the electrical originated from upper left artium (sinoatrial or SA node), travels down to the boundary between the atrium and ventricle (atrioventricular or AV node) and then to the entire venticle.
From this simple description, one can infer that: