Normal heart rhythm is also called the sinus rhythm (SR). During each heart beat, the two upper chambers of the heart (atria) contract, followed by the two lower chambers (ventricles). This mechanical action is directed by the heart's electrical conduction system.

The normal heart beats in the same constant rhythm, about 60 to 100 times per minute at rest and appropriately goes up or down with activity or sleep. The spontaneous electrical impulses begin in the sinoatrial (SA node), located in the right atrium. When the SA node fires an impulse, electrical activity spreads through the right and left atria, contracting them and forcing blood into the ventricles. From the atria the impulse travels to the atrioventricular (AV) node which is the electrical junction that allows the impulses to travel from the atria to the ventricles. The impulse then travels through the walls of the ventricles, causing them to contract and force blood out of the heart to the lungs and body. Impure blood or unoxygenated blood is collected from various parts of the body and is drained into the right atrium from where blood is pumped into the right ventricle. The right ventricle in turn pushes the blood into the lung through the pulmonary artery. The lungs purify (oxygenate) the blood and transfer the oxygenated blood into the left atrium through the pulmonary veins