The heart's electrical system
The heart is, in the simplest terms, a pump made up of muscle tissue. Like all pumps, the heart requires a source of energy in order to function. The heart's pumping energy comes from an intrinsic electrical conduction system.
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How does the heart beat?
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An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small mass of specialized tissue located in the right atrium (right upper chamber) of the heart. The sinus node generates an electrical stimulus periodically (60-100 times per minute under normal conditions). This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's chambers to contract and pump out blood.
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The right and left atria (the 2 upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the 2 lower chambers of the heart). The electrical impulse travels from the sinus node to the atrioventricular (AV) node, where it stops for a very short period, then continues down the conduction pathways via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to both ventricles.
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Normally, as the electrical impulse moves through the heart, the heart contracts about 60 to 100 times a minute. Each contraction represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract.
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Under some conditions, almost all heart tissue is capable of starting a heartbeat, or becoming the pacemaker. An arrhythmia occurs when:
the heart's natural pacemaker develops an abnormal rate or rhythm
the normal conduction pathway is interrupted another part of the heart takes over as pacemaker
