The heart is a pump responsible for maintaining blood supply to the body. It has four chambers. The two upper chambers (the right atrium and left atrium) are the chambers which receive blood as it returns from the body via the veins. The lower chambers (the right and left ventricle) are the chambers responsible for pumping the blood out to the body via the arteries. Like any pump, the heart has an electrical system that controls how it functions.
Normal Heart Rhythm.
In order for the heart to do its work (pumping blood throughout the body), it needs a sort of spark plug or electrical impulse to generate a heartbeat. Normally this electrical impulse begins in the upper right chamber of the heart (in the right atrium) in a place called the sino-atrial (SA) node. The SA node is the natural pacemaker of the heart. The SA node gives off electrical impulses to generate a heartbeat in the range of 60 to 100 times per minute. If you are exercising, doing strenuous work or you are under a lot of stress, your heart rate may be faster. When you rest or sleep your heart rate will slow down. If you take certain medications, your heart rate may be slower. All of this is appropriate.
From the SA node, the electrical impulse is relayed along the heart’s conduction system. It spreads throughout both the right and left atria causing them to contract evenly. When the impulse spreads over the right atrium it reaches the atrio-ventricular (AV) node. This is a very important structure in the heart because it is the only electrical connection between the top chambers and the bottom chambers. It is therefore the only way in which an electrical impulse can reach the pumping chambers (the ventricles). The impulse spreads through the AV node and down into the lower chambers or ventricles of the heart. This causes them to contract and pump blood to the lungs and body.
The Normal ECG
An electrocardiogram or ECG is a quick, painless test to see the activity of the heart. It is used to determine if your heart has abnormal rhythms, if it is getting enough blood, or if areas of your heart are abnormally thick. The ECG records the electrical activity of the heart. The pattern of electrical activity that can be seen on an ECG is called a sinus rhythm.
A normal heartbeat produces regular, identifiable patterns that are called the P wave, the QRS complex, and the T wave. These segments show 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 as the P wave. The signal then reaches the AV node which produces an electrical signal which continues through the Bundle of His, the Bundle Branches, and Purkinje fibers. This causes the ventricles, the lower chambers of the heart, to contract. Electrical activity in the ventricles is seen as QRS and the T wave. The heart refills with blood and the cycle begins again, with the SA node.