The sinoatrial node is the part of the heart muscle where each heartbeat, or contraction, originates. Sometimes known as the SA node, it is the pacemaker that effectively controls the heart rate, although it is influenced by input from the nervous system. The heartbeat spreads from the sinoatrial node through the rest of the cardiac conduction system to reach all parts of the heart muscle, allowing contraction to occur in a coordinated manner. Problems with the sinoatrial node may be corrected using an artificial pacemaker.
In one of the upper chambers of the heart, known as the right atrium, the sinoatrial node can be found within the muscle that forms the wall. Like the rest of the conducting system of the heart, it consists of specialized heart muscle fibers that rapidly transmit electrical impulses. The conducting system is important because it ensures that different parts of the heart beat together in the correct sequence.
Adult humans at rest normally exhibit a heart rate of about 70 beats per minute. Heart contraction occurs first in the upper chambers, or atria, followed after a short delay by the lower chambers, or ventricles. This delay allows time for the atria to empty into the ventricles before they contract and send blood throughout the body.
Once the cardiac impulse has originated in the sinoatrial node, it travels down through the muscle of the atrial walls, causing the atria to contract, until it reaches an area called the atrioventricular node. Conduction in the atrioventricular node is relatively slow. This causes the necessary pause before impulses continue downward through the ventricle walls and ventricular contraction occurs.
Sometimes, an illness such as a heart attack can stop the conduction of impulses from the atria to the ventricles, causing a condition known as heart block. In third degree, or complete, heart block, the atria and ventricles beat independently of one another. The ventricles beat slower as they now receive impulses solely from the atrioventricular node. With incomplete heart block — classed as first or second degree — conduction of impulses from the atria to the ventricles is delayed. First degree heart block causes a slow heartbeat, and second degree leads to conditions where the atria contract but the ventricles do not always follow.
Treatment is not generally required for first degree heart block. It is not always caused by disease and can be found in young people and athletes. Second degree heart block, where the ventricles contract intermittently, is sometimes treated using an artificial pacemaker. Third degree heart block is more severe with symptoms of tiredness, dizzy spells, and fainting; in this case, an artificial pacemaker is usually essential. The operation to insert the pacemaker into the heart is a minor procedure and can be carried out in a cardiology unit under local anesthesia.