The sinus node controls how fast the heart beats under normal circumstances. It causes the heart to beat faster during exercise or in the setting of fever or illness. The electrical energy generated by the sinus node spreads thru the top chambers of the heart to the AV node, the electrical connection between the top and bottom chambers of the heart. The electrical signal then goes thru specialized electrical conduction tissue to make the lower chambers of the heartbeat.
Disturbances of electrical conduction can occur all along this electrical pathway and can also occur in the sinus node itself. If the sinus node or specialized electrical conduction tissue do not function properly, the heart can beat too slowly and cause symptoms of light headedness or even loss of consciousness.
Pacemakers are implanted to prevent the heart from beating too slowly. Pacemakers have two parts, a generator which houses the battery and electrical components and the leads which are thin wires which pass thru the veins into the heart. The generator sends electrical impulses to the heart thru the leads to cause it to beat. The leads also carry information from the heart back to the generator so it can tell when the heart is beating on its own. Pacemakers are described as either single chamber, one lead in either the atrium or ventricle, or dual chamber, leads in both the atrium and ventricle. A new type of pacemaker to treat heart failure is also available which paces not only the right atrium and right ventricle but also the left ventricle. (See Cardiac Resynchronization Therapy)