The electrical function of the heart is a complex interconnected system which is ultimately responsible for coordinating the muscular contractions of the upper chambers (atria) and lower chambers (ventricles) of the heart. The electrical system of the normal heart is made up of many components:
- sinus node ("bandleader")
- atrial muscle
- AV (atrioventricular) node ("gatekeeper")
- His-Purkinje system
- ventricular muscle
In the normal heart, the electrical impulse is initiated in a small group of cells located in the top of the right atrium, called the sinus node. The rate at which the sinus node "fires" is what typically dictates the heart rate (hence, the "bandleader" of the heart). Activities such as sleeping cause the sinus node to fire less frequently hence lowering the heart rate. Activities such as running cause the sinus node to fire more frequently, hence raising the heart rate. As the heart ages, scarring of the sinus node tissue can result in an abnormally slow heart rate which can cause symptoms such as fatigue, shortness of breath, lightheadedness, dizziness, or passing out (syncope). This can be treated successfully with a pacemaker.
The electrical impulse travels out of the sinus node and into the atrial muscle where it is conducted through the right atrium and the left atrium. The amount of time it takes the impulse to travel through the atria depends on their size and any scarring of the tissue that might be present. Chronically high blood pressure or heart attacks can cause the atria to enlarge or form scar and therefore can slow conduction through the atria.
The impulse then travels through the only electrical connection between the upper and lower chambers of the heart, the AV node. The AV node allows only a certain number of impulses to travel from the atria to the ventricles over any given amount of time (hence, the "gatekeeper" of the heart). Typically, when an individual exercises or walks, the number of impulses that the AV node allows through per period of time increases. Conversely, when an individual rests, the number of impulses that the AV node allows through per period of time decreases. Scarring of the AV node from long-standing high blood pressure, infections of the heart, or heart attacks can result in conduction abnormalities leading to bradycardia and the possible need for a pacemaker.
The electrical impulse then continues on its trip through the heart into the His-Purkinje system, which is a specialized collection of electrical fibers that conduct the impulse into the right and left ventricular muscle. In patients with heart disease this system of fibers can become scarred resulting in conduction abnormalities (bundle branch block) or complete lack of conduction to the ventricles (complete heart block). Many forms of heart block require a pacemaker to prevent symptoms.
Once the impulse travels through the His-Purkinje system it enters into the right and left ventricular muscle. The impulse typically travels from the bottom of the ventricles (apex) to the top of the ventricles (base) thereby coordinating the muscular contraction of the ventricles. The entire trip through the electrical system typically takes less than a second to complete. When any of these components of the heart's electrical conduction system malfunction, the results can manifest themselves as bradycardia (a slow heart beat) or tachycardia (a fast heart beat). Please see the Arrhythmias section for more details.