Electrical conduction system of the heart

Understanding the electrical conduction system of the heart is essential to EKG interpretation. This article outlines the conduction system of the heart.

The sinoatrial node (SA node, sinus node) functions as the heart’s pacemaker. Electrical stimulus is generated in the sinoatrial node and from there impulses spread to the rest of the heart causing it to contract. The SA node is located in the right atrium near the entrance of the superior vena cava. It consist of specialized cardiac myocytes (heart muscle cells).

From the sinus node impulses first propagate to the left atrium and to the atrioventricular node (AV node). The AV node is the electrical connection between the atria and the ventricles of the heart. It is located in the septum between the atria.

The AV node slightly delays the impulses from the atria to the ventricles, which importantly ensures atrial contraction and ejection of blood into the ventricles before ventricular contraction. The AV node has decremental conduction, which means that the faster it is stimulated the slower it conducts. This property plays an important role in atrial arrhythmias as the conduction of rapid atrial rhythms to the ventricles are prevented.

From the AV node the electrical conduction continues through the bundle of His which further split into the right and left bundle branches in the septum between the ventricles. The left bundle branch split into an anterior and a posterior fascicle. The two bundle branches finally ends in numerous purkinje fibers in the ventricles.

Notice that all cells in the conduction system have the ability to generate electrical impulses. They are said to have pacemaker potential. In the normal heart the sinus node will override the other cells of the conduction system and drive the heart contractions. This is because of the higher frequency of the SA node (60-100 beats per minute) compared to cells in the AV node (40-60 beats per minute) and the purkinje fibers (20-40 beats per minute).

However, if the sinus node fails or a conduction block is present, a more distal focus in the AV node or the purkinje fibers will (ideally) take over the pacemaker function. The resulting rhythm or beats are referred to as escape rhythms/beats.

Check out the links below to read more articles on conduction defects and see EKG examples.

– Mathias