The Normal Cardiac Conducting System

The heart consists of three types of specialized tissue: (1) pacemaker cells that undergo spontaneous depolarization and can initiate an electric impulse (this property is called automaticity); (2) cells that conduct electrical waves more rapidly than other cardiac cells, causing a very rapid propagation of the electric impulse throughout the heart, and (3) contractile cells, which contract when electrically depolarized.

The sinus [sinoatrial (SA)] node is normally the dominant cardiac pacemaker unless disease or drugs depress its activity. The SA node is located near the junction of the superior vena cava and right atrium. Blood supply is from the sinus node artery, which arises from either the proximal few centimeters of the right coronary artery (in about 55 percent of individuals) or from the proximal few millimeters of the left circumflex artery (in the other 45 percent). Both sympathetic and parasympathetic nerves, which are the primary controls of the heart rate, innervate the SA node. The normal sinus discharge rate is 60 to 100 beats per minute.

The electric impulse generated by the SA node spreads in waves through the cardiac muscles of the atria, activating atrial contraction. Additionally, specialized atrial conduction tracts (anterior, middle, and posterior internodal tracts) serve to propagate the electric impulse through the atria and between the sinus node and the atrioventricular (AV) node.

The atria and ventricles are insulated electrically from each other by the fibrous connective tissue of the AV ring (annulus fibrosus). Normally, electric impulses from the atria can reach the ventricles only by passing through the AV node and infranodal conducting system.

The AV node is under the surface of the right atrial endocardium and directly above the insertion of the septal leaflet of the tricuspid valve. The AV node receives its blood supply from the right coronary artery as it turns to form the posterior descending artery in 90 percent of individuals and, in the other 10 percent, as it comes off the left circumflex artery. This accounts for the common occurrence of AV conduction disturbances with acute inferior myocardial infarctions. The AV node is innervated by both sympathetic and parasympathetic fibers. It has two important electrophysiologic characteristics: a slow conduction velocity and a long refractory period. The slow conduction velocity through the AV node allows time for atrial contraction to give an extra 10 percent ventricular filling, which increases stroke volume according to the Frank-Starling principle. This "atrial kick" is most important for patients with ventricular failure. The long refractory period of the AV node protects the ventricles from excessively rapid stimulation; very rapid heart rates decrease the diastolic filling period and thereby reduce cardiac output, which may cause deterioration into ventricular fibrillation or cardiac failure. Cells near the AV node have automaticity and will escape from the control of the SA node if its rate becomes too low, normally below 60 beats per minute.

Electric impulses leave the inferior pole of the AV node along the bundle of His, which travels downward along the posterior margin of the membranous portion of the intraventricular septum to reach the top of the muscular portion. The bundle of His consists of Purkinje cells, which are the most rapidly conducting cells of the heart. The common bundle is only 1 to 2 cm in length before it divides at the crest of the muscular intraventricular septum into the right and left bundle branches (RBB and LBB). The RBB is a compact group of fibers that travels down to the apex of the right ventricle before separating into smaller branches. The LBB travels 2 to 3 cm before fanning out into a virtual sheet of fibers to cover the left ventricle. There are two relatively distinct pathways to the base of the papillary muscles, the left anterior superior fascicle (LASF) and the left posterior inferior fascicle (LPIF). These fascicles are distinguished more readily by electrical means in humans than by anatomy, but they can be seen clearly in animals.

The blood supply to the RBB and LASF is from the same sources: about half the time from both the AV nodal artery and branches from the left anterior descending coronary artery and the other half from the left anterior descending artery alone. The LPIF is supplied about half the time from the AV nodal artery and the other half by both the AV nodal artery and left anterior descending artery. Infarction in the region supplied by the left anterior descending artery is capable of affecting the RBB and LASF but very rarely the LPIF.

Accessory tracts are embryologic remnants of myocardium found in the AV annulus that can transmit electric impulses between the atria and ventricles, bypassing all or part of the AV node and intranodal system. These bypass tracts conduct at different rates and are the anatomic basis for the preexcitation syndrome.

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