Normal And Abnormal Cardiac Conduction And Electrophysiology

The heart functions via both mechanical and electrical activity. The mechanical activity of the heart refers to atrial and ventricular contraction, the mechanism by which blood is delivered to tissue. When circulated blood returns to the heart via the venous circulation, the blood enters the right atrium. Right atrial contraction and changes in right-ventricular pressure result in delivery of blood to the right ventricle through the tricuspid valve. Right-ventricular contraction pumps blood through the pulmonic valve through the pulmonary arteries to the lungs, where the blood becomes oxygenated. The blood then flows through the pulmonary veins into the left atrium. Left-atri-al contraction and changes in left-ventricular (LV) pressure result in delivery of blood through the mitral valve into the left ventricle. Contraction of the left ventricle results in pumping of blood through the aortic valve and to the tissues of the body.

The mechanical activity of the heart (contraction of the atria and ventricles) occurs as a result of the electrical activity of the heart. The heart possesses an intrinsic electrical conduction system (Fig. 9-1).1 Normal myocardial contraction cannot occur without proper and normal function of the heart's electrical conduction system. Electrical depolarization of the atria results in atrial contraction, and ventricular depolarization is followed by ventricular contraction. Malfunction of the heart's electrical conduction system may result in dysfunctional atrial and/or ventricular contraction and may reduce cardiac output.

The Cardiac Conduction System

Under normal circumstances, the sinoatrial (SA) node (also known as the sinus node), located in the upper portion of the right atrium, serves as the pacemaker of the heart and generates the electrical impulses that subsequently result in atrial and ventricular depolarization (Fig. 9-1).1 The SA node serves as the heart's pacemaker because it has the greatest degree of automaticity, which is defined as the ability of a cardiac fiber or tissue to initiate depolarizations spontaneously. In adults at rest, the normal intrinsic depolarization rate of the SA node is 60 to 100 per minute. Other cardiac fibers also possess the property of automaticity, but normally the intrinsic depolarization rates are slower than that of the SA node. For example, the normal intrinsic depolarization rate of the atrioventricular (AV) node is 40 to 60 per minute, while that of the ventricular tissue is 30 to 40 per minute. Therefore, because of greater automaticity, the SA node normally serves as the pacemaker of the heart. However, if the SA node fails to generate depolarizations at a rate faster than that ofthe AV node, the AV node may take over as the pacemaker. Similarly, if the SA node and AV node fail to generate depolarizations at a rate greater than 30 to 40 per minute, ventricular tissue may take over as the pacemaker.

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