Cardio Phys Background I.pdf - 42 Aero Camino Goleta CA 93117 I Richard Pflanzer Ph.D Biopac Student Lab® Lesson 5

Cardio Phys Background I.pdf - 42 Aero Camino Goleta CA...

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42 Aero Camino, Goleta, CA 93117 Biopac Student Lab ® Lesson 5 ELECTROCARDIOGRAPHY (ECG) I Introduction Rev. 12292017 Richard Pflanzer, Ph.D. Associate Professor Emeritus Indiana University School of Medicine Purdue University School of Science William McMullen Vice President, BIOPAC Systems, Inc. Page I-1 ©BIOPAC Systems, Inc. I. I NTRODUCTION The main function of the heart is to pump blood through two circuits: 1. Pulmonary circuit : through the lungs to oxygenate the blood and remove carbon dioxide; and 2. Systemic circuit : to deliver oxygen and nutrients to tissues and remove carbon dioxide. Because the heart moves blood through two separate circuits, it is sometimes described as a dual pump. In order to beat, the heart needs three types of cells: 1. Rhythm generators, which produce an electrical signal (SA node or normal pacemaker); 2. Conductors to spread the pacemaker signal; and 3. Contractile cells (myocardium) to mechanically pump blood. The Electrical and Mechanical Sequence of a Heartbeat The heart has specialized pacemaker cells that start the electrical sequence of depolarization and repolarization . This property of cardiac tissue is called inherent rhythmicity or automaticity . The electrical signal is generated by the sinoatrial node (SA node) and spreads to the ventricular muscle via particular conducting pathways: internodal pathways and atrial fibers , the atrioventricular node (AV node ,) the bundle of His , the right and left bundle branches , and Purkinje fibers (Fig. 5.1). When the electrical signal of a depolarization reaches the contractile cells, they contract a mechanical event called systole . When the repolarization signal reaches the myocardial cells, they relax a mechanical event called diastole . Thus, the electrical signals cause the mechanical pumping action of the heart; mechanical events always follow the electrical events (Fig. 5.2). The SA node is the normal pacemaker of the heart, initiating each electrical and mechanical cycle. When the SA node depolarizes, the electrical stimulus spreads through atrial muscle causing the muscle to contract. Thus, the SA node depolarization is followed by atrial contraction. The SA node impulse also spreads to the atrioventricular node (AV node ) via the internodal fibers . (The wave of depolarization does not spread to the ventricles right away because there is nonconducting tissue separating the atria and ventricles.) The electrical signal is delayed in the AV node for approximately 0.20 seconds when the atria contract, and then the signal is relayed to the ventricles via the bundle of His, right and left bundle branches , and Purkinje fibers . The Purkinje fibers relay the electrical impulse directly to ventricular muscle , stimulating the ventricles to contract (ventricular systole ). During ventricular systole, ventricles begin to repolarize and then enter a period of diastole (Fig. 5.2).
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