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Electrocardiography

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INTRODUCTION

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Normal cardiac rhythm is initiated by cells within the sinoatrial node, located in the lateral groove of the right atrium at its junction with the superior vena cava. These cells depolarize spontaneously, and the wave of activation spreads through the right and left atria (Fig. 487-1). In the right atrium the wave of depolarization passes inferiorly, and the left atrium is activated via the Bachmann bundle, which also results in an inferiorly directed activation front. This atrial activation generates an electrical potential that is detected on the surface electrocardiogram (ECG) as the P wave. Conduction then propagates to the atrioventricular (AV) node, which is located in the medial floor of the right atrium, within the triangle of Koch. Conduction through the AV node is slow, providing a delay between atrial activation and subsequent depolarization of the ventricles. Physiologically, this delay allows ventricular filling to be completed before ventricular contraction begins, providing AV synchrony. Conduction through the atrium and the AV node is manifest on the ECG as the PR interval. Beyond the AV node, the impulse moves rapidly down through the bundle of His, which penetrates the otherwise electrically insulated AV annulus and divides into the right and left bundle branches. The right bundle branch runs subendocardially on the right ventricular septum and emerges in the moderator band, while the left bundle branch fans out broadly on the left ventricular endocardial surface. As the impulse passes down the septum, it activates septal muscle predominantly from the left side, so that the initial direction of ventricular depolarization passes from left to right and anteriorly and superiorly (Fig. 487-2A). On surface ECG, this corresponds to an early Q wave in the left-sided chest leads and the initial R wave in the right-sided chest leads. After reaching the apex, the impulse activates the ventricular free walls from endocardium to epicardium and from apex toward the base through the Purkinje network. In total, depolarization of the ventricles produces the QRS complex on the surface ECG. The morphology of this QRS complex is dependent on the pattern and the rate of ventricular depolarization through the myocardium. After ventricular depolarization has occurred, there is a slower repolarization process that produces the ST segment and T wave. Thus, the cardiac cycle is borne out on surface electrocardiography with the P, QRS, and T waves.

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Figure 487-1

Diagram of conduction system. AV, atrioventricular node; His, bundle of His, dividing into right and left bundles; LA, left atrium; RV and LV, right and left ventricles; SA, sinoatrial node; SVC and IVC, superior and inferior venae cavae.

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Figure 487-2

Normal ventricular depolarization, starting with 1 and ending with 5. A: Section through ventricles with thicker-walled left ventricle and thinner-walled right ventricle. Arrows are vectors, indicating direction and magnitude of electrical forces at each time. B: Vectors ...

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