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CHAPTER 9
Fundamental Principles, 000 Recording Electrodes and Leads, 000 Clinical Electrocardiographic Lead Systems, 000 The Electrical Axis, 000 The Normal Electrocardiogram, 000 Atrial Activation and the P Wave, 000 Atrioventricular Node Conduction and the PR Segment, 000 Ventricular Activation and the QRS Complex, 000 Ventricular Recovery and the ST-T Wave, 000 The Abnormal Electrocardiogram,000 Atrial Abnormalities, 000 Ventricular Hypertrophy and Enlargement, 000 Intraventricular Conduction Delays and Preexcitation, 000 Myocardial Ischemia and Infarction, 000 Drug Effects, 000 Electrolyte and Metabolic Abnormalities, 000 Clinical Issues in Electrocardiographic Interpretation, 000 References, 000 Guidelines: Electrocardiography, 000

Electrocardiography
David M. Mirvis • Ary L.Goldberger

The electrocardiogram (ECG), as used today, is the product of a series of technological and physiological advances pioneered over the past two centuries.1 Early demonstrations of the heart’s electrical activity reported during the last half of the 19th century, for example, by Marchand and others, were closely followed by direct recordings of cardiac potentials by Waller in 1887. Inventionof the string galvanometer by Willem Einthoven in 1901 provided a reliable and direct method for registering electrical activity of the heart. By 1910, use of the string galvanometer had emerged from the research laboratory into the clinic. Subsequent achievements built on the very solid foundation supplied by the early electrocardiographers. The result has become a widely used and invaluableclinical tool for the detection and diagnosis of a broad range of cardiac conditions, as well as a technique that has contributed to the understanding and treatment of virtually every type of heart disease. Electrocardiography remains the most direct method for assessing abnormalities of cardiac rhythm. Furthermore, the ECG is essential in the management of major metabolic abnormalities such ashyperkalemia and certain other electrolyte disorders, as well as in assessing drug effects and toxicities such as those caused by digitalis, antiarrhythmic agents, and tricyclic antidepressants. More than 7 million ECGs are performed in the United States each year, making ECG the most commonly performed as well as the oldest cardiovascular laboratory procedure.

findings. Each of these steps influencesthe final product—the clinical ECG—and are considered in detail in this chapter to provide a foundation for considering the common abnormalities found in clinical practice and as a basis for further learning. GENESIS OF THE CARDIAC ELECTRICAL FIELD CARDIAC ELECTRICAL FIELD GENERATION DURING ACTIVATION. Transmembrane ionic currents are ultimately responsible for the cardiac potentials that arerecorded as an ECG. Current may be analyzed as though carried by positively charged or negatively charged ions. Through a purely arbitrary choice, electrophysiological currents are considered to be the movement of positive charge. A positive current moving in one direction is equivalent to a negative current of equal strength moving in the opposite direction. The process of generating the cardiacelectrical field during activation can be illustrated by considering the events in a single cardiac fiber, 20 mm in length, that is activated by a stimulus applied to its left-most margin (Fig. 9–2A).Transmembrane potentials (Vm) are recorded as the difference between intracellular and extracellular potentials (fe and fi, respectively). Figure 9–2B plots Vm along the length of the fiber at the instantduring the propagation (to) at which activation has reached the point designated as X0. As each site is activated, the polarity of the transmembrane potential is converted from negative to positive.Thus, sites to the left of the point X0, which have already undergone excitation, have positive transmembrane potentials (that is, the inside of the cell is positive relative to the outside of the cell),...