The electrocardiographic machine is, in effect, a voltmeter which records from the body surface the electrical forces generated during depolarization and repolarization of the heart. Unlike conventional voltmeters in which the output is displayed on a dial, the output of the electrocardiogram is displayed on paper moving at a fixed rate. This rate is usually 25mm per second, but can be made faster or slower.
The electrocardiogram is one of the most commonly performed, most noninvasive and least expensive medical tests. It is easy to generate, does not cause discomfort or place the patient at risk and the results are immediately available. When correctly interpreted it provides a wealth of information about a variety of physiologic, pharmacologic and pathologic entities that influence the electrical activity of the heart and is of inestimable help in establishing the diagnosis and evaluating the treatment of diseases that effect the heart.
Studies focusing on the electrical activity of the heart date to the latter half of the 19th century. The development of the electrocardiogram and its use as a clinical tool was accomplished by Willem Einthoven, a Dutch physiologist who lived from 1860 to 1927 and is often referred to as "the Father of the Electrocardiogram". In 1924 he was awarded the Nobel Prize for his pioneering work which utilized the string galvanometer. In this device, a thin silver coated quartz filament of minimal mass was stretched between the poles of an electromagnet and moved in proportion to the electrical fields generated by the heart. This movement of the filament cast a shadow which was photographed, thereby producing the electrocardiogram. The string galvanometer is obviously a long way from the automated digital machines that are currently in use, however many of the principles which underlie the recording of the body surface electrocardiogram remain the same. Since the early part of the 20th century our understanding of the factors responsible for the electrocardiographic waveform and its correlation to the electrical activity of the heart and to a wide variety of cardiac and non-cardiac diseases has resulted from the work of many clinical and basic scientists. There is now an impressively large and continually growing database relating the electrocardiographic waveform to the clinical setting.
The key which unlocks the information contained in the 12 lead body surface electrocardiogram is the understanding of the various factors which contribute to its waveform. Memorization of ECG patterns alone will permit the correct electrocardiographic diagnosis in many situations. However, reliance on pattern memorization and recognition does not permit an understanding or differentiation of the diverse clinical situations which may produce similar electrocardiographic patterns. Nor does it permit the distinction of benign and non-specific changes from those which are diagnostic of pathological conditions.
What follows is a discussion of the cellular events responsible for the depolarization and repolarization of the heart, the transformation of these events into the waveform recorded on the body surface by the 12 leads used in routine clinical practice, the changes in wave form induced by a variety of anatomic, physiologic, pharmacologic, metabolic and pathologic factors and the electrophysiologic alterations responsible for those changes. It is our belief that the body surface electrocardiogram is effectively a bridge which unites the electrical events occurring at the cellular level to the clinical presentation of the patient. It is also our belief that by understanding the factors which lead to the generation of the electrocardiographic waveform and its alteration the information contained in the waveform will be maximized and will prove to be of an inestimable value in understanding and diagnosing diseases which influence the electrical activity of the heart.
This project is designed to be an interactive guide to an understanding and interpretation of the body surface electrocardiogram. We hope that by stressing electrophysiologic concepts and showing how these concepts are transformed into the body surface electrocardiogram, the mystique surrounding the electrocardiogram and the anxiety associated with its interpretation will be reduced.
References are included only when specifically alluded to. The texts by Surawicz, Electrophysiologic Basis of ECG and Cardiac Arrhythmias: William and Wilkens, Phildelphia 1995, by Chou Electrocardiography in Clinical Practice: Fourth Edition, W. P. Saunders, Philadelphia 1996, and by Zipes and Jalife Cardiac Electrophysiology from Cell to Bedside, Second Edition, Saunders, Philadelphia 1995, served as primary reference sources. Each is extensively referenced and was of inestimable value.
I am indebted to Dr. Jose Jalife for providing the demonstration of spiral waves shown in chapter seven and to Dr. John Michael Criley, Dr. David Criley, and Conrad Zalace for allowing me to reproduce the heart murmurs and phonocardiograms in chapter nine. They are from their CD-ROM The Physiological Origins of Heart Sounds and Murmurs, Lippencott-Raven, 1997.
This work could not have been accomplished without an unrestricted educational grant from Marquette Electronics, Inc. and without the help of Lynn Evans, Philmer Brooks and Carmeria Alston from the ECG laboratory at UNC hospitals. My thanks to Doctors Ann Gettes and Elieser Kaplinsky who reviewed each chapter and provided valuable critiques. My thanks also go out to Anne Middleton who provided expert secretarial assistance and to Jacques Strauss, Steve Korn and Carole Shanley at Futura Pulishing Company Inc. for their patience and support.
This work is dedicated to Ann, Edith, Gretchen and Susi, and to the medical students, house officers and cardiac fellows at the University of North Carolina at Chapel Hill who were the inspiration for this project.