Electrocardiography

The electrocardiogram (ECG) records from the body surface and registers the differences in electrical potential generated by the heart. The signal recorded is determined by action potentials generated by millions of individual cells and their sequence of activation. A multitude of factors, both cardiac and extracardiac, alter the final electrical signal. For instance, the electrical forces generated by the heart are subsequently altered by the position of the heart within the body, the nature of the intervening tissue, and the distance to the recording electrode. Since the final, recorded electrical signal does not faithfully reflect the electrical activity of individual cells, the student should not expect the ECG necessarily to provide an exact anatomic or physiologic: "picture" of the heart. Nevertheless, as the result of careful correlation of electrocardiographic patterns with observed anatomic, pathologic, and physiologic data, it is now possible to deduce, with a high grade of accuracy, the state of the heart from the surface ECG. While it is true that the ECG may be normal despite an abnormal heart, or abnormal with a normal heart, it is equally true that the ECG often provides an important indication of a cardiac abnormality, and even allows a fairly accurate appraisal of the anatomic and physiologic significance of that abnormality. Furthermore, the ECG is by far the best method of analysis of disturbances of the cardiac rhythm.