The Electrocardiogram as a Representation of the Heart Function
This chapter does not pretend to be a source of medical knowledge in cardiology, but to give some insight about ECG generation and recording, and its main characteristics, and highlight the clinically meaningful information that can be extracted from the ECG.
The electrocardiogram is the paper or digital record of the cardiac electrical activity. In most cases it is taken at the body’s surface via a noninvasive and painless procedure, not implying discomfort to the patient and extremely cheap compared with other methods to assess heart function. The intracardiac recordings and other modalities are reserved for use in specialized cases and will not be discussed in this book.
The electrocardiogram has been extensively used in clinical medicine for more than 80 years, and is now a primary diagnostic tool for many cardiac and other diseases (Fisch, 2000; Van Mieghem, Sabbe, & Knockaert, 2004).
It is recorded as a temporal representation of the electrical field resulting from the electrical activity of the heart muscle issue at the cell level. By using several electrodes placed on the skin, it is possible to access several simultaneous aspects of spatial phenomena, known as electrocardiographic leads.
Due to their electrical nature, some congenital heart abnormalities and thickness or damage in the heart muscle are just two examples of the many diseases that may be detected and diagnosed prior to or during heart attacks (myocardial infarction). Cardiologists may also notice evidence of acutely impaired blood flow in the heart.
Abnormal electrical activity of the heart is detected as an unusual representation of particular elements of the heart cycle in time or amplitude. Such variations of externally measurable electrical parameters are caused by too fast/slow or irregular rhythms and abnormal generation or conduction of the cardiac electrical impulses. Specified changes in some of these parameters may represent life-threatening conditions, therefore the evaluation of the cardiac rhythms is a problem of major importance.
As far as the importance of the cardiac diagnosis being recognized, the systematic diagnosis concerned the considerable percentage of populations, and new disease-oriented modalities of the ECG were implemented into clinical practice. The use of automatic analysis systems was found essential in two ways:
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interpretation efficiency, namely in cases of very long records and for automatic preselection of events and suspicious signal segments for further analysis performed by a cardiologist; and
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standardization of the interpretation process which was systematized in the global scale in order to avoid the inter and intra observer variability.
The electrocardiogram of today is interpreted manually (or better said, visually) only in rare cases in which the software fails to give the right solution due to unusual disease, unstable recording conditions, or inconsistent results. The interpretation software is currently implemented as firmware in a wide range of stand-alone bedside ECG recorders and as general-purpose operation system-based interpretive workstations in the cardiologists’ offices. The second implementation benefits from the large computational capacity usually available on personal workstations and gives the cardiologists an opportunity for interaction.