Harvard-MIT Division of Health Sciences and Technology
HST.582J: Biomedical Signal and Image Processing, Spring 2007
Course Directors: Dr. Julie Greenberg
The Physiological Basis of the
Andrew T. Reisner, Gari D. Clifford, and Roger G. Mark
Courtesy of Artech House. Used with permission.
Artech House, 2006. All rights reserved.
Before attempting any signal processing of the electrocardiogram it is important
to first understand the physiological basis of the ECG, to review measurement
conventions of the standard ECG, and to review how a clinician uses the ECG
for patient care. The material and figures in this chapter are taken from [1, 2], to
which the reader is referred for a more detailed overview of this subject. Further
information can also be found in the reading list given at the end of this chapter.
The heart is comprised of muscle (
) that is rhythmically driven to
contract and hence drive the circulation of blood throughout the body. Before every
normal heartbeat, or
a wave of electrical current passes through the entire
heart, which triggers myocardial contraction. The pattern of electrical propagation
is not random, but spreads over the structure of the heart in a coordinated pattern
which leads to an effective, coordinated systole. This results in a measurable change
in potential difference on the body surface of the subject. The resultant amplified
(and filtered) signal is known as an electrocardiogram (ECG, or sometimes EKG).
A broad number of factors affect the ECG, including abnormalities of cardiac con
ducting fibers, metabolic abnormalities (including a lack of oxygen, or
of the myocardium, and macroscopic abnormalities of the normal geometry of the
heart. ECG analysis is a routine part of any complete medical evaluation, due to the
heart’s essential role in human health and disease, and the relative ease of recording
and analyzing the ECG in a noninvasive manner.
Understanding the basis of a normal ECG requires appreciation of four phe
nomena: the electrophysiology of a single cell, how the wave of electrical current
propagates through myocardium, the physiology of the specific structures of the
heart through which the electrical wave travels, and last how that leads to a mea
surable signal on the surface of the body, producing the normal ECG.
1.1 Cellular Processes That Underlie the ECG
Each mechanical heartbeat is triggered by an
which originates from
a rhythmic pacemaker within the heart and is conducted rapidly throughout the or
gan to produce a coordinated contraction. As with other electrically active tissues
Diastole, the opposite of systole, is defined to be the period of relaxation and expansion of the heart