9
Electrocardiogram
9.1 INTRODUCTION AND OVERVIEW
Electrocardiogram (ECG) is the most commonly used biomedical signal in clinical
diagnostics of the heart. The word “electrocardiogram” is a combination of three
words: electro, pertaining to electric signal; cardio, which translates into heart; and
gram, which stands for recording. The recording of the electric activity of the heart
is called ECG.
A cardiac muscle contraction is a direct result of the cellular electric excitation
described by the ECG. The depolarization initiates the shortening of each individual
muscle cell. The electric activation of each cell is an indication of the functioning
of that cell. Therefore, the ECG is the result of depolarization of the heart muscle
in a controlled repetitive fashion. By tracking the process of electric depolarization
of the cardiac muscle cells, an impression of the heart’s functionality can be formed
and used to recognize regions in the heart structure that are not functioning to specifications and may require medical attention. Any deviation from the typical ECG
observed in the recorded electric depolarization signal is analyzed and classified as
a certain cardiac disorder.
The principal concepts of the biological cell and the electric potential across the
cell membrane were discussed in Chapter 8. In this chapter, first, the function of the
heart as a pump will be discussed. Then, in order to fully understand the electric
signals generated by the heart with respect to each contraction, some basic phenomena involved in the contraction process will be described. Finally, the formation,
measurement, and processing of ECG will be discussed.
9.2 FUNCTION AND STRUCTURE OF THE HEART
The heart is the structure comprised of cardiac muscles that are responsible for circulating blood through the body. The anatomy and conduction system of the heart is
outlined in Figure 9.1. The heart has four major functions: collecting the blood that
needs to be refined from all parts of the body (through veins), pumping this collected
blood to the lungs, collecting the refined blood from the lungs, and pumping the
refined blood back to all parts of the body.
As can be seen in Figure 9.1, the heart has four chambers: two atria and two ventricles. The atria work in unison and so do the ventricles. The atrium is separated
from the venous system by a valve so that flow is only possible in one direction. The
superior vena cava and the inferior vena cava lead into the right atrium in combination with the coronary sinus, while the pulmonary veins supply the left atrium.
When the atrium contracts, it pumps the retained blood into the ventricle that is
separated by a valve as well. The valve only allows flow from the atrium to the ventricle and not in the opposite direction. This valve is called the atrioventricular valve.
171
Electrocardiogram
9.1 INTRODUCTION AND OVERVIEW
Electrocardiogram (ECG) is the most commonly used biomedical signal in clinical
diagnostics of the heart. The word “electrocardiogram” is a combination of three
words: electro, pertaining to electric signal; cardio, which translates into heart; and
gram, which stands for recording. The recording of the electric activity of the heart
is called ECG.
A cardiac muscle contraction is a direct result of the cellular electric excitation
described by the ECG. The depolarization initiates the shortening of each individual
muscle cell. The electric activation of each cell is an indication of the functioning
of that cell. Therefore, the ECG is the result of depolarization of the heart muscle
in a controlled repetitive fashion. By tracking the process of electric depolarization
of the cardiac muscle cells, an impression of the heart’s functionality can be formed
and used to recognize regions in the heart structure that are not functioning to specifications and may require medical attention. Any deviation from the typical ECG
observed in the recorded electric depolarization signal is analyzed and classified as
a certain cardiac disorder.
The principal concepts of the biological cell and the electric potential across the
cell membrane were discussed in Chapter 8. In this chapter, first, the function of the
heart as a pump will be discussed. Then, in order to fully understand the electric
signals generated by the heart with respect to each contraction, some basic phenomena involved in the contraction process will be described. Finally, the formation,
measurement, and processing of ECG will be discussed.
9.2 FUNCTION AND STRUCTURE OF THE HEART
The heart is the structure comprised of cardiac muscles that are responsible for circulating blood through the body. The anatomy and conduction system of the heart is
outlined in Figure 9.1. The heart has four major functions: collecting the blood that
needs to be refined from all parts of the body (through veins), pumping this collected
blood to the lungs, collecting the refined blood from the lungs, and pumping the
refined blood back to all parts of the body.
As can be seen in Figure 9.1, the heart has four chambers: two atria and two ventricles. The atria work in unison and so do the ventricles. The atrium is separated
from the venous system by a valve so that flow is only possible in one direction. The
superior vena cava and the inferior vena cava lead into the right atrium in combination with the coronary sinus, while the pulmonary veins supply the left atrium.
When the atrium contracts, it pumps the retained blood into the ventricle that is
separated by a valve as well. The valve only allows flow from the atrium to the ventricle and not in the opposite direction. This valve is called the atrioventricular valve.
171