Download Unit One: Introduction to Physiology: The Cell and General Physiology

Chapter 11: The Normal Electrocardiogram
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Characteristics of the Normal ECG
Fig. 11.1 The Normal Electrocardiogram (ECG)
Characteristics (cont.)
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P-Wave-electrical potentials generated when the atria
depolarize before atrial contraction
•
QRS Complex-potentials generated when the ventricles
depolarize before contraction
•
T wave-caused by repolarization of the ventricles
Fig. 11.2 Recording the depolarization wave (A and B) and the
repolarization wave (C and D) from a cardiac muscle fiber
Relationship of the Monophasic AP of Ventricular Muscle to the
QRS and T Waves in the Standard EEG
Fig. 11.3
Characteristics of the Normal EEG (cont.)
•
From Figure 11.1
a. Relationship of atrial and ventricular contraction
to the waves of the EEG
b. Voltage and time calibration of the EEG
c. Normal voltages in P-Q or P-R interval
d. Normal voltages in the Q-T interval
e. Rate of Heartbeat
Flow of Current Around the Heart During the Cardiac Cycle
Fig. 11.4 Instantaneous potentials develop on the surface of a cardiac muscle
mass that has been depolarized in its center
Fig. 11.5 Flow of current in the chest around partially depolarized ventricles
Flow of Current (cont.)
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The average current flow occurs with negativity
toward the base of the heart and with
positivity toward the apex.
•
In normal hearts, the current flows from
negative to positive in the direction from the
base to the apex.
Electrocardiographic Leads
Fig. 11.6 Conventional arrangement of electrodes;
Einthoven’s Triangle is superimposed
Fig. 11.7 Normal EEG’s recorded from the three
standard leads
EEG Leads (cont.)
•
Lead I-Negative terminal to the right arm and positive
terminal to the left arm
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Lead II-Negative terminal to the right arm and positive
terminal to the left leg
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Lead III-Negative terminal to the left arm and positive
terminal to the left leg
•
Einthoven’s Triangle-depicts how the arms and left
leg form the apices of a triangle around the
heart
Chest Leads
Fig. 11.9 Normal EEGs recorded from the six
chest leads
Fig. 11.8 Connections for recording chest leads
Augmented Unipolar Limb Leads
Fig. 11.10 Normal EEGs from the three augmented unipolar limb leads