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The electrocardiogram (ECG) records the electrical activity (depolarization and
repolarization) of cardiac muscle from the body surface and provides information on
heart rate, rhythm, and intracardiac conduction. Electrocardiography is considered a gold
standard for the diagnosis of arrhythmias.
The standard 12-lead ECG traditionally consists of tracings obtained from the bipolar
limb leads (I, II, and III), unipolar limb leads (aVR, aVL, and aVF) and usually six
unipolar chest or precordial leads (V1 through V6) (Fig. 1). Leads I, II, and III are
formed by connecting the right arm (RA) to the left arm (LA), the right arm to the left leg
(LL), and LA to LL, respectively. Arrows indicate the axes of these leads in relation to
the theoretical electrical center (EC) of the heart. The indifferent electrode of the unipolar
system is obtained by connecting RA, LA, and LL into a central terminal (Fig. 2).

Fig. 1: Electrocardiograph leads

The exploring electrode (called positive) can
then be placed on one of the three extremities to
register the potentials transmitted to that
particular limb. The letter V denotes a unipolar
lead. The letter “a” in aVR (aVL or aVF) means
that the potential difference was electrically

Fig. 2: Frontal lead axes

The P wave represents depolarization of the atria; the QRS complex is produced by
depolarization of the ventricles; and the T wave represents repolarization of the
ventricles at the beginning of diastole. An incompletely understood U wave sometimes
follows the T wave (Fig.3).

Fig. 3: The normal electrocardiogram (ECG). (a) A labeled drawing, and (b) an ECG recording

1. Describe the normal cyclical pattern of electrical impulse production in the heart and
conduction along specialized tissues of the heart.
2. Describe the normal electrocardiogram (ECG) and explain how it is produced.
3. Obtain an electrocardiogram using the limb leads, identify the waves, determine the PR interval, and measure the cardiac rate, etc.
4. Describe the common ECG abnormalities or arrhythmias (if possible).

1. Electrocardiograph ECG-8820 NIHON KOHDEN, Japan
2. Electrode plates, rubber straps, electrolyte gel or paste, cotton and alcohol 900

1. With the subject comfortably reclining (lying down), rub a silver dollar-sized
amount of electrolyte gel on the medial surface, about 2 inches above the wrists
and ankles. Using a alcohol – soaked cotton to clean the electrod...
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