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CHAPTER 13
The Standard 12-ECG System
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Standard 12-ECG System
• Consists of four limb electrodes and
six chest electrodes
• Collectively, view the heart from
12 different positions
– Six standard limb leads
– Six precordial (chest) leads
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ECG Lead Systems
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Standard 12-ECG System
• Each lead:
1. Views the electrical activity of the heart from
a different angle
2. Has a positive and negative component
3. Monitors specific portions of the heart from
the point of view of the positive electrode in
that lead
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STANDARD LIMB LEADS
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Standard Limb Leads
Fig. 13-1. The standard limb
leads—leads I, II, III, aVR,
aVL, and aVF. Each of the
standard limb electrodes can
function as either a positive
or negative electrode.
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Einthoven’s Triangle
Fig. 13-2. Leads I, II, and III
axes form Einthoven’s
triangle.
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Einthoven’s Triangle Around the Heart
Fig. 13-3. Einthoven’s
triangle around the heart.
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Einthoven’s Triangle Around the Heart
Fig. 13-4. In the normal
heart, the dominant electrical
current in the heart flows from
the base to the apex in a right
to left direction.
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Frontal Plane and Limb Leads
Fig. 13-5. The frontal plane
and the limb leads.
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PRECORDIAL
(CHEST) LEADS
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Electrodes and Precordial Leads
Fig. 13-6. (A) The position of
the electrodes on the rib
thorax, and (B) the precordial
leads as they reflect the
surface of the myocardium.
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Axis of Six Precordial Leads
Fig. 13-7. The axis of the six
precordial leads.
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Horizontal Plane and Its Leads
Fig. 13-8. The horizontal
plane and its leads.
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Electrodes for the Monitoring System MCL
Fig. 13-9. The position of the
electrodes for the monitoring
system MCL.
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THE ECG PAPER
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ECG Monitoring Paper
Fig. 13-10. The ECG
monitoring paper, with the
blocks enlarged to illustrate
the minimum units of
measurement.
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ECG Paper
•
•
•
•
Small square = 0.04 sec.
Large square = 5 sm. Sq. = 0.20 sec.
Speed = 5 lg. Sq./sec.
Vertical portion of sm. Sq. =
– Amplitude (voltage) of 0.1 millivolt (mV)
– 1 millimeter (1mm) in distance
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ECG Monitoring Paper
Fig. 13-10. The ECG
monitoring paper, with the
blocks enlarged to illustrate
the minimum units of
measurement.
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ECG Monitoring Paper
Fig. 13-11. ECG monitoring
paper showing markers
indicating 3- and 6-second
intervals. There are 15 blocks
in 3 seconds and 30 blocks in
6 seconds.
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Normal ECG Configurations
Fig. 13-12. Normal ECG
configurations.
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ECG Configuration
Fig. 13-13. The duration
of the normal ECG
configuration.
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The P Wave
• Represents atrial depolarization
• Usually symmetrical and upright
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The PR Interval
• Represents the total atrial
electrical activity
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The QRS Complex
• Represents the ventricular depolarization
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The QRS Complex
Fig. 13-14. (A) Q waveform
of the QRS. (B) R waveform
of the QRS complex. (C) S
waveform of the QRS
complex.
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The ST Segment
• Represents the time between ventricular
depolarization and repolarization
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ST Segment Highlighted within Cardiac Complex
Fig. 13-15. (A) The ST
segment highlighted within
cardiac complex. Note
variation. (B) ST above the
baseline. (C) Shows 3 mm
ST segment increase. (D)
shows 3 mm ST segment
decrease.
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The T Wave
• Represents ventricular repolarization,
rest, and recovery
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The T Wave
Fig. 13-16. (A) T wave
shows ventricular
depolarization. (B) The T
wave with ST segment
elevation. (C) Measuring an
inverted T wave and with ST
segment depression.
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The U Wave
• Its origin and mechanism are not known
• Often prominent in certain
– Electrolyte disturbances
– Medications
– Heart diseases
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The U Wave
Fig. 13-17. The U wave
highlighted (arrow) within
the cardiac complex.
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Summary of Normal ECG Configurations and Heart Activity
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Summary of Normal ECG Configurations and Heart Activity
Table 13-2
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