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Copyright © 2008 Thomson Delmar Learning
CHAPTER 13
The Standard 12-ECG System
Copyright © 2008Thomson Delmar Learning
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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