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

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


ECG Lead Systems


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


STANDARD LIMB LEADS


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.


Einthoven’s Triangle

Fig. 13-2. Leads I, II, and III axes form Einthoven’s triangle.


Einthoven’s Triangle Around the Heart

Fig. 13-3. Einthoven’s triangle around the heart.


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.


Frontal Plane and Limb Leads

Fig. 13-5. The frontal plane and the limb leads.


PRECORDIAL(CHEST) LEADS


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.


Axis of Six Precordial Leads

Fig. 13-7. The axis of the six precordial leads.


Horizontal Plane and Its Leads

Fig. 13-8. The horizontal plane and its leads.


Electrodes for the Monitoring System MCL

Fig. 13-9. The position of the electrodes for the monitoring system MCL.


THE ECG PAPER


ECG Monitoring Paper

Fig. 13-10. The ECG monitoring paper, with the blocks enlarged to illustrate the minimum units of measurement.


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



Fig. 13-10. The ECG monitoring paper, with the blocks enlarged to illustrate the minimum units of measurement.



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.


Normal ECG Configurations

Fig. 13-12. Normal ECG configurations.


ECG Configuration

Fig. 13-13. The duration of the normal ECG configuration.


The P Wave

• Represents atrial depolarization• Usually symmetrical and upright


The PR Interval

• Represents the total atrial electrical activity


The QRS Complex

• Represents the ventricular depolarization


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.


The ST Segment

• Represents the time between ventricular depolarization and repolarization


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.


The T Wave

• Represents ventricular repolarization, rest, and recovery


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.


The U Wave

• Its origin and mechanism are not known• Often prominent in certain

– Electrolyte disturbances– Medications– Heart diseases


The U Wave

Fig. 13-17. The U wave highlighted (arrow) within the cardiac complex.


Summary of Normal ECG Configurations and Heart Activity


Summary of Normal ECG Configurations and Heart Activity

Table 13-2

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