The Standard 12-ECG System CHAPTER 13 The Standard 12-ECG System

Consists of four limb electrodes and six chest electrodes 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: Views the electrical activity of the heart from a different angle Has a positive and negative component 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.

Large square = 5 sm. Sq. = 0.20 sec. Speed = 5 lg. Sq./sec. 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

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

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.

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.

Its origin and mechanism are not known Often prominent in certain 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