About ECG

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Overview of ECG

Cardiovascular disease (CVD) is the world’s leading source of death and disability. In the US alone, costs for CVD diagnostic tests approached several billion dollars. While a variety of technologically advanced diagnostic tests are available, the simple, well established 12 lead electrocardiogram (ECG) is cardiology's most widely used screening tool. Over 200 million ECGs are performed annually in developed nations. The electrocardiogram (ECG) is the most commonly conducted cardiovascular diagnostic procedure and a fundamental tool of clinical practice. It is indeed considered the "first choice" procedure in the evaluation of patients with chest pain, dizziness, or syncope.

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Clinical practice usage

   * Diagnosis and prompt initiation of therapy in patients with acute coronary syndromes (myocardial infarction included)
   * The most accurate means of diagnosing intraventricular conduction disturbances and Arrhythmia
   * May lead to the recognition of electrolyte abnormalities, particularly of serum potassium and calcium
   * Permits the detection of some forms of genetically mediated electrical or structural cardiac abnormalities
   * Routinely used to monitor patients treated with antiarrhythmic and other drugs
   * Routinely used to monitor patients in the preoperative assessment of patients undergoing noncardiac surgery
   * Used for screening individuals in high-risk occupations, and sports.


Introduction

In the section about the anatomy and physiology we explained how the cardiac impulse passes through the heart, but while doing so electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram.

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Placement of electrodes for 12 lead ECG

Ten electrodes are used for a 12-lead ECG. They are labeled and placed on the patient's body as follows:

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Electrode label (in the USA) Electrode placement

RA On the right arm, avoiding bony prominences.

LA In the same location that RA was placed, but on the left arm this time.

RL On the right leg, avoiding bony prominences.

LL In the same location that RL was placed, but on the left leg this time.

V1 In the fourth intercostal space (between ribs 4 & 5) just to the right of the sternum (breastbone).

V2 In the fourth intercostal space (between ribs 4 & 5) just to the left of the sternum.

V3 Between leads V2 and V4.

V4 In the fifth intercostal space (between ribs 5 & 6) in the mid-clavicular line (the imaginary line that extends down from the midpoint of the clavicle (collarbone).

V5 Horizontally even with V4, but in the anterior axillary line. (The anterior axillary line is the imaginary line that runs down from the point midway between the middle of the clavicle and the lateral end of the clavicle; the lateral end of the collarbone is the end closer to the arm.)

V6 Horizontally even with V4 and V5 in the midaxillary line. (The midaxillary line is the imaginary line that extends down from the middle of the patient's armpit.)

The electrocardiogram is composed of waves and complexes. Waves and comlexes in the normal sinus rhythm are the P wave, PR Interval, PR Segment, QRS Complex, ST Segment, QT Interval and T wave.

ECG trace with grid.gif


P wave: During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG. Duration :80ms

PR segment: The PR segment connects the P wave and the QRS complex.Duration:150 to 200ms

QRS complex :The QRS complex is a recording of a single heartbeat on the ECG that corresponds to the depolarization of the right and left ventricles.70 to 110ms

ST segment:The ST segment connects the QRS complex and the T wave.80 to 120ms

T wave: The T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period (or vulnerable period).160ms

PR interval :The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.120 to 200ms

ST interval :The ST interval is measured from the J point to the end of the T wave.320ms

QT interval :The QT interval is measured from the beginning of the QRS complex to the end of the T wave. 300 to 440ms

U wave: The U wave is not always seen. It is typically small, and, by definition, follows the T wave.



The P Wave P waves are caused by atrial depolarization. In normal sinus rhythm, the SA node acts as the pacemaker. The electrical impulse from the SA node spreads over the right and left atria to cause atrial depolarization. The P wave contour is usually smooth, entirely positive and of uniform size. The P wave duration is normally less than 0.12 sec and the amplitude is normally less than 0.25 mV. A negative P-wave can indicate depolarization arising from the AV node.

Note that the P wave corresponds to electrical impulses not mechanical atria contraction. Atrial contraction begins at about the middle of the P wave and continues during the PR segment. The PR Segment PR segment is the portion on the ECG wave from the end of the P wave to the beginning of the QRS complex, lasting about 0.1 seconds. The PR segment corresponds to the time between the end of atrial depolarization to the onset of ventricular depolarization. The PR segment is an isoelectric segment, that is, no wave or deflection is recorded. During the PR segment, the impulse travels from the AV node through the conducting tissue (bundle branches, and Purkinje fibers) towards the ventricles. Most of the delay in the PR segment occurs in the AV node. Although the PR segment is isoelectric, the atrial are actually contracting, filling the ventricles before ventricular systole.

The QRS Complex In normal sinus rhythm, each P wave is followed by a QRS complex. The QRS complex represents the time it takes for depolarization of the ventricles. The Q wave is not always present. The R wave is the point when half of the ventricular myocardium has been depolarized. The normal QRS duration range is from 0.04 sec to 0.12 sec measured from the initial deflection of the QRS from the isoelectric line to the end of the QRS complex.

Normal ventricular depolarization requires normal function of the right and left bundle branches. A block in either the right or left bundle branch delays depolarization of the ventricles, resulting in a prolonged QRS duration.

The ST Segment The ST segment represents the period from the end of ventricular depolarization to the beginning of ventricular repolarization. The ST segment lies between the end of the QRS complex and the initial deflection of the T-wave and is normally isoelectric. Although the ST segment is isoelectric, the ventricules are actually contracting.

The T Wave The T wave corresponds to the rapid ventricular repolarization. The wave is normally rounded and positive.

You might ask where is the the wave representing repolarization of atria? It is there, but you just don't see it because it occurs at the same time as the QRS complex and T wave.

References

1. Guyton and Hall - Textbook of Medical Physiology (external link)

2. Bullock, Boyle, Wang. Physiology. 4th edition.

3. ganfyd.org (external link) - The free medical knowledge base

4. WikiMD (external link) - the free medical encyclopedia

5. OpenECGproject

6. http://library.med.utah.edu/kw/ecg/ecg_outline/Lesson1/lead_dia.html

7. Electrocardiography. (2009, December 13). In Wikipedia, The Free Encyclopedia. Retrieved 06:41, December 15, 2009, from http://en.wikipedia.org/w/index.php?title=Electrocardiography&oldid=331384422


ECG Leads

Introduction

When we talk about ECGs, we usually refer to 12-lead ECG. This ECG is most useful to doctors, but sometimes in emergencies or for monitoring, only 3-lead ECG is recorded. It basically depends what you want to know. If it is only diagnosis of rhythm problems, then any lead will do. But when one wants to diagnose damage in the ventricular or atrial muscle or in the Purkinje conducting system, 12-lead ECG is the king.

The standard 12-lead ECG consists of 3 bipolar limb leads (I, II and III), 3 augmented limb leads (aVR, aVL, and aVF), and 6 chest leads (V1-V6).

Three Bipolar Limb Leads

The term "bipolar" means that the electrocardiogram is recorded from two electrodes located on different sides of the heart, in this case, on the limbs. Thus, a "lead" is not a single wire connecting from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph.

Lead I In recording limb lead I, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left arm. Therefore, when the point where the right arm connects to the chest is electronegative with respect to the point where the left arm connects, the electrocardiograph records positively, that is, above the zero voltage line in the electrocardiogram. When the opposite is true, the electrocardiograph records below the line.

Lead II To record limb lead II, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left leg. Therefore, when the right arm is negative with respect to the left leg, the electrocardiograph records positively.

Lead III To record limb lead III, the negative terminal of the electrocardiograph is connected to the left arm and the positive terminal to the left leg. This means that the electrocardiograph records positively when the left arm is negative with respect to the left leg.


Leads.gif

Recordings of the electrocardiograms in leads I, II, and III.

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It is obvious that the electrocardiograms in these three leads are similar to one another because they all record positive P waves and positive T waves, and the major portion of the QRS complex is also positive in each electrocardiogram.

Chest Leads

Often electrocardiograms are recorded with one electrode placed on the front surface of the chest directly over the heart. This electrode is connected to the positive terminal of the electrocardiograph, and the negative electrode, called the indifferent electrode, is connected through equal electrical resistances to the right arm, left arm, and left leg all at the same time. Usually six standard chest leads are recorded, the chest electrode being placed sequentially at the six points. The different recordings are known as leads V1, V2, V3, V4, V5, and V6.

Leads3.jpg


Recordings of the electrocardiograms in leads V1-V6.

Leads4.jpg'


Because the heart surfaces are close to the chest wall, each chest lead records mainly the electrical potential of the cardiac musculature immediately beneath the electrode. Therefore, relatively minute abnormalities in the ventricles, particularly in the anterior ventricular wall, can cause marked changes in the electrocardiograms recorded from individual chest leads.

Augmented Limb Leads Another system of leads in wide use is the augmented unipolar limb lead. In this type of recording, two of the limbs are connected through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive terminal. When the positive terminal is on the right arm, the lead is known as the aVR lead; when on the left arm, the aVL lead; and when on the left leg, the aVF lead.

Normal recordings of the augmented unipolar limb leads.

Leads5.jpg

They are all similar to the standard limb lead recordings, except that the recording from the aVR lead is inverted.

References

  1. Guyton and Hall - Textbook of Medical Physiology (external link)
  2. Bullock, Boyle, Wang. Physiology. 4th edition.
  3. OpenECGProject.
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