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Electrocardiogram (ECG)

Definition:

Electrocardiogram is graphic recording of the heart electrical potential spread over the

surface of the body

It is based on a concept named Dipole.

A Galvanometer can record the potential of the dipole.

Amount of the potential is related to:

1. surface of the dipole

2. kind of the charge face the recording electrode

3. conductivity of the medium

4. orientation of the electrode with respect to a line parallel to the dipole

5. the angle of axis of recording electrode to the surface of the dipole

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Depolarization

Depolarization and repolarization

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Electrocardiogram is recorded by:

1. Unipolar leads: VR, VL, VF and precordial V1 to V6

2. Bipolar leads: I, II and III

3. Augmented leads: aVR, aVL and aVF

a. aVR = RA - 2

LALL

b. aVR = RA +

2

LALL

c. RA + LA + LL = 0

d. Thus RA = -(LA + LL)

e. b&d aVR = RA + 2

RA= 3/2RA ( 3/2 VR)

Diagnostic value:

1. Hypertrophy of ventricle and atrium.

2. Conduction velocity.

3. Myocardial infarction and ischemia.

4. Disturbances of electrolyte balance.

5. Nodal conduction velocity.

6. Effects of antiarrhythmic agents.

The items which could be explored by ECG:

1. characteristics ( amplitude and duration) of each wave, complex, segment and interval

2. heart rate

3. heart rhythm

4. vector of the heart

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Waves, segments or

intervals Amplitude(mv) Duration(sec) Significance

P wave 0.15 0.08 Depolarization of atria

QRS complex 1.5 0.11 Depolarization of ventricles

T wave 0.3 0.16 Repolarization of ventricles

PQ(PR) segment 0 0.08 Atria stay in depolarization

ST segment 0 0.16 Ventricles stay in depolarization

PQ interval ---- 0.12-0.20 Time passing current from SA node to the

ventricular septum

QT interval ---- 0.36 Total action potential time of ventricles

QTc (corrected)1 ---- 0.42 in men

0.43 in women Power and performance of the heart

Vector of the heart:

Definition: sum of cardiac loop in the frontal plane

Normal values: -30 to +110º mostly 59º

Diagram of frontal leads

aVR aVL

I

III II

aVF

1 QTc =

RR

QT int

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Instruments:

Electrocardiograph and related accessories consist of wires and electrolyte gel.

Procedure:

1. Explain completely about the experiment to the volunteer.

2. The best time for taking ECG is 2 hours after meal. Too hunger or right after meal

may simulate some pathological conditions.

3. Optimal temperature of the room is about 25oC

.

4. Ask about probable medicines that the volunteer may has been used.

5. The volunteer should remain lying down.

6. Remove all metal instruments from the volunteer.

7. Clean the surface of the electrodes and extremities using a piece of alcoholic cotton.

8. Connect the electrodes to the extremities using a drop of electrolyte gel.

9. Record leads respectively.

Questions:

1. Explain about the experiment

2. Explain about right axis deviation and left axis deviation.

3. Draw a table and measure the waves, segments and intervals from the sample ECG,

and compare them with normal values.

4. Calculate and draw vector of the heart.

5. How important is QTc?

Dr. H. Khorrami Ph.D.