INTRODUCTION
-ECG refers to graphical recording of electrical activities of heart-electrocardiogram.
-The electrical activity occurs prior to mechanical component of cardiac cycle.
-Electrocardiography is the technique and electrocardiograph is the instrument used to record electrical activities.
• Uses of ECG :
1) To record heart rate ( when heart rate is very high)
2) To record rhythm of heart rate
3) Abnormal condition in the heart ( ectopic phase makers )
4) Cardiac ischemia
5) Heart attack
6)Coronary artery diseases
7) Hypertrophy of ventricles.
•Principle of ECG:
– It is the summated electrical activity of heart. It is recorded from the surface of body by using electrodes which are then connected to a amplifier (ECG machine) and recorded on the paper.
-The graph paper has small squares on the x-axis it shows the time in mili seconds and in y-axis it shows amplitude in mili volts .
On x-axis 1mm=0.04sec
On y-axis 1mm=0.1mv.
•ECG leads :
There are 2 types of leads jn ECG
2)Unipolar leads – 1 electrode is indifferent / inactive and 2 nd electrode is active.
-These leads are made by placing series of electrode on the surface of body.
-These electrodes are connected to ECG machine.
-There are total 12 leads including bipolar and unipolar leads.
*Bipolar leads :
In bipolar leads there are 3 limb leads : lead 1, lead 2 , lead 3.
Einthoven’s principle :
-Here bipolar leads are used for the first time by Einthoven.
-He developed lead 1, lead 2 and lead 3.
-The electrodes are connected to right arm, left arm, and left foot.
-The electrodes are connected which makes a triangle heart is assumed as the centre of triangle.
Lead 1 :
It is between left arm and right arm
Left arm is positive
Right arm is negative.
Lead 2 :
It is between right arm and left foot
Left foot is positive
Right arm is negative.
Lead 3 : It is between left arm and left foot.
Left foot is positive
Left arm is negative.
*Unipolar leads :
In unipolar leads there are three unipolar augmented limb leads and unipolar chest leads.
Augmented limb leads :
These are aVL , aVR , and aVF.
They are recorded by the same leads but recordings are amplified.
The indifferent electrode is connected through any 2 limb and the active electrode is connected to the respective limb.
Example:
In aVR – active electrode is connected to right arm.
In aVF – active electrode is connected to left foot.
In aVL – active electrode is connected to left arm.
Chest leads :
They are unipolar leads also known as precardial chest leads.
There are 6 chest leads beginning from V1 to V6.
Here indifferent electrode is connected through all the 3 limb leads.
V1 – It is located in the right 4 th intercostal space just on the right border of sternum.
V2 – lt is located in the left 4 th intercostal space just on the left border of sternum.
V3 – It is located in between V2 and V4.
V4 – It Is located in the left 5 th intercostal space at the midclavicular line.
V5 – It is located in the left 5 th intercostal space in the anterior axillary line.
V6 – lt is located in the left 5 th intercostal space in the mid axillary line.
• Waves of ECG :
There are S waves on ECG.
They are Q, R, S, T.
Sometimes 6 th wave is seen on ECG.
Q, R and S waves together form QRS complex.
P wave :
It is a first positive wave in ECG.
I represents atrial depolarization.
The duration is 0.1 sec .
The amplitude is 0.12 mV.
Significance – P wave is absent in atrial fibrillation , SA node block.
P wave is increased in amplitude in hypokalemia.
QRS complex :
It has both positive and negative waves
Q wave is initial , small, negative.
R wave is large and positive.
S wave is negative wave following R wave .
In QRS complex Q represents depolarization of base of ventricles.
R wave is produced due to depolarization of ventricular muscle and apex of heart .
S wave represents depolarization of base of ventricles at the AV ring.
Duration of QRS is 0.1 sec .
Amplitude in Q wave is 0.1 mV, R wave is 1 mV, S wave is 0.4 mV.
Significance – QRS complex is prolonged in hyperkalemia QRS complex us prolonged also in bundle branch block.
U wave :
It is not usually seen when it is seen it is a positive wave.
It indicates bradycardia, thyrotoxicosis, hypercalcemia, and hypokalemia.
• Intervals of ECG :
(1) P-R interval :
It is the duration of time between beginning of P wave to the beginning of QRS complex.
Normal duration is about 0.18 sec.
P-R interval represents arterial depolarization and conduction through AV node.
P-R interval shorten when heart rate increases.
P-R interval increases in decrease in heart rate , first degree heart block and in AV nodal delay.
(2) Q-T interval :
It is the time between onset of Q wave to the end of T wave.
Normal duration is 0.4 sec.
It indicates depolarization and repolarization.
Q-T interval is prolonged in myocardial infarction, hypothyroidism , and hypocalcemia .
Q-T interval is shortened in hypocalcemia.
(3) S-T segment :
It represents ventricular repolarization .
It is a time between end of S wave and the onset of T wave.
Duration is 0.08sec.
S-T segment is shortened in hypercalcemia.
S-T segment is prolonged in hypocalcemia.
S-T segment is depressed in acute myocardial infarction.
(4) R-R interval :
It is the duration between 2 successive R waves.
R-R interval is used to count heart rate when it increases more than 100 per minute .
R-R interval signifies duration of cardiac cycle .
Normal R-R interval is 0.8 sec.
When R-R interval decreases by 0.5 sec , cardiac cycle significantly reduced and heart rate tremendously increased.
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