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المنتدى :
المنتدى الطبي - Medical Forum
ECG.. part 1 (normal ECG)
What is meant by ECG
Electrocardiography is the recording of the electric potential that occurs during the cardiac cycle.The changes are conducted by extracellular fluid to the whole body so can be taken by electrodes from the surface of the body
The most common two methods of putting electrodes
bipolar leads) OR standard limb leads)
One positive and one negative lead or connection
Types
lead 1: the positive is connected to left and the negative to the right arm
leadII: the negative is connected to the right arm while the positive to the left leg
LeadIII: the negative to the left arm and the positive to the left leg
unipolar leads; only one electrodes is positive exploring waves and the other electrode is zero potential and this method is also called V
Types
Augmented limb leads: one positive electrodes attached to a limb and the other electrode electrode attached to the other two limbs instead of a zero electrode.this method of connection increases the size of the potentials by about 50% without affecting the configuration of the record. The augmented limb leads are,Avr, Avl, and Avf
unipolar chest leads: there are 6 unipolar chest leads designated v1tov6.they are
V1 and v2: right ventricular chest leads, v3andv4: Septal, interventricular and transitional chest leads and v5and6: left ventricular chest leads
esophageal lead's sometimes used to record the electric potential changes of the posterior surface of the atria
And the recording taken by these leads then by special machine is printed on strip paper or seen on screen like that seen in intensive care unit
The drawing seen is as fig 1
The question now is what PQRST, why some of them are longer and other are inverted
The amount of electric activity is proportional to the mass of muscle from which it originates. Therefore atrial electric activity is smaller than ventricular one
The time which an electric activity takes depends on the type of conduction which occurs. Conduction through the ventricular mass is through conducting system which shortens its time very much. While the small atrial mass takes longer time
The sum of electric activity in any mass (not fibre) of muscle can be representated by an arrow or vector
The vector of atrial activity is oblique downwards and to the left running in the middle of the direction of the two atria, because the two atria are nearly equal in masses
The vector of the septal activity is obliquely upwards and to the right because it is supplied by separate branch of conducting fibres from the left bundle branch
The vector of the ventricular activity is oblique downwards and to the left. it is representing the synchronous activity of the two ventricles. It is more inclined to the left than the direction of the atrial vector, because the left ventricular mass is larger than the right one
And the direction of vector I described above make us draw the curve upwards or downwards...Forexample the B wave is up wards but the Q is downwards
The base can be collected in the following rules
if we know that
Depolarization from right to left is up in drawing, depolarization from left to right is down in drawing,repolarization from right to left is down and repolarization from right to left is up so
If I said to you that B wave is due to depolarization from SAN through both atria and the direction of vector is to the left and down wards so the curve of p wave is upwards (positive) and in the same role u can see rest
R, T, U upwards or positive
Q, S downwards or negative
And we must know that
P…depolarization through atria from SAN.
Q…is through conduction of the depolarization through the interventricular septum.
R…is due to conduction of depolarization through the two ventricles simultaneously.
S…through conduction of the depolarization over the latest fibres of left ventricle.
T…is the positive isoelectric wave following repolarization of ventricles through the myocardium.
the amplitude of waves is through muscle mass and this is clear in R wave in comparison to others
THE BIG QUESTION NOW HOW TO READ ECG
We first must know how to read normal ECG
In the drawing of ECG we must examine 9 points
Heart rate rhythm wave voltage
p wave , pr interval , ST segment , QRS complex
QT interval , T wave
-heart rate. in the paper in which THE ECG is drown it is devised into small divisions each space equals 0.04 second thus each 1500 small space equal one minute. The normal heart rate is range 72 cycle per minute (below 60 is bradycardia and above100 and up to 150 is tachycardia)
rhythm..Means that there is regular beating of heart
voltage..Horizontally each 10 spaces are equal 1 mv
p wave…0.008 second
PR interval from the beginning of P to the beginning of QRS complex...0.18 sec
QRS complex.. Small QS and large R
QT interval from beginning of Q to end of T
ST segment. From end of S to begening of T...0.16 SEC
T wave..0.16 sec.
If we applied the nine previous points on the following ECG fig2:
We will find that
So heart rate is 60/(4.5x5x0.04)
The rhythm is normal
P wave is normal……etc
All other points we defined above (the nine points are normal)
So this is normal patient
look at the first fig
look at the 2nd fig
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