1. By Dr. Mudassar Ali Roomi (MBBS, M. Phil)
ECG By
Dr. Mudassar Ali Roomi (MBBS, M. Phil)

2. Certain conventions to record ECG.
For 1mV input there must be 1 cm deflection of stylus. 1 ss on Y-axis= 0.1 mV
Speed of the ECG paper is 25 mm/sec. 1 ss on x-axis= 0.04sec. we can double the speed in tachycardias.
Placement of ECG leads at certain places on the body.

3. NORMAL ECG

4. Components of normal ECG
Waves of ECG:
P-wave
QRS complex
T-wave
U-wave
Segments of ECG:
PQ or PR segment
ST segment
Intervals of ECG:
PR interval
QT interval
RR interval

5. P Wave
it represents atrial depolarization which occurs just before atrial contraction.
The peak of the p wave coincides with time when cardiac impulse reaches the AV node.
Amplitude of P wave is 0.1 – 0.2 mV and duration is 0.1 sec (2.5 smallest squares).

6. QRS Complex
Due to ventricular depolarization which occurs just before ventricular contraction.
Voltage is variable in different leads. We take the voltage from the peak of R wave to the bottom of S wave.
Duration: 0.08 to 0.1 sec (2 – 2.5 smallest squares)
Q wave is due to depolarization of upper part of ventricular septum,
R wave is due to depolarization of lower part of septum and apex of heart.
S wave is due to depolarization of wall and base of the ventricle.
The peak of the R wave coincides with the beginning of the ventricular systole.

7. Vent. REPOLARIZATION
Vent. DEPOLARIZATION

8. T Wave a positive wave, due to ventricular repolarization
Voltage = 0.2 – 0.3 mV
Duration = sec.
End of the T wave approximately coincides with the end of ventricular systole.

9. U WAVE
Sometimes there is U wave after T wave which is due to slow repolarization of papillary muscle
U wave becomes prominent in hypokalemia.
Note: Wave of atrial repolarization is masked by QRS complex. (Viva Q.)

10. SEGMENTS OF ECG:
PR OR PQ segment: End of P wave to beginning of Q wave sec
ST Segment: End of S WAVE to beginning of T wave sec
These two segments are isoelectric because there is no current flow in heart.
In PR segment comes when atria are completely depolarized.
In ST segment ventricles are completely depolarized.
CLINICAL: In acute myocardial infarction (MI) there is elevation of ST segment due to flow of injury current which flows between the area of infarction and normal myocardium. *****

11. INTERVALS OF ECG

12. PR Interval between beginning of P wave and beginning of QRS complex.
It represents the atrial depolarization plus conduction through AV node. AV conduction is also included in PR interval.
Duration: On average it is 0.16 sec. range is 0.12 to 0.2 sec.
It is prolonged in rheumatic fever and different types of AV blocks and also in hypokalemia.*****
PR interval is shortened in accelerated AV conductions and in WPW syndrome.
It is approximately equal to interval between the beginning of atrial contraction and beginning of ventricular contraction.

13. QRS Interval duration of QRS complex: 0.08 to 0.12 sec.
Prolonged in bundle branch blocks and ventricular extra systoles.

14. VAT (VENTRICULAR ACTIVATION TIME):
Time taken by cardiac impulse to pass from the endocardium to epicardium i.e sec.
this time is between beginning of Q wave and beginning of R wave.

15. QT Interval From beginning of Q wave to end of T wave.
It represents ventricular depolarization and ventricular repolarization
duration = 0.36 to 0.4 sec.
it is approximately equal to the duration of ventricular systole.
Shortened in hypercalcemia.
Prolonged in hypocalcaemia and also in ventricular extra systole.

16. RR INTERVAL interval between two successive R waves.
It is equal to duration of one cardiac cycle i.e. 0.8 sec.

17. Calculation of heart rate from ECG
If the heart rate is regular then by RR interval, we can find out the heart rate by the following formulae:
60/ RR interval or
300/no. of large squares b/w two successive R waves. or
1500/no. of smallest squares between two successive R waves.
In case of irregular heart rate we can’t use this formula. In that case we count the number of heart beats in 6 seconds and multiply it with ten.