ECGBy

Dr. Mudassar Ali Roomi (MBBS, M. Phil)

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.

NORMAL ECG

Components of normal ECG

Waves of ECG:

1. P-wave

2. QRS complex

3. T-wave

4. U-wave

Segments of ECG:

1. PQ or PR segment

2. ST segment

Intervals of ECG:

1. PR interval

2. QT interval

3. RR interval

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).

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.

Vent. DEPOLARIZATI

ON

Vent. REPOLARIZATI

ON

T Wave

a positive wave, due to ventricular repolarization

Voltage = 0.2 – 0.3 mV

Duration = 0.16-0.20 sec.

End of the T wave approximately coincides with the end of ventricular systole.

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.)

SEGMENTS OF ECG:

1. PR OR PQ segment: End of P wave to beginning of Q wave. 0.05-0.12 sec

2. ST Segment: End of S WAVE to beginning of T wave. 0.08-0.12 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. *****

INTERVALS OF ECG

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.

QRS Interval

duration of QRS complex: 0.08 to 0.12 sec.

Prolonged in bundle branch blocks and ventricular extra systoles.

VAT (VENTRICULAR ACTIVATION TIME):

Time taken by cardiac impulse to pass from the endocardium to epicardium i.e. 0.03 sec.

this time is between beginning of Q wave and beginning of R wave.

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.

RR INTERVAL

interval between two successive R waves.

It is equal to duration of one cardiac cycle i.e. 0.8 sec.

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:

1. 60/ RR interval or

2. 300/no. of large squares b/w two successive R waves. or

3. 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.