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-wave2. QRS complex3. T-wave4. U-wave Segments of ECG:

1. PQ or PR segment2. ST segment Intervals of ECG:

1. PR interval2. QT interval3. 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 or2. 300/no. of large squares b/w

two successive R waves. or3. 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.

Standard Bipolar limb leads.

Lead I: Right arm negative terminal, left arm positive terminal

Lead II: right arm negative, left foot positive

Lead III: left arm negative, left foot positive

Einthoven’s law

Einthoven’s triangle.

if we know the voltage of two bipolar leads, the voltage of 3rd lead can be found by adding the remaining two leads.

i.e. II = I + III

AUGMENTED unipolar LIMB LEADS

Lead aVR: active electrode is on right arm and is connected to positive terminal and the other two limbs are connected through a high resistance to the negative terminal of machine to make it inactive.

Lead aVL: active electrode is on LEFT arm and is connected to positive terminal and the other two limbs are connected through a high resistance to the negative terminal of machine to make it inactive.

Lead aVF: active electrode is on LEFT FOOT and is connected to positive terminal and the other two limbs are connected through a high resistance to the negative terminal of machine to make it inactive.

Unipolar Chest leads

the active electrode is placed on the standard point on the chest and is connected to the positive terminal. Three limbs are connected to the negative terminal through high resistance of 5000 ohms.

12 lead ECG