lecture on basics of ECG for 1st year MBBS by Dr. Roomi.pptx

8/12/2019 lecture on basics of ECG for 1st year MBBS by Dr. Roomi.pptx

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ECG

By

Dr. Mudassar Ali Roomi (MBBS, M. Phil)Assistant professor physiology


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Certain conventions to

record ECG.

For 1mV input there mustbe 1 cm deflection ofstylus. 1 ss on Y-axis= 0.1mV

Speed of the ECG paper is25 mm/sec.1 ss on x-axis= 0.04sec. we candouble the speed intachycardias.

Placement of ECG leads atcertain places on thebody.


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NORMAL ECG


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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


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P Wave

it represents atrialdepolarization whichoccurs just before atrial

contraction. The peak of the p wave

coincides with time whencardiac impulse reachesthe AV node.

Amplitudeof P wave is0.10.2 mV and durationis 0.1 sec (2.5 smallestsquares).


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QRS Complex

Due to ventricular depolarization

which occurs just beforeventricular contraction.

Voltage is variable in differentleads. We take the voltage fromthe peak of R wave to the bottomof S wave.

Duration: 0.08 to 0.1 sec (22.5smallest squares)

Q wave is due to depolarizationof upper part of ventricularseptum,

R wave is due to depolarization of

lower part of septum and apex ofheart.

S wave is due to depolarization ofwall and base of the ventricle.

The peak of the R wave coincideswith the beginning of the

ventricular systole.


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Vent.

DEPOLARIZATION

Vent.

REPOLARIZATION


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T Wave

a positive wave, due toventricularrepolarization

Voltage = 0.20.3 mV Duration= 0.16-0.20

sec.

End of the T waveapproximately coincideswith the end ofventricular systole.


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U WAVE

Sometimes there is Uwave after T wave whichis due to slowrepolarization of papillary

muscle U wave becomes

prominent inhypokalemia.

Note: Wave of atrialrepolarization is maskedby QRS complex. (Viva Q.)


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SEGMENTS OF ECG:

1. PR OR PQ segment:End of P waveto 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 isoelectricbecause 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. *****


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INTERVALS OF ECG


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PR Interval

between beginning of P waveand beginning of QRScomplex.

It represents the atrialdepolarization plusconduction 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 inrheumaticfever and different types of AVblocks and also inhypokalemia.*****

PR interval is shortened inaccelerated AV conductionsand in WPW syndrome.

It is approximately equal tointerval between thebeginning of atrial contractionand beginning of ventricular

contraction.


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QRS Interval

durationof QRS

complex: 0.08 to 0.12

sec.

Prolonged inbundle

branch blocks and

ventricular extra

systoles.


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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.


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QT

Interval

From beginning of Q wave toend of T wave.

It represents ventricular

depolarization and ventricularrepolarization

duration= 0.36 to 0.4 sec.

it is approximately equal tothe duration of ventricularsystole.

Shortened in hypercalcemia.

Prolonged in hypocalcaemiaand also in ventricular extrasystole.


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RR INTERVAL

interval between two

successive R waves.

It is equal to duration

of one cardiac cycle i.e.0.8 sec.


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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 followingformulae:

1. 60/ RR interval or

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

3. 1500/no. of smallest squaresbetween two successive Rwaves.

In case of irregular heart rate wecant use this formula. In thatcase we count the number ofheart beats in 6 seconds and

multiply it with ten.


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Einthovens law

Einthovens triangle.

if we know the voltage

of two bipolar leads,

the voltage of 3rdleadcan be found by adding

the remaining two

leads.

i.e. II = I + III


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AUGMENTED unipolar LIMB LEADS

Lead aVR: active electrode is on rightarmand 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 LEFTarm and is connected to positive




make it inactive.

Lead aVF: active electrode is on LEFT

FOOTand is connected to positive




make it inactive.


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Unipolar Chest leads

the active electrode is

placed on the standard

point on the chest and

is connected to thepositive terminal. Three

limbs are connected to

the negative terminal

through high resistanceof 5000 ohms.


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12 lead ECG

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lecture on basics of ECG for 1st year MBBS by Dr. Roomi.pptx