ELECTRO CARDIOGRAM ECG

BASIC PRINCIPLE

LEARNING OBJECTIVES

At the end of the lecture the student should be able to know

About ECG

Basic principle in generation of ECG waves

About ECG paper and its calibration

About different leads and their arrangement

About different components of ECG

ELECTROCARDIOGRAM ECG

Graph of electrical activity of heart

Gold standard for diagnosis of cardiac arrhythmias

Helps detect electrolyte disturbances (hyper- &

hypokalemia)

Allows for detection of conduction abnormalities

Screening tool for ischemic heart disease during stress

tests

Helpful with non-cardiac diseases (e.g. pulmonary embolism or

hypothermia

PROPAGATING ACTIVATION WAVEFRONT

At rest, cells have a negative transmembrane

voltage surrounding media is positive

When cells depolarize, they switch to a positive

transmembrane voltage surrounding media

becomes negative

This leads to a propagating electric vector

(pointing from negative to positive)

PROPAGATING ACTIVATION WAVEFRONT

BASIC PRINCIPLE OF ECG RECORDING

Wave of depolarization traveling towards a positive

electrode causes an upward deflection on the ECG

Wave of depolarization traveling away from a

positive electrode causes a downward deflection on

the ECG.

COMPONENT OF ECG

WAVES

P atrial depolarization

QRS complex ventricular depolarization

T ventricular repolarization

SEGMENTstraight line between waves

S-T segment

end of ventricular depolarization to start of vent.

Repolariztion

INTERVAL---wave+segment

P-R interval

Q-T interval

RECORDING ECG

ECG PAPER

Recorded on a calibrated graph paper

Smallest divisions are 1mm square in both X and Y axis

Runs at a paper speed of 25 mm/sec

X axis represent time

Y axis represents voltage

On Y axis 1 mm=0.1 mV

On X axis 1mm =0.04 sec

VERTICAL AXIS

1 small square = 0.1 mV

10 small square =10 mV

1 big square = 5 small square = 0.5 mV

2 big square = 10 smallsquare= 1 mV

ECG PAPER HORIZONTAL AXIS

1 inch = 1 second

Each inch is divided by dark black lines into 5 big square

Each big square= 1/5=0.2 sec

Each big square is further divided into 5 small square

1 small square = 0.2/5= 0.04 sec

1smallsquare = 0.04 sec =1mm

1 second =25 smallsquare

60second =1min =25 X 60=1500 small square =300 big square

ECG PAPER CALIBRATION

ECG LEADS SYSTEM

Lead

two electrodes which are placed on body surface and connected to

ECG machine for measuring the potential fluctuations between two

points

Standard ECG has 12 lead system

6 limb leads

6 chest leads

Another classification is

Bipolar leads---- standard limb leads

Unipolar leads---- augmented limb leads, chest leads

BIPOLAR LIMB LEADS

1 positive 1 neagtive elecrode

Right arm always negative

Left leg always positive

L1 between LA(+) and RA(-)

L2 between LF(+) and RA(-)

L3 between LF(+) and LA(-)

UNIPOLAR LEADS

1 positive electrode

1negative reference point

Summation of 2 negative leads

Augmented unipolar limb leads

aVL,aVF, aVR

Verticalplane

Precordialchest leads

V1-V6

horizontalplane

AUGMENTED LIMB LEADS

aVR

from Right Arm Positive

Other 2limbs negative

aVL

from Left Arm Positive

RA and LF negative

aVF

from Left Foot Positive

RA and LA negative

PRECORDIAL (CHEST) LEAD

POSITION

V1 Fourth ICS, right sternal border

V2 Fourth ICS, left sternal border

V3 Equidistant between V2 and V4

V4 Fifth ICS, left Mid clavicular Line

V5 Fifth ICS Left anterior axillary line

V6 Fifth ICS Left mid axillary line

ECG INFORMATIONIN DIFFERENT LEADS

The 12 leads allow tracing of electric vector in all three

planes of interest

SUMMARY OF LEADS

Limb Leads Precordial Leads

Bipolar I, II, III

(standard limb leads)

-

Unipolar aVR, aVL, aVF (augmented limb leads) V1-V6

ANATOMIC GROUPS

P WAVE

Depolarization of both atria;

First upward deflection

Duration less than 0.1 sec

Followed by QRS complex

Shape and duration of P may indicate atrial

enlargement

PR INTERVAL

From onset of P wave to onset of QRS

Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

Represents atria to ventricular conduction time (through His bundle)

Prolonged PR interval may indicate a 1st degree heart block

QRS COMPLEX

Represents ventricular depolarization

Larger than P wave because of greater muscle mass

of ventricles

Normal duration = 0.08-0.12 seconds

Composed of Q, R ,and S wave

Q wave

negative deflection after P wave

Depolarization of septum

Not always seen

R wave

First positive wave after Q or P

S wave

Negative

J POINT

Point where QRS complex returns to isoelectric line

Beginning of ST segment

ST SEGMENT

Early repolarization of ventricles

From Jpoint to onset of T wave

T WAVE:

Represents repolarization or recovery of ventricles

Interval from beginning of QRS to apex of T is referred to

as the absolute refractory period

QT INTERVAL

Measured from beginning of QRS to the end of the T

wave

Normal QT is usually about 0.40 sec

QT interval varies based on heart rate

EVENTS IN FORMATION OF ECG WAVES

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