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

SEGMENT—straight 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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