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To determine the ECG of a given subject


Prior to each contraction an electrical impulse is generated in the S. A. node and thus transmitted to AV node, Bundil of this Purkinje fibres, ventricular muscle fibre and lastly to the surrounding tissue. Body is a volume conductor and heart muscle being the electrode generator with 2 opposite poles bathed in it. Underthis condition electrical impulses which is initiated in cardiac muscle will be transmitted throughout body. If suitable electrodes are placed on the body, opposite to the heart and connected to a very sensitive Galvanometer with a recording device. Then the electrical potential can be recorded. The record is electrocardiogram (E. C. G.). The machine is electrocardiograph by which it is recorded.


12 Leads, recording device, ECG graph paper, cardiogelly


When electrocardiographic connections are made between two parts of the body then the specific arrangement of

each pairof connections is designated as lead. 12 leads are used. The different leads connected are

1. Standard limb lead

2. Chest lead

3. Augmented unipolar limb leads

Standard limb lead

Lead I, Lead II and Lead III

Lead I: Combination will be made by connecting the left arm to the +ve pole of the galvanometer and right arm to

-ve pole of galvanometer. Lead II: The combinations of lead II will be made by connecting right arm to the -ve pole and left leg to +ve pole. Lead III : The combinations will be made by connecting left arm to -ve pole and left leg to -ve pole.

Einthoven’s triangle is an equilateral triangle drawn arbitorately around the area of heart. Einthoven’s law states that, if ECG is recorded through standard limb leads then some total voltage of QRS (ventricular complex) in leads land Ill isequivalent to lead II.

Normal ECG recorded in standard limb lead. ECG recorded simultaneously in 3 leads. It shows mostly similar in shape, contour and feature. QPRT waves are positive waves in all the lead.

Interpretation of ECG

It shows the following 5 consecutive waves PQRST. There are two isoelectric period - shortes are between S and T. P, Rand Tare upward deflections, while Q & S are downward origin. The waves are therefore alternatively up and down. P is of atrial origin hence called atrial complex while QRST being of ventricular complex.

P - Wave

This is first upward deflection. It is small but constant wave having a rounded or pointed top. It is depolarised wave of atria, caused by the passage of atrial current over atria. Its average duration is about .15 1 amplitude .25 my, prepolarization wave of atria is submerged within the ventricular complex.

The impulse arrives at theAV node at about the summit P. Normal P indicate. 1. impulse is originating in SA node. 2. spread over atria in usual direction. 3. There is no deflection conduction.


As soon as impulse arrives at muscular part of septum the latter contracts producing the first wave. Q - T interval

- It measure the ventricular total systolic time measured from onset of Q wave to each of T wave. It is about 0.365.

RS - T interval - sagging indicate hypoxia

T - P interval - It is measured from and of T wave to begning of P wave. T - P interval actually measure diastolic period of heart.

Unipolar limb leads

Electrodes are placed respectively on the left arm, left leg and right arm. They are connected together to form a central terminal which could passes through a suitable resistence and kept almost 0 potential. This is indifferent electrode. Other electrode is placed different part of body is called exploring electrodes, records the local unmodified action current.

The following unipolar leads are Lead V R, Lead VL, Lead VF, Lead VR. Right arm is +ve and left arm and foot is -ve.

When lead is vertical,

VL resembles VR. ‘R’ wave is small, followed by a big S wave, T wave is inverted.

Lead VF - Left foot is +ve, right arm and left arm is -ve. Large R wave followed by S wave T wave is erect.

Augmented unipolar limb leads The unipolar limb leads amplitude are small.

Chest lead

There are six chest lead, Vi, V2, V3, V4, V5, V6. In chest leads electrode is placed on the anterior surface of the chest and connected to posture pole of galvanometers and other electrode the undifferent electrode place anywhere in the body is connected to -ve pole of galvanometer. One such combination makes the pre-cardial positions.

They are:

V1 - fourth intercostal space at 2.54 cm away from right sternal border. V2 - fourth intercostal space at 2.54 cm away from left sternal border. V3 -At the midpoint between V2 and V4.

V4 - Fifth intercostal space at left midclavicular line.

VS -At point where anterior axillary line intersects perpendicularly the horizontal line extended from V4. V6 -At point where midaxillary line intersects perpendicularly the horizontal line extended from V4.


The electrographic connections are made according to the different leads and ECG recorded. Significance of various leads and their limitation 1) Standard limb leads are most valuable for diagnosis of arrythma and also functional abnormalities of heart. 2) Pre - cardial chest leads are important fordiagnosis for a) localisation of recent or old ventricular damage b) bundle branch block c) detection of ventricular hypertrophy 3)Augmental limb leads are most valuable for a) determine the position of heart b) confirming the significance of Q and T wave in standard leads. C) Confirming the evidence of ventricular damage or hypertrophy.

Clinical Importance

It gives a fairly accurate information of the condition of atria and ventricles. In cardiac abnormalities characteristic variations occur in the electrocardiogram along with side by side clinical findings may act as a dependable guide to the diagnosis, prognosis and treatment.


1i. Draw and label the normal ECG from lead II?

2. Enumerate the uses of ECG?

3. Drawthe ECG of the following conditions?

a. Myocardial Infarction b. Complete heart block.