The heart is a muscular pump that supplies blood to the whole body. It functions as two separate units, the atrial and the ventricular compartments.
The two compartments function harmoniously and rhythmically to produce alternate contractions and relaxation of the heart muscle. The important properties of the heart muscle includes excitability, rhythm city, conductivity, contractility and distensibility.
Self excitation of the S.A. node is possible because of it's properties listed below : Resting potential is only -55mv.
Increased leakiness to sodium ions.
Opening of Potassium channels -hyper polarization
The impulse thus produced is transmitted to the whole heart through the conducting system of the heart constituted by,
1. Sinoatrial node
2. Internodal pathways
3. Atrioventricular node
4. Atrioventricular bundle (Bundle of His)
5. Right and left bundle of Purkinje fibres

CONDUCTION OF THE IMPULSE
Normally the interior of the cell is lined by the cell membrane and is negatively charged (-90mv) and the outside is positively charged. This is the resting potential or, the cell is said to be polarized. If at a stage, this is lost or the potential rises from -90 to 0, then this excites a further rise of potential, called the action potential. The action potential is transmitted throughout the cell and forms the impulse. During the rise of potential, the membrane becomes permeable to Sodium ions and the potential rises to a positive direction. This phenomena is called depolarization. Within a few 1/10,000 th of a second following depolarization, the Sodium channels close and there is rapid diffusion of K+ ions into the exterior, reestablishing the normally negative resting membrane potential. This is called re-polarisation. Depolarisation is followed by muscle contraction and repolarisation is followed by muscle relaxation.

An action potential excited at any point on an excitable membrane usually excites adjacent portions of the membrane resulting in the propagation of the potential . The new depolarized areas caused local circuits of current flow still further along the membrane causing more and more depolarization. If the action potential reaches a non-excitable part of the membrane , the spread of depolarization will stop. Repolarisation first begins at the original point of stimulation, moving in the same direction that depolarization had originally spread. Depolarisation is a rapid process whereas repolarisation is a slow process.The transmission of the depolarization wave, called the cardiac impulse is rather by transmission of electrochemical radiation.
As the wave passes through the heart, it sets up electric current which spread into the tissues surrounding the heart and surface of the body. If electrodes are placed on the skin on opposite sides of the heart, these potentials can be read ,thus giving a good account on the functioning of the heart. This record is the Electro Cardio Gram. When the direction of the flow of current is towards a positive electrode, a positive deflection is obtained, and vice versa. If the positive electrode is at right angles to the direction of the current, the wave touches the baseline.
The first ECG was obtained by Wallen in 1887 with the help of a Capillary Electrometer. However in 1903 Einthoven, for the first time used the string galvanometer for recording ECG and came to be known as the Father of Electrocardiography.

USES OF E.C.G.
To analyse abnormal rhythms of the heart .
Detects changes in the myocardium as in IHD
Assess thickness of myocardium - thrombus , growths, calcification
Mitral valve diseases especially mitral stenosis.
Electrical activity of the heart can be ascertained
General metabolic changes - Hypokalaemia, Thyrotoxicosis
Abnormal movements of ventricular walls - Aneurysms.

RECORDING DEPOLARIZATION AND REPOLARISATION WAVES
Depolarisation of Atria
Conduction upto AV node
Depolarisation of Septum
Depolarisation of left ventricle
Depolarisation of right ventricle
Repolarisation of Ventricles
The left ventricle exerts more influence on the ECG pattern when compared to the right ventricle because of the large muscle mass.

RECORDING THE ELECTROCARDIOGRAM - THE E.C.G PAPER
ECG machines record changes in electrical activity by drawing a trace on a moving paper strip. All ECG machines run at a standard rate and use paper with standard sized squares . The electrocardiograph uses thermal paper, which is a graph paper & runs at a speed of 25mm per sec. Time is plotted on the X axis & voltage is plotted on the Y axis. In X axis, 1 second is divided into 5 large squares each of which represents 0.2 sec. Each large square is further divided into 5 small squares which represents 0.04 sec.
The ECG machine is calibrated in such a way that an increase of voltage by 1 mVolt should move the stylus vertically by 1cms. The calibration signal should be included with every record. 1 small square = 1mm = 0.1 mv

ELECTROCARDIOGRAPHIC LEADS - CONVENTIONAL
The electrical signal from the heart is detected at the surface of the body through positive and negative electrodes which are connected to the ECG recorder by wires. The ECG recorder compares the electrical activity detected in the different electrodes, and the electrical picture so obtained is called a lead. The different leads look at the heart from different angles.
In clinical practice there are 12 conventional leads, physiologically divided into two groups.
1. Frontal plane leads.
2. Horizontal plane leads.

Einthoven assumed that the shoulders and groin are points equidistant from each other and form an equilateral triangle with the heart in the center. These points form the sites for placing the electrodes. The heart is situated in the center of the electrical field which it generates by itself. The intensity of this electrical field diminishes algebraically with the distance from the center. Thus the electrical intensity recorded by the electrode diminishes rapidly when the electrode is moved a short distance from the heart. With distances greater than 15 cms from the heart , the intensity of the electrical field is hardly noticeable.Hence, in an electrical sense, electrodes placed at a distance greater than 15 cms away from the heart, may be considered to be equidistant from the heart.

FRONTAL PLANE LEADS
a. Bipolar leads : Standard limb leads
b. Unipolar leads : Augmented unipolar leads are used.
Bipolar leads : These record the actual difference in potential across the two electrodes. There are three standard limb leads.
Positive negative
Lead I Left arm Right arm
Lead II Left foot Right arm
Lead III Left foot Left arm

The lead axes form the sides of an equilateral triangle with the heart at the center ( Einthoven's triangle). The sum total of the potential in the three leads equals zero and mathematically it could be demonstrated that the potential in L II equals sum of the potentials in L I and L III i.e, Einthoven's law.
Unipolar limb leads : Constituted by the indifferent electrode which forms the negative electrode and the exploring electrode which forms the positive electrode. The indifferent electrode is constituted by connecting all limb lead electrodes together through an electrical resistance there by maintaining the zero potential . The positive electrode records the true potential at a given point. Here the record is of low voltage. Goldberger augmented these leads by omitting the connection of the neutral terminal to the limb which is being tested and allowing it to hang free. These leads came to be known as augmented unipolar limb leads, represented by aVR, aVF, aVL leads.

Positive                 Negative
aVR Right arm Left arm + Left foot
aVF Left foot Left arm + Right arm
aVL Left arm Left foot + Right arm
Unipolar chest leads are constituted by an indifferent electrode resulting from a connection between all three standard limb leads and an exploring electrode placed on 6 different points on the chest wall. The indifferent electrode forms the negative terminal &the exploring electrode forms the positive terminal.

Placement of precordial leads.
V 1 - 4th intercostal space , right of sternum.
V 2 - 4th ICS left of sternum
V 4 - 5th ICS midclavicular line
V 3 - Midway between V2 and V4
V 5 - 5th ICS anterior axillary line.
V 6 - 5th ICS mid axillary line.

SIGNIFICANCE OF VARIOUS LEADS AND THEIR LIMITATIONS:Standard limb leads are most valuable for diagnosis of arrhythmia and to study the functional abnormalities of the heart.
Precordial leads are important in the diagnosis of
1. Localisation of recent or old ventricular damage
2. Bundle branch block
3. Detection of ventricular hypertrophy.

Augmented unipolar limb leads help in
1. Determining the position of the heart
2. Conforming the significance of Q and T waves in L 1, II and III
3. Conforming evidence of ventricular damage or hypertrophy.

Leads II,III,aVF-Record changes in the interior or diaphragmatic surface of
the heart.
Lead I & AVL : Record changes from the base &superior left lateral wall
Chest leads : Record changes in the interventricular septum and anterior wall
Conventionally there is no lead which helps in asessing conditions of the posterior wall of the heart. In short, the different leads look at the heart from various angles.

V1,V2, V3, V4 - Antero septal leads
V5, V6 - Apical or lateral leads.

FUNDAMENTAL PRINCIPLES OF ELECTROCARDIOGRAPHY.
1 An electromagnetic force, current or vector has both magnitude and direction. When this force is directed to the positive electrode of a lead, the ECG will record an upward or positive deflection.
2 When the vector is directed away from the positive electrode the ECG will record a downward or negative deflection, if at 90degree to the electrode the wave touches the baseline.
3 Anatomically, the left ventricle is the dominant structure, physiologically, left ventricle and inter ventricular septum constitutes the dominant part,and hence maximally influences variations in ECG.

The amplitude of the wave in any lead is influenced by the myocardial mass, net vector of depolarization, thickness and properties of intervening tissue and the distance between electrode and the myocardium.

HOW TO RECORD THE E.C.G
Good contact between chest wall and electrode is necessary. It might be essential to shave the chest and apply electro cardio graphic jelly.
The patient must lie down and relax to prevent muscle tremor.
Connect up the limb electrodes to the correct limb.
Calibrate the record with 1mv signal.
Record six standard leads : Three or four complexes.
Record six chest leads.

BASIC ELECTROCARDIOGRAPHIC DEFLECTIONS
Letters P, Q, R, S, T & U are used arbitrarily to represent ECG waves. This gives chances to represent new waves if detected in course of time, before or after the waves which have now been detected.

EVENTS RESPONSIBLE FOR AN ECG COMPLEX IN LEAD II
Atrial depolarization : P wave
Spread of impulse to AV node : PQ interval
Septal depolarisation : Q wave
Left ventricular depolarisation : R wave
Right ventricular depolarisation : S wave.
Ventricular repolarisation : T wave
Slow polarisation of I.V. conducting system : U wave.

NOMENCLATURE OF THE WAVE: Generally based on L II
First positive deflection - P wave
First downward deflection - Q wave
First upward deflection in QRS - R wave
Down ward deflection following
R wave whether there is a
preceeding 'Q' or not. - S wave

CHARACTERISTIC WAVE PATTERNS
P WAVE

GENESIS : The P wave is a composite deflection of right and left atrial activation. But, since SA node is situated in the right atrium, right atrium contracts first , followed by the left atrium. But both these overlap and hence produce a single wave. The characters of the P wave are best studied in L II , because the P wave axis coincides with L II where the wave is pyramidal with rounded apex. The duration is not greater than 0.11 sec.( three small squares) and the amplitude is not higher than 2mms ( 2small squares).

QRS COMPLEX
Due to ventricular activation . The relative size of the QRS deflections are usually reflected by upper &lower case lettering i.e. capital and small letters.

r S
R S
R s
R
q R s
Q r
Q S
r S r'
r s R'
R R'

GENESIS
1. Activation of ventricles begin in the lower third of interventricular septum. Then the impulse spreads transversely from left to right where it is opposed by a small force from right to left .The resultant is from left to right.
2. Activation of free walls of both ventricles from subendocardium to subepicardium of both ventricles. The left ventricle dominates.
Net resultant from right to left.
In short ventricular activation is depicted by a small initial vector from left to right through interventricular septum, followed by a large vector from right to left through free walls of the left ventricle.

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