Electrocardiography is concerned with the recording and analysis of the electrical activity of the heart. The instrument used for recording the electrical activity is a sophisticated galvanometer called as electrocardiograph. The graphic record is called as the electrocardiogram (ECG)

HISTORY:
The first ECG was obtained by Wallen in 1887 with the help of a capillary electrometer. Einthoven in 1903 introduced the string galvanometer for recording ECG, which was largely supplanted by vacuum tube amplifier driven oscillograph n 1930s. Cathode ray oscilloscopes are presently being used in ECG monitors for continuous visual display of ECG.

ELEMENTARY ELECTROPHYSIOLOGY:
In a healthy resting muscle cell, certain molecules dissociate into positive and negative ions. The positively charged ions are on the outer surface and the negatively charged ions on the inner surface of the cell membrane. The positive charges are exactly equal in number to the negative charges. When this occurs the cell is in a state of electrical balance and is said to be POLARIZED.

When two electrical charges of equal and opposite directio are juxtaposed on either side of a membrane they constitute a dipole. When two charged ions of equal and opposite direction are situated next to each other on the surface of an excitable tissue, they constitue a doublet.

When the cell is stimulated or injured, the negative ions migrate to the outer surface of the cell and the positive charges pass into the cell i.e. the polarity is reversed. This process is termed as DEPOLARISATION. With recovery, positive charges migrate into the cell. This process is termed REPOLARISATION, i.e. the polarity or the electrical balance of the cell is reestablished.

The chief extra cellular cation (+) is sodium and the chief intracellular cation is potassium. Potassium and Calcium ions and to a less extend the sodium ions have influence in the contractility and excitability of the heart muscles. Depolarization wave in myocardial cells and cells of Purkinge system is brought about by fast inward movement of sodium where as in the SA node and proximal region of AV node it is brought about by slow inward movement of Calcium.
Injured cells emit a continous negative charge to and electrode oriented to its surface.

MEASUREMENT OF THE ELECTRICAL ACTIVITY:

THE INSTRUMENT:
Electrocardiograph is a sophisticated galvanometer, a sensitive electromagnet, which can detect and record changes in electromagnetic potential. It has a positive pole and a negative pole. The wire extensions from these poles have electrodes at each end; a positive electrode at the end of the extension from the positive pole, and a negative electrode at the end of the extension from the negative pole. The paired electrodes together constitute a electrocardiographic lead.

THE RECORD:
The record is called as Electrocardiogram. It is a paper that is divided into smaller and larger squares, which are formed by fainter and darker lines respectively. The squares form a grid which facilitates the measurement of
1. Time parameter (horizontal measurement)

2. Deflexion amplitudes (vertical measurement)
The distance between the fainter lines is 1mm and the distance between two darker lines is 5mm. Conventionally ECG is recorded at the speed of 25mm per second.

THE METHOD:

ELECTRICAL FIELD OF THE HEART
The heart is situated at the centre of the electrical field which it generates. The electrical intensity recorded by an electrode diminishes rapidly when the electrode is moved a short distance from the heart, and less and less as the electrode is moved still further away from the heart. With distances greater than 15 cm from the heart the decrement is hardly noticeable, so all the leads placed at a distance greater than 15 cm are considered to be equidistant from the heart.


ELECTOCARDIOGRAPHIC LEADS (CONVENTIONAL)
There are 12 conventional lead which may be physiologically divided into 2 groups:
1. The frontal plane leads: These are oriented to the frontal or coronal plane of the body. The standard leads I,II,III and leads AVR, AVL AND AVF belongs to this category

2. The horizontal plane leads: these are oriented in the transverse or horizontal plane of the body and are formed by the precordial leads- leads V1 to V6.

1. FRONTAL PLANE LEADS:
THE STANDARD LEADS
Einthoven deliberately placed the electrodes of the three standard limbs as far as away from the heart as possible i.e. on the right arm, left arm and left leg. These three electrodes thus electrically equidistant from the heart.

The leads derived from these three electrodes are:
1. Standard lead I: negative electrode on the right arm and positive electrode on left arm
2. Standard lead II: negative electrode on the right arm and the positive electrode on the left foot
3. Standard lead III: negative electrode on the left arm and the positive electrode on the left foot.
The three lead axis forms an equilateral triangle called as the Einthoven’s triangle.


UNIPOLAR LIMB LEADS
The electrode of a unipolar lead constitutes the exploring electrode which is the positive electrode of the lead. The negative electrode is so constructed that it is considered to be at zero potential. All unipolar leads are termed V leads. Extremity leads are of low electrical potential and are therefore instrumentally augmented. These augmented extremity leads are thus prefixed by the letter “A”.

1. Lead AVR is the augmented unipolar right arm lead
2. Lead AVL is the augmented unipolar left arm lead
3. Lead AVF is the augmented unipolar left leg lead


THE HORIZONTAL PLANE LEADS:
PRECORDIAL (CHEST) LEADS
They are designated by the alphabet ‘V’. There are 6 chest leads (V1 to V6)

1. Lead V1 is placed over the fourth intercostal space immediately to the right of the sternum.
2. Lead V2 is placed over the fourth intercostal space immediately to the left of the sternum.
Please note that the next electrode to be placed is V4
3. Lead V4 is placed over the fifth intercostal space in the mid clavicular line
4. Lead V3 is placed on the chest exactly midway between the lead V2 and V4 electrode positions
5. Lead V5 is placed at the same horizontal level as lead V4 on the anterior axillary line
6. Lead V6 is placed at the same horizontal level as leads V4 and B5 on the mid axillary line

THE BASIC ELECTROCARDIOGRAPHIC DEFLEXIONS
Electrocardiology is based on one essential and fundamental principle, which can be succinctly reflected by two statements

1. When an electromagnetic force (current, vector, activation front, depolarization front) flows, or is directed, towards the positive electrode of a lead, the electrocardiograph will record an upward or positive deflexion.
2. When an electromagnetic force flows, or is directed, away, from the positive electrode of a lead and thus towards the negative electrode of the lead, the electrocardiograph will record a downward or negative deflexion.
Galvanometer writing on paper in zero position; paper moving from left to right.
Representation of vector principle

P WAVE
Definition:
It is the deflection produced by atrial depolarization. It is the sum of the right and the left atrial activation, the right preceding the left because SA node is located in the right atrium.

Features:
1. It is upright in most of the leads except lead AVR. It is best seen in leads L2 and V1. In lead V1 , the P wave is generally biphasic with a small terminal negative deflection produced by atrial activation.
2. The P wave normally does not exceed 0.11 sec in duration or width. The P wave notch is not easily visible.
3. The P wave amplitude or height does not exceed 2.5mm normally
4. The normal P wave axis is in the range of +40 degrees to +60 degrees.


QRS COMPLEX
Definition:
The major positive deflection, from the beginning of the Q wave (or the R wave if no Q wave is visible) to the termination of the S wave is known as the QRS complex. It represents the sequence, time and synchronization of total ventricular muscle depolarization. It is upright in most leads except aVR.

FEATURES:
1. The normal duration on the horizontal axis is .04 to .08sec.
2. The amplitude in the limb leads except aVR should be at least 5mm
S wave is prominent in the right-sided chest leads and R is prominent in the left-sided chest leads. The amplitude of R wave should exceed in the left sided chest leads.
3. The ventricular activation time (V.A.T) should not exceed 0.035 sec in leads V1, V2 and should not exceed 0.055 sec in leads V5 and V6


Q WAVE
Definition:
The Q wave is the initial negative deflection of the QRS complex, which precedes the first positive deflection, the R wave.

FEATURES:
1. They represent septal activation from left to right. It may be observed in the following leads – L1, aVL, V5-V6 with a horizontal heart position. In L2, L3 and aVF with a vertical heart position.

T WAVE
Definition:
The T wave is a deflection following the QRS complex. It is produced by ventricular depolarization. The normal T wave is upright in most leads except aVR, and taller in lead V6 than V1

U WAVE
The U wave is a small positive deflection after the T wave produced by slow and late repolarization of the ventricular Purkinje fibers. It is best appreciated in the precordial transition zone V2 to V4, during a slow rhythm and when the Q-T interval is short in which case it is clearly separable from the T wave.

P – R SEGMENT
Definition:
It is the portion tracing form the end of the P wave to the onset of the QRS complex. The P-R segment is at the same level as the S-T segment, which is the isoelectric line.

S – T SEGMENT
Definition:
It is the portion tracing from the J point (termination of S wave) to the onset of the T wave. Normally it is in the isoelectric line at the same level as P-R segment.

P – R INTERVAL
Definition:
The interval between beginning of P wave and the onset of the QRS complex, irrespective of whether it begins with a Q wave or R wave. The normal P-R interval is in the range 0.12 –0.20 sec.

Q – T INTERVAL
Definition:
It is the interval between the onset of Q wave and the end of the T wave duration represents ventricular repolarization time. It measures the total duration of electrical activity of the ventricles


STANDARDIZATION
The electrocardiograph is conventionally standardized so that one millivolt will result in 10mm vertical deflexion.

THE ELECTRICAL AXIS
The interpretation of the ECG deflexions in terms of axis direction and deviation constitute a most important diagnostic aid to the accurate and deductive evaluation of electrocardiogram. ECG interpretation is then removed from the realm of pure empiricism and becomes elevated to a logical and deductive discipline.

Using leads I and aVF the axis can be calculated to within one of the four quadrants at a glance.
If the axis is in the "left" quadrant take your second glance at lead II.
• both I and aVF +ve = normal axis
• both I and aVF -ve = axis in the Northwest Territory
• lead I -ve and aVF +ve = right axis deviation
• lead I +ve and aVF -ve
o lead II +ve = normal axis
o lead II -ve = left axis deviation

Causes of a Northwest axis (no man's land)
a. Emphysema
b. Hyperkalemia
c. Lead transposition
d. Artificial cardiac pacing
e. Ventricular tachycardia

Causes of right axis deviation
a. Normal finding in children and tall thin adults
b. Right ventricular hypertrophy
c. Chronic lung disease even without pulmonary hypertension
d. Anterolateral myocardial infarction
e. Left posterior hemiblock
f. Pulmonary embolus
g. Wolff-Parkinson-White syndrome - left sided accessory pathway
h. Atrial septal defect
i. Ventricular septal defect
Causes of left axis deviation
a. Left anterior hemiblock
b. Q waves of inferior myocardial infarction
c. Artificial cardiac pacing
d. Emphysema
e. Hyperkalaemia
f. Wolff-Parkinson-White syndrome - right sided accessory pathway
g. Tricuspid atresia
h. Ostium primum ASD
i. Injection of contrast into left coronary artery

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