The principle and significance:

The Electrocardiograph (ECG): It is clinically very useful, as it shows the electrical activity within the heart, simply by placing electrodes at various points on the body surface. This enables clinicians to determine the state of the conducting system and of the myocardium itself, as damage to the myocardium alters the way the impulses travel through it.


An electrocardiograph is an instrument that measures and records the electrocardiogram (ECG), the electrical activity generated by the heart. Electrodes placed on various anatomical sites on the body help in conducting the ECG to the electrocardiograph.

The ECG alone is not sufficient to diagnose all abnormalities possible in the pacing or conduction system of the heart. The interpretation of the 12-lead ECG provides a differential diagnosis for many arrhythmias.

Lead systems

Twelve leads usually comprise a diagnostic ECG recording six limb leads (three bipolar and three unipolar) and six unipolar precordial leads. The instantaneous cardiac scalar voltages resulting from the electrical activity in the heart is measured in each of the 12 leads. Since the cardiac vector varies in magnitude with time over a three-dimensional space, it is important to know its presentation (i.e. appearance or projection) in each of the 12 leads of the ECG. The lead placement to acquire the 12-lead ECG. The leads can be categorized into the frontal leads (I, II, III, aVR, aVL and aVF) and the transverse leads (V1, V2, V3, V4, V5 and V6). The frontal leads measure the projection of the cardiac vector on the frontal plane of the body. The frontal plane is parallel to the floor when lying supine. The transverse or precordial leads measure the projection of the cardiac vector on the horizontal plane (i.e. the plane that is parallel to the floor when standing). Leads I, II and III of the frontal plane are bipolar. They record the differences between two points on the body.

Leads aVR (on the right arm), aVL (on the left arm) and aVF (on the foot) are unipolar leads. They measure the potential difference on the limbs with respect to a reference point formed by the two resistors between limb electrodes. For example, lead aVR is measured between an electrode on the right arm and a reference point formed via a resistor to the left arm and another resistor to the left foot. These leads show the cardiac vector projection on the frontal plane and are amplified by about 50% (i.e. augmented) so that their amplitudes are comparable to those of the bipolar leads. The six precordial leads, V1 to V6, are unipolar and horizontal plane. These precordial leads are measured with respect to the Wilson central terminal, which is formed by a three resistor network. V1 and V2 are placed on the fourth intercostal space to the right and left, respectively, of the sternum. The V4 electrode is placed on the fifth intercostal space at the left mid-clavicular line. The V3 electrode lies between V2 and V4. Electrode V5 is placed to the left of V4 on the anterior axillary line and V6 is placed on the same level as V5 on the mid-axillary line. It is important to account for the position of these electrodes when interpreting the ECG on leads V1 through V6. The precordial leads measure the potential between each of V1 through V6 and the Wilson‘s central terminal formed. The 12-lead ECG provides various viewpoints of the three-dimensional instantaneous cardiac vector that are somewhat redundant and this is helpful in providing discriminatory information for diagnosing abnormalities in the pacing and conduction system of the heart.

Different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricle. The P wave represents atrial depolarization there is little muscle in the atrium so the deflection is small. The Q wave represents depolarization at the bundle of His; again, this is small as there is little muscle there. The R wave represents the main spread of depolarization, from the inside out, through the base of the ventricles. This involves large amounts of muscle so the deflection is large. The S wave shows the subsequent depolarization of the rest of the ventricles upwards from the base of the ventricles. This S wave of the normal human ECG originates due to late depolarization of the ventricular walls moving back towards the AV junction (junction between the atrioventricular node nd the nonbranching portion of the bundle of His. The T wave represents repolarization of the myocardium after systole is complete. This is a relatively slow process- hence the smooth curved deflection.

When looking at an ECG, it is often helpful to remember that an upward deflection on the ECG represents depolarization moving towards the viewing electrode and a downward deflection represents depolarization moving away from the viewing electrode

Significance Electrocardiogram captures the electrical activity of the heart, so it can show the following conditions

  1. Myocardial Infarction (Electrical changes: ST elevation or ST depression).
  2. Re-entry pathways in the electrical system (generate different kinds of waves in the ECG).
  3. Location of the heartbeat initiator (there are many which can create a heartbeat).
  4. Types of cardiac arrest (Ventricular Fibrillation, A Systole, Pulse less Ventricular Tachycardia) and other different changes in the electrical activity of the heart.