Electrocardiogram (ECG)

The recording of the electrical activity associated with the functioning of the heart is known as Electrocardiogram. ECG is quasi-periodical, rhythmically repeating signal synchronized by the function of the heart; which acts as a generator of bioelectric events. This generated signal can be described by means of a simple electric dipole (pole consisting of a positive electric dipole and negative pair of charge). The dipole generates a field vector, charging periodically in time and space. Its effects are measured on the surface. The waveforms recorded have been standardized in terms of amplitude and phase relationships and any deviations from this would reflect the presence of an abnormality hence the need to understand the electrical activity and the associated mechanical sequences performed by the heart in providing the driving for the circulation of the blood.

The heart has its own system of generating and conducting potentials through a complex change of ionic concentration across the cell membrane. Located in the top right atrium near the entry of Vena cava; are a group of cells known as the Sino-atrial node (SA) that initiate the heart activity and acts as the primary pace maker of the heart. The SA node is 25 to 30 mm in length and 2 to 5 mm thick. It generates impulses at the normal rate of the heart, about 72 beats per minute at rest.

The diagram above shows the position of the Sino-atrial node in the heart, from where the impulses responsible for the electrical activity of the heart originates. The arrow shows the path of the impulse.

The numbers like 0.025, 0.15, 0.16, etc. indicate the time taken for the impulse to travel from the SA node to various paths of the heart.

Because the body acts as a purely resistive medium, the potential field generated by the SA node extends to the other parts of the heart. The wave propagates through the right and left atria at a velocity of about 1 m/s. About 0.1s are required for the excitation of the atria to complete.  The action potential contracts the atrial muscle and the impulse spreads through the atrial wall about 0.04s to the atrio-ventricular node (AV). This node is located in the lower part of the wall between the two atria.

The AV node delays the spread of excitation for about 0.12s, due to the presence of a fibrous barrier of non-excitable cells that effectively prevent its propagation from continuing beyond the limits of the atria. Then, a special conduction system, known as the bundle of His, carries the action potential to the ventricles. The atria and ventricles are thus functionally linked only by the AV node and the conduction system. The AV node delay ensures that the atria complete their contraction before there is any ventricular contraction. The impulse leaves the AV node via the bundle of His. The fibres in this bundle, known as purkinje fibres, after a short distance, split into two branches to initiate action potentials simultaneously in the ventricles.

Conduction velocity in the purkinje fibres is about 1.5 to 2.5 m/s. Since the direction of the impulses propagating in the bundle of His is from the apex of the heart, ventricular contraction begins at the apex and proceeds upward through the ventricular walls. This results in the contraction of the ventricles producing a squeezing action which forces the blood out of the ventricles into the arterial system.

To the trained Clinician, the ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system. The ECG can be used to measure the rate and rhythm of heart beats, the size and position of the heart chambers, the presence of any damage to the heart’s muscle cells or conduction system, .the effects of cardiac drugs and the function of implanted pacemakers.

The PR and PQ interval measured from the beginning of the P wave to the onset of the R or Q wave respectively, marks the time which an impulse leaving the SA node takes to reach the ventricles. The PR interval normally lies between 0.12 to 0.2s, the QRS interval, which represents the time taken by the heart impulse to travel first through the interventricular system and then through the free walls of the ventricles, normally varies from 0.05 to 0.1s.

The T wave represents repolarization of both ventricles. The QT interval therefore, is the period for one complete ventricular contraction (systole). Ventricular diastole, starting from the end of the T wave extends to the beginning of the next Q wave. Typically amplitude of QRS is 1 mV for a normal human heart, when recorded in lead 1 position.

You can also read: How Electrical Potentials are generated in the body

Electrodes which are used to trace the voltage difference at any two areas due to electrical activity of the heart are called leads. There are 2 types of leads:

• Bipolar leads
• Unipolar leads

Bipolar leads: In bipolar leads, ECG is recorded by using 2 electrodes such that the potential difference between the electrodes gives the electrical potential existing between them.

Lead I: Electrodes placed on right arm and left arm

Lead II: Electrodes placed on right arm and left arm

Lead III: Electrodes placed on left arm and left leg

The reference lead taken is the right leg

The block diagram of the ECG shows how the leads (electrodes) that picks up the signals that are fed to the ECG amplifier, which can be then taken to the Digital Storage Oscilloscope (DSO) which stores and analyses the signals digitally.

Related: Principle Parts and Types of ECG Recorders