Electrocardiogram (ECG) deals with the study of electrical activity of the heart. The instrument used to record the activity is called electrocardiograph. It was developed by a Dutch physiologist Einthoven in the year 1903. The recording was known as electrokardiogram (EKG). Both ECG and EKG are valid terms that can be used for the recording. The study of ECG, tells us the heart rate, rhythm, conduction in the heart and presence of any abnormalities in them known as arrhythmias. It is also useful to know the presence of infarction in the myocardium and the effect of drugs, electrolytes on the heart.
The ECG waves represent the sum total of tiny action potentials developed from the cardiac muscle. The electrical activity is spread to the surface of the body through the body fluid, which acts as a volume conductor. These electrical potentials from the surface of the body can be recorded by placing surface electrodes or leads, on certain conventional positions in the body. They are amplified and connected to a string galvanometer, which records them on a moving strip of paper or displayed on the screen in cathode ray oscilloscope.
There are 12 leads used in the recording of ECG. Einthoven recorded the electrical activity of the heart by using bipolar limb leads. He considered right arm, left arm and left leg as the regions for surface recording and showed that, when these points are joined, an equilateral triangle could be obtained. In the center of this triangle, the heart is situated. The equilateral triangle obtained by this method is called Einthoven’s triangle. The bipolar limb leads record the potential difference between two limbs. Accordingly, there are three types of leads present.
- Lead I (between right arm and left arm)
- Lead II (between right arm and left leg)
- Lead III (between left arm and left leg).
In the bipolar limb leads, if we know the potentials in any two leads, the potential in the third lead can be determined. According to Einthoven’s law, the sum of the potentials in lead II.
Lead I + Lead III = Lead II
In Unipolar augmented limb leads method, there is an indifferent electrode (V), which is obtained by connecting the three limb leads and passing through 5000 ohms resistance to get 0 potential (Wilson’s terminal). Recording between one limb and the other two limbs increases the size of the potential by 50%. The two limbs are connected through electrical resistance to the negative terminal and the other limb is connected to the positive terminal. There are three types of leads such as aVR, aVL and aVF present in this category.
There is an indifferent electrode (V) and exploring electrode is placed on the anterior chest wall in six positions. They are given numerical numbers from 1 to 6. The leads are V1, V2, V3, V4, V5 and V6. In ECG recording, positive deflection is recorded, when the wave of excitation moves towards the positive or exploring electrode. If the depolarization wave moves away from the exploring electrode, a negative deflection is recorded. In aVR lead, the exploring electrode is facing the cavity of the ventricles and the wave of excitation moves away from the recording electrode and hence in this lead, all the deflections of ECG are negative.