The passage of cardiac impulse through the heart spreads electrical current into the tissues around the heart, and a small portion spreads throughout the surface of the body. These electrical charges can be recorded all along the cardiac cycle. The recording of electrical potential generated by the spread of cardiac impulse, is called electrocardiogram (ECG). In fact, ECG is a graphic record of the electric current produced by the excitation of the cardiac muscles.
A normal electrocardiogram is composed of a P wave, a QRS complex and a T wave. The QRS complex has three separate Q, R and S waves. The P wave is a small upward wave that indicates the depolarization of the atria, or the spread of impulse from the sinus node throughout the atria. The second wave, i.e. the QRS complex, begins after a fraction of a second of the P wave. It begins as a small downward deflection (Q) and continues as large upright (R) and triangular wave, ending as downward wave (S) at the base. This is the expression of the ventricular depolarization. The potential generated by the recovery of the ventricle from the depolarization state is, called the repolarisation wave.
In electrocardiography, P-Q interval (also called PR interval) is the time taken by the impulse to travel through the atria, AV node and the rest of the conducting tissues. During rheumatic fever, and in arteriosclerotic heart disease (i.e. the formation of plaques and calcification), the P-Q interval lengthens. This is due to the inflammation of atria and atrioventricular node. The normal PR interval lasts for 0.16 second. The enlarged Q and R waves are the indication of myocardial infarction. The ST interval is the representation of time between the end of the spread of impulse through ventricles, and its repolarisation. The S-T segment is elevated in acute myocardial infarction, and depressed in a condition when the when the heart muscles receive insufficient oxygen. The ventricular repolarisation is represented as T wave. When the heart muscle receives insufficient oxygen then the T wave is flattened.
For the purpose of recording, mental electrodes or leads are attached in each arm and leg with the help of straps after cleaning and putting a special jelly, which improves electrical conduction. An additional electrode is placed on the chest with the help of a rubber suction cup. Then the electrocardiograph is switched on. The electrical current of the heart is detected and amplified by the machine, and it is transmitted to the recording pen that draws a wavy line, called the deflection waves (electrocardiogram).
Pacemaker or the SAN (Sino- Auricular Node), is the rhythmic center, which establishes a pace of activity. Sometimes, the component of the impulse conduction system is disrupted, causing irregularity in the heart rhythm, like failure of receiving the atrial impulse by ventricle or completely independent contraction of the atria and the ventricles. Such types of patients are provided with an artificial electronic device, which regularly sends small amount of electrical charge for maintaining the rhythmicity of the heart. This device is known as artificial pacemaker, which is implanted subcutaneously in the upper thoracic region having a connection with the heart. In the patients having the symptoms of ventricular escape (Strokes - Adams syndrome), in which the atrial impulse suddenly fails to be transmitted to the ventricle, which may last for a few seconds to a few hours even, the artificial pacemaker is connected to the right ventricle for controlling its rhythm. The artificial pacemaker consists of a pulse-generator containing a cell (solid state lithium cell) to produce electrical impulse, the lead in the form of a wire, which transmits the impulse and an electrode, which is connected to the portion of the heart where impulse is to be transmitted.