The heart muscles generate electrical impulses in the form of depolarization and repolarization that are transmitted throughout the body. If suitable electrodes are placed on the body opposite to the heart and connected to Galvanometer with a recording device, then the electrical potential of the cardiac cycle can be recorded. This recording of electrical potential generated by the spread of cardiac impulse is known as an electrocardiogram (ECG) and the instrument by which electrocardiogram is recorded is called an electrocardiograph. Electrocardiograph was discovered by Einthoven (1903), commonly called “Father of Electrocardiography”, while ECG was first recorded by Waller (1887). ECG taken when the patient is lying down is called resting ECG while ECG taken when a patient is exercising is called stress ECG. The electrocardiograph provides vital information about the rate and rhythm of the heart and the conditions of the heart muscles.
Each cardiac cycle (or in very common terms we may say each heartbeat) produces a P wave, a QRS complex and a T wave. The QRS complex has three separate Q, R and S waves.
It is a small upward wave that appears first. It indicates atrial depolarization, during which excitation spreads from the SA node to all over atria. About 0.1 sec after P wave begins, atria contracts. Hence P wave represents atrial systole. Enlarged P-Wave indicates enlarged atrium i.e. it may occur in a condition called mitral stenosis in which due to narrowing of the mitral valve, the blood backs up into the left atrium).
It is the second wave that begins as a little downward wave (Q) but continues as a large upright triangular wave (R) and ends as a downward wave (S). It represents the ventricular depolarisation, i.e., the spread of excitation over ventricles. Just after this wave begins, ventricles start to contract. Hence QRS is equivalent to ventricularsystole.
It is the third small wave in the form of a dome-shaped upward deflection. It indicates ventricular repolarisation. It also represents the beginning of ventricle diastole.
Atrial diastole gets merged with the large QRS Wave.
P-Q Interval (also called P-R Interval):
This is the interval from the onset of the P wave to that of QRS. It measures the conduction time of the impulse from the SA node to 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 sec.
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 (heart attack) and depressed in a condition when the heart muscles receive insufficient oxygen.
It measures the diastolic period of heart.
By counting the number of QRS complexes that occur in a given time period, one can determine the heartbeat rate of an individual. The ECG obtained from different individuals has roughly the same shape for a given lead configuration, any deviation form this shape indicates the abnormality or disease. Hence, it is of great clinical significance.