The QRS wave (complex) begins after a fraction of 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 its base. It represents ventricular
depolarisation (ventricular contraction). It is caused by the impulses of the contraction from AV node throughout the bundle
of His and Purkinje fibres and the contraction of the ventricular muscles. Thus this wave is due to the spread of electrical impulse
through the ventricle.
The T wave is dome-shaped which indicates ventricular repolarisation (ventricular relaxation). The potential generated by the
recovery of the ventricle from the depolarisation state is called the repolarisation wave. The end of the T-wave marks the end
of systole.
Table : ECG intervals
Intervals Normal duration (in sec.) Events in the heart during interval
Average Range
PR interval^1 0.18^2 0.12 – 0.20 Atrial depolarisation and conduction through AV node
QRS duration 0.08 < 0.12 Ventricular depolarisation and atrial repolarisation
QT interval 0.40 < 0.44 Ventricular depolarisation plus ventricular repolarisation
ST interval (QT minus QRS) 0.32 – Ventricular repolarisation(^1) Measured from the beginning of the P wave to the beginning of the QRS complex.
(^2) Shortens as heart rate increases from average of 0.18 at a rate of 70 beats/min to 0.14 at a rate of 130 beats/min.
Clinical significance
Any deviation from the normal ECG shape indicates a possible abnormality or disease. Enlargement of the P wave indicates
enlargement of the atria. During atherosclerotic heart disease and rheumatic fever, the P–R interval is lengthened. This is
due to the inflammation of atria and AV node.
The enlarged Q and R waves indicate a myocardial infarction (heart attack). The S–T segment is elevated in acute myocardial
infarction and depressed when the heart muscle receives insufficient oxygen.
T wave is flat when the heart muscles receive insufficient oxygen as in atherosclerotic heart disease. It may be elevated
when the body’s potassium level is increased.
Rate of heartbeat as determined from the electrocardiogram
The rate of heartbeat can be determined easily from an electrocardiogram because the heart rate is the reciprocal of the time
interval between two successive heartbeats. If the interval between two beats as determined from the time calibration lines is 1
second, the heart rate is 60 beats per minute. The normal interval between two successive QRS complexes in the adult person
is about 0.83 second. This is a heart rate of 60/0.83 times per minute or 72 beats per minute.
BLOOD PRESSURE
Blood pressure is defined as the force or pressure which the blood exerts on the walls of the artery in which it is contained. The
arterial blood pressure is the result of the discharge of the blood from the left ventricle into the already full aorta.
When the left ventricle contracts pushing the blood into the aorta, the pressure produced is known as systolic blood pressure
(120 mmHg). When the complete diastole occurs and the heart is resting, the pressure within the blood vessels is called as the
diastolic blood pressure (80 mmHg).
The blood pressure is expressed as BP = 120/80 mmHg. The difference between the systolic and diastolic pressure is called pulse
pressure. The average of blood pressure levels is called mean blood pressure. Its normal value is 40 mmHg but becomes
more in case of hypertension. Pulse pressure is lower in case of kids.
Measurement of blood pressure
Both systolic and diastolic blood pressures are readily measured in human beings with the use of a device called sphygmo-
manometer, in terms of height in millimeters of a column of mercury. An inflatable cuff containing a pressure gauge is wrapped
around the upper arm and a stethoscope is placed in a spot on the arm just below the cuff where the brachial artery lies.
The cuff is then inflated with air to a pressure greater than systolic blood pressure. The high pressure in the cuff is transmitted through
the tissue of the arm and completely compresses the artery under the cuff, thereby preventing blood flow through the artery. The air in