Blood Pressure: What is It?

The term 'blood pressure' (BP) refers to the outward pressure exerted by the blood on the walls of arteries and is more properly named 'systemic arterial blood pressure'. Measurements are taken with the aid of a manual or, increasingly, an electronic, sphygmomanometer. Measurements are given in 'millimetres of mercury' (mmHg) and are two in number: systolic and diastolic. This refers to the cardiac cycle: 'systolic pressure' being the highest and 'diastolic pressure' the lowest (i.e. in the resting phase).

An average healthy person, if there is such an animal, has a typical reading of 120/80 mmHg, although pressures change naturally from beat to beat and can be affected by the time of day (circadian cycle), stress, drugs, exertion, disease, nutrition, coffee and a myriad of other things.

Hospitals generally use a more accurate form of measurement, especially during surgery. 'Invasive Arterial blood Pressure' is measured in 'IV Cannulas' by inserting a cannula into a blood vessel. The cannula is then connected to a fluid-filled, electronic measuring device, which sends its output to a monitor or screen in the form of a continuous wavy line, which represents each heart beat rather than a mean average, which is particularly important for critically ill patients. It also provides valuable information for anaesthetists during operations. These monitors can then be linked to alarm systems.

Doctors and 'do-it-yourselfers' tend to prefer to use the cuff method, sacrificing a small amount of accuracy for the sake of comfort. These devices are generally known as auscultatory (from the Latin for 'listening'). Some have mercury-filled barometer-type devices called aneroid manometers (to be used with a stethoscope) and others are electronic, but they all work on the same principle. Most important is that the cuff is the correct size: too small and the reading will be high; too large and it will be low. There are usually markings on the cuff indicating approximate fitting, although a general rule is that the user should be able to insert two fingers under the deflated cuff comfortably. Upper-arm cuffs provide a more reliable reading than wrist cuff monitors. If a wrist cuff has to be used, the hand should be rested at heart height.

The patient should be relaxed and seated comfortably on a chair with feet flat on the floor. Readings should be taken at the same time of the day every day to provide greater accuracy. One of the best times is on waking, but before bathing and breakfast, while the body is still 'resting'. This can provide a more accurate reading than in a doctor's surgery due to the 'white coat' affect, which refers to the fact that most people are nervous around doctors, which tends to increase blood pressure.

Children tend to have lower blood pressure than adults and the elderly somewhat higher. It is now widely believed that higher than normal blood pressure is not beneficial. Long-term lower blood pressure tends to indicate greater cardiovascular health. There are various prescription drugs and homeopathic remedies available to reduce blood pressure, but the best start, after visiting the doctor, is probably to normalise your weight, be more active, drink more water and eat less red meat.

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Michael Russell Your Independent guide to Blood Pressure

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