Taking your blood pressure readings

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Measuring blood pressure

The most accurate way to get blood pressure readings would be to connect a vertical tube directly to an artery and see how high up the blood goes. (In fact this is how blood pressure was first ever measured, although it was in a horse which subsequently died).

A more practical alternative is to see how much pressure it takes to squeeze an artery shut and this is best achieved by inflating a cuff strapped to the arm. The brachial artery, on the inside of the elbow, is then listened to with a stethoscope.

Blood pressure readings are always expressed as two values, one high and one low, representing the squeeze (systolic) and relax (diastolic) phases of the heart's pumping.

The units of blood pressure are millimetres of mercury - that might seem a strange kind of measurement but it’s really just seeing how far up a tube the pressure can push a column of mercury. So the full expression is, for example, 120 millimetres of mercury or 120 mmHg. (Hg is the symbol for mercury. It is an abbreviation of the Latin phrase Hydragyrum which means silvery liquid). Mercury is used because its density means that a conveniently short tube can be used.

The mercury-based machine is called a sphygmomanometer (pictured right). It's a mouthful so most people say sphyg (pronounced ssfig!). The sphygmomanometer is as easy to use as it is to pronounce; you have to listen carefully to the blood flow, with a stethoscope, and know exactly at what point to note the readings. It takes significant dexterity to use one on yourself as you need one arm and hand to be still, one hand to hold the stethoscope in place and a third hand to pump up the cuff!

Thankfully, monitoring your own blood pressure is now a simple procedure because of the availability of affordable automated monitors which work at the press of a button. They don't use mercury although their readings are still expressed in mmHg. (See picture on left)

If you are serious about controlling your blood pressure you should own a monitor - you wouldn't try to control your weight without a set of scales would you? Click here for more about monitors.

The correct way to take blood pressure readings

Too many people do not have their blood pressure checked properly and this can lead to incorrect diagnosis, anxiety and even unnecessary treatment. The following tips will ensure you get accurate readings. (When you have your blood pressure measured at the doctor's, do comment if proper procedure is not followed).

• You should have your blood pressure read in a calm setting, seated with your arm at the level of your mid-chest, resting, so that your arm muscles are relaxed.

• If your garment is not loose enough at the sleeve to be comfortably folded clear of your upper arm your top should be removed. (If the material is thin it can be left on.)

• You should have left enough time (at least ten minutes before taking the measurement) after anything that can temporarily alter your blood pressure such as exercise or stress.

• For some things like coffee, cigarettes, alcohol or a large meal leave at least 30 minutes.

• Do go and empty your bladder first - (squirming and crossing your legs while trying to keep from an 'accident' will most likely affect your BP!)

• Use the correct cuff size. If your arms are larger than normal (due to muscle or fat) a small cuff will give an artificially high reading, so a large cuff should be used. That advice applies if you are taking your own readings at home.

Recommended cuff sizes according to arm measurement 18-22 cm - Small cuff 22-32 cm - Medium cuff 32-45 cm - Large cuff Measure your arm mid-way between shoulder and elbow. Most monitors are supplied with a medium cuff however other cuff sizes are usually available separately.

• Additional checks include checking at least once that there is no difference in the readings between the left and right arms.

Despite proper preparation, some people have a high reading only when they are at the doctor's office. This is called 'white coat hypertension'. You can click here. to read more about it.

Looking at the above, you can see that your home environment is conducive to getting accurate blood pressure readings.

Remember, its your blood pressure - control it!

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This page was last modified on : May 22, 2011.