Most people have had their blood pressure taken at some time. It is a simple and painless procedure that gives a lot of useful information about the heart and the condition of the blood vessels.

What is measured?

The doctor measures the maximum pressure (systolic) and the lowest pressure (diastolic) made by the beating of the heart.

The systolic pressure is the maximum pressure in an artery at the moment when the heart is beating and pumping blood through the body.

The diastolic pressure is the lowest pressure in an artery in the moments between beats when the heart is resting.

Both the systolic and diastolic pressure measurements are important - if either one is raised, it means you have high blood pressure (hypertension).

How is blood pressure measured?

To take a blood pressure reading, you need to be relaxed and comfortably seated, with your arm well supported. Alternatively, you can lie on an examination couch.

A cuff that inflates is wrapped around your upper arm and kept in place with Velcro. A tube leads out of the cuff to a rubber bulb.

Another tube leads from the cuff to a reservoir of mercury at the bottom of a vertical glass column. Whatever pressure is in the cuff is shown on the mercury column. The mercury is held within a sealed system – only air travels in the rubber tubing and the cuff.

Air is then blown into the cuff and increasing pressure and tightening is felt on the upper arm.

The doctor puts a stethoscope to your arm and listens to the pulse while the air is slowly let out again.

The systolic pressure is measured when the doctor first hears the pulse.

This sound will slowly become more distant and finally disappear.

The diastolic pressure is measured from the moment the doctor is unable to hear the sound of the pulse.

The blood pressure is measured in terms of millimetres of mercury (mmHg).

Blood pressure can be measured in other ways, such as using an automatic blood pressure gauge that can also be used at home.

Electronic measuring devices

Electronic blood pressure measuring devices are becoming the norm now mercury is being phased out because of its hazardous nature.

Most of these are now accurate enough for routine clinical use and are relatively inexpensive.

They eliminate many of the errors in blood pressure measurement that human beings can generate.

Ambulatory blood pressure monitoring

Ambulatory blood pressure monitoring (ABPM) involves measuring your blood pressure for 24 hours as you go about your daily routine and when asleep.

You wear a device that measures your blood pressure at regular intervals. The information is recorded on a chip in the device and allows the doctor to get a detailed picture of blood pressure variation in a normal environment.

Average daytime ABPM blood pressure is lower than equivalent blood pressure readings.

A high reading using ABPM is:

above 135/85 for the general population

above 130/80 for people with diabetes.

ABPM may be used:

when blood pressure levels show unusual variability

when high blood pressure is resistant to drug treatment - three or more drugs

when symptoms suggest the possibility of low blood pressure due to over-treatment

to aid the diagnosis of high blood pressure related to anxiety in the clinical setting, known as 'white coat hypertension'.