Blood Pressure

So how do we measure blood pressure?

Blood pressure is recorded using two numbers. The first of these numbers represents your systolic blood pressure while the second represents your diastolic blood pressure.

A doctor or nurse will obtain these numbers by following a well-recognised procedure. They will first compress your arm and the artery within it using an inflatable cuff. They will then listen through a stethoscope to hear firstly the systolic or high blood pressure number and secondly the diastolic or low blood pressure number.

Your systolic blood pressure is measured at the point at which the pressure is strong enough to force your artery open for a moment. Your diastolic blood pressure is recorded at the point at which your blood flows smoothly in your artery without making any noise and coincides with your heart relaxing for a few moments.

Measurements

For example, your blood pressure may be recorded as 120/80 mm/Hg. The first of these numbers, 120, represents your systolic blood pressure while the second number, 80, represents your diastolic blood pressure. The abbreviation "mm/Hg" indicates the number of millimetres that your blood pressure will push a column of Mercury up a glass tube using a device called a sphygmomanometer.

Mercury sphygmomanometers are in the process of being replaced by machines that use a pressure sensor rather than the very useful but potentially toxic Mercury. We traditionally use millimetres of Mercury (mmHg) as in times past, all sphygmomanometers utilised mercury for convenience because it is so dense. If we used a water column to measure our blood pressures, the sphygmomanometer would have to be about 15 feet tall - not a very practical proposition!

We also sometimes use a device that is a spin off from NASA's space programme to check blood pressure. This is called a 24 Hour Blood Pressure Monitor. This monitor is normally used to confirm the diagnosis that a person has raised blood pressure or to verify that their blood pressure is properly controlled. It involves the person in question wearing a small box on their belt connected to a cuff on their arm. This checks blood pressure every half hour or so and prints out the results using a computer. We use this machine because we know that some people's blood pressure rises when they visit a doctor because they are nervous - so called 'White Coat Hypertension'. This effect is eliminated when they have their blood pressure checked with the 24-hour monitor while they are going about a normal day's activities out of the doctor's surgery.

What should my blood pressure be?

Ideally, blood pressure should be around 120/80 mmHg or lower. That is the level at which we know that most things work pretty well. If your blood pressure is much lower, you may feel a little faint or dizzy, especially when rising up off a chair or getting out of bed. This is obviously not pleasant but is usually not dangerous unless you are subject to 'drop attacks.' These occur when you suddenly fall over with little or no warning and are more common as we get older.

"I've heard that your blood pressure should be 100 plus your age. Is this true?"

Not only is this not true but it is dangerous as it implies that your blood pressure should increase as you get older. While it is true that some people's blood pressure does rise with age, this is not necessarily a safe situation as neither the condition of the heart nor the blood vessels improve as we age. In general, as we get older, we become less tolerant of rising blood pressure. For this reason, we should all become more rigorous about monitoring blood pressure, especially once we pass 40 years of age.