Low blood pressure (hypotension) can be easily diagnosed by measuring your blood pressure.
Measuring blood pressure
A blood pressure reading is taken using two measurements. The first measurement is known as systolic, which is the pressure in your arteries when your heart contracts and pushes the blood around your body. The second measurement is known as diastolic, which is the pressure in your arteries when your heart refills with blood in between heart beats. Both systolic and diastolic blood pressures are measured in millimetres of mercury (mmHg).
Sphygmomanometer
Your GP, or practice nurse, will use a device known as a sphygmomanometer to measure your blood pressure. This device has an inflatable cuff and a scale of mercury, like a thermometer, as a pressure gauge. The cuff is placed around your upper arm and inflated to restrict the flow of blood in your arm. The air is then slowly released from the cuff.
Your GP, or practice nurse, will watch the mercury pressure gauge and listen to your blood flow in the main artery of your arm using a stethoscope. Upon hearing your heart beat, the systolic pressure will be recorded. When the sound disappears, the diastolic pressure will be recorded.
Alternatively, a digital sphygmomanometer may be used. This measures your pulse using electrical sensors and takes blood pressure readings automatically. Blood pressure testing kits are also commercially available.
After you have had your blood pressure taken, your GP, or nurse, will give you your systolic reading first, followed by your diastolic reading. If your systolic blood pressure is 120 mmHg, and your diastolic blood pressure is 80 mmHg, you will be told that your blood pressure is 120 over 80, which is commonly written as 120/80.
What is low blood pressure?
As a general guide, low blood pressure is a reading of 90/60. However, it is not necessary for both your systolic and diastolic readings to be in this range for it to be considered low blood pressure. For example, a reading of 80/65 would be considered low because the systolic number is in the low range, and 100/55 would also be considered as low because the diastolic number is in the low range.
If you have low blood pressure according to this guide, you do not need to worry. Having low blood pressure is considered healthy as it protects you from the risks and diseases of high blood pressure. You will only need to have treatment if you are experiencing symptoms as a result of your low blood pressure.
Mean arterial blood pressure
As well as measuring your blood pressure, your GP, or practice nurse, may also calculate your mean arterial pressure (MAP). This is the average pressure required to push blood through your body. The reading takes account of blood flowing away from your heart and to it, and it can be a better indication of whether your blood pressure is too low.
Your MAP can be calculated from you normal blood pressure reading using the following formula:
MAP = [ (2 x diastolic figure) + systolic figure] divided by 3
For example, if your blood pressure is reading 120/80, your systolic reading is 120 and your diastolic reading is 80. Your MAP is:
MAP = [ ( 2 x 80 ) + 120 ] / 3 = [ 160 + 120 ] / 3 = 280 / 3 = 93.33 mmHg
If your MAP is below 65 mmHg, it is possible that your brain and vital organs are not receiving enough oxygen. According to this calculation, a low blood pressure reading of 90/60, gives an MAP of 70 mmHg. This is therefore still unlikely to cause health problems.
Postural or orthostatic hypotension
If your symptoms of low blood pressure mostly occur when you change position, (postural or orthostatic hypotension), then your blood pressure may be measured before and after you move. For example, your blood pressure may be measured while you are sitting down and again while you are standing up.
Depending on what your seated blood pressure was, if your systolic reading drops by between 15-30 mmHg when you stand up, you may have orthostatic hypotension.
Underlying causes
Your GP, or practice nurse, will usually be able to diagnose low blood pressure very easily. However, determining the reason for low blood pressure can be more difficult.
If you have an underlying condition that is causing low blood pressure, it is likely that you will have other symptoms as well. You should discuss these with your GP who may recommend that you have further tests.
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