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The facts about blood pressure
Q. What is low blood pressure?
A. Low blood pressure is defined as having a blood pressure that is below 90/60mmHg.
Blood pressure can vary over a wide range. For instance, when the heart is pushing the blood round the body, your blood pressure can reach a top or maximum (systolic) level of anywhere between 90 to 240 mmHg and, when your heart is relaxing, your blood pressure can fall to a bottom or minimum (diastolic) level of between 40 to 160 mmHg.
About a hundred years ago it was first recognised that high blood pressure was a major cause of strokes and heart attacks. At this time, it was also thought that low blood pressure was a major cause of depression and tiredness. Many people were quite wrongly treated for low blood pressure in the UK in the early 1900s. However, there is no evidence that low blood pressure causes either of these symptoms.
It is only in a tiny minority of people with low blood pressure, ie, readings below 90/60 mmHg, where there is an underlying problem that is causing their blood pressure to be at this level and treatment may be needed. Most people with low blood pressure are extremely lucky because on average they are going to live longer and need to take out a good pension policy!
However, there are a few people with a blood pressure at these levels who require treatment. In nearly all of these cases, your blood pressure is much lower when you stand up, particularly if you stand up suddenly. This fall in blood pressure on standing is likely to cause symptoms of fainting and/or dizziness.
Normally, when you stand up blood tends to pool in the veins in your legs. This is stopped by nerves, which contract the veins in your legs ensuring that enough blood returns to your heart and that there is no reduction in the amount of blood that your heart pumps. This maintains the blood supply to your brain.
However, if there is pooling of blood in your veins, less blood returns to your heart and less is pumped out which means a reduction in the amount of blood going to your brain. This causes symptoms of dizziness and some people may faint. This is exactly what happens to guards on parade when they faint because they have been standing in one position for too long. You can mimic this effect when you squat down for a while, for instance when looking at books on a low shelf, and then stand up suddenly.
Therefore, if you have a low blood pressure and you also have symptoms of dizziness or faintness when standing up suddenly you should have your blood pressure checked by your doctor, both when you are lying down and when you then stand up. This drop in blood pressure when you stand up is called postural hypotension.