Low blood pressure

Blood pressure can vary over a wide range. For instance, the top pressure when the heart contracts (systolic) can vary from 90 to 240 mmHg and the bottom pressure when the heart relaxes (diastolic) can vary from 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 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 cause 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! This page on low blood pressure is designed to reassure those with low blood pressure and at the same time ensure that, in those rare situations where there is an underlying cause, you seek appropriate medical advice.

What is blood pressure?

Your heart pumps blood around your body through a network of tubing called arteries and capillaries and then back to your heart via your veins. As your heart contracts it pushes blood into your arteries and this causes an increase in pressure. When your heart contracts, the pressure is at its highest in your arteries and this is known as systolic pressure. As your heart relaxes and refills with blood, the pressure in your arteries falls and this is known as diastolic pressure. When blood pressure is measured in your arm, both of these pressures are measured.

For further information about measuring your blood pressure visit the dedicated home blood pressure monitoring site.

Your blood pressure varies by large amounts, depending on what you are doing. The lowest blood pressures occur when you are asleep or if you relax all your muscles. Standing up, exercising or anxiety all cause an increase in blood pressure. In a single day your blood pressure may vary by 30 to 40 mmHg systolic with similar proportionate changes in diastolic pressure. This is why when you have your blood pressure level assessed it is so important to have it measured under the same conditions every time.
The higher your blood pressure, the higher your risk of heart disease and stroke, whatever your level is. This means that someone with a blood pressure of 120/80 mmHg is at greater risk than someone with a blood pressure of 110/70 mmHg. This is why it is so important for everyone to lead a healthy lifestyle to make sure that their blood pressure is as low and as healthy as it can be. Current evidence suggests that if your blood pressure is less than 100/70 mmHg, your risk of a stroke or heart attack is very small and therefore the best blood pressure to have would be below 100/70 mmHg.
What is normal blood pressure?
Until recently, normal blood pressure was considered to be the average blood pressure of the population, ie, in the UK the average adult blood pressure is approx 125/80 mmHg. However, this is not an ideal blood pressure. For instance, the Yamomamo Indians who live in the tropical jungle of Venezuela and still lead the same lifestyle that all humans led up to 1,000 years ago, with a diet consisting of fruit, vegetables and roots with only small amounts of meat. They have an average blood pressure of 96/62 mmHg and this does not rise as they get older. They also have an average cholesterol of around 3.1 mmol/L and do not develop any disease of the arteries.


What is low blood pressure?

If the Yamomamo Indians have an average blood pressure of 96/62 mmHg and are extremely fit and well, it is difficult to define what is meant by low blood pressure. Most people with blood pressures at these levels, ie, around 90/60 mmHg are completely normal and are very lucky that for some reason they are protected from all the bad things we do that increase our blood pressure, like eating too much salt, not eating enough fruit and vegetables, being overweight and inactive etc. However, there are a few people with a blood pressure at these levels who have an underlying cause for their low blood pressure. In nearly all of these cases, 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, 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.


What causes low blood pressure?

The commonest causes are:

Acute illness, for example, severe blood loss and sudden severe infectionMany acute illnesses associated with severe blood loss, fluid loss, infection or damage to the heart may cause low blood pressure and very low blood pressures when sitting up or standing. When this happens blood pressure levels are an important measure of the severity of the illness and that is why blood pressure is measured so often in acutely ill people.

Blood pressure lowering drugs Nearly all of the modern blood pressure lowering drugs do not cause a drop in blood pressure when you stand up. The only class of drugs that is commonly used nowadays that may cause a drop in standing pressure are alpha blockers – the commonest one used in the UK is doxazosin. If you are taking doxazosin and you feel dizzy or faint when you stand up you must get your blood pressure measured when standing to check whether it is dropping.
If you are older, and particularly if you have diabetes, there may already be a tendency for your blood pressure to fall when you stand and even the modern tablets may make this slightly worse.

If you get severe food poisoning and are sick and have diarrhoea, you may lose large amounts of fluid from your body. This will make any tablets lower your blood pressure further. If you do get severe food poisoning it is important to check your blood pressure and stop the tablets if your blood pressure is low, i.e. less than 110/70mmHg.

If you have a drop in blood pressure when you stand up, but you still need to continue with the tablets for high blood pressure, then it will help if you get up slowly. For example, when you are lying flat in bed, sit up slowly, then stand up slowly and avoid sudden attempts to stand up without support.Rare diseases of the nerves that control the reflexes in the veinsSome people develop problems with the nerves supplying the reflexes in their legs so that when they stand up the veins do not contract and they develop a severe drop in their blood pressure. This is very rare. If you have a severe drop in blood pressure when you stand and you are otherwise well (ie, not on any blood pressure lowering drugs) then you should have further investigations. You will need to have tests on a ‘tilt table’ where you lie down and then have your blood pressure and heart rate measured whilst the table tilts to see exactly what is causing the drop in blood pressure.

Diabetes

Diabetes can also cause damage to the nerves supplying blood vessels and can therefore be associated with a drop in pressure when standing up.

Increasing age

As you get older, your arteries stiffen and are less supple and this may occasionally cause a fall in blood pressure when you stand.

Failure of the adrenal gland

The adrenal glands are two very small glands just above the kidney that produce important hormones, one of which is aldosterone. This hormone is very important in controlling the amount of salt in your body. If your adrenal glands become damaged this can mean that there is not enough aldosterone in your body. This leads to a loss of salt from your body and this may cause low blood pressure, with dizziness or faintness when you stand up suddenly. This condition is very rare, but important to pick up as, it can easily be treated by replacing the missing hormones.


Will I need any investigations?

Investigations are only conducted if you have symptoms that suggest a fall in blood pressure when you stand up, ie, dizziness or faintness. If you do have these symptoms or a fall in blood pressure when you stand then your doctor should conduct further tests or refer you to a specialist. This will mean having a tilt test where you will be strapped to a table and tilted, with careful measurements of heart rate and blood pressure taken. Hormone levels in your blood may also be measured to check that your adrenal glands and nerves are working normally.

How is low blood pressure treated?

This will very much depend on the cause. If you are well with no drop in blood pressure when standing then you should congratulate yourself – you do not need any treatment. However, if you do have symptoms then treatment will depend on the underlying cause. If you are taking blood pressure lowering drugs they may need to be changed, particularly if you are taking an alpha blocker such as doxazosin. You will need to discuss this with your doctor or practice nurse. Many people find that by getting up slowly their symptoms can be avoided.
Failure of the adrenal glands can be treated by replacement of the missing hormones. If you have a disease of the nerves then this can be more difficult to treat, but you may respond to drugs that stimulate the nervous system. You may also find that wearing elastic stockings or an anti-gravity suit, or taking hormones that cause retention of salt and/or eating more salt can help. Talk to your doctor for more information.


Is low blood pressure associated with tiredness or depression?

Many years ago it was thought that blood pressures below a systolic of 90 mmHg could lead to tiredness and depression and many women were wrongly treated for low blood pressure with drugs that stimulate the nervous system. Large studies have now suggested that there is no relationship, provided there is no underlying cause for the low blood pressure.

What should I do if I think I have got low blood pressure?

The first thing to do is to have it measured either by your doctor or practice nurse, or get a machine to measure it yourself. It is important to measure your blood pressure not just when you are sitting, but also when you are lying down and standing up. If when you stand there is a large fall in blood pressure and you feel dizzy, you should then seek further advice.

If your systolic pressure is above 80mmHg with no drop when you stand up, and you feel quite well, then you are one of the few lucky people who naturally have very low blood pressure. For reasons that are not fully understood you are protected from the effects of salt and other factors that can cause raised blood pressure levels. On average, you will live longer than people with higher blood pressures.