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Low blood pressure

There is no set figure at which blood pressure is said to be low, as this can vary from person to person.

About low blood pressure

Having a naturally low blood pressure (normally thought of as being lower than 90/60 mmHg) is unlikely to mean there is anything wrong with you. It may be normal for you and in fact is beneficial in many ways, as it can reduce your chance of getting diseases such as coronary heart disease and stroke.

Low blood pressure is only considered to be a problem when it causes symptoms such as fainting. This usually only happens if there is some underlying problem which means your blood pressure is not being controlled properly.

What is blood pressure?

Blood pressure is a measure of the force that the blood applies to the walls of your arteries as it flows through them.

Blood carrying oxygen and nutrients is pumped around your body by your heart. As a result of the pumping action of your heart and the size and flexibility of the arteries that carry blood, the blood is under pressure. This blood pressure is an essential and normal part of the way your body works.

Blood pressure is expressed as two numbers, such as 120/80 mmHg (one hundred and twenty over eighty millimetres of mercury).

  • The top figure - the systolic blood pressure - is a measure of the pressure when your heart muscle is contracted and pumping blood. This is the maximum pressure in your arteries.
  • The bottom figure - the diastolic blood pressure - is the pressure between heart beats when the heart is resting and filling with blood. This is the minimum pressure in your arteries.

A high blood pressure is associated with various diseases, such as coronary heart disease and stroke. So it's normally good to have as low a blood pressure as possible. The British Hypertension Society advises that the ideal blood pressure for adults is less than 120/80 mmHg.


When is low blood pressure a problem?

If your blood pressure only decreases in certain situations, such as after standing up, there may be an underlying problem which is preventing your blood pressure from being controlled properly. The sudden drop in blood pressure can cause the blood supply to the brain to be temporarily interrupted, leading to fainting or dizziness. This is often due to one of the following problems:

  • Postural hypotension (also called orthostatic hypotension). This is when your blood pressure drops when you stand up from a sitting or lying position. Normally, the veins in your legs will automatically constrict (become narrower) when you stand up, preventing blood from pooling in your legs and forcing it back to your heart. If you have postural hypotension however, this doesn't happen. This means the blood remains in your legs, leading to a lack of blood reaching your brain.
  • Postprandial hypotension. This is when your blood pressure drops after eating a meal. It is thought to be due to blood pooling in the blood vessels within your abdomen as your meal is being digested.

Standing up for too long and strong emotions, such as fear, can also trigger this kind of reaction in otherwise healthy people.

Symptoms of low blood pressure

If your blood pressure is naturally low, you are unlikely to get any symptoms and will probably feel completely well.

If you have postural hypotension, you may feel faint or dizzy when you stand up. This may happen after eating if you have postprandial hypotension. You should visit your GP for advice if this happens.

Causes

If you consistently have a low blood pressure, this may be completely normal for you. However if you have postural or postprandial hypotension, there is often an underlying cause. Causes include:

  • taking drugs to treat high blood pressure (hypertension) - especially drugs called diuretics and alpha blockers
  • acute illnesses that cause severe blood loss or damage to the heart
  • diseases involving the nerves that control the veins in your legs
  • diabetes - as diabetes can also damage the nerves
  • a disease of the adrenal gland (Addison's disease) - this can cause a loss of salt from your body, resulting in low blood pressure
  • early stages of pregnancy
  • loss of blood due to serious injury or loss of fluid due to burns

You are also more likely to get postural hypotension as you get older. This is because your arteries become less supple, which can mean they don't respond as quickly when you stand up.

Postprandial hypotension almost always occurs in elderly people. You are also more likely to get it if usually, your blood pressure is too high (hypertension) or you have a disease affecting your nerves.

Diagnosis

If you have a naturally low blood pressure, you will probably only find out when having your blood pressure measured as part of a general checkup, or while having tests for some other reason. If it's not causing you any symptoms, you are unlikely to need any further tests or treatment.

Measuring blood pressure

Blood pressure is measured using a device called a sphygmomanometer. This may be either a manual device (a column of mercury) or an automated, digital device. The sphygmomanometer is attached to a cuff, which will be placed around your upper arm.

With a manual sphygmomanometer, your doctor or nurse will slowly inflate the cuff using a pump. He or she will listen to the sound of your blood flow in an artery in your arm using a stethoscope, before deflating the cuff again. Your doctor or nurse will look at the height of the column of mercury to work out your blood pressure.

With a digital monitor, your doctor or nurse will press a button to inflate the cuff, and it will automatically slowly deflate. A sensor in the cuff detects your blood pressure and the result is shown on a display screen.

Further tests

You will only need further tests if you have symptoms such as dizziness and fainting. Your GP will ask about your symptoms and examine you. He or she may also ask you about your medical history.

Your GP may measure the change in your blood pressure as you stand up after lying or sitting down, if you have symptoms of postural hypotension. Or if you have symptoms of postprandial hypotension, your blood pressure will be measured before and after you have a big meal.

If your GP thinks you could have postural hypotension, you may also be asked to have a tilt test. This is carried out in hospital and involves lying on a table, where you are secured with safety belts, and slowly tilted upwards as your blood pressure and heart rate are monitored.

Treatment

You are unlikely to need any treatment if your blood pressure is naturally low.

If you have postural hypotension, your treatment will depend on the underlying cause. For example, if you are taking drugs for the treatment of high blood pressure and these are causing your blood pressure to drop too much, your doctor may switch you to a different type of drug. This is normally only the case if you are taking drugs called diuretics or alpha blockers - most drugs for high blood pressure only lower blood pressure to a level that is beneficial.

Self-help

If you have postural hypotension, your GP may advise you to:

  • stand up slowly, especially when you first wake up
  • try not to stand still for a long time
  • make sure your baths/showers are not too hot
  • avoid strenuous physical activity
  • increase the salt in your diet and amount of fluids you drink - however, it is important to remember that too much salt in your diet can lead to high blood pressure, so you should follow your doctor's advice
  • wear compression stockings

If you have postprandial hypotension, your GP may advise you to:

  • lie down after eating
  • eat regular meals with lower levels of carbohydrate, rather than large meals with lots of carbohydrate

These measures may not be suitable for everyone - talk to your doctor about what is right for you.

Medicines

Occasionally, your doctor may also prescribe medicines if the self-help measures described above are not stopping your symptoms. These may include:

  • drugs to increase your retention of salt
  • drugs to make your veins constrict

Further information

Blood Pressure Association

Sources

  • Low blood pressure. Blood Pressure Association. www.bpassoc.org.uk, accessed 18 February 2008
  • Williams B, Poulter NR, Brown MJ, et al. Guidelines for management of hypertension: report of the fourth working party of the British Hypertension Society. Journal of Human Hypertension 2004; 18:139-185
  • Kumar P, Clark M, Clinical Medicine. 6th ed: Elsevier, 2005
  • Simon C, Everitt H, and Kendrick T, Oxford Handbook of General Practice. 2nd ed. Oxford: Oxford University Press, 2005:280
  • Beers MH, Fletcher AJ, Jones TV, et al., The Merck Manual of Medical Information. 2nd ed. New York: Pocket Books, 2003
  • Task force on syncope - European Society of Cardiology, Guidelines on management (diagnosis and treatment) of syncope. European Heart Journal 2001; 25(22):1256-1306

Related topics

  • High blood pressure
  • Cardiovascular system
  • Compression stockings