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Blood pressure (low)

Introduction

Low blood pressure, also known as hypotension, is where the blood pressure in your arteries is abnormally low.

It's usually the case that the lower your blood pressure, the healthier you are. 

Naturally low blood pressure is unlikely to cause any symptoms and is normally nothing to worry about. However, if your blood pressure drops too low, it can restrict the amount of blood flowing to your brain and other vital organs, which can cause fainting or dizziness and lightheadedness.  

See your GP if you experience any symptoms of low blood pressure and you are concerned.  

All adults should have their blood pressure checked at least every five years. If you haven't had yours measured or don't know what your reading is, ask your GP to check it.

What is low blood pressure?

The heart pumps a constant supply of blood around the body through arteries, veins and capillaries.   

Blood pressure is a measure of the force of the blood on the walls of the arteries as the blood flows through them.

It is measured in millimetres of mercury (mmHg) and recorded as two measurements:

  • systolic pressure: the pressure when your heart beats and squeezes blood into your arteries
  • diastolic pressure: the pressure when your heart rests between beats 

For example, if your systolic blood pressure is 120 mmHg and your diastolic blood pressure is 80 mmHg, your blood pressure is 120 over 80, which is commonly written as 120/80.

A blood pressure reading below 130/80 is generally considered normal. If you have a reading of 140/90 or more, you have high blood pressure (hypertension), which puts you at greater risk of developing serious health conditions, such as heart attack or stroke.

People with a blood pressure reading of around 90/60 or less are usually regarded as having low blood pressure.

Why do I have low blood pressure?

You can have blood pressure for many reasons, including the time of day, age, the temperature, any medication you may be on, an injury and some illnesses. Find out more about the causes of low blood pressure.

Treatment and self-help

Naturally low blood pressure does not usually need to be treated unless it is causing symptoms such as dizziness or recurrent falls. If it is causing you to have symptoms, your GP will look at what the cause might be.

There are various things you can do to help limit the symptoms of low blood pressure, inlcluding:

  • standing up gradually
  • wearing support stockings 
  • avoiding caffeine at night and limiting your alcohol intake
  • including more salt in your diet
  • eating smaller meals, more often

Find out more about treating low blood pressure.

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Symptoms

If your blood pressure is naturally low, it's unlikely that it will cause you any symptoms or require treatment.

However, low blood pressure can sometimes mean that there is not enough blood flowing to your brain and other vital organs, which can lead to symptoms such as:

  • dizziness
  • fainting (a sudden, temporary loss of consciousness) 
  • lightheadedness
  • blurred vision
  •  a rapid, or irregular heartbeat (palpitations) 
  • confusion
  • feeling like you are going to be sick (nausea)
  • general weakness

If you experience the symptoms of low pressure after changing positions, such as standing up, it is known as postural, or orthostatic, hypotension.

The symptoms should only last for a few minutes as your blood pressure adjusts to your new position. This type of low blood pressure tends to affect people more as they get older, when it can lead to more frequent falls. Similar symptoms may also occur after exercise.

If you experience symptoms after eating, it is known as postprandial hypotension and occurs more often in older people, particularly in those who have high blood pressure or conditions such as Parkinson's disease and diabetes

After a meal, your intestines need a large amount of blood for digestion. If your heart rate does not increase enough to maintain blood pressure, your blood pressure will fall, causing symptoms. See your GP if you notice any.

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Causes

Throughout the day, your blood pressure can vary by between 30-40 mmHg (both systolic and diastolic) depending on what you are doing. Having a stressful week at work, the temperature outside and even what you had for lunch could affect your blood pressure reading.

Each time that you have your blood pressure measured, it is important that the test is carried out under similar conditions to ensure that the results are consistent. If you have a low blood pressure reading, your GP will first consider the everyday causes that might have affected it, before considering the possible underlying causes. 

Everyday causes

Many factors have a daily, or even hourly, effect on your heart and circulation. Below are things that could affect your blood pressure and, in some cases, may cause low blood pressure.

  • The time of day – your blood pressure falls overnight so it will be low in the morning.
  • Your age – typically, blood pressure rises as you get older, although postural, or orthostatic, and postprandial hypotension are also more likely in the elderly.
  • How stressed or relaxed you are – if you are stressed, your heart will beat faster and your blood pressure will increase, and the opposite if you are relaxed.
  • How much exercise you do – initially, exercise will raise your blood pressure, but if you are healthy and exercise regularly, your blood pressure will be low when you are resting.
  • Your temperature – if you are cold, your heartbeat will slow down and your blood pressure will fall.
  • If you have recently eaten – blood will be used for digesting food in your stomach, so the blood pressure elsewhere in your body will fall.

Underlying causes

If your blood pressure is still considered low after taking into account everyday factors, such as those listed above, there may be another cause. Some possibilities are explained below.

Medication

Some medication may cause hypotension as a side effect. This tends to be orthostatic or postural hypotension (low blood pressure when you stand up, or change position). Examples of medication that can cause hypotension include:

  • beta-blockers – which may be prescribed for a problem with your heart
  • alpha-blockers – a medicine that is prescribed to lower blood pressure for people with hypertension (high blood pressure)
  • some antidepressants

Your GP will discuss any possible side effects with you when prescribing medication. While you are taking medication, your blood pressure will be carefully monitored if you are considered to be at risk of hypotension. 

Serious illnesses or conditions

If you have an acute (short-term) illness, your blood pressure will be measured regularly because it is a good indicator of the severity of your illness. A heart condition, such as heart disease or a heart attack, can also cause low blood pressure, as your heart may not be able to pump blood around your body.

Autonomic disorders

Autonomic disorders affect your autonomic nervous system and can cause hypotension. Your autonomic nervous system is part of your nervous system (the network of cells that carry information around your body). It controls the bodily functions that you do not actively think about, such as sweating, digestion and the beating of your heart.

The autonomic nervous system also controls the widening and narrowing of your blood vessels. If there is a problem with it, your blood vessels could remain too wide, causing low blood pressure. In particular, autonomic disorders tend to cause orthostatic hypotension.

Some examples of autonomic disorders are:

  • diabetes mellitus – a long-term (chronic) condition caused by too much glucose (sugar) in the blood
  • Parkinson’s disease – a chronic condition that affects the way the brain co-ordinates body movements
  • multiple system atrophy – a disorder that causes the brain signals to the muscles and limbs responsible for movement to deteriorate

Adrenal glands

The adrenal glands are two small glands that are located just above your kidneys. They produce hormones that control your blood pressure and maintain the balance of salt and water in your body. One of the hormones they produce is aldosterone, which is responsible for controlling the amount of salt in your body.

If your adrenal glands become damaged – for example through an infection or a tumour – the production of aldosterone may be reduced, resulting in a loss of salt from your body. This can cause dehydration which, in turn, leads to low blood pressure.

If a problem with your adrenal glands is diagnosed, it can be treated by increasing the amount of aldosterone in your body. This could also be a symptom of Addison’s disease (a condition in which the adrenal glands cannot produce enough of the hormones cortisol and aldosterone). Addison’s disease can also be treated with medication.

Serious injuries and shock

Low blood pressure can also be caused by serious injuries or burns, particularly if you have lost a lot of blood. This can mean that there is less blood being pumped around your body. Low blood pressure can also occur if you go into shock after having a serious injury.

Other kinds of shock are described below.

Septic shock and toxic shock syndrome

Septic shock and toxic shock syndrome are caused by bacterial infections. The bacteria attack the walls of the small blood vessels, causing them to leak fluid from the blood into the surrounding tissues. This causes a significant drop in blood pressure (severe hypotension).

Anaphylactic shock

Anaphylactic shock, or anaphylaxis, is caused by an allergic reaction to something such as a wasp sting or a peanut. During an allergic reaction, your body produces a large amount of a chemical called histamine, which causes your blood vessels to widen and leading to a sudden, severe drop in blood pressure.

Cardiogenic shock

Cardiogenic shock occurs when your heart cannot supply enough blood to your body, so your blood pressure drops. This can happen during a heart attack.

Other causes

Other possible causes of low blood pressure are listed below.

  • Rare nerve conditions – if the nerves in your legs are affected, you may experience a severe drop in blood pressure when you stand up (postural or orthostatic hypotension).
  • Increasing age – as you get older, your arteries can become stiffer. If they do not constrict (get smaller), your blood pressure may drop, particularly when you stand up.
  • Pregnancy – during the early to middle stages of pregnancy, low blood pressure is fairly common.
  • Prolonged bed rest – low blood pressure may possibly occur as a result of moving less and having overall less nervous system activity.
  • Dehydration – low blood pressure may occur following particularly severe dehydration from vomiting and diarrhoea because the lack of water and salt in your body will reduce the volume of your blood.
  • Your genes – some research has suggested that low blood pressure is genetic. If your parents have low blood pressure, it is possible that you could inherit it from them.
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Diagnosis

Low blood pressure (hypotension) can be easily diagnosed by measuring your blood pressure.

A blood pressure reading is taken using two measurements. The first is known as systolic, which is the pressure in your arteries when your heart gets 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 heartbeats. Both systolic and diastolic blood pressures are measured in millimetres of mercury (mmHg).

How blood pressure is measured

Blood pressure is often measured using a sphygmomanometer, a device which consists of a stethoscope, arm cuff, dial, pump and valve.

The cuff is placed around your arm and pumped up to restrict the blood flow. The pressure is then slowly released as your pulse is checked using the stethoscope.

Hearing how your pulse beats after the cuff is released allows a measurement to be taken on the mercury scale, giving an accurate reading of your blood pressure.

Many GP surgeries now use digital sphygmomanometers, which measure your pulse using electrical sensors.

Before having your blood pressure taken, you should rest for at least five minutes and empty your bladder.

To get an accurate blood pressure reading, you should be sitting down and not talking when the reading is taken.

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, written as 120/80.

Low blood pressure

As a general guide, low blood pressure is a reading below 90/60. 

If you have low blood pressure according to this guide, you do not need to worry. Naturally low blood pressure rarely causes symptoms or needs treating. Having low blood pressure is considered healthy because it protects you from the risks and diseases of high blood pressure.

Postural or orthostatic hypotension

If your symptoms of low blood pressure mostly occur when you change position (postural or orthostatic hypotension), 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 is, if your systolic reading falls by between 15 to 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 often 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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Treatment

If you have naturally low blood pressure, it does not usually need treating.

But if your low blood pressure is causing symptoms, your GP will try to establish the underlying cause to determine what treatment is necessary. 

Medication

If your GP suspects that your medication is causing low blood pressure, they may change it or alter your dose. This includes medication to treat high blood pressure (hypertension), and medication to treat Parkinson’s disease.

Your blood pressure should be monitored while you're taking medication and any changes noted. Tell your GP if you are experiencing side effects from taking medication.

Underlying illnesses or conditions

If your GP suspects that your low blood pressure is being caused by a health condition, you may be referred to hospital for further tests and treatment.

If adrenal gland failure is found to be the cause, fludrocortisone may be prescribed to replace the missing hormone, aldosterone. This will usually be in tablet form and will need to be taken for the rest of your life.

If a nerve condition is causing your low blood pressure, it can be more difficult to treat. You may be prescribed medication to help stimulate your nervous system.

Fluids and salt

Dehydration can cause low blood pressure. This can be easily treated by increasing your fluid and salt intake. Ensuring that you drink enough fluid will help by increasing the volume of your blood. Having more blood in your arteries will also increase your blood pressure.

People who have high blood pressure are usually advised to restrict their salt intake. If you have low blood pressure, you may be advised to include more salt in your diet. Your GP will be able to advise you about how much additional salt you need and whether you can add salt to your usual food or if you need to take salt tablets.

General advice

The advice outlined below will help to limit your symptoms of hypotension, particularly postural or orthostatic hypotension (where blood pressure falls after a sudden movement).

  • Stand up gradually: particularly first thing in the morning. It may also be useful to try some other physical movements first to increase your heart rate and the flow of blood around your body. For example, stretching in bed before you get up or crossing and uncrossing your legs if you are seated and about to stand. 
  • Wear support stockings: sometimes called compression stockings, these are tight-fitting elastic socks or tights. They provide extra pressure to your feet, legs, and abdomen, which will help stimulate your circulation and increase your blood pressure.
  • Raise the head of your bed or use extra pillows under your head: this will increase the flow of blood in your body and will also make it easier when you need to get up. 
  • Avoid caffeine at night, and limit your alcohol intake: this will help you to avoid becoming dehydrated, which can also cause low blood pressure.
  • Eat small, frequent meals rather than large ones: this will help you to prevent postprandial hypotension (low blood pressure after eating). Lying down after eating or sitting still for a while may also help.

Very few people are prescribed medication for low blood pressure. The symptoms of hypotension can be usually be treated by making these small changes to your lifestyle and, in particular, by increasing your fluid and salt intake.

If medication is necessary, it will usually be medicines to expand the volume of your blood or to constrict (narrow) your arteries. By increasing your blood, or decreasing your arteries, your blood pressure will increase because there will be more blood flowing through a smaller space.

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