Low blood pressure
Low blood pressure, or hypotension, generally refers to a blood pressure reading below an average of about 90/60 mmHg.
There are many reasons that someone may have low blood pressure. Diet, medication or an underlying health condition may be the cause. Low blood pressure in a healthy person is often of little significance, unless it causes symptoms, and is often only identified as a result of a routine medical examination.
Sudden low blood pressure can be associated with serious medical conditions. People who are very sick can have a sudden, severe drop in their blood pressure (shock) as well as other symptoms relating to the condition that is making them unwell. These people need urgent medical attention.
How low is too low?
In general, it is much better to have low blood pressure rather than high blood pressure, and it may even prolong life expectancy. However, sometimes there are symptoms of low blood pressure, such as:
- light-headedness (sometimes described as dizziness);
- pale, clammy skin;
- blurry vision; and
If you have these symptoms, you should lie down and, if possible, elevate your feet and legs (for example on a cushion). This should help relieve symptoms within a few minutes.
An episode of dizziness and fainting in young, healthy people is usually nothing to worry about. Such symptoms may be more serious in older people as they may have a number of problems that make it difficult to maintain constant blood pressure.
Many cases of low blood pressure get better without any treatment. However, if symptoms continue to occur, the underlying cause should be investigated by a doctor.
Factors affecting blood pressure
Blood pressure is determined by 3 main factors.
- Cardiac output (the rate at which blood is pumped from the heart).
- Volume of blood in the blood vessels. A decreased amount of blood in the body is called hypovolaemia.
- Capacity of the blood vessels (as determined by their internal diameter, or width).
Your blood pressure changes throughout the day and is affected by your:
- activities (e.g. your blood pressure is lower when you are asleep);
- fitness level;
- food intake;
- stress levels; and
- environment (e.g. temperature can affect your blood pressure).
Types of low blood pressure
There are 3 main types of low blood pressure.
Orthostatic hypotension, also called postural hypotension, is an inability to regulate blood pressure quickly. People with this type of low blood pressure have a sudden fall in blood pressure upon changing positions – usually standing up – which causes fainting (or feeling faint, or light-headed or dizzy).
There are many causes of orthostatic hypotension, including dehydration, pregnancy and certain medicines.
One type of orthostatic hypotension is low blood pressure that happens after eating – this is called postprandial hypotension and usually affects older people.
Neurally mediated hypotension
Neurally mediated hypotension is low blood pressure that is often brought on by stress, pain or prolonged standing (especially in a hot environment).
In this type of low blood pressure, there is a simultaneous drop in heart rate and dilatation (widening) of blood vessels that results in fainting or feeling faint.
Severe low blood pressure due to shock
Shock is a condition where there is a severe drop in blood pressure related to a serious problem, such as massive blood loss, sepsis (an overwhelming bacterial infection) or anaphylaxis (a severe allergic reaction). This is an emergency and needs immediate treatment.
What causes low blood pressure?
Low blood pressure that does not result in symptoms in otherwise healthy, active people often runs in families. This type of low blood pressure – chronic asymptomatic hypotension – is sometimes known as ‘naturally low blood pressure’.
Low blood pressure that causes symptoms may result from a number of different causes, including the following.
- Dehydration due to fluid loss either from sweating in very hot weather or from vomiting and diarrhoea.
- Certain medicines, such as medicines used to treat high blood pressure, some angina medicines, diuretics, other heart medicines, some medicines used for erectile dysfunction and some antidepressants.
- Conditions affecting the heart and lungs, such as a faulty heart valve, abnormal heart rhythms, pulmonary embolism, the weakening of the heart muscle following a heart attack (myocardial infarction), or generalised weakness of the heart muscle due to conditions such as cardiomyopathy.
- Anaemia (a reduced number of red blood cells or a lack of haemoglobin, the substance in red blood cells which transports oxygen around the body).
- Addison’s disease – a condition affecting the adrenal glands, which normally secrete hormones involved in blood pressure control.
- Conditions that can affect the autonomic nervous system, which is involved in controlling blood pressure (such as diabetes, spinal cord injuries or Parkinson’s disease).
- Prolonged immobility (bed rest for an extended period due to illness or injury).
- Significant blood loss.
- Septic shock (a severe infection).
- Anaphylaxis (a severe allergic reaction).
Your doctor will ask about your symptoms and perform a physical examination, including a blood pressure check. They may ask to take your blood pressure while you are sitting and again while you are standing.
It’s important that you give your doctor as much information as possible about the circumstances under which your symptoms occur. Also, tell them about any medical conditions you may have and any prescription or non-prescription medicines you may be taking (including herbs, supplements and eye drops), so that an accurate diagnosis can be made.
Depending on your symptoms and physical examination, initial tests your doctor may recommend include:
- blood tests; and
- an electrocardiogram (ECG) to check your heart rate and rhythm.
Your doctor may refer you to a specialist for further evaluation and treatment if necessary.
Treatment of low blood pressure
Low blood pressure may or may not require treatment, depending on the cause.
Healthy people with low blood pressure that is not causing any symptoms generally do not need any treatment.
If low blood pressure is causing symptoms, the aim of treatment is to bring the blood pressure back to normal to relieve the symptoms, as well as treat the cause. If the underlying cause is a particular type of medicine, your doctor may advise you to stop taking it or reduce the dose.
Treatment of orthostatic hypotension
Orthostatic hypotension may occasionally require treatment with medicines. However, the following self-care measures can often help relieve symptoms.
- Stand up slowly, especially if you have been sitting or lying down for a long time.
- Move your legs around before changing positions.
- Drink plenty of fluids to avoid becoming dehydrated.
- Reduce your intake of alcohol. (Alcohol can contribute to low blood pressure.)
- Avoid very hot showers and baths.
- If you get low blood pressure after eating, sit still or lie down after meals, and try eating smaller amounts more frequently.
- If you have had a period of prolonged bed rest, slowly increase the amount of time you spend sitting up.
In addition, there are steps you can take to prevent symptoms occurring, for example, tilting the bed head upwards, taking salt supplements and wearing supportive (compression) stockings.
Treatment of neurally mediated hypotension
Lifestyle measures can help treat this type of low blood pressure. These include:
- avoiding situations that tend to cause symptoms (such as standing for long periods);
- sitting or lying down when symptoms first start to avoid fainting; and
- keeping well hydrated.
Sometimes medicines are prescribed to normalise blood pressure.
Treatment of sudden severe hypotension (shock)
Serious conditions that cause a sudden drop in blood pressure need urgent medical treatment in hospital. Treatment will depend on the cause, but will often include measures to raise blood pressure. Depending on the cause and severity of the condition, these may include:
- fluids given via a drip into a vein;
- a blood transfusion; or
- medicines to elevate blood pressure.
When to see the doctor
If you have had episodes where you feel faint or have fainted, see your doctor. In many cases, an episode of low blood pressure is nothing to worry about, but some people may have an underlying problem that needs treatment.
Treating low blood pressure will help reduce symptoms and lower the chances of you fainting or falling and injuring yourself.
2. National Heart, Lung and Blood Institute. Hypotension (updated 1 Nov 2010). https://www.nhlbi.nih.gov/health/health-topics/topics/hyp (accessed May 2016).
3. NHS Choices. Low blood pressure (hypotension) (updated 22 May 2015). http://www.nhs.uk/conditions/blood-pressure-(low)/Pages/Introduction.aspx (accessed May 2016).