Low blood pressure, or hypotension, generally refers to a blood pressure reading below an average of about 100/60 mmHg. However, it is often of little significance, unless it causes symptoms, and is often only identified as a result of a routine medical examination.

How low is too low?

It is much better to have low blood pressure rather than high blood pressure, and it may even prolong life expectancy. However, extremely low blood pressure can be associated with dizziness, light-headedness or fainting. Many cases get better without any treatment. However, if they 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).

- The volume of blood in the blood vessels. A decreased amount of blood in the body is called hypovolaemia.

- The capacity of the blood vessels (as determined by their internal diameter).

What causes excessively low blood pressure?

Excessively low blood pressure may result from a number of different causes. It may be due to reduced cardiac output resulting from 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 cardiomyopathy.

It may also result from reduced blood volume (such as might occur through dehydration or massive blood loss); or it might be through excessive dilation of the blood vessels (as could occur in response to septic shock, anaphylaxis (a severe allergic reaction), exposure to heat, conditions of the nervous system or to certain medications). Pregnant women also often have low blood pressure.

Specific diseases associated with low blood pressure

Abnormally low blood pressure may be associated with a number of different causes, such as:

- Vasovagal syncope (the common type of fainting brought on by stress or pain);

- Hypoaldosteronism (deficiency of a steroid hormone produced by the adrenal glands, as can occur with Addison's disease);

- Parkinson’s disease.

Addison’s disease may also be associated with low blood pressure as a result of excessive diarrhoea, urination or sweating. Untreated diabetes or spinal cord injuries may cause hypotension as a result of damage to the nerves regulating blood vessel diameter.

Medicines causing low blood pressure

Hypotension may also be caused by certain medicines, such as antidepressants, diuretics and drugs that dilate the blood vessels (for example, angiotensin converting enzyme [ACE] inhibitors, calcium channel blockers and nitrates).

Orthostatic hypotension

Orthostatic hypotension is an inability to regulate blood pressure quickly. It is characterised by a sudden fall in blood pressure upon standing up, which causes fainting due to a reduced blood flow to the brain. Fatigue, exercise, or excessive food or alcohol intake may make the symptoms worse.

Orthostatic hypotension is more common in the elderly, and can be caused by an overuse of diuretics, vasodilators, or other types of medications (including pain relievers). It can also result from dehydration, or excessive bed rest.

Treatment of low blood pressure

Low blood pressure may or may not require treatment, depending on the cause. If the underlying cause is a particular type of medication, your doctor may advise you to stop taking it or reduce the dose.

Symptoms of dizziness and fainting in young people without heart disease are usually nothing to worry about. Such symptoms may be more serious in elderly people as they may have a number of interrelated problems that make it difficult to maintain constant blood pressure.

Orthostatic hypotension may occasionally require treatment with medications. However, non-medical management of the condition is often sufficient. These measures may include avoiding things that may make the condition worse — such as standing up too quickly, dehydration, hot showers, or prolonged bed rest — as well as taking steps to prevent symptoms occurring by, for example, tilting the bed head upwards, taking salt supplements or wearing supportive hose.

Conditions such as Addison’s disease or heart failure require specific treatment.

Correct diagnosis

It is important that you provide your doctor with as much information as possible about the circumstances under which your symptoms occur, any medical conditions you may have, and any prescription or non-prescription drugs you may be taking (including herbs, supplements and eye drops), so that an accurate diagnosis can be made.