Your blood pressure is the force exerted by your heart, against the resistance created by the arteries, to keep blood flowing through your body. Blood pressure measurement consists of two parts – systolic and diastolic.
The systolic pressure represents the pressure during heart contraction (heart beat) and the diastolic pressure the pressure when the heart relaxes between beats.
Doctors typically define normal blood pressure as 120/80 mm Hg. The first figure is the systolic pressure and the second the diastolic ('120 over 80'). The measurement unit, millimetres of mercury (symbol Hg), refers to the distance that pressure raises a column of mercury in the measurement instrument.
In general, the lower your blood pressure, the better – as long as you feel well. It's 'normal' for some petite women to have blood pressure around 90/60 mm Hg.
If the force with which your heart contracts or the resistance in your arteries decreases, your blood pressure will also decrease as a result. Should your blood pressure drop below a certain level, the blood supply to vital organs, such as the brain, kidneys and heart, may decrease. This may lead to the malfunctioning of these organs, because they depend on oxygen and other nutrients carried to them in the blood. Blood pressure that's abnormally low or that causes light-headedness or fainting is called hypotension.
Cause and risk factors
Acute causes of low blood pressure comprise:
- Shock, which may be caused by:
- Severe infection
- Heart attack
- Severe dehydration, which may be caused by:
- Heat exposure
- Overuse of diuretic medication (water tablets)
There are several more chronic causes of low blood pressure:
- Conditions affecting the autonomic nervous system – this may include diabetes and its complications. However, autonomic (automatic/reflex) nervous system dysfunction may sometimes cause low blood pressure in people who are otherwise healthy, as in the case of orthostatic hypotension. This is an excessive drop in blood pressure upon rising, which may lead to light-headedness or even fainting. Gravity normally causes your blood pressure to drop slightly when you stand up from a sitting or lying position. Your autonomic nervous system normally accommodates this effect by increasing the tone (constriction) of your blood vessels and by increasing your heart rate. There are many possible reasons why this adjustment may fail.
- Heart failure – this condition is usually accompanied by shortness of breath, ankle swelling or other physical signs.
- Excessive treatment with anti-hypertensive medication – drugs for high blood pressure, such as calcium channel blockers and beta-blockers. Beta-blockers can cause dizziness if combined with exercise, since they prevent the body's usual response to exercise, which is to increase the heart rate.
- Chronic dehydration – a result of prolonged fasting or fad diets.
- Pooling of blood in the peripheral venous system – after exercise, when a large proportion of blood pools in the muscles, or in people with varicose veins.
- Obstruction of the aorta (outflow vessel of the heart) – due to a diseased heart valve or an overgrowth of heart muscle.
- Addison's syndrome – a deficiency of adrenal steroids (rare).
- Lack of upright activity – prolonged bed rest during illness or disability may cause inadequate stimulation of the autonomic reflexes that control blood pressure. It may take some time to readjust to an upright position. (Astronauts who return to earth after a period of near-zero gravity commonly experience low blood pressure upon standing once they return to earth.)
If you suffer from any of the conditions above or are on anti-hypertensive treatment, you are at risk of developing hypotension. These are only some of the possible causes of low blood pressure; other causes depend on the age and gender of the affected person.
Hypotension can result in light-headedness or fainting. It's not unusual to
feel light-headed when you suddenly stand up after sitting or lying down for a
long time. But if these sensations persist while standing, your heart and blood
vessels aren't responding quickly enough.
In many people, blood pressure under 90 mm Hg systolic can cause occasional
faintness, dizziness and loss of consciousness.
More severe forms of hypotension associated with a serious underlying disease
process can lead to prolonged loss of consciousness and symptoms related to
organ failure, such as renal and liver failure.
The outcome of low blood pressure is closely related to its cause.
Severely low blood pressure of sudden onset is life threatening because of the associated decreased blood and oxygen supply to the vital organs. Tissue death in areas of the affected organs may lead to stroke, heart attack or renal failure.
Severely low blood pressure of sudden onset is usually related to a serious underlying disease process such as massive haemorrhage (bleeding), extensive heart attack or serious infection.
Less severe hypotension, resulting in dizziness, fainting spells and lethargy, usually has no serious consequences, although these symptoms can be troublesome.
When to see a doctor
- Severe bleeding or symptoms of a heart attack or heart failure need emergency attention – contact the nearest emergency service immediately.
- If there is any loss of consciousness or fainting, a health care advisor must be consulted as a matter of urgency.
- Persistent tiredness and lack of energy might be related to low blood pressure and should be brought to the attention of your doctor or health care provider.
A health care provider can easily diagnose hypotension by taking your blood pressure with a blood pressure monitor/cuff. It may be necessary to take several blood pressure readings in various positions – lying, sitting and standing – to establish whether there is a drop in blood pressure upon standing.
Blood pressure does, however, normally vary during the course of a 24-hour period and the reading taken at any given time might differ from the actual level during the symptomatic period.
A 24-hour ambulatory blood pressure recording might be required. A portable blood pressure monitor will be fixed to your arm and your blood pressure will be recorded at regular intervals over 24 hours.
Blood pressure measurement is invariably part of the medical assessment of someone who is ill and often gives an indication of the severity of the underlying disease. Extremely low blood pressure of sudden onset is a poor prognostic sign.
The treatment of low blood pressure often depends on the underlying cause:
- The dosage of antihypertensive medication may be adjusted if it is too high.
- Fluid and blood may be administered intravenously if there has been massive blood loss.
- Wearing elastic stockings can prevent pooling of blood in varicose veins.
- Various drugs are available for the improvement of tone in peripheral vessels when there is autonomic nervous system dysfunction – but they are not very effective.
- If you forcefully contract your abdominal muscles as you rise, it will 'pump' blood from the periphery to the central circulation system and to the brain and may prevent orthostatic hypotension.
(Reviewed by Dr Mark Abelson)
The information provided in this article was correct at the time of publishing. At Mediclinic we endeavour to provide our patients and readers with accurate and reliable information, which is why we continually review and update our content. However, due to the dynamic nature of clinical information and medicine, some information may from time to time become outdated prior to revision.