Low Blood Pressure
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Your blood pressure
Your heart pumps blood around your body through a circulatory system that is made up of tubes called arteries and capillaries. The blood flows back to your heart through a network of veins.
When your heart contracts (squeezes in) it pushes blood into your arteries, causing an increase in pressure. At this stage, the pressure in your arteries is at its highest, and is known as the systolic pressure. When your heart relaxes and refills with blood, the pressure in your arteries decreases. This is known as the diastolic pressure. Both your systolic and diastolic pressures are measured in terms of millimetres of mercury (mmHg).
Throughout the day, your blood pressure can vary by between 30-40 mmHg (both systolic and diastolic) depending on what you are doing. When you are asleep, or relaxed, your blood pressure will be at its lowest. When you exercise, or you are stressed or anxious, your blood pressure will increase. Therefore, each time you have your blood pressure measured, it is important that the test is carried out under similar conditions so that the results are consistent.
The highs and lows
For a young, healthy adult, normal blood pressure is about 110/70, but generally, the lower your blood pressure is, the better. If you have a reading of 140/90 or more, you have high blood pressure, also known as hypertension. If your blood pressure is high, your GP may give you advice about how to lower it by making some simple lifestyle changes, such as altering your diet, giving up smoking, and taking regular exercise. Alternatively, you may be prescribed drugs that will help to reduce your blood pressure.
Low blood pressure is also known as hypotension. People with a reading of around 90/60, or less, are commonly regarded as having low blood pressure. If you have low blood pressure, to a certain degree you have some protection against factors that increase blood pressure, such as eating too much salt, not eating enough fruit and vegetables, or being overweight. However, some people who have low blood pressure may experience symptoms, and there may be an underlying cause.
Symptoms
On its own, low blood pressure does not always cause symptoms. If you have low blood pressure, and do not have any symptoms, you do not require treatment. However, sometimes if your blood pressure is too low, there may not be enough blood flowing to your brain, and other vital organs. As a result, you may experience symptoms such as dizziness and fainting. If you do, your GP will try to find out whether there is an underlying cause for your symptoms.
As well as dizziness and fainting, other symptoms of hypotension may include:
- light-headedness,
- blurred vision,
- palpitations,
- confusion,
- nausea,
- general weakness, and a
- temporary loss of consciousness.
Postural, or orthostatic hypotension
You may feel dizzy, or faint, after changing posture - for example, when you sit up from a lying position, or stand up from a sitting position. However, the fall in blood pressure usually only lasts for a few minutes as it adjusts to your new posture. This is known as postural, or orthostatic hypotension, and can affect you more as you get older. Similar symptoms may also occur after exercise.
Postprandial hypotension
Dizziness, light-headedness, faintness, and falls, are symptoms that can sometimes occur after eating, as a result of low blood pressure. This condition, known as postprandial hypotension, tends to occur more often in older people, particularly in those who have high blood pressure, or a condition such as Parkinsons disease, or diabetes.
After a meal, the intestines need a large amount of blood for digestion, and so the heart rate increases, and the blood vessels in other parts of the body constrict (narrow) to help maintain blood pressure. However, the heart rate of some elderly people may not increase enough, and their blood vessels may not constrict enough to maintain blood pressure. As a result, their blood pressure falls. Lying down after eating, and eating frequent, small, low-carbohydrate meals may help to reduce the effects of postprandial hypotension.
Causes
There are a number of things that can cause low blood pressure. They include:
- the use of certain medications, such as anti-depressants, and medicines to treat high blood pressure,
- diabetes mellitus which can cause damage to the nerves that supply your blood vessels, resulting in a fall in blood pressure when you stand up (postural or orthostatic hypotension),
- serious injuries, such as burns, or those that cause severe blood loss, and lead to shock, and a reduction in blood volume,
- serious illnesses, or conditions, such as a heart attack, or adrenal gland failure,
- rare nerve conditions that affect the nerves in your legs can cause a severe drop in blood pressure when you stand up (postural or orthostatic hypotension),
- increasing age - as you get older, your arteries become stiffer which can cause your blood pressure to drop, particularly when you stand up, and
- pregnancy during the early to mid stages of pregnancy, low pressure is fairly common.
Blood pressure lowering medicines
Nowadays, most blood pressure lowering medicines do not cause a drop in blood pressure when you stand up. Alpha-blockers, such as doxazosin, are the only type of medicine that may cause a decrease in your standing blood pressure. You should have your standing blood pressure checked, if you are taking doxazosin, and you feel dizzy, or faint, when you stand up.
In the past, it was thought that low blood pressure could cause tiredness, depression, and anxiety. However, recent studies have found no strong evidence to suggest that low blood pressure causes these symptoms.
Adrenal gland failure
The adrenal glands are two small glands that are located just above your kidneys. They produce a number of hormones, including aldosterone, which controls the amount of salt in your body. If your adrenal glands become damaged, the production of aldosterone may be reduced, resulting in a loss of salt from your body. This can lead to low blood pressure, and may make you feel dizzy, or faint, if you stand up too quickly. Adrenal gland failure is rare but, if it is diagnosed, it can be treated by increasing the amount of aldosterone.
Serious illnesses
Sometimes, severe illnesses that cause infection, blood loss, or heart damage, may result in low blood pressure, particularly when sitting up or standing. If you have an acute illness, your blood pressure will be measured regularly because it is a good indicator of the severity of your illness.
