Postural Hypotension 
Low Blood Pressure when you stand up

Every time the heart beats, it pumps blood through youreries.  Your blood pressure is at its highest when the heart is beating and forcing blood into the arteries.  This is called the systolic pressure. 

When the heart is resting between these beats, your blood pressure then falls. This is known as the diastolic pressure. Blood pressure is always given as two numbers, the systolic and diastolic pressures and both are important. The systolic pressure is the top number, and the diastolic pressure is the bottom number (for example, 120/80). So if your blood pressure is 120/80, you say it is "120 over 80".

When you stand up, your blood at once tries to pool in the veins in your legs and if this is allowed to occur it would be a bad idea, because there would then be a shortage of blood in the rest of your body.

This pooling of blood in the leg veins is stopped by your nerves which contract the leg veins. These nerves ensure enough blood returns to your heart and there is no decrease in the quantity of blood your heart is pumping. This then maintains the blood supply needed to your brain and other parts of your body.

However, if there is pooling of blood in your veins and less blood returns to your heart, less is then pumped out which means a reduction in the amount of blood going to your brain and other parts of your body. This reduction will cause symptoms of dizziness and some people may also faint as a result.

This is precisely what happens to guards on parade when they collapse. It is because they have been standing in one position for too long. You can copy this effect when you sit on your heels for a while, for instance when looking at books on a low shelf, and then stand up suddenly. (You will feel very giddy).

If you have a low blood pressure and you also show symptoms of dizziness or faintness when standing suddenly you should have your blood pressure checked both when you are lying down and when you then stand up. This drop in blood pressure when you stand up is called postural hypotension.

The commonest causes for postural hypotension are:

• Acute sickness, such as a severe blood loss, sudden severe infection or damage to the heart might cause low blood pressure when sitting up or standing. When this happens blood pressure levels are a significant measure of the severity of the illness and are why blood pressure is measured so often in acutely ill people.
  
• Blood pressure lowering drugs. Many modern blood pressure lowering drugs do not cause a fall in blood pressure when you stand up. The only class of drugs commonly used nowadays that may cause a slump in standing pressure are alpha blockers, such as doxazosin.
  
• If you are an older person, and particularly if you have diabetes, you may already have a tendency for your blood pressure to go down when you stand, but your high blood pressure tablets may make this condition slightly worse.
  
• If you get severe food poisoning with sickness and diarrhoea, you will probably lose large amounts of fluid from your body. This encourages high blood pressure tablets to lower your blood pressure even further.  If you do get severe food poisoning it is important to check your blood pressure and stop the tablets if your blood pressure is lower than 110/70.
  
• If you suffer a drop in your blood pressure when you stand up, but still need to continue with your high blood pressure tablets, then you will have to learn to slow down. For instance, when lying flat in bed, sit up slowly, then stand up slowly and avoid sudden attempts to stand up without support.
  
• Rare diseases of the nerves controlling reflexes in the veins can prevent the veins from contracting when you stand up. This can result in a severe drop in blood pressure but thankfully this is very rare. If you have a severe drop in blood pressure when you stand and you are otherwise well (ie, not on any blood pressure lowering drugs) then you should see your doctor to have further investigations.
  
• Diabetes can also cause a lot of damage to the nerves which supply the blood vessels; they can therefore be associated with a drop in pressure when standing up.
  
• Increasing age is another factor.  As you get older, your arteries tend to stiffen up slightly and become less supple. This may occasionally cause a fall in blood pressure when you stand.
  
• A malfunction of the adrenal glands can be another problem.  The adrenal glands are two very small glands just above the kidneys that produce hormones, one of which is called aldosterone. This hormone controls the salt content in your body.
  
• If the adrenal glands become injured or fail, it can mean a lack of aldosterone in your body. This then leads to a loss of salt from your body which can factor in low blood pressure, with accompanying dizziness or faintness when you stand up suddenly. This condition is very uncommon, but an important to pick up as; it can easily be treated by replacing the absent hormones. Investigations will only be completed if you suffer symptoms suggesting a fall in blood pressure when you stand up, such as dizziness or faintness. If you do complain of these symptoms then your doctor will conduct further tests or refer you to a specialist.

If you are taking prescribed blood pressure lowering drugs, they may need to be changed, particularly if you are prescribed an alpha blocker such as doxazosin. 

You will need to discuss this with your doctor or practice nurse first though to see if this is best for you. Many people find just by getting up slowly their symptoms can be avoided.