What is Low Blood Pressure?

What is low blood pressure (hypotension)? 

Low blood pressure is typically defined as a systolic blood pressure (top number, represents the pressure in the blood vessels when the heart muscle squeezes) of less than 90 mmHg, or a diastolic pressure (bottom number, represents the pressure in the blood vessels when the heart relaxes) below 60 mmHg.

However, some people (especially very fit athletes) may have a blood pressure that sits around or below this level naturally without ever having any symptoms or problems.

Others may have symptoms of low blood pressure at levels well above 90/60 mmHg. A common example of this is a condition called postural hypotension (or ‘orthostatic hypotension’). This is when the blood pressure falls quickly when someone stands up from being seated or lying down. To officially be diagnosed with postural hypotension, there should be a fall of at least 20 mmHg in the systolic reading, or at least 10 mmHg in the diastolic reading. The blood pressure itself may still be above 90/60 mmHg but usually the person gets symptoms due to the drop in pressure that occurs.

For example, if your sitting blood pressure is 125/82 mmHg and this drops to 100/75 mmHg when you stand, even though 100/75 mmHg is well above the cut off of 90/60 mmHg and so not officially ‘low’, this fall in blood pressure could still make you feel quite wobbly and would be enough of a drop to be classified as postural hypotension.

Symptoms of low blood pressure:

If your blood pressure drops lower than normal for you, you may experience one of the following symptoms:

• Feeling light-headed, dizzy, or faint
• Blurred vision
• Nausea
• Fainting
• Sweating

There are serious medical conditions that can lead to a blood pressure dropping, such as dehydration, sepsis and other types of shock, and endocrine disorders, not to mention medication side effects.

What causes low blood pressure?

There are many causes of low blood pressure. It might be easiest to think of them in terms of short-term causes and long-term causes. Short-term causes often involve a loss of fluid from the body (think tummy bugs, excessive bleeding, dehydration due to heat or exercise). Problems that tend to be more long term include:

• Problems with how well the heart can pump, e.g., heart failure.
• Problems with the valves inside the heart, e.g., aortic stenosis, mitral valve prolapse.
• Problems with the blood vessels and nerves, e.g., autonomic insufficiency (when the nerves don’t tell the right blood vessels to squeeze, leading to blood pooling in lower half of the body with gravity instead of being pumped to the brain). This can happen in parkinson’s disease and diabetes, as well as with ageing.
• Problems with hormones, e.g., an underactive thyroid gland, diabetes, addison’s disease.
• Medications, e.g., over-treating high blood pressure, or side effects of other medications such as viagra, opioid painkillers, water tablets or even drinking alcohol.

When should I worry about low blood pressure?

If you don’t usually have low blood pressure, but your blood pressure is currently either less than 90 mmHg on the top, or less than 60 mmHg on the bottom, OR it’s a lot lower than normal AND you have any symptoms (including dizziness or feeling light-headed, nausea, visual changes, or you just feel generally unwell), please get medical attention.

A blood pressure that has suddenly dropped low can be due to serious causes, such as sepsis. This is where an infection travels through the bloodstream, damaging multiple organs and making you feel very unwell. Usually, there is a history of a fever and there may also be other symptoms such as confusion or shortness of breath. Other serious causes of a sudden low blood pressure include shock from heart problems like abnormal heart rhythms or even a heart attack.

Low blood pressure, generally, isn’t a serious problem for most people and can even be a sign of good fitness. However, for some it could be a sign of more serious health conditions such as sepsis, hormonal problems, or issues with the heart or blood vessels. 

If you are feeling unwell and have a blood pressure that is newly low or suddenly dropped, please make sure you get the medical attention you need urgently, by seeing your GP (or seeking an after-hours GP), or even ringing 111 for life-threatening symptoms. If you are unsure, you can phone Healthline on 0800 611 116 where a registered nurse can advise you on the assistance you need. As long as it’s safe to do so (i.e. a doctor hasn’t told you to limit how much you drink), having a good drink of fluids at the same time may help.