Heart disease health centre

Understanding low blood pressure: Symptoms, diagnosis and treatment

What is low blood pressure?

Hypotension is the medical term for low blood pressure (less than 90/60 mm/Hg).

A blood pressure reading appears as two numbers. The first and higher of the two is a measure of systolic pressure, or the pressure in the arteries when the heart beats and fills them with blood. The second number measures diastolic pressure, or the pressure in the arteries when the heart rests between beats.

Normal blood pressure is usually in the range of 120/80 mmHg (systolic/diastolic). In healthy people, especially athletes, low blood pressure is a sign of good cardiovascular (heart and blood vessel) health. But low blood pressure can be a sign of an underlying problem - especially in elderly people - where it may cause inadequate blood flow to the heart, brain and other vital organs.

Chronic low blood pressure with no symptoms is almost never serious. But health problems occur when blood pressure drops suddenly, and the brain is deprived of an adequate blood supply. This can lead to dizziness or light-headedness. Sudden drops in blood pressure most commonly occur in someone who's rising from a prone or sitting position to standing. This kind of low blood pressure is known as postural hypotension, orthostatic hypotension, or neurally mediated orthostatic hypotension.

Postural hypotension is considered a failure of the autonomic nervous system - the part of the nervous system that controls involuntary vital actions, such as heartbeat - to react appropriately to sudden changes. Normally, when you stand up, some blood pools in your lower extremities. Uncorrected, this would cause your blood pressure to fall. But your body normally compensates by sending messages to your heart to beat faster and to your blood vessels to constrict. This offsets the drop in blood pressure. If this does not happen, or happens too slowly, postural hypotension results.

The risk of both low and high blood pressure normally increases with age, due in part to normal changes during ageing. In addition, blood flow in the brain declines with age, often as a result of plaque build-up in blood vessels. An estimated 10 to 20% of people over age 65 have postural hypotension.

What causes low blood pressure?

The cause of low blood pressure isn't always clear. It may be associated with the following:

• Pregnancy
• Hormonal problems, such as an underactive thyroid (hypothyroidism), overactive thyroid (hyperthyroidism), diabetes, or low blood sugar (hypoglycaemia)
• Over-the-counter medication
• Overdose of high blood pressure medication
• Heart failure
• Heart arrhythmias (abnormal heart rhythms)
• Widening, or dilation, of the blood vessels
• Heat exhaustion or heat stroke
• Liver disease

What causes a sudden drop in blood pressure?

Sudden drops in blood pressure can be life-threatening. Causes of this type of hypotension include:

• Loss of blood from bleeding
• Low body temperature
• High body temperature
• Heart muscle disease causing heart failure
• Sepsis, a severe blood infection
• Severe dehydration from vomiting, diarrhoea, or high temperature
• A reaction to medication or alcohol
• A severe allergic reaction, called anaphylaxis