Health Topics A-Z

What Is Low Blood Pressure?

Hypotension is the medical term for low blood pressure (less than 90/60). Normal blood pressure is usually in the range of 120/80 (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 signal of an underlying problem — especially in the elderly. In this population, low blood pressure may cause inadequate blood flow to the heart, brain and other vital organs.



Chronic low blood pressure 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 lightheadedness. It most commonly occurs in someone who's rising from a prone or sitting position to a standing position. In such cases, it 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 the heartbeat — to react appropriately to sudden changes. When you stand up, some blood pools in your lower extremities. Uncorrected, this would cause your blood pressure to fall. However, normally your body 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 incidence of both low and high blood pressure normally increases with age, due in part to normal changes. In addition, blood flow in the brain declines with age, often as a result of plaque buildup in blood vessels. So the prevalence of postural hypotension also increases with age; an estimated 10-20% of elderly people have postural hypotension.

What Causes It?
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, and low blood sugar (hypoglycemia).
• Many over-the-counter medications.
• Overdose of high blood pressure drugs.
• Heart failure.
• Heart arrhythmias (abnormal heart rhythms).
• Widening, or dilation, of the blood vessels.
• Heat exhaustion or heat stroke.
• Liver disease.

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

• Loss of blood (bleeding).
• Low body temperature.
• High body temperature.
• Heart muscle disease causing heart failure.
• Sepsis, a severe blood infection.
• Severe dehydration.
• A reaction to medication.
• Severe allergic reaction (anaphylaxis).

Postural hypotension can happen to anyone for a variety of reasons, such as dehydration, lack of food, prolonged standing in the heat, or being generally run down. It can also be influenced by genetic make-up, aging, medication, dietary and psychological factors, and acute triggers, such as infection and allergy.

Postural hypotension occurs most frequently in people who are taking drugs to control high blood pressure (hypertension). It can also be related to pregnancy, strong emotions, hardening of the arteries (atherosclerosis), or diabetes. The elderly are particularly affected, especially those who have high blood pressure or autonomic nervous dysfunction. Postprandial (after meals) hypotension is estimated to affect up to one-third of elderly people and is a common cause of dizziness and falls after eating.

Several drugs are commonly associated with postural hypotension. These medications can be divided into two major categories 1) medications and 2) other common causes:

• Drugs used to treat high blood pressure, such as diuretics, beta-blockers, calcium channel blockers, and angiotensin-converting enzyme (ACE) inhibiting drugs.
• Drugs that have hypotension as a side effect, including nitrates, drugs for Parkinson's disease, antipsychotics, neuroleptics, anti-anxiety agents, sedative-hypnotics, and tricyclic antidepressants.

Common causes of naturally occurring postural hypotension include:

• Dehydration and electrolyte loss, which may result from another condition, such as diarrhea or vomiting or excessive blood loss during female menstruation.
• Age-associated decline in blood pressure regulation, which may be worsened by certain health conditions or medications.

Common causes of pathologic (attributable to a disease) postural hypotension include:

• Central nervous system disorders, such as Shy-Drager syndrome (multiple system neuropathy).
• Peripheral neuropathy and autonomic neuropathy (nerve problems), such as may be caused by diabetes.
• Cardiovascular disorders.
• Alcoholism.
• Nutritional diseases.

Less common causes include diseases such as amyloidosis (caused by deposits of a waxy substance called amyloid in the body), vitamin deficiencies, spinal cord injuries, and neuropathies associated with cancer, particularly lung cancer or pancreatic cancer.