Low blood pressure (hypotension)
Mayo Clinic staff
Blood pressure is a measurement of the pressure in your arteries during the
active and resting phases of each heartbeat. Here's what the numbers mean:
- Systolic pressure. The first (top) number in a blood
pressure reading, this is the amount of pressure your heart generates when
pumping blood through your arteries to the rest of your body.
- Diastolic pressure. The second (bottom) number in a blood
pressure reading, this refers to the amount of pressure in your arteries
when your heart is at rest between beats.
Current guidelines identify normal blood pressure as equal to or lower than
120/80 — many experts think 115/75 is even better.
Although you can get an accurate blood pressure reading at any given time,
blood pressure isn't always the same. It can vary considerably in a short amount
of time — sometimes from one heartbeat to the next, depending on body
position, breathing rhythm, stress level, physical condition, medications you
take, what you eat and drink, and even time of day. Blood pressure is usually
lowest at night and rises sharply on waking.
Blood pressure: How low can you go?
What's considered low blood pressure for you may be normal for someone else.
Most doctors consider chronically low blood pressure too low only if it causes
Some experts define low blood pressure as readings lower than 90 systolic or
60 diastolic — you need to have only one number in the low range for your
blood pressure to be considered lower than normal. In other words, if your
systolic pressure is a perfect 115, but your diastolic pressure is 50, you're
considered to have lower than normal pressure.
A sudden fall in blood pressure can also be dangerous. A change of just 20 mm
Hg — a drop from 110 systolic to 90 systolic, for example — can cause
dizziness and fainting when the brain fails to receive an adequate supply of
blood. And big plunges, especially those caused by uncontrolled bleeding, severe
infections or allergic reactions, can be life-threatening.
Athletes and people who exercise regularly tend to have lower blood pressure
and a slower heart rate than do people who aren't as fit. So, in general, do
nonsmokers and people who eat a healthy diet and maintain a normal weight.
But in some rare instances, low blood pressure can be a sign of serious, even
Conditions that can cause low blood pressure
Some medical conditions can cause low blood pressure. These include:
- Pregnancy. Because a woman's circulatory system expands
rapidly during pregnancy, blood pressure is likely to drop. During the first
24 weeks of pregnancy, systolic pressure commonly drops by five to 10 mm Hg
and diastolic pressure by as much as 10 to 15 mm Hg. This is normal, and
blood pressure usually returns to your pre-pregnancy level after you've
- Heart problems. Some heart conditions that can lead to
low blood pressure include extremely low heart rate (bradycardia), heart
valve problems, heart attack and heart failure. These conditions may cause
low blood pressure because they prevent your body from being able to
circulate enough blood.
- Endocrine problems. An underactive thyroid
(hypothyroidism) or overactive thyroid (hyperthyroidism) can cause low blood
pressure. In addition, other conditions, such as adrenal insufficiency
(Addison's disease), low blood sugar (hypoglycemia) and, in some cases,
diabetes, can trigger low blood pressure.
Dehydration. When you become dehydrated, your body loses
more water than it takes in. Even mild dehydration can cause weakness,
dizziness and fatigue. Fever, vomiting, severe diarrhea, overuse of
diuretics and strenuous exercise can all lead to dehydration.
Far more serious is hypovolemic shock, a life-threatening complication of
dehydration. It occurs when low blood volume causes a sudden drop in blood
pressure and a reduction in the amount of oxygen reaching your tissues. If
untreated, severe hypovolemic shock can cause death within a few minutes or
- Blood loss. Losing a lot of blood from a major injury or
internal bleeding reduces the amount of blood in your body, leading to a
severe drop in blood pressure.
- Severe infection (septicemia). Septicemia can happen when
an infection in the body enters the bloodstream. These conditions can lead
to a life-threatening drop in blood pressure called septic shock.
- Severe allergic reaction (anaphylaxis). Anaphylaxis is a
severe and potentially life-threatening allergic reaction. Common triggers
of anaphylaxis include foods, certain medications, insect venoms and latex.
Anaphylaxis can cause breathing problems, hives, itching, a swollen throat
and a drop in blood pressure.
- Lack of nutrients in your diet. A lack of the vitamins
B-12 and folate can cause anemia, a condition in which your body doesn't
produce enough red blood cells, causing low blood pressure.
Medications that can cause low blood pressure
Some medications you may take can also cause low blood pressure, including:
- Diuretics (water pills)
- Alpha blockers
- Beta blockers
- Drugs for Parkinson's disease
- Certain types of antidepressants (tricyclic antidepressants)
- Sildenafil (Viagra), particularly in combination with the heart
Types of low blood pressure
Doctors often break down low blood pressure (hypotension) into different
categories, depending on the causes and other factors. Some types of low blood
Low blood pressure on standing up (postural or orthostatic
hypotension). This is a sudden drop in blood pressure when you
stand up from a sitting position or if you stand up after lying down.
Ordinarily, gravity causes blood to pool in your legs whenever you stand.
Your body compensates for this by increasing your heart rate and
constricting blood vessels, thereby ensuring that enough blood returns to
your brain. But in people with postural hypotension, this compensating
mechanism fails and blood pressure falls, leading to symptoms of dizziness,
lightheadedness, blurred vision and even fainting.
Postural hypotension can occur for a variety of reasons, including
dehydration, prolonged bed rest, pregnancy, diabetes, heart problems, burns,
excessive heat, large varicose veins and certain neurological disorders. A
number of medications can also cause postural hypotension, particularly
drugs used to treat high blood pressure — diuretics, beta blockers,
calcium channel blockers and angiotensin-converting enzyme (ACE) inhibitors
— as well as antidepressants and drugs used to treat Parkinson's disease
and erectile dysfunction.
Postural hypotension is especially common in older adults, with as many
as 20 percent of those over age 65 experiencing postural hypotension. But
postural hypotension can also affect young, otherwise healthy people who
stand up suddenly after sitting with their legs crossed for long periods or
after working for a time in a squatting position.
Low blood pressure after eating (postprandial hypotension).
Postprandial hypotension is a sudden drop in blood pressure after eating. It
affects mostly older adults.
Just as gravity pulls blood to your feet when you stand, a large amount
of blood flows to your digestive tract after you eat. Ordinarily, your body
counteracts this by increasing your heart rate and constricting certain
blood vessels to help maintain normal blood pressure. But in some people
these mechanisms fail, leading to dizziness, faintness and falls.
Postprandial hypotension is more likely to affect people with high blood
pressure or autonomic nervous system disorders such as Parkinson's disease.
Lowering the dose of blood pressure drugs and eating small, low-carbohydrate
meals may help reduce symptoms.
Low blood pressure from faulty brain signals (neurally mediated
hypotension). This disorder causes blood pressure to drop after
standing for long periods, leading to signs and symptoms such as dizziness,
nausea and fainting.
Neurally mediated hypotension mostly affects young people, and it seems
to occur because of a miscommunication between the heart and the brain. When
you stand for extended periods, your blood pressure falls as blood pools in
your legs. Normally, your body then makes adjustments to normalize your
blood pressure. But in people with neurally mediated hypotension, nerves in
the heart's left ventricle actually signal the brain that blood pressure is
too high, rather than too low. As a result, the brain lessens the heart
rate, decreasing blood pressure even further. This causes more blood to pool
in the legs and less blood to reach the brain, leading to lightheadedness
- Low blood pressure due to nervous system damage (multiple system
atrophy with orthostatic hypotension). Also called Shy-Drager
syndrome, this rare disorder causes progressive damage to the autonomic
nervous system, which controls involuntary functions such as blood pressure,
heart rate, breathing and digestion. Although this condition can be
associated with muscle tremors, slowed movement, problems with coordination
and speech, and incontinence, its main characteristic is severe orthostatic
hypotension in combination with very high blood pressure when lying down.