Low blood pressure is generally considered a blood pressure reading lower than 90 millimeters of mercury (mm Hg) for the top number (systolic) or 60 mm Hg for the bottom number (diastolic).
What's considered low blood pressure for one person might be OK for someone else. Low blood pressure might cause no noticeable symptoms, or it might cause dizziness and fainting. Sometimes, low blood pressure can be life-threatening.
The causes of low blood pressure range from dehydration to serious medical conditions. It's important to find out what's causing low blood pressure so that it can be treated, if necessary.
Types of low blood pressure include:
- Orthostatic hypotension (postural hypotension). This is a sudden drop in blood pressure when standing from a sitting position or after lying down. Causes include dehydration, long-term bed rest, pregnancy, certain medical conditions and some medications. This type of low blood pressure is common in older adults.
- Postprandial hypotension. This drop in blood pressure occurs 1 to 2 hours after eating. It's most likely to affect older adults, especially those with high blood pressure or autonomic nervous system diseases such as Parkinson's disease. Eating small, low-carbohydrate meals, drinking more water, and avoiding alcohol might help reduce symptoms.
- Neurally mediated hypotension. This is a blood pressure drop that happens after standing for long periods. This type of low blood pressure mostly affects young adults and children. It might result from miscommunication between the heart and the brain.
- Multiple system atrophy with orthostatic hypotension. Also called Shy-Drager syndrome, this rare disorder affects the nervous system that controls involuntary functions such as blood pressure, heart rate, breathing and digestion. It's associated with having very high blood pressure while lying down.
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Low blood pressure (hypotension) symptoms may include:
- Blurred or fading vision
- Dizziness or lightheadedness
- Trouble concentrating
For some people, low blood pressure may be a sign of an underlying health condition, especially when it drops suddenly or occurs with symptoms.
A sudden fall in blood pressure can be dangerous. A change of just 20 mm Hg — a drop from 110 mm Hg systolic to 90 mm Hg systolic, for example — can cause dizziness and fainting. And big drops, such as those caused by uncontrolled bleeding, severe infections or allergic reactions, can be life-threatening.
Extreme low blood pressure can lead to a condition known as shock. Symptoms of shock include:
- Confusion, especially in older people
- Cold, clammy skin
- Decrease in skin coloration (pallor)
- Rapid, shallow breathing
- Weak and rapid pulse
When to see a doctor
If you have symptoms of extreme low blood pressure (hypotension) or shock, seek emergency medical help.
Most health care providers consider blood pressure to be too low only if it causes symptoms. Occasional minor dizziness or lightheadedness can be caused by many things, such as spending too much time in the sun or in a hot tub. It's important to see a health care provider to get a correct diagnosis.
If you have consistently low blood pressure readings but feel fine, your provider may just monitor you during routine health checkups. It can be helpful to keep a record of your symptoms, when they occur and what you're doing at the time.
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Blood pressure is determined by the amount of blood the heart pumps and the amount of resistance to blood flow in the arteries. A blood pressure measurement is given in millimeters of mercury (mm Hg). It has two numbers:
- Systolic pressure. The first (upper) number is the pressure in the arteries when the heart beats.
- Diastolic pressure. The second (bottom) number is the pressure in the arteries when the heart rests between beats.
The American Heart Association categorizes ideal blood pressure as normal. An ideal blood pressure is usually lower than 120/80 mm Hg.
Blood pressure varies throughout the day, depending on:
- Body position
- Food and drink
- Physical condition
- Time of day
Blood pressure is usually lowest at night and rises sharply on waking. Certain health conditions and use of medications may cause low blood pressure.
Conditions that can cause low blood pressure
Medical conditions that can cause low blood pressure include:
- Pregnancy. Changes during pregnancy cause blood vessels to expand rapidly. The changes may cause blood pressure to drop. Low blood pressure is common in the first 24 weeks of pregnancy. Blood pressure usually returns to pre-pregnancy levels after giving birth.
- Heart and heart valve conditions. A heart attack, heart failure, heart valve disease and an extremely low heart rate (bradycardia) can cause low blood pressure.
- Hormone-related diseases (endocrine disorders). Conditions affecting the parathyroid or adrenal glands, such as Addison's disease, may cause blood pressure to drop. Low blood sugar (hypoglycemia) and, sometimes, diabetes also may lower blood pressure.
- Dehydration. When the body doesn't have enough water, the amount of blood in the body (blood volume) decreases. This can cause blood pressure to drop. Fever, vomiting, severe diarrhea, overuse of diuretics and strenuous exercise can lead to dehydration.
- Blood loss. Losing a lot of blood, such as from an injury or internal bleeding, also reduces blood volume, leading to a severe drop in blood pressure.
- Severe infection (septicemia). When an infection in the body enters the bloodstream, it can lead to a life-threatening drop in blood pressure called septic shock.
- Severe allergic reaction (anaphylaxis). Symptoms of a severe allergic reaction include a sudden and dramatic drop in blood pressure.
- Lack of nutrients in the diet. Low levels of vitamin B-12, folate and iron can keep the body from producing enough red blood cells (anemia), which can lead to low blood pressure.
Medications that can cause low blood pressure
Some medications can cause low blood pressure, including:
- Water pills (diuretics), such as furosemide (Lasix) and hydrochlorothiazide (Microzide)
- Alpha blockers, such as prazosin (Minipress)
- Beta blockers, such as atenolol (Tenormin) and propranolol (Inderal, Innopran XL, Hemangeol)
- Drugs for Parkinson's disease, such as pramipexole (Mirapex) or those containing levodopa
- Certain types of antidepressants (tricyclic antidepressants), including doxepin (Silenor) and imipramine (Tofranil)
- Drugs for erectile dysfunction, including sildenafil (Revatio, Viagra) or tadalafil (Adcirca, Alyq, Cialis), particularly when taken with the heart medication nitroglycerin (Nitrostat, Nitro-Dur, Nitromist)
Anyone can have low blood pressure (hypotension). Risk factors for hypotension include:
- Age. Drops in blood pressure on standing or after eating occur primarily in adults older than 65. Neurally mediated hypotension primarily affects children and younger adults.
- Medications. Certain medications, including some blood pressure drugs, increase the risk of low blood pressure.
- Certain diseases. Parkinson's disease, diabetes and some heart conditions may increase risk of low blood pressure.
Potential complications of low blood pressure (hypotension) include:
- Injury from falls
Severely low blood pressure can reduce the body's oxygen levels, which can lead to heart and brain damage.