Causes

Blood pressure is a measurement of the pressure in your arteries during the active and resting phases of each heartbeat.

• Systolic pressure. The top number in a blood pressure reading is the amount of pressure your heart produces when pumping blood through your arteries to the rest of your body.
• Diastolic pressure. The bottom number in a blood pressure reading refers to the amount of pressure in your arteries when your heart is at rest between beats.

Current guidelines identify normal blood pressure as lower than 120/80 mm Hg.

Blood pressure varies throughout the day, depending on:

• Body position
• Breathing rhythm
• Stress level
• Physical condition
• Medications you take
• What you eat and drink
• 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 blood pressure too low only if it causes symptoms.

Some experts define low blood pressure as readings lower than 90 mm Hg systolic or 60 mm Hg diastolic. If either number is below that, your pressure is lower than normal.

A sudden fall in blood pressure can be dangerous. A change of just 20 mm Hg — a drop from 110 systolic to 90 mm Hg systolic, for example — can cause dizziness and fainting when the brain fails to receive enough blood. And big drops, such as those caused by uncontrolled bleeding, severe infections or allergic reactions, can be life-threatening.

Conditions that can cause low blood pressure

Medical conditions that can cause low blood pressure include:

• Pregnancy. Because the circulatory system expands rapidly during pregnancy, blood pressure is likely to drop. This is normal, and blood pressure usually returns to your pre-pregnancy level after you've given birth.
• 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.
• Endocrine problems. Parathyroid disease, adrenal insufficiency (Addison's disease), low blood sugar (hypoglycemia) and, in some cases, diabetes can trigger low blood pressure.
• Dehydration. When your body loses more water than it takes in, it can cause weakness, dizziness and fatigue. Fever, vomiting, severe diarrhea, overuse of diuretics and strenuous exercise can lead to dehydration.
• Blood loss. Losing a lot of blood, such as 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). 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). Common triggers of this severe and potentially life-threatening reaction include foods, certain medications, insect venoms and latex. Anaphylaxis can cause breathing problems, hives, itching, a swollen throat and a dangerous drop in blood pressure.
• Lack of nutrients in your diet. A lack of the vitamin B-12, folate and iron can keep your body from producing enough red blood cells (anemia), causing 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, others)
• Alpha blockers, such as prazosin (Minipress)
• Beta blockers, such as atenolol (Tenormin) and propranolol (Inderal, Innopran XL, others)
• 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, others)

Types of low blood pressure

Doctors often break down low blood pressure (hypotension) into categories, depending on the causes and other factors. Some types of low blood pressure include:

• Low blood pressure on standing up (orthostatic or postural) hypotension). This is a sudden drop in blood pressure when you stand up from a sitting position or after lying down.

  Gravity causes blood to pool in your legs when you stand. Ordinarily, your body compensates by increasing your heart rate and constricting blood vessels, thereby ensuring that enough blood returns to your brain.

  But in people with orthostatic hypotension, this compensating mechanism fails and blood pressure falls, leading to dizziness, lightheadedness, blurred vision and even fainting.

  Orthostatic hypotension can occur for various reasons, including dehydration, prolonged bed rest, pregnancy, diabetes, heart problems, burns, excessive heat, large varicose veins and certain neurological disorders.

  A number of medications also can cause orthostatic 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.

  Orthostatic hypotension is especially common in older adults, but it also affects young, otherwise healthy people who stand up suddenly after sitting with their legs crossed for long periods or after squatting for a time.

• Low blood pressure after eating (postprandial hypotension). This drop in blood pressure occurs one to two hours after eating and affects mostly older adults.

  Blood flows to your digestive tract after you eat. Ordinarily, your body increases your heart rate and constricts 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.

  Eating small, low-carbohydrate meals; drinking more water; and avoiding alcohol might help reduce symptoms.

• Low blood pressure from faulty brain signals (neurally mediated hypotension). This disorder, which causes a blood pressure drop after standing for long periods, mostly affects young adults and children. It seems to occur because of a miscommunication between the heart and the brain.
• Low blood pressure due to nervous system damage (multiple system atrophy with orthostatic hypotension). Also called Shy-Drager syndrome, this rare disorder has many Parkinson disease-like symptoms. It causes progressive damage to the autonomic nervous system, which controls involuntary functions such as blood pressure, heart rate, breathing and digestion. It's associated with having very high blood pressure while lying down.