Causes of Low Blood Pressure

 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 lower than 120/80 mm Hg.
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 noticeable symptoms. Some experts define low blood pressure as readings lower than 90 mm Hg systolic or 60 mm Hg 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 also 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 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 life-threatening disorders.

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. 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.
  These conditions may cause low blood pressure because they prevent your body from being able to circulate enough blood.
   
  
• Endocrine problems. Thyroid conditions — such as parathyroid disease — 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 hours.
  
  
• 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. This condition 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 a condition in which your body doesn't produce enough red blood cells (anemia), 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), such as furosemide (Lasix) and hydrochlorothiazide (Microzide, Oretic)
• Alpha blockers, such as prazosin (Minipress) and labetalol
• Beta blockers, such as atenolol (Tenormin), propranolol (Inderal, Innopran XL, others) and timolol
• Drugs for Parkinson's disease, such as pramipexole (Mirapex) or those containing levodopa
• Certain types of antidepressants (tricyclic antidepressants), including doxepin (Silenor), imipramine (Tofranil), protriptyline (Vivactil) and trimipramine (Surmontil)
• Sildenafil (Viagra) or tadalafil (Cialis), particularly in combination with the heart medication nitroglycerin

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 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 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 orthostatic hypotension, this compensating mechanism fails and blood pressure falls, leading to symptoms of dizziness, lightheadedness, blurred vision and even fainting.
  
  Orthostatic 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 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, with as many as 20 percent of those older than age 65 experiencing orthostatic hypotension.
  But orthostatic 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 and fainting.
  
  
• 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.
  
  
  Source:
  
  http://www.mayoclinic.org/diseases-conditions/low-blood-pressure/basics/causes/con-20032298