Low Blood Pressure - When Blood Pressure Is Too Low

How low is too low for blood pressure?
Within certain limits, the lower your blood pressure reading is, the better. There is also no specific number at which day-to-day blood pressure is considered too low, as long as none of the symptoms of trouble are present.

Symptoms of low blood pressure 
Most doctors will only consider chronically low blood pressure as dangerous if it causes noticeable signs and symptoms, such as:

Dizziness or lightheadedness
Nausea
Fainting (syncope)
Dehydration and unusual thirst
Dehydration can sometimes cause blood pressure to drop. However, dehydration does not automatically signal low blood pressure. Fever, vomiting, severe diarrhea, overuse of diuretics and strenuous exercise can all lead to dehydration, a potentially serious condition in which your body loses more water than you take in. Even mild dehydration (a loss of as little as 1 percent to 2 percent of body weight) can cause weakness, dizziness and fatigue.
Lack of concentration
Blurred vision
Cold, clammy, pale skin
Rapid, shallow breathing
Fatigue
Depression
Underlying causes of low blood pressure 
Low blood pressure can occur with:

Prolonged bed rest
Pregnancy
During the first 24 weeks of pregnancy, it’s common for blood pressure to drop.
Decreases in blood volume
A decrease in blood volume can also cause blood pressure to drop. A significant loss of blood from major trauma, dehydration or severe internal bleeding reduces blood volume, leading to a severe drop in blood pressure.

Certain medications

A number of drugs can cause low blood pressure, including diuretics and other drugs that treat hypertension; heart medications such as beta blockers; drugs for Parkinson’s disease; tricyclic antidepressants; erectile dysfunction drugs, particularly in combination with nitroglycerine; narcotics and alcohol. Other prescription and over-the-counter drugs may cause low blood pressure when taken in combination with high blood pressure medications.

Heart problems
Among the heart conditions that can lead to low blood pressure are an abnormally low heart rate (bradycardia), problems with heart valves, heart attack and heart failure. Your heart may not be able to circulate enough blood to meet your body’s needs.

Endocrine problems
Such problems include complications with hormone-producing glands in the body’s endocrine systems; specifically, an underactive thyroid (hypothyroidism), parathyroid disease, adrenal insufficiency (Addison’s disease), low blood sugar and, in some cases, diabetes.

Severe infection (septic shock)
Septic shock can occur when bacteria leave the original site of an infection (most often in the lungs, abdomen or urinary tract) and enter the bloodstream. The bacteria then produce toxins that affect blood vessels, leading to a profound and life-threatening decline in blood pressure.

Allergic reaction (anaphylaxis)
Anaphylactic shock is a sometimes-fatal allergic reaction that can occur in people who are highly sensitive to drugs such as penicillin, to certain foods such as peanuts or to bee or wasp stings. This type of shock is characterized by breathing problems, hives, itching, a swollen throat and a sudden, dramatic fall in blood pressure.

Neurally mediated hypotension
Unlike orthostatic hypotension, this disorder causes blood pressure to drop after standing for long periods, leading to symptoms such as dizziness, nausea and fainting. This condition primarily affects young people and occurs because of a miscommunication between the heart and the brain.

Nutritional deficiencies
A lack of the essential vitamins B-12 and folic acid can cause anemia, which in turn can lead to low blood pressure.
If you notice a sudden decline in blood pressure

A single lower-than-normal reading is not cause for alarm, unless you are experiencing any other symptoms or problems. If you experience any dizziness, lightheadedness, nausea or other symptoms, it’s a good idea to consult with your healthcare provider. To help with your diagnosis, keep a record of your symptoms and activities at the time they occurred.

Source- 

http://www.heart.org/HEARTORG/Conditions/HighBloodPressure/AboutHighBloodPressure/Low-Blood-Pressure_UCM_301785_Article.jsp#