Blood pressure lower than 120/80 mm Hg is considered "normal," and the term “low blood pressure” is relative. (Note: Your blood pressure is usually lowest at night and rises sharply upon waking.) Low blood pressure is generally considered dangerous when it drops suddenly or is accompanied by symptoms, such as dizziness or fainting. Severely low blood pressure can indicate serious heart, endocrine or neurological disorders and can deprive the brain and other vital organs of oxygen and nutrients, leading to shock, which can be a life-threatening condition.
Symptoms of low blood pressure to watch for include:
* Dizziness or lightheadedness
* Fainting (called syncope)
* Lack of concentration
* Blurred vision
* Cold, clammy, pale skin
* Rapid, shallow breathing
* Unusual thirst
There is no specific number at which blood pressure is considered to be too low. Most doctors consider chronically low blood pressure dangerous only if it causes noticeable signs and symptoms. However, a sudden fall in blood pressure can be dangerous -- even a change of just 20 mm Hg can cause dizziness or fainting. Some rapid falls in blood pressure indicate a deeper underlying problem such as uncontrolled bleeding, severe infections or allergic reaction.
Factors that can contribute to low blood pressure:
Pregnancy. During the first 24 weeks of pregnancy, it's common for blood pressure to drop.
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; Viagra®, particularly in combination with nitroglycerine; narcotics and alcohol. Other prescription and over-the-counter medications may cause low blood pressure when taken in combination with high blood pressure drugs.
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. These are conditions in which your heart may not be able to circulate enough blood to meet your body's needs.
Endocrine problems. These include an underactive or overactive thyroid (hypothyroidism and hyperthyroidism), adrenal insufficiency (Addison's disease), low blood sugar and, in some cases, diabetes.
Dehydration. 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.
Blood loss. A significant loss of blood from major trauma or severe internal bleeding reduces blood volume, leading to a severe drop in blood pressure.
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 your 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 characterised by breathing problems, hives, itching, a swollen throat and a sudden, dramatic fall in blood pressure.
Postural (orthostatic) hypotension. In some people, blood pressure drops rapidly when standing from a sitting or prone position, causing dizziness, lightheadedness, blurred vision and even fainting. Causes can include dehydration, prolonged bed rest, diabetes, heart problems and excessive heat. Medications like diuretics, beta blockers, calcium channel blockers, ACE inhibitors, antipsychotics, antidepressants and drugs for Parkinson's disease can also cause this condition. In some cases, sitting for long periods of time with legs crossed or squatting can be the cause.
Postprandial hypotension. A sudden drop in blood pressure after a meal usually affects older adults with high blood pressure or autonomic nervous system disorders such as Parkinson's disease. Lowering the dose of blood-pressure-lowering medication and eating small, low-carbohydrate meals may help reduce symptoms.
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.
When to see the doctor
If you experience any dizziness or lightheadedness, you may want to see your doctor. If you have gotten dehydrated, have low blood sugar or spent too much time in the sun or a hot tub, it's more important how quickly your blood pressure drops than how low it drops. Keep a record of your symptoms and your activities at the time your symptoms occurred.
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