Usually a systolic blood pressure reading below 90 mm of Hg or a diastolic blood pressure reading below 60 mm of Hg is considered a low blood pressure or hypotension. Some people may normally have a low blood pressure, especially if they do regular exercises. Presence of symptoms is more important than just a low blood pressure reading. Symptoms signify inadequate supply of oxygen to vital organs like brain and heart. If there is no underlying condition causing it and the person is not having any symptoms, no therapy is indicated for low blood pressure.

Low blood pressure can be caused by numerous conditions. Some of the most commonly encountered ones are dehydration, heart disease, acute blood loss, pregnancy, shock, anti-hypertensive drugs and endocrine diseases.

Heart Disease

• Numerous heart diseases can result in low blood pressure. Most common ones are heart failure, heart attack, pericardial diseases, cardiac arrhythmias (heart rate too low or too high, other arrhythmias), valvular dysfunctions (e.g. Aortic Stenosis)

Dehydration

• One of the most common causes of low blood pressure. Loss of fluids and electrolytes due to any cause (e.g. heat stroke) can lower the blood pressure.

Pregnancy

• The blood pressure of a pregnant woman drops during the first and second trimester. This is normal physiological phenomenon and requires no intervention unless the drop is significant and symptomatic. The blood pressure normalizes again after childbirth.

Shocks (various types)

• Shock is a serious condition characterized by rapid fall in blood pressure to very low levels and inadequate supply of oxygen to the organs of the body. There are various subtypes of shocks depending upon the cause, but all share the above feature. Acute blood loss can reduce the circulating volume of blood and cause hypovolemic shock. Similarly internal bleeding can also result in hypovolemic shock. Severe infection in the blood can result in septicemic shock. Anaphylaxis is the shock associated with severe allergic reaction. Any shock is a life-threatening medical emergency and requires urgent medical care.

Drugs

• Hypotension is an important adverse effect of many drugs. Anti-hypertensive drugs can sometimes lower the blood pressure too much, resulting in hypotension. There are numerous other drugs like sildenafil, some Anti-Parkinson drugs, etc. that can cause hypotension.

Endocrine Diseases

• Some endocrine disorders like hypothyroidism and hyperthyroidism, adrenal insufficiency, etc. can result in low blood pressure.

There is another somewhat different type of low blood pressure condition known as orthostatic hypotension or postural hypotension. In this, the low blood pressure is temporary and the blood pressure falls only when the person is standing up from a lying down or sitting position. This sudden fall in blood pressure may cause dizziness or even fainting. Similarly, there is postprandial hypotension, in which the blood pressure falls after a meal. The fall in blood pressure may give rise to symptoms of low blood pressure. Both of these conditions are more common in old age.

General approach to management of low blood pressure is first an evaluation for an underlying cause. Treatment is given only if there are symptoms or the underlying condition is associated with increased risk of complications in the future. Asymptomatic low blood pressure with no underlying serious cause requires no treatment. As already mentioned, any type of shock is a serious life-threatening emergency and requires immediate medical care.