What causes very low blood pressure?

Blood pressure is measured in millimeters of mercury. The first or upper number is known as the systolic blood pressure. It is the pressure that occurs in the arteries when the heart muscle contracts and pumps blood.

A normal systolic is 120 or less. Blood pressure of 120 to 140 is considered pre-hypertensive. A systolic over 140 is considered hypertensive.

The diastolic is the second or lower number. It is the pressure in the arteries when the heart relaxes and fills with blood. A diastolic of less than 90 is considered normal.

If you are using an automatic blood pressure cuff, it may be reading the diastolic lower than it actually is. This is often a problem if the blood pressure cuff is too big or too small for your arm.

Low blood pressure is only a concern when a patient has symptoms such as being lightheaded, a change in vision, losing consciousness, shortness of breath, or poor exercise tolerance. Patients most commonly complain of these symptoms when standing up from a sitting position or sitting up after lying down for awhile. This is called orthostatic hypotension and usually lasts only a short time. Patients with these symptoms need to seek medical attention.

Hypotension (low blood pressure) is most commonly caused by medications. Blood pressure medications, antidepressants and other drugs can impair the nerves of blood vessels in the legs. Normally these nerves make blood vessels in the leg contract when one is rising. When the blood vessels do not contract when changing position, blood is pooled in the legs. This deprives some blood volume from the heart, and the symptoms occur due to decreased blood perfusion of the brain.

Some patients have symptomatic low blood pressure due to a low amount of fluid in their body. This is called hypovolemia. It can be due to dehydration, caused by sweating from exercise, decreased oral intake, or use of a diuretic. Diuretics are commonly used to treat high blood pressure, but one of the most common diuretics that people encounter is caffeine.

Uncommon reasons for a low blood pressure include problems of the heart. One way that most people’s bodies deal with a small amount of dehydration is to increase the heart rate, but some patients have a heart problem that does not allow them to increase their heart rate. This is often treated with a pacemaker.

Congestive heart failure is often associated with high blood pressure but can also be associated with symptomatic low blood pressure. New onset congestive heart failure is an emergency.

Very rarely, the cause of the symptomatic low blood pressure cannot be determined. There are some drug therapies that are used in these cases. Increased salt intake is often a simple solution.

Low blood pressure without any symptoms merits a medical evaluation, but rarely needs to be treated.

Conditions Expert 
Dr. Otis Brawley Chief Medical Officer,
American Cancer Society