What Is Hypotension?

Hypotension is abnormally low blood pressure. Normal blood pressure is a reading of less than 120/80 mmHg (mmHg = millimeters of mercury, a unit for measuring pressure). Hypotension is blood pressure that is lower than 90/60 mmHg.

Some people have low blood pressure all the time. They have no signs or symptoms and their low readings are normal for them. In other people, blood pressure drops below normal because of some event or medical condition. Hypotension is a medical concern only if it causes signs and/or symptoms such as dizziness, fainting, or, in extreme cases, shock.

What Is Blood Pressure?

Blood is carried from the heart to all parts of your body in vessels called arteries. Blood pressure is the force of the blood pushing against the walls of the arteries. Each time the heart beats (about 60–70 times a minute in adults at rest), it pumps blood out into the arteries. Your blood pressure is at its highest when the heart beats, pumping the blood. This is called systolic (sis-TOL-ik) pressure. When the heart is at rest, between beats, your blood pressure falls. This is the diastolic (di-a-STOL-ik) pressure.

Blood pressure is always given as these two numbers, the systolic and diastolic pressures. Both are important. Usually they are written one above or before the other, such as 120/80 mmHg. When the two measurements are written down, the systolic pressure is the first or top number, and the diastolic pressure is the second or bottom number (for example, 120/80). If your blood pressure is 120/80, you say that it is “120 over 80.”

Blood pressure changes during the day. It lowers as you sleep and rises when you wake up. It also can rise when you are excited, nervous, or active.

The body is very sensitive to changes in blood pressure. Special cells in the arteries, called baroreceptors (BAR-o-re-SEP-ters), can sense if blood pressure begins to rise or drop. When the baroreceptors sense a rise or drop in blood pressure, they cause certain responses to occur throughout the body in an attempt to bring the blood pressure back to normal.

For example, if you stand up quickly, the baroreceptors will sense a drop in your blood pressure. They quickly take action to make sure that blood continues to flow to the brain, kidneys, and other important organs. The baroreceptors cause the heart to beat faster and harder. They also cause the small arteries (arterioles) and veins (the vessels that carry blood back to the heart) to narrow.

Most forms of hypotension happen when the body can’t bring blood pressure back to normal or can’t do it fast enough.

Outlook

In a healthy person, hypotension without signs or symptoms is usually not a problem and requires no treatment. Doctors will want to identify and treat any underlying condition that is causing the hypotension, if one can be found. Hypotension can be dangerous if a person falls because of dizziness or fainting.

Shock, a severe form of hypotension, is a life-threatening condition that is often fatal if not treated immediately. Shock can be successfully treated if the cause can be found and the right treatment provided in time.

August 2006