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Low blood pressure is a term for blood pressure that is abnormally low.

What is going on in the body?

Blood pressure is expressed in millimeters of mercury, abbreviated as mm Hg. The systolic blood pressure is the top number of a blood pressure reading. This shows the maximum pressure in the blood vessels as the heart contracts and circulates blood throughout the body. The diastolic blood pressure is the bottom number of a blood pressure reading. It shows the lowest pressure in the blood vessels between heartbeats, when the heart is at rest.

Many conditions can cause low blood pressure.

What are the signs and symptoms of the condition?

Low blood pressure may or may not cause symptoms. Many symptoms get worse when the person stands. Common symptoms include:

• lightheadedness or dizziness
• excessive sweating
• fainting
• an abnormal awareness of the heartbeat, called palpitations

Other symptoms depend on the cause. In general, if a person's blood pressure is high enough for the person not to experience symptoms, it is high enough.

What are the causes and risks of the condition?

There are many possible causes of this condition, including:

• medications, such as those used to treat high blood pressure. In this case, the medication may work too well and make the blood pressure too low. Many other medications, such as diazepam (i.e., Valium) or amitriptyline (i.e., Elavil), can cause low blood pressure in some people.
• heart conditions, such as abnormal heartbeats called arrhythmias or congestive heart failure. Cardiac tamponade, a condition in which fluid builds up around the heart and prevents it from pumping well, is another possible cause.
• excessive blood loss
• dehydration, which may be due to vomiting, diarrhea, or not getting enough fluids
• a very serious blood infection known as sepsis
• hypothermia, or an abnormally low body temperature
• severe allergic reactions of any kind, often called anaphylaxis
• low levels of oxygen in the blood, called hypoxia, which may be due to severe asthma, pneumonia, or other conditions
• a hormone imbalance known as adrenal insufficiency, known as Addison disease
• high levels of acid in the blood, such as respiratory acidosis or metabolic acidosis
• nervous system conditions. One example is diabetic neuropathy, or nerve damage caused by high glucose levels when diabetes is not well controlled. Serious head injuries or other nervous system damage may also cause low blood pressure.

Other causes are also possible. In some cases, no cause can be found. Some people always have a blood pressure that is low compared to other people, but normal for them. They may not have any symptoms from their low blood pressure because it is normal for them.

What can be done to prevent the condition?

Prevention is related to the cause. Avoiding dehydration can prevent cases due to this cause. Avoiding medications known to cause low blood pressure can prevent cases due to this cause. A person who takes medication to treat high blood pressure is instructed not to double up on medication if he or she misses a dose, because this can lead to low blood pressure.

How is the condition diagnosed?

Blood pressure is usually measured with a tool called a blood pressure cuff. Other special devices are sometimes used. Sometimes, the reason for low blood pressure is obvious to the healthcare professional from the history and physical exam. In other cases, further tests are needed. For example, a complete blood count (CBC) can help diagnose infections. Low oxygen in the blood can be detected by arterial blood gases. A heart tracing (ECG) can help diagnose abnormal heartbeats. A chest x-ray can help diagnose pneumonia.

What are the long-term effects of the condition?

Severely low blood pressure may result in temporary or permanent damage to different organs, known as shock. The damage occurs when the blood pressure drops below the pressure needed to maintain blood circulation. Death may occur in serious cases.

Other long-term effects are related to the cause. For example, diabetes can cause damage to many areas of the body, including the heart, eyes, and kidneys. Low blood pressure due to medications often goes away as soon as the medications are stopped, and may have no long-term effects.

What are the risks to others?

Low blood pressure is not contagious and poses no risks to others.

What are the treatments for the condition?

In most cases, treating the cause will correct the low blood pressure. Stopping or reducing the dose of a medication may end medication-related low blood pressure. Treating infections with antibiotics or surgery may stop low blood pressure from this cause. Giving fluids will stop low blood pressure due to dehydration. Blood transfusions may be needed to treat cases due to extensive blood loss.

Extremely low blood pressure is often treated directly to try to raise the blood pressure. This is done to prevent organ damage from lack of proper blood flow. Fluids and medications are given through an intravenous line, a tube connected to a vein in the arm or other area.

What are the side effects of the treatments?

Side effects are related to the treatments used. For example, antibiotics may cause allergic reactions or stomach upset. Medications used to raise blood pressure may cause irregular heartbeats, or arrhythmias. Surgery can be complicated by bleeding, infection, or reactions to anesthesia. Blood transfusions carry a risk of allergic reaction and infection.

What happens after treatment for the condition?

In cases of extensive blood loss or severe infection, death may occur if treatment is delayed or ineffective. Permanent organ damage may occur with severely low blood pressure. This may cause a person to be disabled. In other cases, such as those due to medications, stopping the medication cures the low blood pressure and no further treatment is needed.

How is the condition monitored?

Blood pressure can be rechecked as often as needed. Symptoms can also be monitored. Blood and other tests may be used to monitor for organ damage from very low blood pressure. Other monitoring depends on the cause. For example, a person with diabetes is advised to monitor blood sugar levels regularly.