Hypotension is caused by conditions or events that interfere with the body’s ability to control blood pressure.

Orthostatic Hypotension

Orthostatic hypotension has many causes. Sometimes, two or more causes combined will result in hypotension.

Dehydration is the most common cause of orthostatic hypotension. Dehydration occurs when the body loses more water than it takes in. People can become dehydrated because of:

• Not drinking enough fluids
• Fever
• Vomiting
• Severe diarrhea
• Excessive sweating from strenuous exercise

Some medicines used to treat high blood pressure and heart disease can make it more likely that a person will develop orthostatic hypotension. These medicines include:

• Diuretics
• Calcium channel blockers
• Angiotensin-converting enzyme (ACE) inhibitors
• Angiotensin II receptor blockers
• Nitrates
• Beta blockers

Also, medicines used to treat certain other medical conditions, such as anxiety, depression, erectile dysfunction, and Parkinson disease, can make it more likely that a person will develop orthostatic hypotension.

Other substances that can contribute to orthostatic hypotension include alcohol, barbiturates, and some prescription and over-the-counter medicines, when taken in combination with high blood pressure medicines.

Certain medical conditions can increase a person's chances of having orthostatic hypotension. Some of these conditions are:

• Anemia (low red blood cell count).
• Heart conditions leading to heart failure, such as a heart attack or viral infection of the heart. These conditions reduce the heart’s ability to pump enough blood around the body.
• Heart valve disorders.
• Severe infections.
• Endocrine conditions, such as hypothyroidism (underactive thyroid), hyperthyroidism (overactive thyroid), Addison’s disease (adrenal insufficiency), low blood sugar, and diabetes.
• Disorders of the central nervous system, such as Parkinson disease, multiple systems atrophy (Shy-Drager syndrome), and amyloidosis.
• Pulmonary embolism (a sudden blockage in a lung artery).

Finally, other events or conditions that can contribute to orthostatic hypotension include:

• Being out in the heat for a long time
• Having to stay in bed for a long time because of a medical condition
• Being pregnant
• Getting older (the body doesn’t manage changes in blood pressure as well as it gets older)

Neurally Mediated Hypotension

Neurally mediated hypotension (NMH) occurs when the brain and heart don’t communicate with each other properly. For example, when a person stands for a long time, blood begins to pool in the legs. This causes the person’s blood pressure to drop. Instead of telling the brain that blood pressure is low, the body mistakenly tells the brain that blood pressure is high. In response, the brain slows the person’s heart rate, which makes the blood pressure drop even further, causing dizziness and other symptoms.

Severe Hypotension Associated With Shock

Severe hypotension associated with shock can be caused by many conditions or events. Some of these conditions and events also are causes of orthostatic hypotension. The difference in shock is that the blood pressure doesn’t return to normal by itself, and it is at dangerously low levels. Shock is a medical emergency that must be treated immediately.

Certain severe infections can cause shock. This is known as septic shock. This type of shock can occur when bacteria enter the bloodstream. The bacteria release a toxin (a poison) that leads to a dangerous drop in blood pressure.

Shock can be caused by a severe decrease in the amount of blood or fluids in the body. This is known as hypovolemic shock. Hypovolemic shock can happen as a result of:

• Major bleeding on the outside of the body (for example, from an injury)
• Major bleeding inside the body (for example, from a ruptured blood vessel)
• Significant loss of body fluids from severe burns
• Severe inflammation of the pancreas
• Severe diarrhea
• Severe kidney disease
• Excessive use of diuretics

A major decrease in the heart’s ability to pump blood can cause shock. This is known as cardiogenic shock. It can be caused by a heart attack, pulmonary embolism, or arrhythmia.

A sudden and extreme relaxation of the muscles of arteries, which leads to dilation (widening) of the arteries and a drop in blood pressure, can cause shock. This is known as vasodilatory shock. It can happen because of:

• Severe head injury
• Reaction to some medicines
• Liver failure
• Poisoning
• Severe allergic reaction (anaphylactic shock)