Blood pressure that is too low is known as hypotension. The similarity in pronunciation with hypertension can cause confusion.

Low arterial pressure may be a sign of severe disease and requires urgent medical attention.

When arterial pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness and fainting.

However, people who function well, while maintaining low arterial pressures have lower rates of cardiovascular disease events than people with normal arterial pressures.

Reduced blood volume, called hypovolemia, is the most common mechanism producing hypotension. This can result from hemorrhage, or blood loss; insufficient fluid intake, as in starvation; or excessive fluid losses from diarrhea or vomiting. Hypovolemia is often induced by excessive use of diuretics. (Other medications can produce hypotension by different mechanisms.)

Decreased cardiac output despite normal blood volume, due to severe congestive heart failure, large myocardial infarction, or bradycardia, often produces hypotension and can rapidly progress to cardiogenic shock. Arrhythmias often result in hypotension by this mechanism. Beta blockers can cause hypotension both by slowing the heart rate and by decreasing the pumping ability of the heart muscle. Varieties of meditation and/or other mental-physiological disciplines can create temporary hypotension effects, as well, and should not be considered unusual.

Excessive vasodilation, or insufficient constriction of the resistance blood vessels (mostly arterioles), causes hypotension. This can be due to decreased sympathetic nervous system output or to increased parasympathetic activity occurring as a consequence of injury to the brain or spinal cord or of dysautonomia, an intrinsic abnormality in autonomic system functioning. Excessive vasodilation can also result from sepsis, acidosis, or medications, such as nitrate preparations, calcium channel blockers, angiotensin II receptor inhibitors or ACE inhibitors. Many anesthetic agents and techniques, including spinal anesthesia and most inhalational agents, produce significant vasodilation.

Syndromes

Orthostatic hypotension, also called “postural hypotension”, is a common form of low blood pressure. It occurs after a change in body position, typically when a person stands up from either a seated or lying position. It is usually transient and represents a delay in the normal compensatory ability of the autonomic nervous system. It is commonly seen in hypovolemia and as a result of various medications. In addition to blood pressure-lowering medications, many psychiatric medications, in particular antidepressants, can have this side effect. Simple blood pressure and heart rate measurements while lying, seated, and standing can confirm the presence of orthostatic hypotension.

Neurocardiogenic syncope is a form of dysautonomia characterized by an inappropriate drop in blood pressure while in the upright position. Neurocardiogenic syncope is related to vasovagal syncope in that both occur as a result of increased activity of the vagus nerve, the mainstay of the parasympathetic nervous system.

Another, but rarer form, is Postprandial hypotension, which occurs 30–75 minutes after eating substantial meals. When a great deal of blood is diverted to the intestines to facilitate digestion and absorption, the body must increase cardiac output and peripheral vasoconstriction in order to maintain enough blood pressure to perfuse vital organs, such as the brain. It is believed that postprandial hypotension is caused by the autonomic nervous system not compensating appropriately, because of ageing or a specific disorder.

Treatment

The treatment for hypotension depends on its cause. Asymptomatic hypotension in healthy people usually does not require treatment. Severe hypotension needs to be aggressively treated because reduced blood flow to critical organs including the brain, heart and kidneys may cause organ failure and can ultimately lead to death. Treatment options include systemic vasoconstrictors and other drugs. Adding sodium to a diet can also relieve symptoms of mild hypotension.