Hypotension


Hypotension, also known as low blood pressure, is a cardiovascular condition characterized by abnormally reduced blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood and is indicated by two numbers, the systolic blood pressure and the diastolic blood pressure, which are the maximum and minimum blood pressures within the cardiac cycle, respectively. A systolic blood pressure of less than 90 millimeters of mercury or diastolic of less than 60 mmHg is generally considered to be hypotension. Different numbers apply to children. However, in practice, blood pressure is considered too low only if noticeable symptoms are present.
Symptoms may include dizziness, lightheadedness, confusion, feeling tired, weakness, headache, blurred vision, nausea, neck or back pain, an irregular heartbeat or feeling that the heart is skipping beats or fluttering, and fainting. Hypotension is the opposite of hypertension, which is high blood pressure. It is best understood as a physiological state rather than a disease. Severely low blood pressure can deprive the brain and other vital organs of oxygen and nutrients, leading to a life-threatening condition called shock. Shock is classified based on the underlying cause, including hypovolemic shock, cardiogenic shock, distributive shock, and obstructive shock.
Hypotension can be caused by strenuous exercise, excessive heat, low blood volume, hormonal changes, widening of blood vessels, anemia, vitamin B12 deficiency, anaphylaxis, heart problems, or endocrine problems. Some medications can also lead to hypotension. There are also syndromes that can cause hypotension in patients including orthostatic hypotension, vasovagal syncope, and other rarer conditions.
For many people, excessively low blood pressure can cause dizziness and fainting or indicate serious heart, endocrine or neurological disorders.
For some people who exercise and are in top physical condition, low blood pressure could be normal.
A single session of exercise can induce hypotension, and water-based exercise can induce a hypotensive response.
Treatment depends on the cause of the low blood pressure. Treatment of hypotension may include the use of intravenous fluids or vasopressors. When using vasopressors, trying to achieve a mean arterial pressure of greater than 70 mmHg does not appear to result in better outcomes than trying to achieve an MAP of greater than 65 mmHg in adults.

Signs and symptoms

For many people, low blood pressure goes unnoticed. For some people, low blood pressure may be a sign of an underlying health condition, especially when it drops suddenly or occurs with symptoms. Older adults also have a higher risk of symptoms of low blood pressure, such as falls, fainting, or dizziness when standing or after a meal. If the blood pressure is sufficiently low, fainting may occur.
Low blood pressure is sometimes associated with certain symptoms, many of which are related to causes rather than effects of hypotension:
Low blood pressure can be caused by low blood volume, hormonal changes, pregnancy, widening of blood vessels, medicine side effects, severe dehydration, anemia, vitamin B12 deficiency, anaphylaxis, heart problems or endocrine problems.
Reduced blood volume, hypovolemia, is the most common cause of hypotension. This can result from hemorrhage; insufficient fluid intake, as in starvation; or excessive fluid losses from diarrhea or vomiting. Hypovolemia can be induced by excessive use of diuretics. Low blood pressure may also be attributed to heat stroke which can be indicated by absence of perspiration, light headedness and dark colored urine.
Other medications can produce hypotension by different mechanisms. Chronic use of alpha blockers or beta blockers can lead to hypotension. Beta blockers can cause hypotension both by slowing the heart rate and by decreasing the pumping ability of the heart muscle.
Decreased cardiac output despite normal blood volume, due to severe congestive heart failure, a large heart attack, heart valve problems, or extremely low heart rate, often produces hypotension and can rapidly progress to cardiogenic shock. Arrhythmias often result in hypotension by this mechanism.
Excessive vasodilation, or insufficient constriction of the blood vessels, 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. Dysautonomia, an intrinsic abnormality in autonomic system functioning, can also lead to hypotension. Excessive vasodilation can also result from sepsis, acidosis, or medications, such as nitrate preparations, calcium channel blockers, or AT1 receptor antagonists. Many anesthetic agents and techniques, including spinal anesthesia and most inhalational agents, produce significant vasodilation.
Lower blood pressure is a side effect of certain herbal medicines, which can also interact with several medications. An example is the theobromine in Theobroma cacao, which lowers blood pressure through its actions as both a vasodilator and a diuretic, and has been used to treat high blood pressure.
Hypotension can be a symptom of mast cell activation syndrome.

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. Taking these measurements is known as orthostatic vitals. Orthostatic hypotension is indicated if there is a drop of 20 mmHg in systolic pressure and a 20 beats per minute increase in heart rate.

Vasovagal syncope

is a form of dysautonomia characterized by an inappropriate drop in blood pressure while in the upright position. Vasovagal syncope occurs as a result of increased activity of the vagus nerve, the mainstay of the parasympathetic nervous system. Patients will feel sudden, unprovoked lightheadedness, sweating, changes in vision, and finally a loss of consciousness. Consciousness will often return rapidly once the patient is lying down and the blood pressure returns to normal.

Other

Another, but rarer form, is postprandial hypotension, a drastic decline in blood pressure that occurs 30 to 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 to maintain enough blood pressure to perfuse vital organs, such as the brain. Postprandial hypotension is believed to be caused by the autonomic nervous system not compensating appropriately, because of aging or a specific disorder.
Hypotension is a feature of Flammer syndrome, which is characterized by cold hands and feet and predisposes to normal tension glaucoma.
Hypotension can be a symptom of relative energy deficiency in sport, sometimes known as the female athlete triad, although it can also affect men.

Pathophysiology

Blood pressure is continuously regulated by the autonomic nervous system, using an elaborate network of receptors, nerves, and hormones to balance the effects of the sympathetic nervous system, which tends to raise blood pressure, and the parasympathetic nervous system, which lowers it. The vast and rapid compensation abilities of the autonomic nervous system allow normal individuals to maintain an acceptable blood pressure over a wide range of activities and in many disease states. Even small alterations in these networks can lead to hypotension.

Diagnosis

For most adults, the optimal blood pressure is at or below 120/80 mmHg. If the systolic blood pressure is <90 mmHg or the diastolic blood pressure is <60 mmHg, it would be classified as hypotension. However, occasional blood pressure readings below 90/60 mmHg are not infrequent in the general population, and, in the absence of some pathological cause, hypotension appears to be a relatively benign condition in most people. The diagnosis of hypotension is usually made by measuring blood pressure, either non-invasively with a sphygmomanometer or invasively with an arterial catheter. Another way to diagnose low blood pressure is by using the mean arterial pressure measured using an arterial catheter or by continuous, non-invasive hemodynamic monitoring which measures intra-operative blood pressure beat-by-beat throughout surgery. A MAP <65 mmHg is considered hypotension. Intra-operative hypotension <65 mmHg can lead to an increased risk of acute kidney injury, myocardial injury or post-operative stroke. While an incidental finding of hypotension during a routine blood pressure measurement may not be particularly worrying, a substantial drop in blood pressure following standing, exercise, or eating can be associated with symptoms and may have implications for future health. A drop in blood pressure after standing, termed postural or orthostatic hypotension, is defined as a decrease in supine-to-standing BP >20 mm Hg systolic or >10 mm Hg diastolic within 3 minutes of standing. Orthostatic hypotension is associated with increased risk of future cardiovascular events and mortality. Orthostatic vitals are frequently measured to assist with the diagnosis of orthostatic hypotension, and may involve the use of a tilt table test to evaluate vasovagal syncope.