In physiology and medicine, hypotension is low blood pressure, especially in the arteries
of the systemic circulation. Blood pressure is the force of blood pushing against the
walls of the arteries as the heart pumps out blood. Hypotension is generally considered
systolic blood pressure less than 90 millimeters of mercury or diastolic less than 60 mm Hg.
However, in practice, blood pressure is considered too low only if noticeable symptoms are present.
Hypotension is the opposite of hypertension, which is high blood pressure. It is best understood
as a physiological state, rather than a disease. It is often associated with shock, though
not necessarily indicative of it. For some people who exercise and are in top
physical condition, low blood pressure is a sign of good health and fitness. For many
people, excessively low blood pressure can cause dizziness and fainting or indicate serious
heart, endocrine or neurological disorders. Severely low blood pressure can deprive the
brain and other vital organs of oxygen and nutrients, leading to a life-threatening condition
called shock.
Signs and symptoms The cardinal symptoms of hypotension include
lightheadedness or dizziness. If the blood pressure is sufficiently low,
fainting and often seizures occur. Low blood pressure is sometimes associated
with certain symptoms, many of which are related to causes rather than effects of hypotension:
Causes Low blood pressure can be caused by low blood
volume, hormonal changes, widening of blood vessels, medicine side effects, anemia, 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
is often induced by excessive use of diuretics. Low blood pressure may also be attributed
to heat stroke. The body may have enough fluid but does not retain electrolytes. Absence
of perspiration, light headedness and dark coloured urine are also indicators.
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,
large myocardial infarction, 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. Some heart conditions can lead to low blood
pressure, including extremely low heart rate, heart valve problems, heart attack and heart
failure. These conditions may cause low blood pressure because they prevent the body from
being able to circulate enough blood. Excessive vasodilation, or insufficient constriction
of the resistance 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 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, or AT1 receptor antagonists. Many
anesthetic agents and techniques, including spinal anesthesia and most inhalational agents,
produce significant vasodilation. Meditation, yoga, or other mental-physiological
disciplines may produce hypotensive effects. Lower blood pressure is a side effect of certain
botanicals, which can also interact with hypotensive 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.
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.
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. Orthostatic hypotension is indicated if there is a drop in 20 mmHg of systolic
pressure and a 20 beats per minute increase in heart rate.
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,
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 caused by the autonomic nervous system not compensating appropriately,
because of aging or a specific disorder. Diagnosis
For most adults, the healthiest blood pressure is at or below 120/80 mmHg. A small drop
in blood pressure, even as little as 20 mmHg, can result in transient hypotension.
Evaluation of neurocardiogenic syncope is done with a tilt table test.
Treatment The treatment for hypotension depends on its
cause. Chronic hypotension rarely exists as more than a symptom. Asymptomatic hypotension
in healthy people usually does not require treatment. Adding electrolytes to a diet can
relieve symptoms of mild hypotension. A morning dose of caffeine can also be effective. In
mild cases, where the patient is still responsive, laying the person in dorsal decubitus position
and lifting the legs increases venous return, thus making more blood available to critical
organs in the chest and head. The Trendelenburg position, though used historically, is no
longer recommended. Hypotensive shock treatment always follows
the first four following steps. Outcomes, in terms of mortality, are directly linked
to the speed that hypotension is corrected. Still-debated methods are in parentheses,
as are benchmarks for evaluating progress in correcting hypotension. A study on septic
shock provided the delineation of these general principles. However, since it focuses on hypotension
due to infection, it is not applicable to all forms of severe hypotension.
Volume resuscitation Blood pressure support with a vasopressor
Ensure adequate tissue perfusion Address the underlying problem for infarction,
steroids for adrenal insufficiency, etc...) Medium-term treatments of hypotension include:
Blood sugar control Early nutrition
Steroid support See also
Dysautonomia Hypotensive transfusion reaction
Orthostatic intolerance References
External links Understanding Low Blood Pressure – the Basics
WebMD Hypotension PubMed Health