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In physiology and medicine, hypotension is abnormally low blood pressure. This is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the opposite of hypertension, which is high blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. If it is lower than normal then it is called low blood pressure or hypotension. Hypotension is generally considered as systolic blood pressure less than 90 millimeters of mercury (mm Hg) or diastolic less than 60 mm Hg. However in practice, blood pressure is considered too low only if noticeable symptoms are present.
It is true that for some people, those who exercise and are in top physical condition, low blood pressure is a sign of good health and fitness. But that's not always the case.
For many people, 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
Low blood pressure is sometimes associated with certain symptoms, many of which are related to causes rather than effects of hypotension:
- Chest pain
- Shortness of breath
- Irregular heartbeat
- Fever higher than 101 °F (38.3 °C)
- Stiff neck
- Severe upper back pain
- Cough with phlegm
- Prolonged diarrhea or vomiting
- Foul-smelling urine
- Adverse effect of medications
- Acute, life-threatening allergic reaction
- Loss of consciousness
- Profound fatigue
- Temporary blurring or loss of vision
- In some cases loss of hair
- Ehlers-Danlos Syndrome
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, heart valve problems, heart attack, heart failure, or extremely low heart rate 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.
Some heart conditions that can lead to low blood pressure include extremely low heart rate (bradycardia), 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 (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 blockers ACE inhibitors. Many anesthetic agents and techniques, including spinal anesthesia and most inhalational agents, produce significant vasodilation.
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.
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 (with a two-minute delay in between each position change) can confirm the presence of orthostatic hypotension. Orthostatic hypotension is indicated if there is a drop in 20 mmHg of systolic pressure (and a 10 mmHg drop in diastolic pressure in some facilities) and a 20 bpm 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, which occurs 30–75 minutes after eating substantial meals. When a great deal of blood is diverted to the intestines (a kind of "splanchnic blood pooling") 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 aging or a specific disorder.
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. In mild cases, where the patient is still responsive, laying the person in dorsal decubitus (lying on the back) position and lifting the legs will increase venous return, thus making more blood available to critical organs at the chest and head. The Trendelenburg position, though used historically, is no longer recommended.
The treatment of hypotensive shock always follows the first four following steps. Outcomes, in terms of mortality, are directly linked to the speed in which hypotension is corrected. In parentheses are the still debated methods for achieving, and 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 (usually with crystalloid)
- Blood pressure support (with norepinephrine or equivalent)
- Ensure adequate tissue perfusion (maintain SvO2 >70 with use of blood or dobutamine)
- Address the underlying problem (i.e. antibiotic for infection, stent or CABG for infarction, steroids for adrenal insufficiency, etc...)
Medium-term (and less well-demonstrated) treatments of hypotension include:
- Blood sugar control (80-150 by one study)
- Early nutrition (by mouth or by tube to prevent ileus)
- Steroid support
 See also
- ^ "Diseases and Conditions Index - Hypotension". National Heart Lung and Blood Institute. September 2008. http://www.nhlbi.nih.gov/health/dci/Diseases/hyp/hyp_whatis.html. Retrieved 2008-09-16.
- ^ "Low blood pressure (hypotension) — Definition". MayoClinic.com. Mayo Foundation for Medical Education and Research. 2009-05-23. http://www.mayoclinic.com/health/low-blood-pressure/DS00590. Retrieved 2010-10-19.
- ^ "Low blood pressure (hypotension) — Causes". MayoClinic.com. Mayo Foundation for Medical Education and Research. 2009-05-23. http://www.mayoclinic.com/health/low-blood-pressure/DS00590/DSECTION=causes. Retrieved 2010-10-19.
- ^ "Low blood pressure - NHS". National Health Service (NHS). ____. http://www.nhs.uk/conditions/Blood-pressure-(low). Retrieved same as submission date.
- ^ Chobanian AV, Bakris GL, Black HR, Cushman WC, Green LA, Izzo JL, Jones DW, Materson BJ, Oparil S, Wright JT, Roccella EJ (December 2003). "Seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure". Hypertension 42 (6): 1206–52. doi:10.1161/01.HYP.0000107251.49515.c2. PMID 14656957. http://hyper.ahajournals.org/cgi/content/abstract/42/6/1206. Retrieved 2009-11-03. .
- ^ "BestBets: Use of the Trendelenburg Position to Improve Hemodynamics During Hypovolemic Shock". http://www.bestbets.org/bets/bet.php?id=1710.
- ^ Rivers E, Nguyen B, Havstad S, Ressler J, Muzzin A, Knoblich B, Peterson E, Tomlanovich M (2001-11-08). "Early goal-directed therapy in the treatment of severe sepsis and septic shock". N Engl J Med. (NEJM) 345 (19): 1368–77. doi:10.1056/NEJMoa010307. PMID 11794169. http://content.nejm.org/cgi/content/full/345/19/1368.