Hypotension is the state of having abnormally low blood pressure. (Below about 90/60 mmHg or lower for particularly athletic patients.) This is normally a minor and transient physiological phenomenon resulting in the sensation of light-headedness. If this dizziness persists or progresses to fainting or seizures, immediate medical attention should be sought as prolonged or severe hypotension can deprive the brain and other organs of oxygen and lead to shock. Hypotension is usually mild and not symptomatic of an ongoing condition requiring such medical attention.
Hypotension can also be associated with blurred vision, fatigue, mild depression, nausea, pallor and sleepiness.
Hypovolemia -- low blood volume, is the common cause of Hypotension. This can be caused by blood loss or dehydration (from too little fluid entering the body or too much leaving). Hypotension can also be caused by abnormally low cardiac output or excessive vasodilation. These situations may result from cardiac, endocrine or nervous dysfunction, possibly due to various medications or anaphylaxis. Anemia or systemic infection may also lead to hypotension.
The common person's experience of hypotension results from minor orthostatic hypotension -- a light-headed sensation experienced when e.g. standing up too quickly from a crouch, or significant vasodilation caused by e.g. lengthy use of a hot tub or sauna. Some people -- particularly older people experiencing hypertension or nervous disorders, experience postprandial hypotension in which the gut requires lots blood to help digest a meal and the neural-hormonal mechanisms that normal regulate blood pressure fail to compensate.
When hypotension is suspected, physicians may use any of the following techniques to obtain diagnostic information: blood test (CBC, electrolyte, cortisol), cardio-stress tests, electrocardiogram, echocardiogram, radiologic examination (x-ray, CT scan), tilt-table test, ultrasonic examination of leg veins, urinalysis and the Valsava maneuver. Common treatments for hypotension include: compression stockings on the lower body, decreased alcohol consumption, increased salt intake, increased water intake and various pharmaceuticals.
Closely related E2 nodes (should some of these be herein merged/moved?): orthostatic hypotension, Postural Hypotension, vaso-vagal syncope
The Mayo Clinic: http://www.mayoclinic.com/health/low-blood-pressure/DS00590
The National Heart Lung and Blood Institute: http://www.nhlbi.nih.gov/health/dci/Diseases/hyp/hyp_whatis.html
MedicineNet: http://www.medicinenet.com/low_blood_pressure/article.htm
Medline (MLN/NIH): http://www.nlm.nih.gov/medlineplus/ency/article/007278.htm
WebMD: http://www.webmd.com/hypertension-high-blood-pressure/guide/understanding-low-blood-pressure-basics