Ischemia

In medicine, ischemia (from Greek ισχαιμία, ischaimía; isch- root denoting a restriction or thinning or to make or grow thin/lean, haema blood) is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia. It also means local anemia in a given part of a body sometimes resulting from congestion (such as vasoconstriction, thrombosis or embolism).

Ischemic means having or showing symptoms of ischemia, while nonischemic means "not related to or showing signs of ischemia".

Mechanism
Rather than hypoxia (a more general term denoting a shortage of oxygen, usually a result of lack of oxygen in the air being breathed), ischemia is an absolute or relative shortage of the blood supply to an organ, i.e. a shortage of oxygen, glucose and other blood-borne fuels. A relative shortage means the mismatch of blood supply (oxygen/fuel delivery) and blood request for adequate metabolism of tissue. Ischemia results in tissue damage because of a lack of oxygen and nutrients. Ultimately, this can cause severe damage because of the potential for a build-up of metabolic wastes.

Ischemia can also be described as an inadequate flow of blood to a part of the body, caused by constriction or blockage of the blood vessels supplying it. Ischemia of heart muscle produces angina pectoris.

This can be due to
 * Atherosclerosis (lipid-laden plaques obstructing the lumen of arteries)
 * Hypoglycemia (lower than normal level of glucose)
 * Tachycardia (abnormally rapid beating of the heart)
 * Hypotension (low blood pressure, e.g. in septic shock, heart failure)
 * Thromboembolism (blood clots)
 * Outside compression of a blood vessel, e.g. by a tumor or in the case of superior mesenteric artery syndrome
 * Embolism (foreign bodies in the circulation, e.g. amniotic fluid embolism)
 * Sickle cell disease (abnormally shaped red blood cells)
 * Induced g-forces which restrict the blood flow and force the blood to the extremities of the body, as in acrobatics and military flying
 * Localized extreme cold, such as by frostbite or improper cold compression therapy
 * Tourniquet application
 * An increased level of glutamate receptor stimulation
 * arteriovenous malformations, and peripheral artery occlusive disease.

The heart, the kidneys, and the brain are among the organs that are the most quickly damaged by loss of blood flow for any period of time.

Consequences
Since oxygen is mainly bound to hemoglobin in red blood cells, insufficient blood supply causes tissue to become hypoxic, or, if no oxygen is supplied at all, anoxic. In very aerobic tissues such as heart and brain, at body temperature necrosis due to ischemia usually takes about 3–4 minutes before becoming irreversible. Administration of "clot-buster" drugs such as Alteplase (given for stroke and heart attack) within this time period, together with some collateral circulation to the ischemic area is a main factor in patient recoveries. However, complete cessation of oxygenation of such organs for more than 20 minutes typically results in irreversible damage.

Ischemia is a feature of heart diseases, transient ischemic attacks, cerebrovascular accidents, ruptured sensitive to inadequate blood supply. Ischemia in brain tissue, for example due to stroke or head injury, causes a process called the ischemic cascade to be unleashed, in which proteolytic enzymes, reactive oxygen species, and other harmful chemicals damage and may ultimately kill brain tissue.

Restoration of blood flow after a period of ischemia can actually be more damaging than the ischemia. Reintroduction of oxygen causes a greater production of damaging free radicals as well as allowing, via removal of the extracellular acidotic conditions, influx of calcium and thus calcium overloading. Overall this results in reperfusion injury which can result in potentially fatal cardiac arrhythmias, also necrosis can be greatly accelerated.

Variations
There are various types of ischemia, organized by the organ experiencing the ischemic insult. The mechanism of ischemia may differ based on the organ involved.

Cardiac ischemia
Cardiac ischemia may be asymptomatic or may cause chest pain, known as angina pectoris. It occurs when the heart muscle, or myocardium, receives insufficient blood flow. This most frequently results from atherosclerosis, which is the long-term accumulation of cholesterol-rich plaques in the coronary arteries. Ischemic heart disease is the most common cause of death in most Western countries and a major cause of hospital admissions.

Bowel
Both large and small bowel can be affected by ischemia. Ischemia of the large intestine may result in an inflammatory process known as ischemic colitis. Ischemia of the small bowel is called mesenteric ischemia.

Brain
Brain ischemia is insufficient blood flow to the brain, and can be acute (i.e., rapid) or chronic (i.e., long-lasting). Acute ischemic stroke is a neurologic emergency that may be reversible if treated rapidly. Chronic ischemia of the brain may result in a form of dementia called vascular dementia.

Limb
Lack of blood flow to a limb results in acute limb ischemia.

Cutaneous
Reduced blood flow to the skin layers may result in mottling or uneven, patchy discoloration of the skin