出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/04/29 07:34:47」(JST)
In medicine, ischemia, also spelled as ischaemia or ischæmia, (/ɪˈskiːmɪə/;[1][2] from Greek language ισχαιμία, ischaimía; isch- root denoting a restriction or thinning or to make or grow thin/lean, haema blood) is a restriction in blood supply to tissues, causing a shortage of oxygen and glucose needed for cellular metabolism (to keep tissue alive).[3] Ischemia is generally caused by problems with blood vessels, with resultant damage to or dysfunction of tissue. It also means local anemia in a given part of a body sometimes resulting from congestion (such as vasoconstriction, thrombosis or embolism).
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An inadequate flow of blood to a part of the body may be caused by any of the following:
Since oxygen is carried to tissues in the blood, insufficient blood supply causes tissue to become starved of oxygen. In the highly aerobic tissues of the heart and brain, irreversible damage to tissues can occur in as little as 3–4 minutes at body temperature. The kidneys are also quickly damaged by loss of blood flow. Tissues with slower metabolic rates may undergo irreversible damage after 20 minutes.
Ischemia results in tissue damage in a process known as ischemic cascade. The damage is the result of the build-up of metabolic waste products, inability to maintain cell membranes, mitochondrial damage, and eventual leakage of autolyzing proteolytic enzymes into the cell and surrounding tissues.
Restoration of blood supply to ischemic tissues can cause additional damage known as reperfusion injury that can be more damaging than the initial ischemia. Reintroduction of blood flow brings oxygen back to the tissues, causing a greater production of free radicals and reactive oxygen species that damage cells. It also brings more calcium ions to the tissues causing further calcium overloading and can result in potentially fatal cardiac arrhythmias and also accelerates cellular self-destruction. The restored blood flow also exaggerates the inflammation response of damaged tissues, causing white blood cells to destroy damaged cells that may otherwise still be viable.[4]
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.[5]
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 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. A brief episode of ischemia affecting the brain is called a Transient Ischemic Attack (TIA), frequently referred to as a mini-stroke.
Lack of blood flow to a limb results in acute limb ischemia.
Reduced blood flow to the skin layers may result in mottling or uneven, patchy discoloration of the skin
Decrease in body temperature reduces the aerobic metabolic rate of the affected cells, reducing the immediate effects of hypoxia. Reduction of body temperature also reduces the inflammation response and reperfusion injury.
For frostbite injuries, limiting thawing and warming of tissues until warmer temperatures can be sustained may reduce reperfusion injury.
For blood clots, administration of "clot-buster" drugs such as Alteplase within the critical time period, together with some collateral circulation to the ischemic area is a main factor in patient recoveries.
Other drug therapies such as cannabinoids and hydrogen sulfide are being researched for limiting ischemic injuries.
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