Article: Ischemia

In medicine, ischemia (Greek ισχαιμία, isch- is restriction, hema or haema is 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.

Mechanism

Rather than in 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. Relative shortage means the mismatch of blood supply (oxygen delivery) and blood request for adequate oxygenation of tissue.

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:

• Tachycardia (abnormally rapid beating of the heart)
• Atherosclerosis (lipid-laden placques obstructing the lumen of arteries)
• 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
• Foreign bodies in the circulation (e.g. amniotic fluid in amniotic fluid embolism)
• Sickle cell disease (abnormally shaped hemoglobin)

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. This can cause necrosis and cell death.

Ischemia is a feature of heart diseases, transient ischemic attacks, cerebrovascular accidents, ruptured arteriovenous malformations, and peripheral artery occlusive disease.

Tissues especially sensitive to inadequate blood supply are the heart, the kidneys, and the brain. 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.

Necrosis due to ischemia usually takes about 10-12 hours.