Ischemia

Overview
In medicine, ischemia (Greek &#953;&#963;&#967;&#945;&#953;&#956;&#943;&#945;, 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 plaques 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)
 * Induced g-forces which restrict the blood flow and force the blood to the extremities of the body, as in aerobatics and military flying

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 (i.e. cell death). In very aerobic tissues such as heart and brain, at body temperature Necrosis due to ischemia usually takes about 3-4 hours before becoming irreversible. This and typically some collateral circulation to the ischemic area accounts for the efficacy of "clot-buster" drugs such as Alteplase, given for stroke and heart-attack within this time period. 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 arteriovenous malformations, and peripheral artery occlusive disease. The heart, the kidneys, and the brain are among the organs that are the most 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, resulting in reperfusion injury. With reperfusion injury, necrosis can be greatly accelerated.

Detection of cardiac ischemia

 * Initial evaluation of chest-pain patients involves a 12 lead electrocardiogram (ECG) and cardiac markers such as troponins. These tests are highly specific but very insensitive and often leave the requirement for further testing to achieve an accurate diagnosis.


 * Magnetocardiography (MCG) imaging utilises superconducting quantum interference devices (SQUIDs) to detect the weak magnetic fields generated by the heart's electrical fields. There might be a direct correlation between abnormal cardiac depolarization or repolarisation and abnormality in the magnetic field map. In July 2004, the Food and Drug Administration (FDA) approved the CardioMag Imaging MCG as a safe device for the non-invasive detection of ischemia.

Related Chapters

 * Cerebral ischemia
 * Infarction
 * Ischemic cascade
 * Reperfusion injury