In medicine, ischemia, also spelled as ischaemia or ischæmia, is a restriction in blood supply
to tissues, causing a shortage of oxygen and glucose needed for cellular metabolism.
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.
Signs and symptoms 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.
Clinical manifestations of acute arterial ischemia include pain, pallor, pulselessness,
paresthesia, paralysis, poikilothermia Without immediate intervention, ischemia may
progress quickly to tissue necrosis and gangrene within a few hours.
Paralysis is a very late sign of acute arterial ischemia and signals the death of nerves supplying
the extremity.
Foot drop may occur as a result of nerve damage.
Because nerves are extremely sensitive to hypoxia, limb paralysis or ischemic neuropathy
may persist after revascularization and may be permanent.
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 or chronic.
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,
often erroneously referred to as a "mini-stroke".
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 Causes
Ischemia is a vascular disease involving an interruption in the arterial blood supply
to a tissue, organ, or extremity that, if untreated, can lead to tissue death.
It can be caused by embolism, thrombosis of an atherosclerosis artery, or trauma.
Venous problems like venous outflow obstruction and low-flow states can cause acute arterial
ischemia.
An aneurysm is one of the most frequent causes of acute arterial ischemia.
Other causes are heart conditions including myocardial infarction, mitral valve disease,
chronic atrial fibrillation, cardiomyopathies, and prosthesis, in all of which thrombi are
prone to develop.
Occlusion The thrombi may dislodge and may travel anywhere
in the circulatory system, where they may lead to pulmonary embolus, an acute arterial
occlusion causing the oxygen and blood supply distal to the embolus to decrease suddenly.
The degree and extent of symptoms depend on the size and location of the obstruction,
the occurrence of clot fragmentation with embolism to smaller vessels, and the degree
of peripheral arterial disease.
Thromboembolism Embolism
Trauma Traumatic injury to an extremity may produce
partial or total occlusion of a vessel from compression, shearing or laceration.
Acute arterial occlusion may develop as a result of arterial dissection in the carotid
artery or aorta or as a result of iatrogenic arterial injury.
Other An inadequate flow of blood to a part of the
body may be caused by any of the following: Thoracic outlet syndrome
Atherosclerosis Hypoglycemia
Tachycardia Hypotension
Outside compression of a blood vessel, e.g. by a tumor or in the case of superior mesenteric
artery syndrome Sickle cell disease
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
rupture of significant blood vessels supplying a tissue or organ.
Anemia vasoconstricts the periphery so that red blood cells can work internally on vital
organs such as the heart, brain, etc., thus causing lack of oxygen to the periphery.
Pathophysiology
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.
Treatment Early treatment is essential to keep the affected
limb viable.
The treatment options include injection of an anticoagulant, thrombolysis, embolectomy,
surgical revascularisation, or amputation.
Anticoagulant therapy is initiated to prevent further enlargement of the thrombus.
Continuous IV unfractionated heparin has been the traditional agent of choice.
If the condition of the ischemic limb is stabilized with anticoagulation, recently formed emboli
may be treated with catheter-directed thrombolysis using intraarterial infusion of a thrombolytic
agent, streptokinase, or urokinase).
A percutaneous catheter inserted into the femoral artery and threaded to the site of
the clot is used to infuse the drug.
Unlike anticoagulants, thrombolytic agents work directly to resolve the clot over a period
of 24 to 48 hours.
Direct arteriotomy may be necessary to remove the clot.
Surgical revascularization may be used in the setting of trauma.
Amputation is reserved for cases where limb salvage is not possible.
If the patient continues to have a risk of further embolization from some persistent
source, such as chronic atrial fibrillation, treatment includes long-term oral anticoagulation
to prevent further acute arterial ischemic episodes.
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.
See also Cerebral ischemia
Infarction Inhibitor protein
Ischemic cascade Organ donation
Reperfusion injury Trauma triad of death
Notes
References
Bibliography Elizabeth.
Oxford Reference: Concise Medical Dictionary.
Oxford University Press: Market House Books, 1987, 2nd ed., pg. 107, ISBN 0-19-281991-7,
ISBN 978-0-19-281991-8