In the opposite situation, increased oxygen, decreased carbon dioxide, and decreased acidity would cause vasoconstriction and increased vascular resistance, thereby diverting blood to another area of the brain. Global ischemia Cerebral ischemia can involve the entire brain, or only portions of the brain. The loss of systemic blood pressure, due to cardiac arrest for example, or the elevation of intracranial pressure can lead to global ischemia. Acute brain swelling, as associated with severe head trauma or massive intracerebral bleeding, can cause such an elevation in intracranial pressure that perfusion pressure, even with autoregulation, may no longer maintain blood flow. Only 5 to 10 min of global ischemia can result in a permanent and irreversible damage to specific brain cells.
Focal ischemia is a local decrease in cerebral blood flow.
The typical cause is the occlusion of a cerebral blood vessel from a clot. Other causes of focal ischemia can be progressive narrowing of blood vessels by accumulation of fat or calcium in vessel walls, blood or infection in the subarachnoid space that act as chemical irritants causing spasm of the blood vessels or local inflammation of blood vessel walls as in vasculitis. These may lead to the formation of multiple clots in vessels, as in disseminated intravascular coagulation. Other causes of clot formation are deficiencies of anticlotting factors, such as antithrombin III, or production of substances that induce hypercoagulable states, such as antiphospholipid antibodies. When enough brain cells become ischemic, from either global or focal processes, the normal functions of the brain are interrupted.