Cerebrovascular Disease Pathophysiology

Standard techniques of cerebral blood flow (CBF) measurement provide information on both global and regional flow in patients with cerebral ischaemia or infarction. Recent availability of positron emission tomography (PET), recording oxygen and glucose metabolism, as well as blood flow and blood volume, gives a more detailed and accurate understanding of pathophysiological changes after stroke.

Changes in cerebral infarction

NON-ISCHAEMIC HEMISPHERE

Mild reduction in global CBF - perhaps due to transneuronal depression of metabolism in the unaffected hemisphere - diaschisis.

In the normal brain, cerebral blood flow to a particular part varies depending on the metabolic requirements, i.e. the supply of 02 and glucose is 'coupled' to the tissue needs. After infarction, between areas of reduced flow and areas of luxury perfusion, lie areas of relative luxury perfusion where reduced flow exceeds the tissue requirements, i.e. 'uncoupling' of flow and metabolism has occurred.

Studies with SPECT imaging suggest that 40% of infarcts are reperfused with blood within 48 hrs.

ISCHAEMIC HEMISPHERE Reduction in global CBF

In the infarcted area and its surroundings, more subtle changes of regional cerebral blood flow (rCBF) are detected.

Areas of reduced flow are - ""bordered by areas of increased flow - luxury perfusion - due to vasodilatation of arteriolar bed in response to lactic acidosis.

These changes in rCBF are transient and revert to normal within days of the onset. The degree of disturbance of rCBF correlates with outcome. Flow of < 28 ml/min/lOOg results in the development of the morphological changes of infarction.

Areas of reduced flow are - ""bordered by areas of increased flow - luxury perfusion - due to vasodilatation of arteriolar bed in response to lactic acidosis.

Pathophysiology of ischaemia

Progression from reversible ischaemia to infarction depends upon the degree and duration of the reduced blood flow.

THRESHOLDS OF CEREBRAL ISCHAEMIA

50"

50"

Electrocortlcal function affected Electrical failure Ionic pump failure Cell death

01 QC

Reversible deficit

Infarction

Duration of ischaemia

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