Cerebral Perfusion

Question 1

% cardiac output to brain

Answer 1

15%

Question 2

Normal cerebral blood flow

Answer 2

averages 55 to 60 mL/100 g brain tissue per minute
Grey matter the blood flow is 75 mL/100 g/ minute
White matter it is around 45 mL/100 g/ minute

Question 3

CBF classes as Ischemia

Answer 3

20 mL/100 g/minute. Permanent damage usually results when the blood flow drops below 10 mL/100 g/minute

Question 4

cerebral perfusion pressure (CPP)

Answer 4

CPP is the effective blood pressure gradient across the brain
CPP = MAP − ICP
MAP = DP + 1⁄3PP or 2⁄3DP + 1⁄3SP

Question 5

Factors that regulate cerebral blood flow under physiological conditions

Answer 5

CPP
Concentration of arterial CO2
Arterial PO2

Question 6

Cerebral autoregulation

Answer 6

The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mm Hg).

Question 7

Under certain pathological conditions cerebral blood flow cannot always be autoregulated

Answer 7

CPP exceeds 150 mm Hg, such as in hypertensive crisis, the autoregulatory system fails. Exudation of fluid from the vascular system with resultant vasogenic edema
Certain toxins such as carbon dioxide can cause diffuse cerebrovascular dilatation and inhibit proper autoregulation
During the first 4 to 5 days of head trauma, many patients can experience a disruption in cerebral autoregulation

Question 8

Vasogenic Oedema

Answer 8

increased capillary permeability, mainly white matter, plasma proteins, increased extracellular fluid. Primary or metastatic tumour, abscess, late stages of infarction, trauma. steriods and mannitol effective

Question 9

cytotoxic oedema

Answer 9

cellular swelling (neuronal, endothelial, glial). grey and white matter. increased intracellular fluid and sodium due to failure of membrane transport. decreased extracellular fluid. early stages of infarction, water intoxication. steriods not effective. mannitol effective.

Question 10

Monro- Kelly Doctrine

Answer 10

The cranium is a rigid structure
Brain, blood and CSF
When a new intracranial mass is introduced, a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant

Question 11

Compliance

Answer 11

Change in volume observed for a given change in pressure

dV / dP

Question 12

Elastance

Answer 12

Inverse of compliance
Change in pressure observed for a given change in volume
dP / dV
It represents the accommodation to outward expansion of an intracranial mass

Question 13

Cushing’s reflex

Answer 13

Vasopressor response
-Hypertension, irregular breathing and bradycardia

Cranial surgery or head injury.

Would expect patients to have a lower GCS.

• Increased ICP more than MAP
• Compression of cerebral arterioles
• Decreased CBF, activation autonomic nervous system
• Sympathetic response: alpha-1 adrenergic receptors -> Hypertension and tachycardia
• Aortic baro-receptors stimulate vagus nerve -> Bradycardia
• Bradycardia also due to mechanical distortion of medulla

Question 14

Management of increased ICP

Answer 14

Head end elevation: facilitate venous return
Mannitol/ Hypertonic saline
Hyperventilation: decrease CBF (temporary measure)
Barbiturate coma: decrease cerebral metabolism, CBF
Surgical decompression