Lecture 40 Clinical Aspects of Cerebral Perfusion and ICP

Question 1

Cerebral blood flow makes up how much of total cardiac output

Answer 1

15%

Question 2

What is normal cerebral flow per 100g brain tissue per minute

Answer 2

55-60 mL

Question 3

Grey matter blood flow per 100g/minute

Answer 3

75mL

Question 4

White matter blood flow per 100g/minute

Answer 4

45 mL

Question 5

At what blood flow does ischaemia occur

Answer 5

20mL/100g/minute

Question 6

At what blood flow does permanent blood flow occur

Answer 6

10mL/100g/minute

Question 7

What equation defines Cerebral Perfusion Pressure

Answer 7

MAP- ICP

Question 8

What is CPP

Answer 8

effective blood pressure gradient across the brain

Question 9

If the ICP increases what happens to the CPP

Answer 9

It decreases

Question 10

Define MAP as an equation

Answer 10

MAP = DP + 1⁄3PP or 2⁄3DP + 1⁄3SP

Question 11

Factors that regulate cerebral blood floor

Answer 11

• CPP
• Concentration of arterial CO2
• Arterial PO2

Question 12

What is cerebral autoregulation

Answer 12

• The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mm Hg) is called cerebral autoregulation

Question 13

When CPP is low how does the body auto regulate

Answer 13

The cerebral arterioles dilate to allow adequate flow at the decreased pressure

Question 14

When CPP is high how does the body auto regulate

Answer 14

The cerebral arterioles constrict to reduce blood volume

Question 15

Under what conditions can CPP not be auto regulated

Answer 15

• CPP exceeds 150 mm Hg, such as in hypertensive crisis, the autoregulatory system fails
• 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 16

Describe the Monro-Kelly Doctrine

Answer 16

• 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 17

What is compliance

Answer 17

• Change in volume observed for a given change in pressure

* dV / dP

Question 18

What is Elastance

Answer 18

• Change in pressure observed for a given change in volume

* dP / dV

Question 19

When the venous system collapses where does the blood go

Answer 19

squeezes venous blood out through the jugular veins or through the emissary and scalp veins

Question 20

Describe the waves in ICP waveforms

Answer 20

• Investigates blood flow within the brain
• P1- systole wave
• P2- wave in the arteries
• P3- aortic valve closure- not always visible

Question 21

What is the Cushing’s Reflex

Answer 21

• Vasopressor response
• Hypertension, irregular breathing and bradycardia
• Indicate insufficient blood flow to the brain as well as the compression of arterioles
• Decreased CBF, activation autonomic nervous system
• Sympathetic response: alpha-1 adrenergic receptors -> Hypertension and tachycardia
• Aortic baro-receptors stimulate vagus nerve -> Bradycardia

Question 22

Management of Increased ICP

Answer 22

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