Clinical Aspects of Cerebral Perfusion and ICP

Question 1

What is normal cerebral blood flow?

Answer 1

Averages around 55-60 ml/100g brain tissue per minute

Question 2

What blood rate and flow does ischaemia and parmanent damage usually occur?

Answer 2

Ischaemia - 20 ml/100g per minute

Permanent damage - 10 ml/100g per minute

Question 3

What is the largest factor in determining the cerebral blood flow?

Answer 3

Cerebral perfusion pressure

Question 4

How do we calculate the cerebral perfusino pressure? (CPP)

Answer 4

MAP - ICP

Question 5

What is the result of an increased ICP on cerebral perfusion pressure?

Answer 5

Increased ICP causes cerebral perfusino pressure to decrease

Question 6

What are the factors that regulate cerebral blood flow inder physiological conditions?

Answer 6

CPP

Concentration of arterial CO2

Arterial PO2

Question 7

What is meant by cerebral autoregulation?

Answer 7

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

Question 8

How is cerebral autoregulation achieved when there is low CPP?

Answer 8

Cerebral arterioles dilate to allow adequate flow at decreased pressure

Question 9

Under what corcumstance would cerebral arterioles constrict?

Answer 9

When the CPP is high

Question 10

When does the autoregulatory system fail?

Answer 10

When the CPP exceeds 150 mm Hg - such as in the hypertensive crisis

During the first 4-5 days of head trauma

Exudation of fluid from the vascular system with resultant vasogenic oedema

Toxins such as carbon dioxide causing cerebrovascular dilation and inhibit proper vasoregulation

Question 11

What is the pathology associated with cerebral oedema?

Answer 11

Prominent cause of subacute to chronic intracranial hypertension

Question 12

Why does cerebral oedema cause increased brain volume?

Answer 12

Because of an increase in water content

Increased capillary permeability, the exudate is plasma filtrate containing plasma proteins and is mainly located in the white matter.

Extracellular fluid is increased

Question 13

What are the possible pathogenic lesions that can cause vasogenic oedema?

Answer 13

Tumour (primary or metastatic)

Absecess

Infarction (later stages)

Trauma

Question 14

What is the structure of the blood brain barrier?

Answer 14

A barrier composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions

(Endothelial tight junctions are the barrier to the passive movement of many substances in order to protect the sensitive neural tissue from toxic materials)

Question 15

How do lipid soluble substances cross the endothelial cells?

Answer 15

Can penetrate all capillary endothelial cell membranes in a passive manner

Question 16

How do amino acids and sugars pass across the capillary endothelium?

Answer 16

By specific carrier mediated mechanisms

Question 17

What are the different compositions within the skull?

Answer 17

Brain (70%) and interstitial fluid (10%) - 1400 ml

Blood (10%) - 150 ml

Cerebrospinal fluid (10%) - 150 ml

Total 1700ml

Question 18

What is the monro - kelly doctrine?

Answer 18

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 19

What is meant by compliance?

Answer 19

Change in volume observed for a given change in pressure

dv/dp

Question 20

What is elastance?

Answer 20

The inverse of compliance, change in pressure observed for a chanfe in volume

dp / dv

It represents the accomdation to outward expansion of an intracranial mass

Question 21

How does venous blood get displaced when the intracranial pressure increases?

Answer 21

Venous system collapses easily

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

Question 22

How is CSF displaced?

Answer 22

CSF is displaced from the ventricular system through the foramina luschka and magendie into the spinal subarachnoid space

Question 23

What is the effect of an increasing pressure when the compensatory mechanisms have been exhausted?

Answer 23

Small changes in volume produce significant increase in pressure

Displacement of CSF and venous blood keeps compliance flat until a critical volume is reached

After this critical volume, small volumetric changes result in precipitous increases in pressurem and intracranial hypertension naturall ensues

Question 24

What are the P1, P2 and P3 waves indicative of?

Answer 24

P1 - Percussion wave

P2- Tidal wave

(Dicriotic notch of the trough between p2 and p3)

P3- Dicrotic wave

Question 25

What are the different types of Lundberg waves?

Answer 25

A waves - abrupt elevation in ICP for 5 - 20 minutes followed by a rapid fall in the pressure to resting levels

B waves - Frequency of 0.5 to 2 waves per minute, are related to rhythmic variations in breathing

C waves - Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude

Question 26

What does the cushings reflex combat?

Answer 26

ICP

Question 27

What are the three main feeatures of cushings reflex?

Answer 27

Hypertension, irregular breathing and bradycardia

Question 28

What is the physiological response to raised ICP?

Answer 28

Compression of cerebral arteries

Decreased cerebral blood flow and therefore activation of the autonomic nervous system

Sympathetic response: Alpha - 1 adrenergic receptors - results in hypertension and bradycardia

Aortic baro-receptors stimulate vagus nerve - bradycardia

Bradycardia also due to mechanical distortion of the medulla

Question 29

What is management of increased ICP?

Answer 29

Head end elevation - facilitates venous return

Mannitol / hypertonic saline (In the case of vasogenic oedema steroids and mannitol are both effective in reducing intracranial pressure)

Hyperventilation: Decrease CBF (temporary measure)

Barbituate coma: decrease cerebral metabolism

Surgical decompression

Question 30

What are the newer concepts in terms of managing intracranial pressure?

Answer 30

Brain tissue oxygen monitoring -

• Probe to monitor oxygenation of tissue
• Detect and treat low oxygenation, increasing CPP

Micro-dialysis:

• Investigate brain metabolism
• Implantation of specially designes catheters
• To collect small - molecular weight substances to help measure and identify neurotransmitters, peptides and other substances