Cerebral Perfusion and Intracranial Pressure

Question 1

What is the normal perfusion of cerebral blood flow?

Answer 1

55 to 60 mL/100g brain tissue per minute

Question 2

What is the perfusion of cerebral blood flow in ischaemia?

Answer 2

20 mL/100 g/minute

Question 3

At what perfusion does permanent damage occur?

Answer 3

When it drops below 10mL/100 g/minute

Question 4

What is the cerebral perfusion pressure (CPP)?

Answer 4

It is the blood pressure gradient across the brain which determines cerebral blood flow

Question 5

How do you work out the CPP?

Answer 5

CPP = MAP - ICP

This represents the pressure gradient driving cerebral blood flow (CBF) and hence oxygen and metabolite delivery.

Question 6

How do you work out MAP?

Answer 6

MAP = DP + 1/3PP

MAP = 2/3DP + 1/3SP

DP - diastolic pressure
PP - pulse pressure
SP - systolic pressure

Question 7

What is the effect of increased ICP on the CPP?

Answer 7

Increase ICP = decrease in CPP

Question 8

Name three factors which regulate cerebral blood flow

Answer 8

• CPP
• Concentration of arterial CO2
• Arterial PO2

Question 9

What is cerebral autoregulation?

Answer 9

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

Question 10

What is the cerebral autoregulation when the CPP is low?

Answer 10

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

Question 11

What is the cerebral autoregulation when the CPP is high?

Answer 11

The cerebral arterioles constrict

Question 12

In what pathological conditions can the cerebral blood flow not be autoregulated?

Answer 12

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

• Exudation of fluid from the vascular system with resultant vasgoenic oedema
• Toxins like CO2 can cause diffuse cerebrovascular dilatation and inhibit proper autoregulation
• During the first 4-5 days of head trauma, many patients can experience disruption

Question 13

What is cerebral oedema and what can it cause?

Answer 13

Increased brain volume as a result of an increase in water content

Prominent cause of subacute to chronic intracranial hypertension

Question 14

What are the two different types of cerebral oedema?

Answer 14

Vasogenic oedema (extracellular) and cytotoxic oedema (intracellular)

Question 15

Describe vasogenic oedema

Answer 15

• Increased capillary permeability
• Mainly white matter
• Extracellular fluid increased
• Plasma filtrate containing plasma protein

Question 16

Describe cytotoxic oedema

Answer 16

• Cellular swelling (neuronal, glial and endothelial cells)
• Grey and white matter
• Increased intracellular after and sodium due to failure of membrane transport
• Extracellular fluid decreases

Question 17

What is the blood brain barrier?

Answer 17

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

It is composed of astrocytic foot processes wrapping around a capillary endothelium which contains tight junctions

Question 18

How are materials transported across the endothelial cells?

Answer 18

• Lipid-soluble substances can penetrate all capillary endothelial cell membranes in a passive manner
• Amino acids and sugars are transported across the capillary endothelium by specific carrier-mediated mechanisms

Question 19

What is the Monro-Kelly Doctrine?

Answer 19

It is the homeostatic intracerebral volume regulation, which stipulates that the total volume of the brain tissue, cerebrospinal fluid, and blood remains constant

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 20

Describe the mechanisms for maintaining constant intracranial volume

Answer 20

Compliance vs elastance

Question 21

Describe the elastance mechanism in maintaining constants intracranial volume

Answer 21

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

Question 22

Describe compliance mechanism in maintaining constant intracranial volume

Answer 22

Change in volume observed for a given change in pressure

Question 23

Describe how the venous system drains blood from the brain

Answer 23

It collapses easily and squeezes blood out through the jugular veins or through the emissary and scalp veins

Question 24

Describe CSF exits the ventricles

Answer 24

Displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space

Question 25

What happens when the compensatory venous and CSF pathways are exhausted?

Answer 25

Small changes in volume produce significant increase in pressure.

Homeostatic pressure-buffering mechanism offered by displacement of CSF and venous blood keeps compliance flat until ‘critical volume’ is reached

After this critical volume, small volumetric changes result in increases in pressure, and intracranial hypertension occurs.

Question 26

Describe the relationship of change in volume and pressure with regards to compliance on the graph

Answer 26

During high compliance, an increase in volume causes a small increase in pressure.

With low compliance, there is a greater change in pressure for the same increase in volume.

After the critical volume, where there is no compliance, a small increase in volume will cause a massive increase in pressure

Question 27

Name mechanism that decreases intracranial pressure

Answer 27

External ventricular drain (EVD) - drains CSF to reduce pressure

Question 28

Describe the ICP waveforms from the pressure transducer of the EVD

Answer 28

P1 percussion wave - the arterial pulse transmitted through the choroid plexus into the CSF

P2 tidal wave - represents cerebral compliance, it can be thought of as a “reflection” of the arterial pulse wave bouncing off the springy brain parenchyma.

P3 dicrotic wave - closure of the aortic valve, which makes the trough prior to P3 the equivalent of the dicrotic notch

Question 29

What are the different stages in lundberg waves?

Answer 29

1. A waves
2. B waves
3. C waves

Question 30

What are the A waves in the lundberg waves?

Answer 30

Abrupt elevation in ICP for 5 to 20 minutes followed by a rapid fall in the pressure to resting levels

The amplitude may reach as high as 50 to 100 mm Hg

Question 31

What are the B waves in the lundberg waves?

Answer 31

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

Question 32

What are the C waves in the lundberg waves?

Answer 32

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

Question 33

What is Cushing’s reflex?

Answer 33

Physiological response to increased intracranial pressure (ICP) that results in Cushing’s triad:

• Hypertension
• Irregular breathing
• Bradycardia

Question 34

How does the increased ICP cause the Cushing’s triad?

Answer 34

• Compression of cerebral arterioles
• Decreased CBF, activation of ANS
• Sympathetic response: A1 adrenergic receptors –> hypertension and tachycardia
• Aortic baroreceptors stimulate vagus nerve –> bradycardia
• Bradycardia also due to mechanical distortion of medulla

Question 35

What are five ways to manage increased ICP

Answer 35

• Head end elevation: facilitate venous return
• Mannitol/ Hypertonic saline
• Hyperventilation: decrease CBF (temporary measure), reduce PCO2
• Barbiturate coma: decrease cerebral metabolism, CBF
• Surgical decompression