Cerebral perfusion and ICP

Question 1

What % of the cardiac output does the brain receive?

Answer 1

15%

Normal cerebral blood flow averages 55 to 60 mL/100 g brain tissue per minute

Question 2

Does Grey or White matter receive more blood?

Answer 2

Grey

Question 3

Factors that regulate cerebral blood flow under physiological conditions (3)

Answer 3

CPP

Concentration of arterial CO2

Arterial PO2

Question 4

What is cerebral autoregulation?

Answer 4

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

Question 5

What is CPP

Answer 5

The net pressure gradient that drives oxygen delivery to cerebral tissue. It is the difference between the mean arterial pressure (MAP) and the Intracranial Pressure (ICP)

Question 6

What happens to the cerebral arterioles when CPP is low?

Answer 6

they dilate to allow adequate flow at decreased pressure

Question 7

Trauma/stroke can have what effect on autoregulation?

Answer 7

Autoregulation mechanism stops working – little bit of change in pressure has bad effects

Question 8

If CPP exceeds 150mmHg such as in hypertensive crisis what can happen?

Answer 8

autoregulatory system fails

Question 9

What is Vasogenic cerebral oedema? (extracellular)

Answer 9

Type of cerebral oedema in which the blood brain barrier (BBB) is disrupted

Increased capillary permeability- intravascular proteins and fluid to penetrate into the extracellular space

Question 10

Which toxin can cause diffuse cerebrovascular dilatation and inhibit proper autoregulation?

Answer 10

CO2

Question 11

Effects of cerebral oedema

Answer 11

prominant cause of subacute to chronic intracranial hypertension

Question 12

What are the 4 types of cerebral oedema

Answer 12

Vasogenic
Cytotoxic
Osmotic
Interstitial

Question 13

What is Cytotoxic oedema (intracellular)

Answer 13

BBB remains intact but a disruption in cellular metabolism impairs functioning of the Na+ and K+ pump in the glial cell membrane, leading to cellular retention of sodium and water.

oedema in G and W matter

Neuronal, glial and endothelial cells swell

Question 14

Causes of cytotoxic oedema?

Answer 14

early stages of infarction, water intoxication, stroke/brain injury

Question 15

Where are the tight junctions situated?

Answer 15

In the endothelium of blood vessels - capillary network

Question 16

What mechanisms exist by which materials can be transported across the endothelial cells: discuss lipid-soluble substances and amino acids and sugars

Answer 16

lipid-soluble substances can usually 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 17

What happens when a new intracranial mass is introduced within the cranial cavity?

Answer 17

A compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant

For example a tumour adds on to brain vol. The vol of CSF decreases or less blood comes to brain as a result. If tumour mass continues to increase - more venous blood or CSF leave and pressures will change

Question 18

Define elastance

Answer 18

Inverse of compliance

Change in pressure observed for a given change in volume

It represents the accommodation to outward expansion of an intracranial mass

Question 19

Define compliance

Answer 19

Change in volume observed for a given change in pressure

Question 20

Homeostatic mechanism in venous system?

Answer 20

venous system collapses easily and squeezes venous blood out through jugular veins or through emissary and scalp veins

Question 21

Homeostatic mechanism relating to CSF displacement

Answer 21

CSF can be displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space

Question 22

How is it that tumours can go undetected until the brain can no longer accommodate for them?

Answer 22

The innate homeostatic pressure-buffering mechanism offered by displacement of CSF and venous blood keeps compliance flat until a “critical volume” is reached. This mechanism carries on in the background as the tumour grows but at some point the changes in vol and pressure are too much for the mechanism to accommodate.

After this critical volume, small volumetric changes result in dangerously high increases in pressure, and intracranial hypertension naturally ensues

Question 23

What is the Monro-Kelly Doctrine?

Answer 23

The cranium is a rigid structure containing the brain, blood and CSF

The Monro-Kellie doctrine states the sum of volumes of brain, CSF and intracerebral blood is constant.

So when a new intracranial mass is introduced there must be a compensatory change in vol

Question 24

Describe the ICP waveform

Answer 24

It has 3 distinct peaks. P1 and P2 = most important.

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

P2 - tidal wave - 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

Question 25

Cushing’s reflex

Answer 25

Also called ‘Vasopressor response’

response to when ICP increases to more than MAP causing compression of cerebral arterioles. The repsonse results in Cushing’s triad.

Aim of cushing’s is to increase arterial pressure in order to overcome the increased ICP.

Question 26

What is Cushing’s triad?

Answer 26

Hypertension
Irregular breathing
Bradycardia

Question 27

How is bradycardia induced?

Answer 27

Aortic baro-receptors stimulate vagus nerve also due to mechanical distortion of the medulla

Question 28

How is hypertension induced?

Answer 28

Sympathetic response: alpha-1 adrenergic receptor

Question 29

How is increased ICP managed? (5)

Answer 29

Head end elevation: facilitate venous return

Mannitol/ Hypertonic saline

Hyperventilation: decrease Cerebral Blood Flow (temporary measure)

Barbiturate coma: decrease cerebral metabolism, CBF

Surgical decompression

Question 30

What is Micro-dialysis

Answer 30

A newer management concept to investigate brain metabolism

Implantation of specially designed catheters

To collect small-molecular-weight substances to help measure and identify neurotransmitters, peptides, and other substances