Intracranial Pressure and cerebral blood flow

Question 1

what is intracranial pressure?

Answer 1

Dynamic balance between these contents

Normal ICP 5 to 15 cm of water

Question 2

what does the intracranial cavity contain?

Answer 2

contains nervous tissue, blood vessels venous channels and CSF

Question 3

when is transient increase or decrease in ICP common?

Answer 3

Transient increase or decrease is common during activities like straining, sneezing or getting up from recumbent position

Question 4

when is increase in ICP considered pathological?

Answer 4

Sustained increase beyond 20 cm of water is pathological

Question 5

what is the monro-kellie hypothesis?

Answer 5

Intracranial volume is fixed due to non-compliant skull

Contents are non-compressible

Change in volume of contents or addition of new space occupying lesion increases pressure unless compensated by reduction of other content

Question 6

how would a large extradural haematoma affect ICP?

Answer 6

Compensation
Reduction of venous blood
Displacement of CSF towards spine
Reduction CSF volume

Brain has minimal compliance

If these cannot compensate for the increase in volume then ICP rises exponentially

Question 7

what are pathologies of raised ICP?

Answer 7

Localized shifts of brain across various compartments
Focal or global reduction of blood flow
Focal or global ischemia
Reduced energy production
Pump failure on cell membrane
Cellular dysfunction
Interstitial microenvironment changes (toxic metabolites)
Membrane damage and cell death

Question 8

what would happen if we put a pressure transducer inside the brain?

Answer 8

we will find that the ICP can be traced as waves.
Under normal circumstances the normal ICP wave consists of three components.
The wave consists of P1, P2 and P3.
These are the three notches that are present in a normal ICP wave and
these respond to the arterial wave of the pulse.

even with respiration, there is a gradual increase and decrease of the ICP.
So essentially, ICP varies based on two components.
One is the arterial pulse, and second is the respiration.

Question 9

what is normal cerebral blood flow?

Answer 9

50ml/100g/min or 700 ml/min

Question 10

what percentage of cardiac output is cerebral blood flow?

Answer 10

14 % of the cardiac output.

Question 11

how does cerebral blood flow range between white and grey matter?

Answer 11

Range from 20ml/100g/min in white matter to 70ml/100g/min in grey matter

Regional variation in flow depending on demand

Question 12

what is mean arterial pressure (MAP)?

Answer 12

diastolic pressure plus one third of the pulse pressure (difference between the systolic and diastolic)

Question 13

what is the cerebral perfusion pressure (CPP)?

Answer 13

Difference between mean arterial pressure (MAP) and intracranial pressure (ICP)

Question 14

what is the cerebro-vascular resistance (CVR)?

Answer 14

Resistance offered by cerebral vasculature to flowing blood

Question 15

what is autoregulation?

Answer 15

ability of brain to maintain constant blood flow over wide range of pressures

Question 16

what four factors does the physiology of cerebral blood flow depend on?

Answer 16

Mean arterial pressure (driving blood into intra-cranial cavity)

Intracranial pressure (creates a gradient against incoming blood)

Vascular resistance (low resistance higher flow or vise-versa)

Autoregulatory mechanisms of the brain

Question 17

what does the brain have the capacity to do in terms of autoregulation?

Answer 17

ability to increase blood flow to specific regions in brain when these are active e.g. temporal lobe during speech

Question 18

what are the different mechanisms of autoregulation?

Answer 18

The autonomic neurogenic theory

Endothelial mechanisms

Myogenic autoregulation

Metabolic autoregulation
PaCO2 has significant influence on CBF
PaO2 influences CBF only if it drops significantly

Question 19

what does brain tissue use exclusively for metabolism?

Answer 19

glucose and ketones

Question 20

what do brain tissues not have?

Answer 20

a stored energy source

Question 21

what effect will impaired blood flow have on energy production?

Answer 21

Impaired blood flow will lead to mitochondrial energy production failure within seconds and cell death within minutes

Question 22

what does loss of the brains ability to autoregulate result in?

Answer 22

Injured brain loses its ability to autoregulate hence hypotension, hypertension, hypoxia or raised ICP can lead to significant secondary insult

Question 23

what effect does head injury have on autoregulation?

Answer 23

Severe traumatic brain injury causes raised ICP

Hypotension and hypoxia is common in polytrauma

Autoregulation is lost

Neuromonitoring involves ICP measurements and maintenance of CPP as therapy

Question 24

what is neuromonitoring?

Answer 24

Continuous monitoring of ICP with surgically implanted pressure transducer

Continuous measurement CPP by measuring MAP and ICP

Optimizing CPP to prevent brain insult by reducing high ICP and preventing high or low CPP