Cerebral Perfusion

 

now another very interesting background

area of physiology that we need to

understand relates to the blood flow to

the brain I'm just going to write an

equation down CPP CPP equals MA BP minus

IC VIN of course that means absolutely

nothing at the moment but CCP stands for

cerebral perfusion pressure that is the

pressure of the blood going through the

brain and that is equal to mean arterial

blood pressure the mean arterial

systemic blood pressure minus the

intracranial pressure

so let's imagine we're looking at the

vault of the school here which as we

know is closed then the blood has got to

get into the vault of the scope and the

vertebral arteries actually go through

the foramen magnum but let's assume that

that's closed with a spinal cord and

brain at the moment and the carotid

arteries going via the holes in the base

of the skull so let's imagine that

they're the internal carotid arteries

carrying blood into the brain into the

cranial cavity now the mean arterial

blood pressure is going to force blood

into the cranial cavity that is the

pressure of the blood in the systemic

arteries force its way into the cranial

cavity and that is going to tend to

perfuse the massive blood vessels

located throughout the throughout the

brain in the intracranial compartment 

CPP = MAP - ICP

100mmHg MAP - 10

so

that's the mean arterial blood pressure

but remember inside the brain there is

an intracranial pressure already so

let's suppose that the mean arterial

blood pressure is around about 100

millimeters and Mercury forcing blood in

then the intracranial pressure is going

to be about 10 millimeters of mercury

actually inside inside the cranial

cavity so you've got a hundred

millimeters of mercury forcing in the 10

tending to force the act that means the

perfusion pressure in the vessels in the

brain is going to be that hundred

pushing in with that 10 force pushing

out so it's going to be a hundred minus

ten so the cerebral perfusion pressure

the pressure of the blood in the blood

vessels in the

your cavity is going to be equal to the

mean arterial blood pressure which is

about 100 minus the intracranial

pressure which is about 10 and 100 minus

perfusion pressure of nitrogen so 98 or

so diffusion pressure equals 90 equals a

hundred minus 10 so the cerebral

perfusion pressure the pressure going

through the blood vessels of the brain

equal to the mean arterial blood

pressure resisted by the intracranial

pressure minus intracranial pressure and

in this relatively normal case we see

that the cerebral perfusion pressure is

going to be about 90 millimeters of

mercury now this graph shows the

Brain blood flow

physiological situation of brain blood

flow the amount of blood flowing through

the brain and this will be 50% of normal

this will be a hundred percent of normal

ie normal blood flow through the brain

and this would represent increase

levels of blood flow through the brain

this is a really beautiful piece of

physiology because we see that through

blood pressures around about 50 up to

blood pressures around about a hundred

and fifty the blood flow to the brain or

through the brain is pretty well

constant at 100 a hundred percent of

normal this means that the brain is not

to hypoperfusion Elevens until we round

about 50 millimeters of mercury mean

arterial blood pressure then has been

our Teva blood pressure rises up to

normal levels and even as it goes beyond

normal levels we still see that the

blood flow actually roundabout the brain

to the brain is is still unchanged at

the normal hundred percent level so this

is very useful because it means we don't

really go fatal with low pressure starts

dropping below say about 60 in an

otherwise healthy

youngish person but if the book trasury

rises for whatever reason there's not a

significantly increased levels of blood

flow around the brain until we start

getting on to very high levels of blood

pressure when the cerebral blood flow is

increased so this is the normal

situation where blood flow is Auto

regulated the brain will regulate its

own perfusion throughout a normal range

throughout a large range of mean

arterial blood pressures but of course

Brain injury

this is describing the normal

physiological situation when there is

brain injury in many individual cases of

brain injury the ability of the brain to

auto regulate itself is actually lost

this means that if the blood pressure

drops then the cerebral perfusion

pressure will drop as well because the

auto regulatory power is lost in brain

injured patients so patients with brain

injuries their cerebral perfusion

pressure is dependent on their mean

arterial blood pressure being high

enough and that intracranial pressure

being low enough to maintain cerebral

perfusion this is why it is so important

in brain injured patients if they've

lost fluid and they are hypotensive to

give them fluids to restore the blood

pressure they must have a rapid

restoration of normal blood pressure if

they're put pressure is low because the

cerebral perfusion pressure in the brain

injured patient is completely dependent

on the mean arterial blood pressure

being sufficient to overcome the

intracranial pressure to give normal

cerebral perfusion pressures it is not

like this normal situation where the

brain is able to auto regulate so in

brain injured patients we absolutely

must maintain good levels of blood

pressure if the blood pressure drops for

a period of time the brain will become

nificant behind Pope refused it will

become hypoxic and that severely worsens

the prognosis so maintain adequate there

was a blood pressure in a brain injured

patient as a matter of some urgency

once the brain injury has occurred the

damage is very difficult to reverse

because the brain cannot actively

regenerate its own tissues so it's very

important to prevent brain injury and to

understand how to do this let's think

about the nature of brain injuries so

what we've got here is brain injuries

this is this is the injuries that can

occur to the brain and there are two

types of these primary brain injury and

secondary brain injury now the primary

brain injury is the injury to the brain

that occurs at the time of the accident

or the trauma so when the head is hit

when the trauma impacts on the head that

will generate a primary brain injury it

occurs at the time so when the patient

is admitted to your care the primary

brain injury has already occurred and

there's nothing you can do about that

but after the primary brain injury

secondary brain injury come to me about

it can develop now a secondary brain

injury is any injury which occurs to the

brain after the initial trauma and these

are caused by complications of the

initial brain injury an our role in

healthcare is to stop the patient from

developing secondary brain injuries so

the primary brain injury we can do

nothing about it's already occurred when

the patient comes into our care our aim

is to prevent secondary brain injuries