Cerebral Circulation: Demand and Delivery

What percentage of body mass is the brain?

~2%

What percentage of resting energy does the brain use?

~20%

What percentage of cardiac output goes to the brain?

~15%

How much power does the brain use at rest?

~20 W

What does almost all of the brain's energy expenditure go towards?

Pumping ions back across cell membranes

Why can the brain not tolerate a prolonged interruption in blood flow?

Neurons have no glycogen reserve and are obligate aerobes, so oxygen and glucose must arrive continuously by blood flow

What happens after ~10 seconds without adequate cerebral blood flow?

Unconsciousness

What happens after ~20 seconds without adequate cerebral blood flow?

EEG goes flat

What happens after ~4–8 minutes without adequate cerebral blood flow?

Neurons start to die

Why don't neurons have a useful energy reserve?

Neurons hold no glycogen

What are the two key substrates that must continuously reach the brain?

Oxygen and glucose

What is normal whole-brain cerebral blood flow (CBF)?

~50 ml/100 g/min

How does cerebral blood flow differ between grey and white matter?

Grey matter ~80 ml/100 g/min; white matter ~20 ml/100 g/min

What happens when CBF falls below ~20 ml/100 g/min?

Tissue stops working but survives; this is the penumbra

What happens when CBF falls below ~10 ml/100 g/min?

Tissue dies; this is the infarct core

What are the three pressures important for cerebral perfusion?

MAP, ICP and CPP

What does MAP stand for?

Mean arterial pressure

What is MAP?

Average pressure in the arteries; it is nearer to diastolic than systolic pressure

What is the normal MAP range given in the lecture?

~70–100 mmHg

What does ICP stand for?

Intracranial pressure

What is ICP?

Pressure inside the closed skull

What is the normal ICP range given in the lecture?

~5–15 mmHg

What can cause ICP to rise?

A bleed, tumour or swelling

What is cerebral perfusion pressure (CPP)?

CPP = MAP − ICP

What actually drives cerebral blood flow: arterial pressure or CPP?

CPP

What is the normal CPP range?

~70–90 mmHg

What happens when CPP falls below ~50 mmHg?

Ischaemia

What is the Monro–Kellie principle?

The brain, blood and CSF occupy a closed box; adding volume causes ICP to rise and CPP to fall

What are the approximate proportions of the contents of the skull?

Brain 80%, blood 10%, CSF 10%

How does a bleed, tumour or oedema affect cerebral perfusion?

It adds volume inside the closed skull, increasing ICP and therefore reducing CPP

What is cerebral autoregulation?

The ability of the cerebral circulation to maintain relatively constant blood flow across a range of MAP

Across what MAP range is cerebral blood flow held relatively flat by autoregulation?

~60–150 mmHg

Which chemical stimulus is particularly powerful in regulating cerebral blood flow?

CO₂

When does O₂ become an important stimulus for cerebral blood flow?

When O₂ falls below ~50 mmHg

A patient with untreated hypertension has a usual MAP of ~130 mmHg. If medication lowers MAP to 75 mmHg, what happens according to the lecture?

Nothing — 75 mmHg is within the normal autoregulatory plateau

What are the two arterial systems supplying the brain?

Anterior circulation via the internal carotids and posterior circulation via the vertebrals → basilar

How many major arteries supply the brain from these two systems?

Four arteries: two internal carotids and two vertebral arteries

What does the anterior circulation mainly supply?

The cerebral hemispheres

Approximately what proportion of cerebral blood flow comes through the internal carotids?

~75%

What does the posterior circulation mainly supply?

The brainstem, cerebellum and occipital regions

What arteries form the posterior circulation?

Two vertebral arteries that join to form the basilar artery

Why is failure of the vertebrobasilar circulation particularly dangerous?

It supplies the brainstem, and failure threatens consciousness

Where is the commonest site of atherosclerosis in the neck?

The carotid bifurcation

Why does atherosclerosis commonly develop at the carotid bifurcation?

Low, oscillating shear stress at the carotid bulb promotes plaque formation

How does carotid bifurcation atherosclerosis mainly cause stroke?

By throwing emboli rather than by simply throttling blood flow

What important sensory structures are located around the carotid bifurcation?

The carotid sinus and carotid body

What does the carotid sinus detect?

Blood pressure via a baroreceptor

What does the carotid body detect?

Chemical changes via a chemoreceptor

What is amaurosis fugax?

A transient “curtain over one eye” caused by a carotid embolus entering the ophthalmic artery

Why should amaurosis fugax be treated as an emergency?

It can indicate a carotid embolus and is a warning of vascular disease/stroke rather than simply an eye complaint

Through what structures do the vertebral arteries ascend?

The transverse foramina from C6 to C1

Through which opening do the vertebral arteries enter the skull?

The foramen magnum

What happens to the two vertebral arteries after entering the skull?

They fuse to form the basilar artery

What are the major branches of the vertebrobasilar system listed in the lecture?

PICA, AICA, pontine perforators and SCA

What does the basilar artery eventually divide into?

The two posterior cerebral arteries

Why can neck movement and trauma affect the vertebral arteries?

They run through a tunnel of bone

What are the key numbers for cerebral blood flow to remember?

Whole brain ~50; grey matter ~80; white matter ~20 ml/100 g/min

What are the two important CBF thresholds?

Below 20 = tissue stops working but survives (penumbra); below 10 = tissue dies (infarct core)

What is the key equation for cerebral perfusion?

CPP = MAP − ICP

What happens to CPP when ICP increases while MAP stays constant?

CPP falls

What happens to CPP if MAP decreases while ICP stays constant?

CPP falls

What are the key autoregulation numbers?

Cerebral blood flow is maintained across MAP ~60–150 mmHg

What are the key arterial supply numbers?

Internal carotids supply the anterior circulation and ~75% of cerebral flow; vertebrals → basilar supply the posterior circulation

What is the key anatomical relationship to remember for the carotids?

The carotid bifurcation is around C4 and is a common site of plaque

What is the key anatomical relationship to remember for the vertebrals?

They ascend through transverse foramina C6→C1, enter via the foramen magnum and fuse into the basilar artery