The document discusses neurophysiology and factors controlling cerebral blood flow (CBF). Some key points:
- The brain has high metabolic needs but no oxygen storage, so it relies on continuous CBF. CBF parallels metabolic activity and averages 50 ml/100g/min.
- CBF is controlled by cerebral perfusion pressure (CPP), which depends on mean arterial pressure and intracranial pressure. Autoregulation normally keeps CBF constant over a wide range of pressures.
- Important factors influencing CBF include carbon dioxide, which causes vasodilation; oxygen; hematocrit; temperature; and anesthetic agents, many of which are cerebral vasodilators. Barbiturates
2. Peculiarities of brain
Has a high metabolic rate
Has no oxygen stores
Unable to maintain its integrity
through anaerobic metabolism
Neurons don’t require insulin for
transport of glucose across cell
membrane
3. Neurophysiology
2% of body weight
17%-20% of Cardiac output consumption at rest
20% of inspired oxygen
60% - for neuronal activity
40% - to maintain cellular integrity
CMR / CMRO2 - 3-3.8ml/100g/min
50ml/min
Cerebral glucose consumption - 5mg/100g/min
4. cerebral blood flow
80% - Internal carotid arteries
20% - Vertebral arteries
Anterior n posterior communicating
arteries
Circle of Willis
Communication between exl & int
carotids – opthalmic arteries
9. Cerebral perfusion pressure
CPP is the difference between Mean
arterial pressure and intracranial pressure
(or cerebral venous pressure, which ever
is greater)
CPP = MAP – ICP
10. CPP = MAP - ICP
Normal CPP – 80 to 100mmHg
More dependent on MAP
ICP > 30mmHg compromise CPP
CPP
< 50mmHg – slowing of EEG
25 – 40 mmHg – flat EEG
< 25 mmHg – irreversible brain damage
11. Cerebral auto regulation
Ability of the cerebral blood vessels to
alter their caliber in order to maintain a
constant flow in face of variations in
blood pressure
12. Cerebral auto regulation
CBF is kept constant over a wide range of
MAP ( 60 – 160 mm Hg )
CPP = MAP – Ven Press = MAP - ICP
↑ MAP Cerebral vasoconstriction
↓ MAP Cerebral vasodilatation
Constant CBF is maintained
14. Auto regulation……
The cerebral vasculature rapidly adapts to
change in CPP. (10 - 60 sec)
In Hypertensive persons cerebral
autoregulation curve shifts to higher pressure
levels : 180 – 200mm Hg and towards
right.
15. Changes in autoregulation
Absent ( Vasomotor paralysis )
brain trauma
surgical retraction
high ICP
brain tumor
seizures
Shift to right
Systemic hypertension
States of sympathetic activation
Shift to left
Volatile anesthetic agents
17. Myogenic factors
Is the intrinsic response of
smooth muscle cells in cerebral
arterioles to changes in MAP
Protective mechanism against
excessive pressure fluctuation
at capillary level
19. Innervation
The sympathetic fibers arise mainly from the
superior cervical ganglion
The parasympathetic from the sphenopalatine
and otic ganglia
Sensory fibers from the trigeminal ganglion
20. Neuronal regulation
α-Adrenergic receptors in arterial smooth muscle
Postganglionic sympathetic fibers release
noradrenaline
Causes smooth muscle contraction and
arterial constriction
Sympathetic innervation is responsible for
vascular tone
21. Sympathetic
Large & Medium sized arteries
normally overridden by autoregulation
Historically thought to have no role in cerebral
circulation
Comes into play in states of excessive circulatory
activity / pathologic states
Role in prevention of cerebral haemorrhage –
cerebral vasospasm
23. Effect of CO2 on CBF
CBF œ PaCO2 between 20 – 80 mmHg
1mmHg ↑↓ PaCO2-↑↓ CBF by 1-2ml/100g/min
After 24 – 48 hrs CSF HCO3-
compensation limits the
effects of hypocapnia/ hypercapnia
Persistent hyperventilation Leftward shift of oxy-Hb
dissociation curve and marked changes in CBF
cerebral impairment
24. Hypercarbia - CBF
The relationship
between PaCO2
and CBF is sigmoid
with plateaus below
25 mmHg and
above 75 mmHg.
The slope is
approximately linear
25. Mechanism of CO2 on CBF
The mechanism of CO2 induced changes in vessel caliber
An increase in perivascular H+ concentration
Associated NOS activation
An increase in intracellular cGMP
K+
efflux
A reduction in intracellular Ca + +
resulting in dilation
NOS inhibition attenuates the
Cyclooxygenase inhibition CBF response to CO2
26. Effect of oxygen
Hyperoxia – minimal decrease in CBF
10%
Severe hypoxia – PaO2 < 50mmHg
Increases CBF
29. Intracranial pressure
“ICP means supra tentorial CSF pressure
measured in the lateral ventricles or over the
cerebral cortex and is normally less than
10mmHg.”
Minor variations may occur depending on site
measurement but, in lateral recumbent position,
lumbar CSFpressure normally approximates
supratentorial pressure.
30. Intracranial pressure
MONRO-KELLIE DOCTRINEMONRO-KELLIE DOCTRINE
The cranial vault is a rigid structure with fixed
volume
Brain 80%
Blood 12%
CSF 8%
Any increase in one component must be offset by an
equivalent decrease in another to prevent rise in ICP
31. Intracranial pressure
ICP normally 10mmHg and less.
Intracranial elastance determined by
measuring the change in ICP in response to
change in intracranial volume
Initially increases in volume are initially
well compensated until it reaches a point
which further increase can cause rise in ICP
33. Intracranial pressure
Major compensatory mechanisms include
a)Displacement of CSF from cranial to spinal
compartment
b)An increase in CSF resorption
c)Decrease in CSF production
d)Decrease in total cerebral blood volume
35. Volatile agents
Volatile agents – dose dependent
dilatation of cerebral vessels
Impair auto regulation
Response to CO2 retained
May increase cerebral blood volume
May result in elevated ICP
36. Halothane
Has greatest effect on
CBF
Con.> 1% - abolishes
auto regulation
Generalized increase in
CBF
At equivalent MAC CBF
up to 200%
Prior hyperventilation
to be initiated
Isoflurane
CBF
Auto regulation
maintained up to 1 MAC
is > in sub cortical than
neocortical areas
At equivalent MAC
CBF up to 20%
Simultaneous
hyperventilation can
prevent in ICP
37. Sevoflurane:
CBF effects similar to isoflurane
Produce slightly less vasodilation
Auto regulation maintained up to 1.5 MAC
Desflurane:
CBF similar to isoflurane
Autoregulation progressively abolished as dose
increases
38. Nitrous Oxide:
When administered on its own- increases both
CBF and metabolism.
when added to a background of another
anesthetic, it increases CBF without changing
metabolism
It is a direct acting and potent cerebral
vasodilator
39. IV induction agents
Intravenous anesthetics reduce CBF in
a dose dependent fashion
coupled to the reduction in metabolism
Once maximal suppression of
metabolism occurs, no further reduction
in CBF occurs
40. Barbiturates
Barbiturates maximal 50% reduction in CBF
and metabolism
CO2 reactivity is maintained but is
quantitatively reduced compared to the awake
response
Cerebral auto regulation maintained
intact
41. Propofol
Propofol produces a coupled dose dependent
reduction in CMRO2 and CBF
High doses vasodilator effect overcomes the
coupling & CBF increases
Both CO2 responses and auto regulation are
maintained intact in the normal brain
In head injured patients static auto regulation
may be impaired by high propofol infusion rates
42. Ketamine
Dilates the cerebral vasculature and
increases CBF ( 50 – 60%)
Increases in CBF, CBV, CSF volume can
increase ICP markedly in patients with
decreased IC compliance
43. Opioids
Opioids at low doses produce very little effect
on CBF (provided CO2 is not allowed to rise)
Auto regulation remains intact
Some opioids in ICP
BP vasodilatation to maintain CBF
cerebral blood volume
increase intracranial pressure.
44. Vasopressors
With intact auto regulation & BBB
in CBF occurs when
MAP<50 -60mmHg
MAP>150 – 160mmHg
In the absence of auto regulation,
vasopressors CBF by direct effect on CPP.
45. Vasodilators
In the absence of hypotension
Cerebral vasodilatation
CBF
With Hypotension
CBF is maintained or increased
CBV & ICP in patients with IC
compliance
46. NMBD
No direct effect on CBF
Histamine releasing agents can cause
hypotension , CPP
47. What is
Luxury perfusion ?
Intra cerebral steal ?
Reverse steal phenomenon ?
48. Luxury perfusion
The combination of a decrease in
CMRO2 and increase in CBF has
been termed luxury perfusion
met. Demand met. Supply
49. Luxury perfusion…
Seen in
Acute cerebral infarction
Vessels – max. dilated
Induced hypotension with isoflurane
50. Intracerebral Steal
In a setting of focal ischemia ,
vasodilatation in a normal area
would shunt blood away from the
diseased area.
ischemic normal
51. Steal
Seen in
in PaCO2 in cerebral ischemia
Volatile anesthetic agents
Results in vasodilatation in normal areas
not in ischemic areas
52.
53. Reverse Steal phenomenon
Diversion or redistribution of blood
flow from normal to ischemic areas in
the brain is termed Reverse Steal /
Robin Hood phenomenon
ischemicnormal