Ch 36 - Anaesthesia for Intracranial Surgery Flashcards

1
Q

What is the Monro-Kellie Doctrine?

A

Viewing the cranium as a rigid box with a nearly incompressible brain. The cranium and its contents (blood, CSF, parenchyma) create a state of volume equilibrium such that any increase in volume of one component must be offset by a decrease in volume of another

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2
Q

How do you calculate cerebral perfusion pressure (CPP)

A

CPP = MAP - (ICP or CVP, whichever is highest)

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3
Q

What is the normal CPP in anaesthetised Beagles?
In humans, what CPP has been associated with increased mortality and poor outcome?

A
  • Beagles 82 +/- 5 mmHg
  • CPP less than 60mmHg associated with poor outcome
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4
Q

What is normal ICP of the dog and cat?
What are broad categories of causes of increased ICP?

A
  • Normal 5 - 12mmHg

Increases
- Expanding tissue or fluid mass
- Depressed skull fracture
- Interference with normal absorption of CSF
- Systemic disturbances causing brain oedema

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5
Q

How is cerebral blood flow (CBF) measured?
What is normal CBF in the dog?
What affects the autoregulation of CBF in dogs?

A

CBF = CPP/cerebral vascular resistance
Normal = 75.9 +/- 10.4ml/min/100g

Autoregulation keeps CBF fairly constant between MAP of 60-180mmHg. Above of below this range, CBF with increase/decrease linearly with MAP.
Autoregulation can be impaired by:
- Trauma
- Hypoxaemia
- hypercapnia
- Large-dose volatine agents

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6
Q

What is this graph called?

A

The intracranial elastance curve

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7
Q

How does CO2 effect CBF?

A

A linear relation exists between PaCO2 and CBF between 20-60mmHg suspected to be due to CO2 mediation changes in the extracellular pH
- Decreased PaCO2 -> vasoconstriction -> decreased CBF (and ICP)
- Increased PaCO2 -> vasodilation -> increased CBF (and ICP)

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8
Q

How does O2 effect CBF?
What is the recommended PaO2?

A

Minimal effect until below 50mmHg at which point there is a significant increase in CBF via vasodilation
Hyperoxia can also worsen outcomes following TBI

  • Aim for PaO2 60 - 250mmHg
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9
Q

What is cerebral metabolic rate for oxygen (CMRO2)?
What is the normal value in dogs?
How much of the total body oxygen requirement is used by the brain?

A

CRMO2 is the volume of metabolised O2 by the brain per unit of time
- Normal = 3.5 +/- 0.3ml/min/100g
- 20% total O2 used by the brain

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10
Q

What increases the CRMO2?
What decreases the CRMO2?

A

Increases
- Pyrexia
- Seizures
- Ketamine

Decreases
- Hypothermia
- Benzodiazepines
- Barbiturates
- Propofol

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11
Q

What receptors cause a reflex bradycardia during the Cushings responce?

A

Pressor receptors in the aortic arch and carotid bodies

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12
Q

Why is dexmedetomidine good for intracranial surgeries?

A
  • Typically does not produce vasodilation or arterial hypotension
  • Good haemodynamic stability
  • Shown neuroprotective activity (decreased cortical ischaemia by 40% in rat experimental model)
  • Shorter emergence and extubation times
  • Lower incidence of post-op nausea and vomiting

The cardiovascular effects may be difficult to distinguish from Cushings reflex!

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13
Q

Why is acepromazine contraindicated?

A

Often results in vasodilation and arterial hypotension (leading to decreased CPP)

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14
Q

Why is ketamine contraindicated?

A
  • Increases the CMRO2 thereby increasing CBF and ICP
  • Prolonged and poor quality recovery
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15
Q

Why are opioids good?

A
  • Do not alter the cerebral vascular response
  • Early recovery
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16
Q

Why are benzodiazepines good?

A
  • Causes a decrease in CMRO2 and CBF until a plateau is reached at about 75%
  • Commonly given combined with fentanyl :)
17
Q

Why is propofol good?

A
  • Reduces CRMO2 and CBF (and therefore ICP)
  • Preserves cerebral autoregulation and vascular reactivity
  • Lower post-op nausea and vomiting

Commonly used an maintenance agent as TIVA

18
Q

Why are inhalants generally contraindicated?

A
  • They are cerebral vasodilators causing increased CPP and increased ICP
  • Impair cerebral autoregulation (Sevoflurane less so)
19
Q

What are key features of anaesthetic monitoring during intracranial surgery?

A
  • Direct arterial blood pressure (ideal 80mmHg)
  • Central venous pressure
  • Capnography (28 - 32mmHg or 30-35mmHg)
  • Pulse ox
  • Oesophageal temp (effects CMRO2)
20
Q

What effect does shivering have on O2 requirement?

A

Increases it by up to 400%

21
Q

What are the main anaesthetic strategies for brain protection?

A

Maintaining BP at a MAP of at least 80mmHg
- Volume resuscitation
- Alpha-2s
- Decrease in depth

Haemoglobin concentration (anaemia)
- Preop Hb conc greater than 120g/L
- Intra-op Hn conc at least 90g/L

Hypothermia
- Decreases CMRO2
- Risks: Bacteraemia, arrhythmais, impaired immune function
- sepsis, pneumonia, electrolyte imbalance, imparied coagulation
- Not recommended

Osmotic agents
- Mannitol
- Hypertonic saline

Barbiturates
-Decrease CMRO2

Lidocaine
- neuroprotective - Na+ channel blocker (Na influx is the first step in ischaemic cascade)

22
Q

Is mannitol or hypertonic saline less permeable through the BBB?

A

Hypertonic saline! May result in a more effect fluid shift out of the brain parenchyma