Ch 36 - Anaesthesia for Intracranial Surgery Flashcards
What is the Monro-Kellie Doctrine?
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
How do you calculate cerebral perfusion pressure (CPP)
CPP = MAP - (ICP or CVP, whichever is highest)
What is the normal CPP in anaesthetised Beagles?
In humans, what CPP has been associated with increased mortality and poor outcome?
- Beagles 82 +/- 5 mmHg
- CPP less than 60mmHg associated with poor outcome
What is normal ICP of the dog and cat?
What are broad categories of causes of increased ICP?
- Normal 5 - 12mmHg
Increases
- Expanding tissue or fluid mass
- Depressed skull fracture
- Interference with normal absorption of CSF
- Systemic disturbances causing brain oedema
How is cerebral blood flow (CBF) measured?
What is normal CBF in the dog?
What affects the autoregulation of CBF in dogs?
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
What is this graph called?
The intracranial elastance curve
How does CO2 effect CBF?
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)
How does O2 effect CBF?
What is the recommended PaO2?
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
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?
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
What increases the CRMO2?
What decreases the CRMO2?
Increases
- Pyrexia
- Seizures
- Ketamine
Decreases
- Hypothermia
- Benzodiazepines
- Barbiturates
- Propofol
What receptors cause a reflex bradycardia during the Cushings responce?
Pressor receptors in the aortic arch and carotid bodies
Why is dexmedetomidine good for intracranial surgeries?
- 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!
Why is acepromazine contraindicated?
Often results in vasodilation and arterial hypotension (leading to decreased CPP)
Why is ketamine contraindicated?
- Increases the CMRO2 thereby increasing CBF and ICP
- Prolonged and poor quality recovery
Why are opioids good?
- Do not alter the cerebral vascular response
- Early recovery
