Chapter 36 Anaesthesia for Intracranial Surgery Flashcards
What does the Monro-Kellie doctrine describe
Relationship between the contents of the cranium and intracranial pressure.
What is normal CPP in dogs and cats?
50 - 90 mmHg
What is the formula for CPP?
CPP = MAP - ICP (or CVP, whichever is higher)
What is normal ICP?
5-12 mmHg
(elsewhere says 8-15…)
What is the formula for cerebral blood flow?
CBP = CPP/Cerebral vascular resistance
What are the hree main consideration sin neuroanaesthesia for intracranial surgery?
- Maintain CPP and oxygeneation
- Preserve neuro function
- Rapid, high quality recovery
What is normal cerebral blood flow in dog?
0.75 mL/min/g
(vs 4 mL/min/g in the kidney!)
Below what MAP does cerebral autoregulation fail?
<60 mm Hg
From this point, cerebral blood flow declines lineraly with MAP
(Also note, if PaO2 <50 mmHg –> rapind increase in CBF)
What is effect of PaCO2 on cerebral blood flow?
Inc PaCO2 –> Inc CBF
What is target paCO@ in intracranial surgery
30 mmHg
(ischaemia below this, risk of inc ICP above)
What is normal cerebral metabolic rate for oxygen in the dog?
What % of total body oxygen consumption does this account for?
3.5 ml/min/100g
20% total body oxygen requirement
Cerebral oxygen consumption used for two things. What are they and what is their proportion of oxygen utilization?
- 60% neuronal activity, (ATP generation)
- 40% maintaining cellular integrity
Hyperglcaemia may worsen cerebral ischaemia. Above what Bg level is treatment recommened?
11 mmol/L
Comment on use of dexmedetomedine in intracranial surgery
- Sedative + analgesic
- Minimal resp depression
- Sympatholytic
- Neuroprotective
What is a unique feature of remifentanyl vs other pure-mu
What is the half life of remifentanyl in dogs?
Hydrolysed by plasma esterases i.e. not dependent on hepatic or renal metabolism
Half life = 7 minutes
HOw does propofol affect CBF and ICP
Reduces both
Outline a reasonable anaesthesia pan for intracranial surgery
- Premed: Fentanyl/midazolam +- dexmedetomedine
- Induction: Propofol coinduction (e.g. lidocaine - has neurprotective effects)
- Maintenance: Ideally propofol TIVA, otherwise sevo (maintain at <1.0 x MAC. Had least effect on cerebral autoregulation) + analgesia (fentanyl or remifentanyl)
How does NO2 affect cerebral vasculature
Potent vasodilator - avoid in intracranial surgery
What adjunctive medication might be added in prep for intracranial surgery and why
Maropitant >1hr before induction as anti-emetic/anti-nausea.
Vomiting –> raised ICP so avoid
Aside from usual premed, induction, maintenance and anti-emetic, what else shoudl be consideredin intra-craial surgery
NMBA
e.g. atracurium
Reverse with edrophonium or neostigmine
Describe 11 steps when troubleshooting “tight brain” = brain swelling
- Check ventilation. Moderate hypocapnia (target PaCO2 30 mm Hg [4.0 kPa]) is recommended.
- Check normal oxygenation.
- Control blood pressure. Target is normotension.
- Ensure adequate venous drainage from the brain.
- Elevate the head (30 degrees optimum). Check for compression of the external jugular veins
- Rule out pneumothorax.
- Maintain adequate neuromuscular relaxation.
- Administer mannitol or hypertonic saline
- Ensure that the concentration of volatile agent is less than 0.5 minimum alveolar concentration.
- If brain swelling does not subside, switch to an intravenous anesthetic technique. A combination of propofol and opioid infusion is ideal.
- If brain swelling does not abate, then the probability that the patient will have protracted increased intracranial pressure during the postoperative period is high. In this event, barbiturates (pentobarbital if available) may be administered until the swelling is reduced.
