Chapter 36 Anaesthesia for Intracranial Surgery

Question 1

What does the Monro-Kellie doctrine describe

Answer 1

Relationship between the contents of the cranium and intracranial pressure.

Question 2

What is normal CPP in dogs and cats?

Answer 2

50 - 90 mmHg

Question 3

What is the formula for CPP?

Answer 3

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

Question 4

What is normal ICP?

Answer 4

5-12 mmHg

(elsewhere says 8-15…)

Question 5

What is the formula for cerebral blood flow?

Answer 5

CBP = CPP/Cerebral vascular resistance

Question 6

What are the hree main consideration sin neuroanaesthesia for intracranial surgery?

Answer 6

• Maintain CPP and oxygeneation
• Preserve neuro function
• Rapid, high quality recovery

Question 7

What is normal cerebral blood flow in dog?

Answer 7

0.75 mL/min/g

(vs 4 mL/min/g in the kidney!)

Question 8

Below what MAP does cerebral autoregulation fail?

Answer 8

<60 mm Hg

From this point, cerebral blood flow declines lineraly with MAP

(Also note, if PaO2 <50 mmHg –> rapind increase in CBF)

Question 9

What is effect of PaCO2 on cerebral blood flow?

Answer 9

Inc PaCO2 –> Inc CBF

Question 10

What is target paCO@ in intracranial surgery

Answer 10

30 mmHg

(ischaemia below this, risk of inc ICP above)

Question 11

What is normal cerebral metabolic rate for oxygen in the dog?

What % of total body oxygen consumption does this account for?

Answer 11

3.5 ml/min/100g

20% total body oxygen requirement

Question 12

Cerebral oxygen consumption used for two things. What are they and what is their proportion of oxygen utilization?

Answer 12

• 60% neuronal activity, (ATP generation)
• 40% maintaining cellular integrity

Question 13

Hyperglcaemia may worsen cerebral ischaemia. Above what Bg level is treatment recommened?

Answer 13

11 mmol/L

Question 14

Comment on use of dexmedetomedine in intracranial surgery

Answer 14

• Sedative + analgesic
• Minimal resp depression
• Sympatholytic
• Neuroprotective

Question 15

What is a unique feature of remifentanyl vs other pure-mu

What is the half life of remifentanyl in dogs?

Answer 15

Hydrolysed by plasma esterases i.e. not dependent on hepatic or renal metabolism

Half life = 7 minutes

Question 16

HOw does propofol affect CBF and ICP

Answer 16

Reduces both

Question 17

Outline a reasonable anaesthesia pan for intracranial surgery

Answer 17

• 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)

Question 18

How does NO2 affect cerebral vasculature

Answer 18

Potent vasodilator - avoid in intracranial surgery

Question 19

What adjunctive medication might be added in prep for intracranial surgery and why

Answer 19

Maropitant >1hr before induction as anti-emetic/anti-nausea.

Vomiting –> raised ICP so avoid

Question 20

Aside from usual premed, induction, maintenance and anti-emetic, what else shoudl be consideredin intra-craial surgery

Answer 20

NMBA

e.g. atracurium

Reverse with edrophonium or neostigmine

Question 21

Describe 11 steps when troubleshooting “tight brain” = brain swelling

Answer 21

1. Check ventilation. Moderate hypocapnia (target PaCO2 30 mm Hg [4.0 kPa]) is recommended.
2. Check normal oxygenation.
3. Control blood pressure. Target is normotension.
4. Ensure adequate venous drainage from the brain.
5. Elevate the head (30 degrees optimum). Check for compression of the external jugular veins
6. Rule out pneumothorax.
7. Maintain adequate neuromuscular relaxation.
8. Administer mannitol or hypertonic saline
9. Ensure that the concentration of volatile agent is less than 0.5 minimum alveolar concentration.
10. If brain swelling does not subside, switch to an intravenous anesthetic technique. A combination of propofol and opioid infusion is ideal.
11. 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.