Elective neurosurgery

Which one of the following statements is TRUE regarding anaesthesia for routine

elective neurosurgery?

a) Desflurane is the agent of choice for many neuroanaesthetists

b) Dense neuromuscular blockade is required for a craniotomy

c) Permissive hypothermia is usually employed for cerebral protection

d) Resection of cortex is profoundly stimulating

e) A central venous catheter is mandatory for a craniotomy in a head-up position

Answer: a

Explanation

This question will be simple for those candidates who have clinical experience of

neuroanaesthesia, but frustrating for those who have used standard textbooks to

prepare for their exam. A consequence of modular training is that it is not uncommon

that a candidate will be sitting their Final FRCA before completion of each of the

anaesthetic sub-specialties. It is worthwhile trying to arrange some study leave accompanying

a neuroanaesthetist for a routine neurosurgery list, as some of the historical

mantras of neuroanaesthesia will be dispelled.

The concern over volatile anaesthetic agents is that they abolish cerebral autoregulation

and cause a vasodilation that in turn increases intracranial pressure. This is

counterbalanced by the fact that they reduce cerebral metabolism and reduce oxygen

demand thus protecting neurones. Historically isoflurane was the agent of choice as its

disruption in these respects was the least. In fact, all agents will diminish autoregulation

in a dose-dependent fashion. Examination of data and graphs for percentage

change in cerebral blood flow when compared to end-tidal agent concentration

shows that at 0.5 minimum alveolar concentration (MAC) of isoflurane, sevoflurane,

desflurane and even halothane there is minimal increase in cerebral blood flow over

baseline. At 1 MAC of halothane, cerebral blood flow is increased to 250% of baseline

and should therefore be avoided for neuroanaesthesia. However, for desflurane, isoflurane

and sevoflurane at concentrations of around 1 MAC there is only a slight

increase in cerebral blood flow (desflurane > isoflurane > sevoflurane). If a remifentanil

infusion is used then analgesia may be titrated to cover particularly stimulating stages

of a craniotomy, it is MAC sparing (so a full MAC of volatile agent is seldom required

and the cerebral blood flow effects not seen) and dense neuromuscular blockade is not

required as with adequate infusion rates spontaneous respiration, coughing and even

response to laryngeal stimulation (by the endotracheal tube if the patient’s head is

moved) is abolished. Given the intricacy of the surgery it is of course mandatory that

there is no movement (or potential movement) of the surgical field. The advantage of

combining remifentanil and desflurane for a neuroanaesthetic is that rapid recovery

allows early neurological assessment for potential deficit.

Application of Mayfield pins, dissection of the scalp (without local anaesthetic) and

incision of the dura are very stimulating. The cortex is insensate and for the large part of

the craniotomy stimulation is minimal.

Although an arterial line is required, a central venous catheter is rarely necessary

(although this seems to be institution dependent). The use of central venous catheters

for emergency aspiration of venous air embolism is attractive theoretically but may be

practically prohibited. Hypothermia may be used in head injuries and some emergency

work, but not for routine elective neurosurgery.