Basic Principles of Neuroanaesthesia

Goals of neuroanaesthesia

  • On top of the triad of anaesthesia (hypnosis; paralysis; and analgesia); there are other goals, some particular to neurosurgery, some important to anaesthesia in general
    1. Haemodynamic stability- adequate cerebral perfusion pressures must be maintained, or there is risk of cerebral ischaemia.  Conversely, hypertension should be avoided as it increases the risk of cerebral oedema or worse, haemorrhage.
      • Many anaesthetic agents, particularly inhaled agents, reduce the cardiac output by causing myocardial depression (decreased rate) and by decreasing systemic vascular resistance.
    2. Maintaining cerebral perfusion
    3. Keep Intracranial pressure down (avoid a ‘full’ skull)
      • Hyperventilation can be used to decrease ICP (causes vasoconstriction- note can lead to hypoperfusion if done inappropriately i.e. should only be used in short bursts for patients with significantly raised ICP unresponsive to mannitol)
      • Intra-operative mannitol
        • An osmotic diuretic, it draws water out of the tissue into the vasculature.  High dose is recommended (2g/kg over ~30 mins) provided the patient is not hypovolaemic or hyperosmolar.
      • Positioning with the head up has been shown to significantly decrease ICP without affecting CPP.
      • The use of dexamethasone prior to surgery is often used to reduce cerebral oedema.  However, be aware that it can increase blood sugars, which can have significant implications for patients in neurocritical care.
    4. Protect the cerebral tissue where possible
      • Some advocate the use of ketamine, propofol and volatile agents for their neuroprotective effects (i.e. preventing cell death)- however, the evidence for this in patients suggest these effects are short lived)
    5. Ensure rapid recovery
      • Managing analgesia with suitable opioid (e.g. remifentanyl) as well as using quick onset/offset volatile agents e.g. sevoflurane, is important for rapid recovery and assessment of function

Drugs

  • Note that the choice of anaesthetic agents used in neurosurgery is controversial, as all have theoretical disadvantages.  The choice of drug is less likely to affect outcome than measures highlighted above.  Raised ICP prior to surgery, degree of midline shift and diagnosis (malignant tumour) are all more likely to cause intraoperative swelling.
  • Inhaled agents
    • These have the potential to increase ICP due to vasodilation.  However, in practice, if given appropriately (low concentrations) and there is no rise in ICP prior to surgery, the effect is minimal.
    • Sevoflurane seems to be superior for its comparative effect on CBF and ICP, causing the least vasodilation and least effect on the autoregulation of blood flow
  • Intravenous agents
    • Propofol is often used in combination with an inhaled agent.  It appears to have a theoretical effect of reducing ICP and increased CPP when compared with sevoflurane alone.
    • Some operations require evoked potential monitoring which can be affected by volatile agents.  In these operations, total IV anaesthesia (TIVA) may be advised

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