Anaesthesia for craniotomy for intracranial aneurysms: principles and precautions

Neil Warwick

Westmead Hospital, Westmead, NSW

Ninety percent of intracranial aneurysms (ICA's) present with subarachnoid haemorrhage (SAH), with two thirds requiring anaesthesia intervention in the "out-of-hours" period. Realistically, therefore, "operative anaesthesia" management may start in the A&E or radiology department.

Note that anaesthesia technique probably does not so much as favourably affect outcome when well managed, as adversely affect outcome when managed less than well! (ie, you probably can't make it better but you certainly can "screw-it-up"!).
What the Surgeon Needs: A small, soft brain; an intact aneurysm; a still operating field; maintained cerebral perfusion; a vigilant colleague who is prepared to make major alterations in management at a moment's notice
A few pertinent "patient factors": These patients are typically young to middle aged (4th to 7th decade), more commonly female and often with few intercurrent pre-existing medical conditions. These patients are acutely afflicted, as per the classical description, and so are seldom fasted. Furthermore, as SAH is a "life-threatening" event, it evokes a powerful fight-flight response. Given this sympathotonic response to SAH, patients are unlikely to have an "empty" stomach for several days.
The hypertension we commonly see in these patients is usually acute and may be in response to an acute rise in intracranial pressure (ICP); be wary about acutely lowering the MAP and cautious about which agent you choose to use, it CAN make a difference to ICP and LOC. Note that the surgeon is very wary of systolic hypertension rather than MAP. Discuss in detail what he requires at particular times during the procedure. Re-rupture prior to dural opening has a high likelihood of POOR outcome. All this circulating catecholamine can adversely affect systemic vascular resistance and coronary perfusion, causing left ventricular dysfunction. This, combined with the possible loss of consciousness at initial rupture which can be associated with pulmonary aspiration, may be responsible for "neurogenic" pulmonary oedema.
Level of consciousness and symptoms determine the Hunt and Hess grade, ultimate outcome being a function of grade at presentation, EXCEPT that if the GCS is low due to acute hydrocephalus. In this case, EVD placement may improve grade which translates into a commensurate improvement in prognosis; ie GCS 3 is not necessarily hopeless!
Which monitors and when? All usual ones but the arterial line is really important during induction, so it must be placed, be reliable and monitored during induction. Central venous pressure is important both intraoperatively and postoperatively, particularly if vasospasm ensues. Since "HHH" therapy can easily last a week, and reliable access is needed, at least a double lumen CVL is required, preferable placed in a subclavian vein. Neck lines must not be placed ipsilateral to the aneurysm (in case carotid access is needed for control) and are more difficult to access intraoperatively and nurse postoperatively. Temperature should be monitored at your favourite reliable site, since none have been shown superior to others. Neurophysiological monitoring of a least bi-cortical EEG, since this gives you an end point if burst-suppression is requested; although SSEP's are more sensitive for focal perfusion deficits. BIS or commercial equivalent if you believe this helps.

How to rupture or re-rupture an aneurysm:
Principal determinants of likelihood of rupture:

1. Size (we can't really affect this)
2. Trans-mural pressure gradient (this we can determine, to a certain extent)

"Sure-fire" approaches to a bad outcome:

1. Overly aggressive, early cerebral volume reduction prior to dural opening accompanied by
2. Poor attention to variability in blood pressure with inadequate control

Receipe for anaesthesia: PLAN THE EMERGENCE NOW! Premedication: sympathostatic, minimally sedating, non-enteroparetic = alpha-2 agonist. Induction: Smooth, rapid or "modified"-rapid sequence. Must have minimal haemodynamic instability prior to and during tracheal intubation. Thio./Sux or Propofol / Roc. unmodified will give poor results. Remifentanil or alfentanil rapidly suppress airway reflex responses and esmolol rapidly decreases sympathotonic effects. Infusions / push doses of fentanyl work slowly and often evoke late hypotension. IV lignocaine 1.5mg.kg-1 still works but needs ~ 2min. Relaxation at intubation should be optimal, thus the only options are suxamethonium or rocuronium, subsequent relaxation should be titrated to 2-4 PTC twitches. Maintenance with up to 1 MAC sevo/Isoflurane will maintain CBF at preinduction values at mild hypocapnia but be wary of ETCO2 as it often correlates poorly with PACO2 in this group. TIVA will give a coupled reduction in CBF in responsive regions and thus usually gives a lower ICP and smaller brain size. Outcome appears unaffected by choice, use the one you know best. Watch TIVA clearance variations with temperature and cardiac output. N2O is better avoided as acute pulmonary pathology plus nimodipine make its use difficult anyway. Pin placement evokes a response of similar magnitude to intubation and will need prophylaxis with opiate +/- esmolol, similarly watch for BP changes during head manipulation of the intubated patient. Temperature target can be anything EXCEPT >37C. Brain shrinkage with Mannitol 1-1.5g.kg-1+/- frusemide (<=20mg) should proceed only when the surgeon is turning the bone flap to minimise the chance of re-bleed. Temporary clipping is managed by raising the BP ~15-20%; occasionally metabolic suppression is requested, the "magic bullet" dose of thiopentone being anything from 250mg - 1.5g! (put on an EEG). Steroids are of no proven benefit and often further elevate blood glucose with attendant adverse risk during ischaemia. The aneurysm ruptures! Put down coffee; add thiopentone to burst suppression and lower BP to ~90 systolic; request blood, which you may need; book an ICU bed and allow passive cooling without rewarming. If regional isolation is achieved, the BP should be raised to minimise collateral damage.
To wake-up or not. Uncomplicated procedures in grade I, II, III patients, preferably above ~ 35C: Extubate, neurological signs are important and useful in these patients. Remember to replace fluids (watch Na+ content) as hypovolaemia is associated with cerebral hypoperfusion. If you consider ICU seriously, that's where the patient should go!

Suggested reading: Handbook of Neuroanaesthesia, 4th Ed. P. Newfield, J.E.Cottrell, 2007