Principles of neuroanaesthesia
CPD matrix matches
2A03, 2F01
Principles of neuroanaesthesia
Many of the principles (though not all) regarding head injury transfer hold true for neuroanaesthesia. Techniques should optimize cerebral metabolism (cerebral metabolic rate of oxygen, CMRO2) and the cerebral blood flow and should not cause rises in ICP. Generally accepted principles are:
◆ To prevent rises in ICP
◆ To maintain an adequate cerebral perfusion pressure (CPP)
◆ To use techniques with minimal interference with cerebral autoregulation
◆ To affect rapid and smooth awakening at the end of the procedure. Slow emergence from anaesthesia may be a sign of an expanding haematoma, and techniques should be used to minimize the confounding effects of anaesthesia.
The importance of an optimal patient positioning in neurosurgery cannot be overemphasized. Cases can be long and complex, with multiple pieces of equipment to negotiate. A good operating theatre set-up will have staff members, familiar with the positioning requirements for a variety of neurosurgical procedures, working together within specified roles.
What are the specific considerations in patient positioning in neuroanaesthesia?
◆ Provision of optimal venous drainage to prevent excessive intraoperative bleeding
◆ Avoidance of prolonged, excessive pressure on the skin and nerve bundles, for example
◆ Special thought will be required for patients with unstable spines, ankylosing spondylitis, scoliosis, spasticity, gross obesity, or indeed cachexia, in consideration of pressure areas, chest excursion, and operative exposure
◆ Use of gravity to optimize the opening of soft tissue planes, thus avoiding or minimizing the requirement for retraction, especially on the brain
◆ The recognition of scenarios that have a risk of air embolism and the provision of appropriate methods of reducing this risk or being able to treat it if it occurs
◆ The increasing use of ancillary equipment poses special problems and may create a crowded operating room:
• A frameless neuronavigation requires an unobstructed line of site between the infrared beam source and the reference star and pointer (newer electromagnetic equipment is beginning to be used that avoids this problem)
• The microscope placement is important for both the operating surgeon and assistant
• Allowing optimal X-ray penetrance from image intensifiers (i.e. ensuring the arms are pulled down so that the shoulders are not obstructing the view when operating on the lower cervical spine)
• Ultrasound guidance equipment is optimally used when a pool of saline may be maintained over the area of interest; therefore, the operative area should be kept horizontal
• Endoscopy generally requires a set-up where two surgeons can comfortably work opposite each other with a good view of a monitor
• The use of the Budde halo (ring for attaching retractors) will require some thought as to how the position will affect its placement and draping. Similar principles may be said of the Mayfield clamps (see Figure 7.1a)
• The introduction of intraoperative MRI in some units will also bring new problems with patient positioning
◆ Awake craniotomies require an extra level of attention to detail. Patients must be comfortable on the operating table, have appropriate screening, but maintain access to theatre personnel. Their neurological functions (speech, limb movement) also need to be assessable.
What is the anaesthetist’s role in preventing a raised intracranial pressure when managing the airway?
The anaesthetist may help by using tape to secure ETTs, not ties which act like a ‘neck tourniquet’. All should be done to optimize venous drainage from the head. Our practice is to use armoured ETTs to minimize kinking and flexing of the neck. Patients are generally positioned head up.
Positioning is critical in neurosurgery.
Proper positioning will provide an optimal venous drainage away from the surgical field. Appropriate positioning will also allow gravity to expose tissue plains and minimize the requirement for surgical retraction of the brain (thus reducing post-operative brain swelling).
There are a number of commonly adopted positions on the operating table (see Figures 7.1b–f):
◆ The supine position (e.g. used for anterior and middle cranial fossae and anterior cervical work) (see Figures 7.1b–c)
◆ The lateral position (e.g. used for posterior fossa, occipital regions, and lumbar peritoneal shunts) ) (see Figure 7.1d)
◆ The prone position (an alternative for posterior fossa, craniocervical junction, posterior spine) (see Figure 7.1e).
Fig. 7.1 (a) Various positions of Mayfield clamps.
Fig. 7.1 (b) Variation on supine positioning; head in Mayfield pins on a non-slip mattress.
Fig. 7.1 (c) Further variation on the supine position.
Fig. 7.1 (d) Variations on the lateral/park bench position.
Fig. 7.1 (e) One version of prone positioning. In addition to being on a non-slip mattress, the patient should have footguard to prevent the body weight from ‘hanging’ from Mayfield pins. Ankles should be kept in a neutral position, e.g. rolled towel under the ankles (not shown).
Fig. 7.1 (f) Version of the sitting position.
The sitting position
This position had been popular as an approach to the posterior fossa and posterior approaches for high cervical work (see Figure 7.1f). The perceived advantages (access to the operative field, drainage of fluid away from the surgical field under gravity, decreased blood loss, and historically, if the patient had been allowed to breathe spontaneously, localization due to changes in the respiratory pattern when operating on the brainstem) are now largely thought by many to be outweighed by the disadvantages.
These disadvantages include:
1 Hypotension (causing stroke, particularly quadriplegia). One of the reasons for the advantage of reduced blood loss was due to the reduced perfusion pressure of the site of surgery being higher than that of the heart. It may result in cardiovascular instability
2 Air embolus, particularly if the patient were breathing spontaneously
3 Macroglossia caused by tongue swelling
4 Pressure sores.
Consequently, the use of the sitting position in neurosurgery has been on the wane in recent years in the UK and other countries. A survey of neurosurgical units in the British Isles in 1991 showed that only 8 (20%) of the UK centres still used this position for posterior fossa surgery, in comparison to 19 (53%) 10 years prior to that. There are still advocates for its retention as a 2002 editorial on the subject notes. However, as centres abandon its use, the number of staff familiar with its use will diminish.
Cases will tend to be longer than in other subspecialties, so due care must be taken over pressure areas to prevent neuropathies.
We acknowledge that a large caseload in neurosurgical units encompasses neck and other spinal surgery. However, given that we are restricted to a small number of cases in each chapter of this book, we have illustrated the teaching points by describing cases with intracranial pathology. This chapter concentrates on the intracranial aspects of neurosurgery.
Greenberg MS (2010). Handbook of neurosurgery, 7th edn. Thieme, New York.
Leonard IE and Cunningham AJ (2002). The sitting position in neurosurgery-not yet obsolete! British Journal of Anaesthesia, 88, 1–3.
Mishra LD, Rajkumar N, and Hancock SM (2006). Current controversies in neuroanaesthesia, head injury management and neurocritical care. Continuing Education in Anaesthesia, Critical Care & Pain, 6, 83–5.
Rhoton AL (2008). Cranial anatomy and surgical approaches. Lippincott Williams and Wilkins, Philadelphia.
St-Arnaud D and Paquin M-J (2008). Safe positioning for neurosurgical patients. AORN Journal, 87, 1156–68.