22 - Anaesthesia for neurosurgery

Summary

The practice of neuroanaesthesia is unique in that the target organ of both the surgeon and the anaesthetist is one and the same. Thus, the surgical goals have a profound impact on the constraints that the anaesthesiologist must work within. In order to appropriately anaesthetise the patient for neurosurgery, an understanding of the interrelationships of neurophysiology, pathophysiology and pharmacology is important. This chapter will review: (1) basic neurophysiological principles, (2) specific approaches to the management of intracranial pressure (ICP) as they relate to clinical neuroanaesthesia, and (3) intraoperative management of the patient with a supratentorial mass lesion.

Basic principles of neurophysiology

There are six interrelated components that are important to the practice of neuroanaesthesia. They are maintenance of cerebral perfusion pressure (CPP), cerebral blood flow (CBF), cerebral blood volume (CBV), intracranial pressure (ICP), CO₂ responsiveness (CO₂R) and cerebral oxygen metabolism (CMRO₂).

Cerebral perfusion pressure

CPP is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP) (CPP = MAP – ICP), although in the occasional patient where central venous pressure (CVP) is higher than ICP, CPP = MAP – CVP. Both intracranial pathology and drugs may compromise CPP through effects on MAP and/or ICP. CPP is usually >70mmHg. An optimal CPP has not been defined but in the context of head trauma, a CPP <60 is associated with a poorer outcome; a benefit of higher CPP has not been shown [1].