This chapter will cover clinical considerations concerning the central and peripheral nervous system as they pertain to the practice of anesthesia. Local anesthetic-mediated actions will be covered in a different chapter.
CLINICAL ANESTHESIA CONSIDERATIONS
Perioperative considerations of the central nervous system (CNS) are critical. This is because many CNS diseases, such as tumors or aneurysms, are amenable to surgical intervention. Also, patients may present for procedures with CNS disease, such as prior strokes or Parkinson’s disease, that is factored into their care. The limited oxygen reserve of the CNS, as well as its highly metabolically active state, makes it particularly susceptible to ischemia and hypoxia even after relatively brief periods of time. This chapter, therefore, will discuss the available clinical anesthetic considerations in neurophysiology and focus on these important principles as they relate to perioperative care of patients with CNS disease.1
As discussed in prior chapters, the cranium is separated into supratentorial and infratentorial parts. The supratentorial component includes the thalamus, hypothalamus, and cerebral hemispheres. The infratentorial component includes the cerebellum and the brainstem. Intracranial lesions may be intra-axial, within the brain parenchyma, or extra-axial, outside the brain parenchyma. The intracranial lesion location has significant anesthetic considerations in terms of patient positioning during surgery. This is especially important in intra-axial mass lesions located in vital areas such as the language or motor cortex.
The brain’s blood supply includes the anterior circulation (left and right internal carotid arteries) and posterior circulation (vertebrobasilar artery system). The circle of Willis is formed through the anastomosis of these blood vessels and protects from brain ischemia via collateral circulation. Nonetheless, some patients may have abnormal collateralization.2–4
The Munro-Kellie doctrine states that the cranial compartment is incompressible, and the volume inside the cranium is a fixed volume. The cranium and its constituents (e.g., blood, CSF, and brain tissue) create a state of volume equilibrium, such that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another. Adverse consequences of increasing intracranial volume include brain herniation, cerebral ischemia, poor surgical exposure, and retractor-induced ischemia (Figure 7-1).
Intracranial elastance (compliance).
Cerebral blood flow (CBF) is 50 mL/100 g/min (12–15% cardiac output). The brain receives a large part of the cardiac output related to its high metabolic rate. CBF is directly proportionate to PaCO2 between 20 and 80 mm Hg (Figure 7-2). In the brain, neurovascular coupling which produces cerebral metabolic rate occurs, as well as cerebral autoregulation (e.g., producing cerebral perfusion pressure) and cerebrovascular reactivity (e.g., producing oxygen and carbon dioxide partial pressure). Cerebral metabolic rate ...