Neurology

Published on 10/04/2015 by admin

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Last modified 22/04/2025

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Chapter 3 Neurology

Neurology and neuromuscular disorders

Neuroanaesthesia

Physiology

CO2

Affects CBF through vasodilation by changing the pH of extracellular fluid (ECF) (Fig. 3.4). Aim for a Paco2 of 3.5 kPa to lower ICP without excessive vasoconstriction and prevent ‘reverse steal’. Volatile agents increase effects of CO2 on vasodilation. A 3% change in CBF occurs for each 0.1 kPa change in Paco2.

Acute changes of hyperventilation return to normal values after 48 h due to normalization of CSF pH and a compensatory increase in CSF volume.

O2

Has less influence on CBF until Pao2 <8 kPa (Fig. 3.5). Hyperoxia causes mild cerebral vasoconstriction of 5–10% at 60 kPa.

Anaesthetic drugs

Alter CBF by effects on vascular smooth muscle, cerebral metabolism and alveolar ventilation (Paco2).

Volatile agents

All volatiles with the exception of desflurane reduce cerebral metabolism and oxygen demand. Effect is most marked with isoflurane. All increase ICP and abolish autoregulation in sufficient doses. Halothane causes most cerebral vasodilation (Table 3.2). At 1 MAC (minimum alveolar concentration), isoflurane has minimal effect on ICP, autoregulation or EEG depression. At 2.5 MAC, the EEG becomes isoelectric. Isoflurane reduces cerebral O2 demand more than any other volatile and reduces cerebral blood flow without evidence of ischaemia. Sevoflurane and desflurane appear similar to isoflurane in their CNS and CVS effects.

N2O is a weak vasodilator causing moderate increases in CBF and cerebral metabolic rate (CMR). Changes in CBF are variable and may be more related to changes in Fio2.

Anaesthesia for neurosurgery

Fluids

Maintain plasma osmolality at the upper end of normal. Avoid fluids with low osmolality which increase free brain water (Table 3.3). Avoid glucose-containing solutions which accelerate anaerobic metabolism and may worsen neurological morbidity.

Table 3.3 Osmolality of intravenous fluids

Solution Osmolality (mOsm.L−1)
Normal saline 310
Mannitol 300
Hartmann’s solution 272
Dextrose saline 262
5% dextrose 250

Treatment of raised ICP

Recommendations for the Safe Transfer of Patients with Brain Injury

Association of Anaesthetists of Great Britain and Ireland 2006

Autonomic Nervous System