CNS effects of inhalational anaesthetic agents

[SH4:p47-50]

Effects of inhaled anaesthetics on CNS

• Does not produce retrograde amnesia or prolonged impairment of intellectual function
• Induces decrease in cerebral activity
• Cerebral metabolic oxygen demand decreases as well
• Induces increase in cerebral blood flow
  --> May increase ICP

EEG

• All inhalational AA increase frequency and voltage on EEG at <0.4 MAC
• Cerebral metabolic oxygen requirement starts to decrease abruptly at about 0.4 MAC

Seizure activity

• Enflurane can produce fast frequency and high voltage on EEG and can lead to seizure-like EEG activity
  * Especially when > 2 MAC or PaCO2 < 30
  * The only inhalational anaesthetic agent to cause frank seizures
• Isoflurane causes burst suppression at 1.5 MAC, and electrical silence at 2.0 MAC
  * i.e. Isoflurane suppresses convulsion
• Desflurane and sevoflurane do not produce convulsive activity
• Sevoflurane can suppress convulsive activity induced with lidocaine

Nitrous oxide

• N2O may increase motor activity with clonus and opisthotonus
• N2O at high concentration (in hyperbaric chamber)
  --> alternating periods of muscle activity and relaxation
• Acute N2O dependency can lead to withdrawal seizure

Awareness

• Volatile anaesthetic agents are not equally effective in preventing awareness

For example,

• Isoflurane at 0.4 MAC prevents recall and response to command
• N2O requires greater than 0.5 to 0.6 MAC to produce similar effect

Cerebral blood flow (CBF)

Two opposing forces:

• Intrinsic vasodilatory effect of AA
• Vasoconstriction due to flow-metabolism coupling

Overall,

• Dose-dependent increase in CBF
  * Despite decreased cerebral metabolic requirement

NB:

• According to [MCQ:Q51]
  * Halothane causes greater cerebral vasodilation than enflurane [???]
  * Halothane also causes a loss of autoregulation [???]

Intrinsic vasodilatory property

• Isoflurane > sevoflurane
• N2O > Desflurane

ICP and CSF

• ICP tends to increase with anaesthetics due to increased CBF

ICP

• Hyperventilation to reduce PaCO2 to 30mmHg opposes the tendency of inhaled anaesthetics to increase ICP
• Desflurane <0.8 MAC does not increase ICP
• Desflurane 1.1 MAC increases ICP by 7mmHg

CSF

• Isoflurane does not affect production, but decrease resistance to absorption
  --> Minimal increase in ICP
• Enflurane increase both rate of production and resistance to absorption
  --> Increased ICP
• N2O does not affect production or absorption
  --> ICP increase with increased CBF