2004A01 Describe the effects of isoflurane on intracranial metabolism, intracranial
haemodynamics, intracranial pressure and the EEG.

 

List:

·      Graphs: CMRO2, CBF, anaesthesia

·      EEG effects

·      CSF circulation effects

 

Metabolism and haemodynamics:

CBF vs CMRO2

·   Coupling impaired (not ablated) due to direct vasodilatory effect

·   Left shift and increased slope

CMRO2 vs MAC

·   Dose-dependent reduction in electrophysiological function (60% of total)

·   No effect on basal function (40% of total) – only reduced by hypothermia

CBF vs MAC

·   0-0.5MAC: ↓CBF below baseline (↓CMRO2 outweighs)

·   0.5-1MAC: ↑CBF back to baseline

·   >1MAC: ↑CBF above baseline (vasodilatation outweighs)

Other

·   Luxury perfusion: due to ↓CMRO2 but ↑CBF

·   Hypoventilation: ↑PaCO2 may cause further vasodilatation (if spont vent)

 

EEG effects:

Changes

·  ↑Amplitude

o Due to relative synchronization -> summation

·  ↓Frequency:

o β (12-30Hz) -> α (8-12Hz)-> θ (4-8Hz)-> δ (0-4Hz)

o Burst suppression at ~1.5MAC

o Electrical silence at ~2 MAC

o Ablation of seizures

o ↓BIS and entropy scores

·  Guedel’s stage 2: opposite changes

o ↑Frequency, ↓Amplitude

Causes

·  ↓Activity of ascending reticular activating system (ARAS)

·  Disinhibition of thalamic pacemaker cells -> rhythmic discharge

·  Shift from complex and quasi chaotic to simple and predictable activity

·  Summation of identical waveforms results in increased amplitude

 

CSF effect:

↓Production

·  If mAP <70mmHg

↑Reabsorption

·  If ↑CBF -> ↑ICP

 

Note: X axis unit mmCSF not mmHg

Note: Y axis zero point not at the X axis

 

 

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