Neurosurgery notes/Trauma/Management of trauma/Neuromonitoring/CBF monitoring

CBF monitoring

View Details
logo
Parent item
Neuromonitoring

Jugular bulb oximetry

  • AVDO2 (Arterio-jugular differences of oxygen)
    • Higher the extraction means brain is in ischaemic state → need more oxygenation
  • Used to estimate the adequacy of CBF from its inverse relationship to AVDO2.
    • Normal AVDO2 ranges from
      • 4 to 9 ml 100 ml-1
    • Low CBF and ischaemic state increase oxygen extraction and increase AVDO2.
  • Only can be used if flow-metabolism coupling is intact

Thermal diffusion flowmetry

  • Measures absolute regional blood flow
  • Permits continuous rCBF monitoring by assessing thermal convection due to tissue blood flow.
  • The probe tip is inserted into the white matter of the brain.
  • Commercially available systems include
  • Hemedex® monitoring system (Codman) utilizing the QFLOW 500® probe,
  • ❌ not MRI compatible.
  • Probe placement:
    • Issues similar to those discussed for pBtO2 (see above).
  • Readout:
    • K value (thermal conduction):
      • Range for white matter is 4.9–5.8 mW/cm-°C (the monitor suppresses CBF readings if the K value is outside this range)
        • K< 4.9: the probe tip is probably out of the brain tissue or white matter—the probe should be advanced 1–2mm
        • K> 5.8: the tip is probably too deep, near a blood vessel, or in the ventricle or epidural or subdural space—the probe should be retracted 1–2mm
    • CBF
      • Normal white matter: 18–25 ml/100 g-min
        • White matter CBF< 15: may indicate vasospasm or ischemia
        • White matter CBF< 10: may indicate infarction
      • Normal gray matter: 67–80 ml/100 g-min
  • Observational data:
    • In a small study of SAH (n=5) and TBI (n=3) there was good correlation between rCBF and pBtO2 91% of the time.
    • Monitoring was not possible 36% of the time due to patient fever (wherein the system prevents monitoring).

Transcranial doppler

  • Non-invasively measures the blood flow velocity of basal cerebral arteries in order to estimate CBF,
  • With the caveat that haemorrhage can cause vasospasm which increases flow velocities irrespective of CBF.
  • It measures relative changes in CBF rather than absolute values
  • It can be used to detect impaired CBF and assess cerebrovascular reactivity after TBI.

PET:

  • Combine oxygen 15- labelled positron emission tomography (15 O PET) with fluorine 18- labelled fluoromisonidazole (18 F FMISO)
  • Comparison of ischaemic brain volume (IBV) and hypoxic brain volume (HBV) demonstrates that the diffusion hypoxia barrier in HBV is associated with microvascular collapse, whereas IBV represents macrovascular ischaemia and irreversible damage.
  • Best method to assess cerebral metabolism quantitatively
  • Measures of CBF using stable xenon and mean transit time after injection of contrast material.