Other neurological monitoring Cerebral blood flow CBF

CBF can be measured by radioisotopic techniques utilising tracers such as xenon-133 given intravenously or by inhalation. This remains a research tool in view of the radioactivity exposure and the usual need to move the patient to a gamma-camera. However, portable monitors are now available. Middle cerebral artery (MCA) blood flow can be determined non-invasively by transcranial Doppler ultrasonography. The pulsatility index (PI) relates to cerebrovascular resistance with a rise in PI indicating a rise in resistance and cerebral vasospasm.

Vasospasm can also be designated when the MCA blood flow velocity exceeds 120cm/s and severe vasospasm when velocities >200cm/s. Low values of common carotid end-diastolic blood flow and velocity have been shown to be highly discriminating predictors of brain death. Impaired reactivity of CBF to changes in PCO2 (in normals 3-5% per mmHg PCO2 change) is another marker of poor outcome.

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