Bispectral index (BIS) is commonly used as an index of depth of anaesthesia and sedation. BIS can provide an additional benefit in detecting a critical neurological event, especially if the change of neurological status is caused by the critical illness itself. We encountered such an incident where the BIS value was decreased in a patient with Guillain Barre Syndrome, possibly due to cerebral hypoperfusion. The present report emphasises that BIS can be used not only as an indicator of cerebral ischaemia, but also as a tool to judge adequacy and efficacy of interventions targeted to restore the same.