A 50-year-old man on ICU has a subarachnoid haemorrhage (WFNS grade 4). He is on day 7 post-ictus. His clinical status deteriorates with a new focal neurological deficit. Transcranial Doppler (TCD) shows mean flow velocities in the middle cerebral artery of 200 cm/s (normal <120 cm/s). What is the most likely complication?