In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Sedation depth was titrated using processed electroencephalography with the bispectral index ( BIS ) , and patients were randomized to receive either deep ( BIS , approximately 50 ) or light ( BIS , > or = 80 ) sedation .
### Output Text:
Sedation depth was titrated using processed electroencephalography with the bispectral index ( BIS ) , and patients were randomized to receive either deep ( BIS , approximately 50 ) or light ( BIS , > or = 80 ) sedation . 