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: Pre - treatment of bupivacaine - induced cardiovascular depression using different lipid formulations of propofol .
### Output Text:
Pre - treatment of <span class="chemical">bupivacaine</span> - induced cardiovascular depression using different lipid formulations of <span class="chemical">propofol</span> . 