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: Epidural blood flow during prostaglandin E1 or trimethaphan induced hypotension .
### Output Text:
Epidural blood flow during <span class="chemical">prostaglandin E1</span> or <span class="chemical">trimethaphan</span> induced hypotension . 