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: We investigated the safety and efficacy of adding different doses of ephedrine to propofol in order to obtund the hypotensive response .
### Output Text:
We investigated the safety and efficacy of adding different doses of <span class="chemical">ephedrine</span> to <span class="chemical">propofol</span> in order to obtund the hypotensive response . 