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: The addition of ephedrine to propofol appears to be an effective method of obtunding the hypotensive response to propofol at all doses used in this study .
### Output Text:
The addition of <span class="chemical">ephedrine</span> to <span class="chemical">propofol</span> appears to be an effective method of obtunding the hypotensive response to <span class="chemical">propofol</span> at all doses used in this study . 