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: Remifentanil - related complications were assessed during the remifentanil infusion , and pain caused by propofol was evaluated using a four - point scale during the propofol infusion .
### Output Text:
<span class="chemical">Remifentanil</span> - related complications were assessed during the <span class="chemical">remifentanil</span> infusion , and pain caused by <span class="chemical">propofol</span> was evaluated using a four - point scale during the <span class="chemical">propofol</span> infusion . 