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: PATIENTS : Sixty patients were pretreated in a randomized double - blinded fashion with remifentanil 1 microg / kg or placebo .
### Output Text:
PATIENTS : Sixty patients were pretreated in a randomized double - blinded fashion with <span class="chemical">remifentanil</span> 1 microg / kg or placebo . 