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: These observations suggest that rigidity should be prevented when alfentanil , and , presumably , other opiates , are used in the anesthetic management of patients with ICP problems .
### Output Text:
These observations suggest that rigidity should be prevented when <span class="chemical">alfentanil</span> , and , presumably , other opiates , are used in the anesthetic management of patients with ICP problems . 