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: Deliberate hypotension induced by labetalol with halothane , enflurane or isoflurane for middle - ear surgery .
### Output Text:
Deliberate hypotension induced by <span class="chemical">labetalol</span> with <span class="chemical">halothane</span> , <span class="chemical">enflurane</span> or <span class="chemical">isoflurane</span> for middle - ear surgery . 