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: It is postulated that this action may have been enhanced during halothane anaesthesia with resultant bradycardia and hypotension .
### Output Text:
It is postulated that this action may have been enhanced during <span class="chemical">halothane</span> anaesthesia with resultant bradycardia and hypotension . 