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: The feasibility of using labetalol , an alpha - and beta - adrenergic blocking agent , as a hypotensive agent in combination with inhalation anaesthetics ( halothane , enflurane or isoflurane ) was studied in 23 adult patients undergoing middle - ear surgery .
### Output Text:
The feasibility of using <span class="chemical">labetalol</span> , an alpha - and beta - adrenergic blocking agent , as a hypotensive agent in combination with inhalation anaesthetics ( <span class="chemical">halothane</span> , <span class="chemical">enflurane</span> or <span class="chemical">isoflurane</span> ) was studied in 23 adult patients undergoing middle - ear surgery . 