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 lack of a similar response to ketamine and xylazine anesthesia revealed that the cardiovascular actions of sodium pentobarbital in barium - treated rats were linked specifically to this anesthetic , and were not representative of a generalized anesthetic response .
### Output Text:
The lack of a similar response to <span class="chemical">ketamine</span> and <span class="chemical">xylazine</span> anesthesia revealed that the cardiovascular actions of <span class="chemical">sodium pentobarbital</span> in <span class="chemical">barium</span> - treated rats were linked specifically to this anesthetic , and were not representative of a generalized anesthetic response . 