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: Controlled hypotension to an average MAP of 50 - 55 mm Hg was induced by increasing the dose of isoflurane , and maintained at an inspired concentration of 2 . 2 + / - 0 . 2 % .
### Output Text:
Controlled hypotension to an average MAP of 50 - 55 mm <span class="chemical">Hg</span> was induced by increasing the dose of <span class="chemical">isoflurane</span> , and maintained at an inspired concentration of 2 . 2 + / - 0 . 2 % . 