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: Hypotension was induced for 236 . 9 + / - 15 . 1 min by increasing the isoflurane inspired concentration to maintain a mean arterial pressure of 59 . 8 + / - 0 . 4 mmHg .
### Output Text:
Hypotension was induced for 236 . 9 + / - 15 . 1 min by increasing the <span class="chemical">isoflurane</span> inspired concentration to maintain a mean arterial pressure of 59 . 8 + / - 0 . 4 mmHg . 