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: Five cm H2O CPAP during nitroprusside did not further alter any of the above - mentioned variables .
### Output Text:
Five cm <span class="chemical">H2O</span> CPAP during <span class="chemical">nitroprusside</span> did not further alter any of the above - mentioned variables . 