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: Ten cm H2O CPAP before nitroprusside infusion produced a further decrease in arterial blood pressure and significantly increased heart rate and decreased cardiac output and QS / QT .
### Output Text:
Ten cm <span class="chemical">H2O</span> CPAP before <span class="chemical">nitroprusside</span> infusion produced a further decrease in arterial blood pressure and significantly increased heart rate and decreased cardiac output and QS / QT . 