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: During nitroprusside infusion low levels of CPAP do not markedly alter cardiovascular dynamics , but high levels of CPAP ( 10 cm H2O ) , while decreasing QS / QT , produce marked decreases in arterial blood pressure and cardiac output .
### Output Text:
During <span class="chemical">nitroprusside</span> infusion low levels of CPAP do not markedly alter cardiovascular dynamics , but high levels of CPAP ( 10 cm <span class="chemical">H2O</span> ) , while decreasing QS / QT , produce marked decreases in arterial blood pressure and cardiac output . 