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: Before nitroprusside infusion , 5 cm H2O CPAP significantly , P less than . 05 , decreased arterial blood pressure , but did not significantly alter heart rate , cardiac output , systemic vascular resistance , or QS / QT .
### Output Text:
Before <span class="chemical">nitroprusside</span> infusion , 5 cm <span class="chemical">H2O</span> CPAP significantly , P less than . 05 , decreased arterial blood pressure , but did not significantly alter heart rate , cardiac output , systemic vascular resistance , or QS / QT . 