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 the infusion of aminophylline , the ventricular fibrillation threshold was reduced by 30 to 40 percent of the control when pH and partial pressures of oxygen ( PO2 ) and carbon dioxide ( CO2 ) were kept within normal limits .
### Output Text:
During the infusion of <span class="chemical">aminophylline</span> , the ventricular fibrillation threshold was reduced by 30 to 40 percent of the control when pH and partial pressures of <span class="chemical">oxygen</span> ( <span class="chemical">PO2</span> ) and <span class="chemical">carbon dioxide</span> ( <span class="chemical">CO2</span> ) were kept within normal limits . 