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: When respiratory failure was produced by hypoventilation ( pH 7 . 05 to 7 . 25 ; PC02 70 to 100 mm Hg : P02 20 to 40 mm Hg ) , infusion of aminophylline resulted in an even greater decrease in ventricular fibrillation threshold to 60 percent of the control level .
### Output Text:
When respiratory failure was produced by hypoventilation ( pH 7 . 05 to 7 . 25 ; PC02 70 to 100 mm Hg : P02 20 to 40 mm Hg ) , infusion of <span class="chemical">aminophylline</span> resulted in an even greater decrease in ventricular fibrillation threshold to 60 percent of the control level . 