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: These experiments suggest that although many factors may contribute to the increased incidence of ventricular arrhythmias in respiratory failure , pharmacologic agents , particularly aminophylline , may play a significant role .
### Output Text:
These experiments suggest that although many factors may contribute to the increased incidence of ventricular arrhythmias in respiratory failure , pharmacologic agents , particularly <span class="chemical">aminophylline</span> , may play a significant role . 