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: The possible additive role of pharmacologic agents in precipitating cardiac disturbances in patients with respiratory failure has only recently been emphasized .
### Output Text:
The possible additive role of pharmacologic agents in precipitating cardiac disturbances in patients with respiratory failure has only recently been emphasized . 