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: INTERVENTION : Patients underwent dobutamine stress tests per standard protocol .
### Output Text:
INTERVENTION : Patients underwent <span class="chemical">dobutamine</span> stress tests per standard protocol . 