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: Among 3 , 129 dobutamine stress echocardiographic studies , a hypertensive response , defined as systolic blood pressure ( BP ) > or = 220 mm Hg and / or diastolic BP > or = 110 mm Hg , occurred in 30 patients ( 1 % ) .
### Output Text:
Among 3 , 129 <span class="chemical">dobutamine</span> stress echocardiographic studies , a hypertensive response , defined as systolic blood pressure ( BP ) > or = 220 mm Hg and / or diastolic BP > or = 110 mm Hg , occurred in 30 patients ( 1 % ) . 