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: Severe cardiovascular complications occurred in eight of 160 patients treated with terbutaline for preterm labor .
### Output Text:
Severe cardiovascular complications occurred in eight of 160 patients treated with <span class="chemical">terbutaline</span> for preterm labor . 