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: Age - related changes in ultrasound production corresponded with changes in cardiovascular variables , including baseline cardiac rate and clonidine - induced bradycardia .
### Output Text:
Age - related changes in ultrasound production corresponded with changes in cardiovascular variables , including baseline cardiac rate and <span class="chemical">clonidine</span> - induced bradycardia . 