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: When analyzed separately , no differences could be found in the prevalence of severe hypotension or bradycardia in either the unmatched or matched cohorts .
### Output Text:
When analyzed separately , no differences could be found in the prevalence of severe hypotension or bradycardia in either the unmatched or matched cohorts . 