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: CONCLUSIONS : Severe hypotension and bradycardia occur at similar prevalence in neurocritical care patients who receive dexmedetomidine or propofol .
### Output Text:
CONCLUSIONS : Severe hypotension and bradycardia occur at similar prevalence in neurocritical care patients who receive <span class="chemical">dexmedetomidine</span> or <span class="chemical">propofol</span> . 