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: The primary objective of this study is to compare the prevalence of severe hemodynamic effects in neurocritical care patients receiving dexmedetomidine and propofol .
### Output Text:
The primary objective of this study is to compare the prevalence of severe hemodynamic effects in neurocritical care patients receiving <span class="chemical">dexmedetomidine</span> and <span class="chemical">propofol</span> . 