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: Diphenhydramine prevents the haemodynamic changes of cimetidine in ICU patients .
### Output Text:
<span class="chemical">Diphenhydramine</span> prevents the haemodynamic changes of <span class="chemical">cimetidine</span> in ICU patients . 