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: We , therefore , investigated the effects of the histamine 1 ( H1 ) receptor antagonist , diphenhydramine , on the haemodynamic changes observed after cimetidine in ICU patients .
### Output Text:
We , therefore , investigated the effects of the <span class="chemical">histamine</span> 1 ( H1 ) receptor antagonist , <span class="chemical">diphenhydramine</span> , on the haemodynamic changes observed after <span class="chemical">cimetidine</span> in ICU patients . 