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 use of phosphodiesterase inhibitors such as milrinone in the treatment of severe heart failure is frequently restricted because they cause vasodilation and hypotension .
### Output Text:
The use of phosphodiesterase inhibitors such as <span class="chemical">milrinone</span> in the treatment of severe heart failure is frequently restricted because they cause vasodilation and hypotension . 