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: Hypotension following the initiation of tizanidine in a patient treated with an angiotensin converting enzyme inhibitor for chronic hypertension .
### Output Text:
Hypotension following the initiation of <span class="chemical">tizanidine</span> in a patient treated with an <span class="chemical">angiotensin</span> converting enzyme inhibitor for chronic hypertension . 