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: Direct thrombin inhibitors are appropriate , evidence - based alternatives to heparin in patients with a history of HIT , who need to undergo percutaneous coronary intervention .
### Output Text:
<span class="chemical">Direct thrombin inhibitors</span> are appropriate , evidence - based alternatives to <span class="chemical">heparin</span> in patients with a history of HIT , who need to undergo percutaneous coronary intervention . 