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: Postthrombectomy continuous CDT with alteplase was commenced while argatroban was withheld , and complete patency of the SVC and central veins was achieved after three days of therapy .
### Output Text:
Postthrombectomy continuous CDT with alteplase was commenced while <span class="chemical">argatroban</span> was withheld , and complete patency of the SVC and central veins was achieved after three days of therapy . 