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: A second percutaneous mechanical thrombectomy was performed six days later and resulted in partial revascularization of the SVC and central veins .
### Output Text:
A second percutaneous mechanical thrombectomy was performed six days later and resulted in partial revascularization of the SVC and central veins . 