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: Finally , percutaneous methods for the exclusion of left atrial appendage are under investigation in high - risk patients .
### Output Text:
Finally , percutaneous methods for the exclusion of left atrial appendage are under investigation in high - risk patients . 