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: The Holter electrocardiogram , which was worn by chance , revealed torsade de pointes with gradually prolonged QT intervals .
### Output Text:
The Holter electrocardiogram , which was worn by chance , revealed torsade de pointes with gradually prolonged QT intervals . 