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: Eighty - seven unipolar electrocardiograms ( ECGs ) distributed over the entire thoracic surface were simultaneously recorded .
### Output Text:
Eighty - seven unipolar electrocardiograms ( ECGs ) distributed over the entire thoracic surface were simultaneously recorded . 