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 contribution of transmural dispersion of repolarization ( TDR ) to transmural propagation of EAD and the maintenance of TdP was also evaluated .
### Output Text:
The contribution of transmural dispersion of repolarization ( TDR ) to transmural propagation of EAD and the maintenance of TdP was also evaluated . 