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: Using a nested case - control design , 477 cases of bradyarrhythmia requiring a permanent pacemaker were matched ( 1 : 4 ) to 1 , 908 controls .
### Output Text:
Using a nested case - control design , 477 cases of bradyarrhythmia requiring a permanent pacemaker were matched ( 1 : 4 ) to 1 , 908 controls . 