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: Further suggesting lack of exaggerated adrenergic response , 13 ( 65 % ) of 20 patients required supplemental atropine to reach their target heart rates .
### Output Text:
Further suggesting lack of exaggerated adrenergic response , 13 ( 65 % ) of 20 patients required supplemental <span class="chemical">atropine</span> to reach their target heart rates . 