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: Atropine 1 mg given intravenously resulted in shortening of all P - P intervals without changing the rhythm .
### Output Text:
<span class="chemical">Atropine</span> 1 mg given intravenously resulted in shortening of all P - P intervals without changing the rhythm . 