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: In the presence of autonomic blockade , verapamil failed to promote AF and increased , rather than decreasing , refractoriness .
### Output Text:
In the presence of autonomic blockade , <span class="chemical">verapamil</span> failed to promote AF and increased , rather than decreasing , refractoriness . 