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 patients with various chest pain syndromes , verapamil neither increased the frequency of junctional rhythms nor suppressed their role as escape rhythms under physiologically appropriate circumstances .
### Output Text:
In patients with various chest pain syndromes , <span class="chemical">verapamil</span> neither increased the frequency of junctional rhythms nor suppressed their role as escape rhythms under physiologically appropriate circumstances . 