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: This study examined the frequency of atrioventricular ( AV ) dissociation and accelerated junctional rhythms in 59 patients receiving oral verapamil .
### Output Text:
This study examined the frequency of atrioventricular ( AV ) dissociation and accelerated junctional rhythms in 59 patients receiving oral <span class="chemical">verapamil</span> . 