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: QRS without preexcitation , caused by junctional escape beats after verapamil or unidirectional antegrade block of accessory pathway after catheter ablation , established frequent AVRT attack .
### Output Text:
QRS without preexcitation , caused by junctional escape beats after <span class="chemical">verapamil</span> or unidirectional antegrade block of accessory pathway after catheter ablation , established frequent AVRT attack . 