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: Potentiation of bupivacaine arrhythmia in myocyte cultures suggests that this effect is at least partly mediated at the myocyte level .
### Output Text:
Potentiation of <span class="chemical">bupivacaine</span> arrhythmia in myocyte cultures suggests that this effect is at least partly mediated at the myocyte level . 