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: A partial recovery of bupivacaine - induced ECG alterations was observed after pretreatment of the rats with BN 52021 .
### Output Text:
A partial recovery of <span class="chemical">bupivacaine</span> - induced ECG alterations was observed after pretreatment of the rats with <span class="chemical">BN 52021</span> . 