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: When BN 52021 ( 20 mg / kg i . v . ) was injected immediately after bupivacaine ( 2 mg / kg ) , a partial reversion of the decrease of MBP and HR was observed , whereas the dose of 10 mg / kg was ineffective .
### Output Text:
When <span class="chemical">BN 52021</span> ( 20 mg / kg i . v . ) was injected immediately after <span class="chemical">bupivacaine</span> ( 2 mg / kg ) , a partial reversion of the decrease of MBP and HR was observed , whereas the dose of 10 mg / kg was ineffective . 