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: The maximum change in threshold was 35 dB at 6 kHz measured at the end of the continuous infusion of bupivacaine .
### Output Text:
The maximum change in threshold was 35 dB at 6 kHz measured at the end of the continuous infusion of <span class="chemical">bupivacaine</span> . 