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 second group consisted almost exclusively of elderly women who developed potentially life - threatening bradyarrhythmias or atrioventricular conduction delay , associated with either therapeutic or modestly elevated carbamazepine serum levels .
### Output Text:
The second group consisted almost exclusively of elderly women who developed potentially life - threatening bradyarrhythmias or atrioventricular conduction delay , associated with either therapeutic or modestly elevated <span class="chemical">carbamazepine</span> serum levels . 