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: Sinus node dysfunction has been reported most frequently among the adverse cardiovascular effects of lithium .
### Output Text:
Sinus node dysfunction has been reported most frequently among the adverse cardiovascular effects of <span class="chemical">lithium</span> . 