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: In the present case , complete atrioventricular ( AV ) block with syncopal attacks developed secondary to lithium therapy , necessitating permanent pacemaker implantation .
### Output Text:
In the present case , complete atrioventricular ( AV ) block with syncopal attacks developed secondary to <span class="chemical">lithium</span> therapy , necessitating permanent pacemaker implantation . 