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: This case suggests there is a role for aggressive high dose intravenous calcium therapy in severe diltiazem overdose , particularly with the onset of asystole .
### Output Text:
This case suggests there is a role for aggressive high dose intravenous <span class="chemical">calcium</span> therapy in severe <span class="chemical">diltiazem</span> overdose , particularly with the onset of asystole . 