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: A 40 - year - old man with leukemia and no history of cardiac disease developed recurrent , brief episodes of apparent sinus arrest while receiving continuous - infusion cimetidine 50 mg / hour .
### Output Text:
A 40 - year - old man with leukemia and no history of cardiac disease developed recurrent , brief episodes of apparent sinus arrest while receiving continuous - infusion <span class="chemical">cimetidine</span> 50 mg / hour . 