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: Bradycardia ( defined as a decrease in heart rate to less than 50 beat min - 1 ) was prevented when the larger dose of either active drug was used .
### Output Text:
Bradycardia ( defined as a decrease in heart rate to less than 50 beat min - 1 ) was prevented when the larger dose of either active drug was used . 