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: Serious adverse effects are uncommon and mainly have been related to the depression of cardiac contractility and conduction , especially when the drug is combined with beta - blocking agents .
### Output Text:
Serious adverse effects are uncommon and mainly have been related to the depression of cardiac contractility and conduction , especially when the drug is combined with beta - blocking agents . 