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: BACKGROUND : Inhibition of cardiac sympathetic tone represents an important strategy for treatment of cardiovascular disease , including arrhythmia , coronary heart disease , and chronic heart failure .
### Output Text:
BACKGROUND : Inhibition of cardiac sympathetic tone represents an important strategy for treatment of cardiovascular disease , including arrhythmia , coronary heart disease , and chronic heart failure . 