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: Overall , the altered cardiac contractility and excitability characteristics , the myocardial metabolic disturbances , and the hypersensitivity of the cardiovascular system to sodium pentobarbital suggest the existence of a heretofore undescribed cardiomyopathic disorder induced by chronic barium exposure .
### Output Text:
Overall , the altered cardiac contractility and excitability characteristics , the myocardial metabolic disturbances , and the hypersensitivity of the cardiovascular system to <span class="chemical">sodium pentobarbital</span> suggest the existence of a heretofore undescribed cardiomyopathic disorder induced by chronic <span class="chemical">barium</span> exposure . 