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: We conclude that previously reported hypoglycemia , hyperbilirubinemia , polycythemia , neonatal apnea , and bradycardia are not invariable and cannot be statistically correlated with chronic propranolol therapy .
### Output Text:
We conclude that previously reported hypoglycemia , hyperbilirubinemia , polycythemia , neonatal apnea , and bradycardia are not invariable and cannot be statistically correlated with chronic <span class="chemical">propranolol</span> therapy . 