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: At the effective dose level of ( + ) - propranolol there was a significant prolongation of the PR interval of the electrocardiogram .
### Output Text:
At the effective dose level of ( + ) - <span class="chemical">propranolol</span> there was a significant prolongation of the PR interval of the electrocardiogram . 