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: Subjects received propranolol either by mouth for 48 hours before PPA or as a rapid intravenous infusion after PPA .
### Output Text:
Subjects received <span class="chemical">propranolol</span> either by mouth for 48 hours before <span class="chemical">PPA</span> or as a rapid intravenous infusion after <span class="chemical">PPA</span> . 