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: Controlled hypotension in groups A and C was induced with PGE1 to maintain mean arterial blood pressure at 55 mmHg for 180 min .
### Output Text:
Controlled hypotension in groups A and C was induced with <span class="chemical">PGE1</span> to maintain mean arterial blood pressure at 55 mmHg for 180 min . 