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: Each patient was subjected to an identical anesthetic protocol and similar drug - induced reductions in mean arterial blood pressure ( BP ) ( 50 to 55 mmHg ) .
### Output Text:
Each patient was subjected to an identical anesthetic protocol and similar drug - induced reductions in mean arterial blood pressure ( BP ) ( 50 to 55 mmHg ) . 