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: The systolic pressure variation ( SPV ) , which is the difference between the maximal and minimal values of the systolic blood pressure ( SBP ) after one positive - pressure breath , was studied in ventilated dogs subjected to hypotension .
### Output Text:
The systolic pressure variation ( SPV ) , which is the difference between the maximal and minimal values of the systolic blood pressure ( SBP ) after one positive - pressure breath , was studied in ventilated dogs subjected to hypotension . 