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 SPV during hypotension was 15 . 7 + / - 6 . 7 mm Hg in the HEM group , compared with 9 . 1 + / - 2 . 0 mm Hg in the SNP group ( P less than 0 . 02 ) .
### Output Text:
The SPV during hypotension was 15 . 7 + / - 6 . 7 mm Hg in the HEM group , compared with 9 . 1 + / - 2 . 0 mm Hg in the <span class="chemical">SNP</span> group ( P less than 0 . 02 ) . 