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 delta down , which is the measure of decrease of SBP after a mechanical breath , was 20 . 3 + / - 8 . 4 and 10 . 1 + / - 3 . 8 mm Hg in the HEM and SNP groups , respectively , during hypotension ( P less than 0 . 02 ) .
### Output Text:
The delta down , which is the measure of decrease of SBP after a mechanical breath , was 20 . 3 + / - 8 . 4 and 10 . 1 + / - 3 . 8 mm Hg in the HEM and <span class="chemical">SNP</span> groups , respectively , during hypotension ( P less than 0 . 02 ) . 