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: Left ventricular filling pressure decreased from 19 + / - 2 to 11 + / - 2 mm Hg ( P less than 0 . 001 ) .
### Output Text:
Left ventricular filling pressure decreased from 19 + / - 2 to 11 + / - 2 mm Hg ( P less than 0 . 001 ) . 