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: During rest we observed 0 . 6 + / - 0 . 08 PVC beats / min + / - SEM and during static and dynamic exercise 0 . 6 + / - 0 . 06 and 0 . 8 + / - 0 . 15 , respectively .
### Output Text:
During rest we observed 0 . 6 + / - 0 . 08 PVC beats / min + / - SEM and during static and dynamic exercise 0 . 6 + / - 0 . 06 and 0 . 8 + / - 0 . 15 , respectively . 