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: These pauses were always preceded by the short P - P intervals and were usually followed by one or two P - P intervals of 0 . 92 - 0 . 95 s representing the basic sinus cycle .
### Output Text:
These pauses were always preceded by the short P - P intervals and were usually followed by one or two P - P intervals of 0 . 92 - 0 . 95 s representing the basic sinus cycle . 