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: Under proper control of respiration , body temperature and end - tidal CO2 , intravenous administration of fentanyl ( 50 or 100 micrograms / kg ) consistently promoted an increase in electromyographic activity recorded from the gastrocnemius and abdominal rectus muscles .
### Output Text:
Under proper control of respiration , body temperature and end - tidal <span class="chemical">CO2</span> , intravenous administration of <span class="chemical">fentanyl</span> ( 50 or 100 micrograms / kg ) consistently promoted an increase in electromyographic activity recorded from the gastrocnemius and abdominal rectus muscles . 