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: PbtO ( 2 ) decreased from 51 . 7 + / - 4 . 5 mm Hg sem to 33 . 8 + / - 5 . 2 mm Hg sem in the NTG group and from 38 . 6 + / - 6 . 1 mm Hg sem to 25 . 4 + / - 2 . 0 mm Hg sem in the NTG + NIMO groups , respectively .
### Output Text:
PbtO ( 2 ) decreased from 51 . 7 + / - 4 . 5 mm Hg sem to 33 . 8 + / - 5 . 2 mm Hg sem in the <span class="chemical">NTG</span> group and from 38 . 6 + / - 6 . 1 mm Hg sem to 25 . 4 + / - 2 . 0 mm Hg sem in the <span class="chemical">NTG</span> + <span class="chemical">NIMO</span> groups , respectively . 