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: SigmaST , the sum of ST - segment elevations in 16 precordial leads , decreased ( P less than 0 . 02 ) with intravenous nitroglycerin .
### Output Text:
SigmaST , the sum of ST - segment elevations in 16 precordial leads , decreased ( P less than 0 . 02 ) with intravenous <span class="chemical">nitroglycerin</span> . 