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: RESULTS : Verapamil caused AF promotion in six dogs , increasing mean duration of AF induced by burst pacing , from 8 + / - 4 s ( mean + / - S . E . ) to 95 + / - 39 s ( P < 0 . 01 vs . control ) at a loading dose of 0 . 1 mg / kg and 228 + / - 101 s ( P < 0 . 0005 vs . control ) at a dose of 0 . 2 mg / kg .
### Output Text:
RESULTS : <span class="chemical">Verapamil</span> caused AF promotion in six dogs , increasing mean duration of AF induced by burst pacing , from 8 + / - 4 s ( mean + / - S . E . ) to 95 + / - 39 s ( P < 0 . 01 vs . control ) at a loading dose of 0 . 1 mg / kg and 228 + / - 101 s ( P < 0 . 0005 vs . control ) at a dose of 0 . 2 mg / kg . 