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: In these experiments , verapamil shortened mean effective refractory period ( ERP ) from 122 + / - 5 to 114 + / - 4 ms ( P < 0 . 02 ) at a cycle length of 300 ms , decreased ERP heterogeneity ( from 15 + / - 1 to 10 + / - 1 % , P < 0 . 05 ) , heterogeneously accelerated atrial conduction and decreased the cycle length of AF ( 94 + / - 4 to 84 + / - 3 ms , P < 0 . 005 ) .
### Output Text:
In these experiments , <span class="chemical">verapamil</span> shortened mean effective refractory period ( ERP ) from 122 + / - 5 to 114 + / - 4 ms ( P < 0 . 02 ) at a cycle length of 300 ms , decreased ERP heterogeneity ( from 15 + / - 1 to 10 + / - 1 % , P < 0 . 05 ) , heterogeneously accelerated atrial conduction and decreased the cycle length of AF ( 94 + / - 4 to 84 + / - 3 ms , P < 0 . 005 ) . 