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: The ventricular fibrillation threshold was measured by passing a gated train of 12 constant current pulses through the ventricular myocardium during the vulnerable period of the cardiac cycle .
### Output Text:
The ventricular fibrillation threshold was measured by passing a gated train of 12 constant current pulses through the ventricular myocardium during the vulnerable period of the cardiac cycle . 