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: Recurrence rates of ventricular tachyarrhythmias are high despite complete suppression of the arrhythmia during programmed stimulation .
### Output Text:
Recurrence rates of ventricular tachyarrhythmias are high despite complete suppression of the arrhythmia during programmed stimulation . 