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: During follow - up , seven ( 20 % ) patients had a nonfatal ventricular tachycardia recurrence , and two ( 6 % ) patients died suddenly .
### Output Text:
During follow - up , seven ( 20 % ) patients had a nonfatal ventricular tachycardia recurrence , and two ( 6 % ) patients died suddenly . 