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: Four hundred two patients with suspected myocardial infarction seen within 6 hours of the onset of symptoms entered a double - blind randomized trial of lidocaine vs placebo .
### Output Text:
Four hundred two patients with suspected myocardial infarction seen within 6 hours of the onset of symptoms entered a double - blind randomized trial of <span class="chemical">lidocaine</span> vs placebo . 