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: However , marked tachycardia associated with the use of ephedrine in combination with propofol occurred in the majority of patients , occasionally reaching high levels in individual patients .
### Output Text:
However , marked tachycardia associated with the use of <span class="chemical">ephedrine</span> in combination with <span class="chemical">propofol</span> occurred in the majority of patients , occasionally reaching high levels in individual patients . 