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: OBJECTIVE : To determine whether pharmacological stress leads to prolonged but reversible left ventricular dysfunction in patients with coronary artery disease , similar to that seen after exercise .
### Output Text:
OBJECTIVE : To determine whether pharmacological stress leads to prolonged but reversible left ventricular dysfunction in patients with coronary artery disease , similar to that seen after exercise . 