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: Refractory cardiogenic shock and complete heart block after verapamil SR and metoprolol treatment .
### Output Text:
Refractory cardiogenic shock and complete heart block after <span class="chemical">verapamil</span> SR and <span class="chemical">metoprolol</span> treatment . 