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: We briefly describe two patients suffering from recent - onset atrial fibrillation , who experienced an acute devastating low back pain a few minutes after initiation of intravenous amiodarone loading .
### Output Text:
We briefly describe two patients suffering from recent - onset atrial fibrillation , who experienced an acute devastating low back pain a few minutes after initiation of intravenous <span class="chemical">amiodarone</span> loading . 