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: OBJECTIVES : The aim of this study was to determine whether the use of amiodarone in patients with atrial fibrillation ( AF ) increases the risk of bradyarrhythmia requiring a permanent pacemaker .
### Output Text:
OBJECTIVES : The aim of this study was to determine whether the use of <span class="chemical">amiodarone</span> in patients with atrial fibrillation ( AF ) increases the risk of bradyarrhythmia requiring a permanent pacemaker . 