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: BACKGROUND : The interruption of oral anticoagulant ( OAC ) administration is sometimes indicated in patients with mechanical heart valves , mainly before noncardiac surgery , non - surgical interventions , and pregnancy .
### Output Text:
BACKGROUND : The interruption of oral anticoagulant ( OAC ) administration is sometimes indicated in patients with mechanical heart valves , mainly before noncardiac surgery , non - surgical interventions , and pregnancy . 