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: Torsades de pointes ( TDP ) is a potentially fatal ventricular tachycardia associated with increases in QT interval and monophasic action potential duration ( MAPD ) .
### Output Text:
Torsades de pointes ( TDP ) is a potentially fatal ventricular tachycardia associated with increases in QT interval and monophasic action potential duration ( MAPD ) . 