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: Unusual features of the arrhythmia are repetitive group beating , progressive shortening of the R - R interval , progressive widening of the QRS complex with eventual failure of intraventricular conduction , and changes in direction of the QRS axis .
### Output Text:
Unusual features of the arrhythmia are repetitive group beating , progressive shortening of the R - R interval , progressive widening of the QRS complex with eventual failure of intraventricular conduction , and changes in direction of the QRS axis . 