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: However , one month after switching over nifekalant to sotalol , a short duration of ST elevation was documented in ECG monitoring at almost the same time for three consecutive days .
### Output Text:
However , one month after switching over <span class="chemical">nifekalant</span> to <span class="chemical">sotalol</span> , a short duration of ST elevation was documented in ECG monitoring at almost the same time for three consecutive days . 