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: In one patient , cardiac pacing was not used , because he converted to normal sinus rhythm by electrical defibrillation within three minutes of initiating CPR .
### Output Text:
In one patient , cardiac pacing was not used , because he converted to normal sinus rhythm by electrical defibrillation within three minutes of initiating CPR . 