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: He was found to have atrial flutter at a ventricular rate of 70 / min which slowed down to 30 - 40 / min when nifedipine ( 60 mg ) in 3 divided doses , during which he was paced at a rate of 70 / min .
### Output Text:
He was found to have atrial flutter at a ventricular rate of 70 / min which slowed down to 30 - 40 / min when <span class="chemical">nifedipine</span> ( 60 mg ) in 3 divided doses , during which he was paced at a rate of 70 / min . 