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: Electrocardiographic changes after dipyridamole infusion ( 0 . 568 mg / kg / 4 min ) were studied in 41 patients with coronary artery disease and compared with those after submaximal treadmill exercise by use of the body surface mapping technique .
### Output Text:
Electrocardiographic changes after <span class="chemical">dipyridamole</span> infusion ( 0 . 568 mg / kg / 4 min ) were studied in 41 patients with coronary artery disease and compared with those after submaximal treadmill exercise by use of the body surface mapping technique . 