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: All patients were studied by repeated physical examinations , chest X - rays , electro - and echocardiography and radionuclide left ventricular ejection fraction ( EF ) determinations .
### Output Text:
All patients were studied by repeated physical examinations , chest X - rays , electro - and echocardiography and radionuclide left ventricular ejection fraction ( EF ) determinations . 