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: The plasma levels of BNP in all the patients with clinical and subclinical heart failure increased above the normal limit ( 40 pg / ml ) before the detection of clinical or subclinical heart failure by radionuclide angiography .
### Output Text:
The plasma levels of BNP in all the patients with clinical and subclinical heart failure increased above the normal limit ( 40 pg / ml ) before the detection of clinical or subclinical heart failure by radionuclide angiography . 