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: Left ventricular end - diastolic pressure and left ventricular end - diastolic volume fell in each patient given ouabain , even though they were initially elevated in only two patients .
### Output Text:
Left ventricular end - diastolic pressure and left ventricular end - diastolic volume fell in each patient given <span class="chemical">ouabain</span> , even though they were initially elevated in only two patients . 