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: No significant change in myocardial oxygen consumption occurred after ouabain administration but this may be related to a greater decrease in mean arterial pressure in the ouabain patients than in the control patients .
### Output Text:
No significant change in myocardial <span class="chemical">oxygen</span> consumption occurred after <span class="chemical">ouabain</span> administration but this may be related to a greater decrease in mean arterial pressure in the <span class="chemical">ouabain</span> patients than in the control patients . 