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: Pulse power analysis ( also called " pulse contour analysis " ) of an arterial pressure wave form allows continuous evaluation of systemic vascular resistance and cardiac output in real time , thereby elucidating the causative factors behind changes in blood pressure .
### Output Text:
Pulse power analysis ( also called " pulse contour analysis " ) of an arterial pressure wave form allows continuous evaluation of systemic vascular resistance and cardiac output in real time , thereby elucidating the causative factors behind changes in blood pressure . 