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: Three time domain indexes of hemodynamic variability were employed : the standard deviation of mean arterial pressure as a measure of blood pressure variability and the standard deviation of beat - to - beat intervals ( SDRR ) and the root mean square of successive differences in R - wave - to - R - wave intervals as measures of heart rate variability .
### Output Text:
Three time domain indexes of hemodynamic variability were employed : the standard deviation of mean arterial pressure as a measure of blood pressure variability and the standard deviation of beat - to - beat intervals ( SDRR ) and the root mean square of successive differences in R - wave - to - R - wave intervals as measures of heart rate variability . 