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 primary response variable was based on central reading of 24 hour ambulatory electrocardiographic recordings and was defined as the occurrence of 30 or more single premature ventricular complexes in any two consecutive 30 minute blocks or one or more runs of two or more premature ventricular complexes in the entire 24 hour electrocardiographic recording .
### Output Text:
The primary response variable was based on central reading of 24 hour ambulatory electrocardiographic recordings and was defined as the occurrence of 30 or more single premature ventricular complexes in any two consecutive 30 minute blocks or one or more runs of two or more premature ventricular complexes in the entire 24 hour electrocardiographic recording . 