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: Standard cardiopulmonary resuscitation was started and electrocardiogram ( ECG ) was monitored for ventricular tachycardia , fibrillation , or QRS prolongation .
### Output Text:
Standard cardiopulmonary resuscitation was started and electrocardiogram ( ECG ) was monitored for ventricular tachycardia , fibrillation , or QRS prolongation . 