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: Digitalis arrhythmia , which is suppressed by Na channel blockers , was induced by intermittent intravenous ( i . v . ) injection of ouabain in pentobarbital - anesthetized dogs .
### Output Text:
<span class="chemical">Digitalis</span> arrhythmia , which is suppressed by <span class="chemical">Na</span> channel blockers , was induced by intermittent intravenous ( i . v . ) injection of <span class="chemical">ouabain</span> in <span class="chemical">pentobarbital</span> - anesthetized dogs . 