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 patients with ischaemic heart disease developed profound cardiac failure , hypotension and bradycardia during combined therapy with verapamil and beta - adrenergic blocking drugs .
### Output Text:
Three patients with ischaemic heart disease developed profound cardiac failure , hypotension and bradycardia during combined therapy with <span class="chemical">verapamil</span> and <span class="chemical">beta - adrenergic blocking drugs</span> . 