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: Although it is difficult to define causality in a case report , it is logical to think that a beta - agonist like salbutamol ( known to induce tachycardia ) may be the trigger of adrenergic reflexes originating in the esophagus while swallowing and that a beta - blocker such as atenolol ( that blocks the adrenergic activity ) may relieve it .
### Output Text:
Although it is difficult to define causality in a case report , it is logical to think that a beta - agonist like <span class="chemical">salbutamol</span> ( known to induce tachycardia ) may be the trigger of adrenergic reflexes originating in the esophagus while swallowing and that a beta - blocker such as <span class="chemical">atenolol</span> ( that blocks the adrenergic activity ) may relieve it . 