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: A 69 - yr - old man , who was concurrently being treated with pilocarpine nitrate and timolol maleate eye drops , developed a bradycardia and became hypotensive during halothane anaesthesia .
### Output Text:
A 69 - yr - old man , who was concurrently being treated with <span class="chemical">pilocarpine nitrate</span> and <span class="chemical">timolol maleate</span> eye drops , developed a bradycardia and became hypotensive during <span class="chemical">halothane</span> anaesthesia . 