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: Several minutes after the GTN the patient experienced a sudden drop in blood pressure and heart rate , this was rectified by atropine sulphate and a fluid challenge .
### Output Text:
Several minutes after the <span class="chemical">GTN</span> the patient experienced a sudden drop in blood pressure and heart rate , this was rectified by <span class="chemical">atropine sulphate</span> and a fluid challenge . 