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: To evaluate the effect of prostaglandin E1 ( PGE1 ) or trimethaphan ( TMP ) induced hypotension on epidural blood flow ( EBF ) during spinal surgery , EBF was measured using the heat clearance method in 30 patients who underwent postero - lateral interbody fusion under isoflurane anaesthesia .
### Output Text:
To evaluate the effect of <span class="chemical">prostaglandin E1</span> ( <span class="chemical">PGE1</span> ) or <span class="chemical">trimethaphan</span> ( <span class="chemical">TMP</span> ) induced hypotension on epidural blood flow ( EBF ) during spinal surgery , EBF was measured using the heat clearance method in 30 patients who underwent postero - lateral interbody fusion under <span class="chemical">isoflurane</span> anaesthesia . 