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: By means of intra - arterial ambulatory monitoring the haemodynamic effects were followed for 18 h after the infusions were stopped .
### Output Text:
By means of intra - arterial ambulatory monitoring the haemodynamic effects were followed for 18 h after the infusions were stopped . 