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: METHODS : For this paper , the current medical literature on LMWH in patients with mechanical heart valves was extensively reviewed .
### Output Text:
METHODS : For this paper , the current medical literature on <span class="chemical">LMWH</span> in patients with mechanical heart valves was extensively reviewed . 