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: Mean left ventricular ejection fraction and atrial dimension ( LAD ) were 66 + / - 11 % and 40 + / - 6 mm , respectively .
### Output Text:
Mean left ventricular ejection fraction and atrial dimension ( LAD ) were 66 + / - 11 % and 40 + / - 6 mm , respectively . 