How to get item relations id

I installed the Item Relations plugin, and built a relationship between two items. Right now the relationship shows up on each item's "browse item" page. I would like to extract the related item's id on my current item's page. The following are my codes, but it does not work well:
$showItemRelations = show_item_metadata(array('show_element_sets' => array('Item Relations' => 'subject_item_id')));
echo $showItemRelations;