RCM – skr. angl. right costal margin pravý rebrový oblúk.

arcus costalis costal margin, costal arch rebrový oblúk
http://www.pdfoz.com/k-2795447.html

en.wikipedia.org/wiki/Costal_margin
The costal margin is the lower edge of the chest (thorax) formed by the bottom edge of the rib cage. Sometimes referred to as the costal arch, it is the medial ...


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Note added at 2 hrs (2015-02-05 12:19:11 GMT)
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or you may prefer to express it in terms of above/below the costal margin:

The comparative value of palpation, light percussion, and auscultatory percussion for detecting the liver below costal margin was studied in 45 normal subjects and 20 patients. The presence of the liver 2 cm or more below the costal margin was considered abnormal,
http://www.ncbi.nlm.nih.gov/pubmed/8115880


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Note added at 2 hrs (2015-02-05 12:23:41 GMT)
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The normal liver is 'within' or 'above' the costal margin:

The normal upper level of the liver is at the level of the right nipple, whereas the lower margin of the liver is at the right costal margin.
http://www.ncbi.nlm.nih.gov/books/NBK421/