German to English Medical Medical: Cardiology Lab Results

This is the first entry in a table of lab test results. It is under the heading of "tested parameter" and the units are mmol/L with a reference range between 135 and 145. It is followed by such parameters as NT Pro BNP, TSH, PTT, INR, Quick, Thrombocyten, so in other words, a fairly standard set of lab tests, but I haven't run across this one before, and it's not in the term base or TM. It's in the context of cardiac arrhythmia. I don't think it's sodium, because both letters are capitalized, and it's not grouped with the other electrolytes. Thanks in advance for any insights.