(0 , 2 , 4, 6, 12 and 24 hrs): Total Magnesium (tMg) and total Calcium (tCa), ionized Ca (iCa)(bloodgas analyzer). In addition, hematocrit, albumin, total protein, ureum and creatinine and parathormone (PTH) are determined in arterial blood at 0 and 24 hrs or at the time of protocol exit and citrate concentrations in postfilter and arterial blood at 1 and 24 hrs or at protocol exit.