The relationship between ionized and total serum magnesium concentrations during abdominal surgery

Abstract

Study objective To determine the necessity for ionized magnesium (iMg) assay by evaluating the effect of abdominal surgery without massive transfusion on total magnesium (Mg) and iMg concentrations. Design Prospective, observational study. Setting Operating rooms at a university teaching hospital. Patients 31 patients without electrolyte abnormalities who were scheduled for abdominal surgery. Interventions None. Measurements Serum electrolytes, iMg, ionized calcium, proteins before and after surgery, and urine Mg were recorded. Main results Total Mg and iMg decreased: Mg 23%, iMg 15%; p < 0.0001. Total and iMg were highly correlated (r = 0.728, p < 0.0001). Urine Mg output remained within normal limits (mean 0.24 mmol/hr). Conclusions During abdominal surgery without massive transfusion, both total and ionized hypomagnesemia occur. Changes in iMg and total serum Mg concentrations are closely correlated. Total serum Mg assay, while overestimating the prevalence of ionized hypomagnesemia, is sufficient to screen for hypomagnesemia.