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J Thorac Cardiovasc Surg 2000;119:891-898
© 2000 The American Association for Thoracic Surgery

Surgery For Congenital Heart Disease

Whole blood ionized magnesium: age-related differences in normal values and clinical implications of ionized hypomagnesemia in patients undergoing surgery for congenital cardiac disease

From the Departments of Cardiology^a and Anesthesia,^b Children’s Hospital, and the Departments of Pediatrics^c and Anaesthesia,^d Harvard Medical School, Boston, Mass.

Supported in part by funds from the Nova Biomedical Corporation (Waltham, Mass) and the Boston Children’s Heart Foundation.

Presented at the American College of Cardiology Forty-eighth Annual Scientific Session, New Orleans, La, March 7-10, 1999.

Address for reprints: Ricardo Munoz, MD, Cardiac ICU Office, FA-105, Children’s Hospital, 300 Longwood Ave, Boston, MA 02115 (E-mail: munoz_r{at}a1.tch.harvard.edu ).

Objectives: We sought to (1) determine reference values for whole blood ionized magnesium concentrations in newborns, children, and young adults and (2) evaluate the frequency and clinical implications of ionized hypomagnesemia in patients undergoing surgery for congenital heart disease.
Method: We prospectively measured ionized magnesium concentrations in 299 subjects (113 control subjects and 186 patients undergoing surgery for congenital heart disease). Subjects were categorized by age. In the surgical group blood samples were obtained before bypass, during bypass (cooling and rewarming), after bypass, and during admission to the intensive care unit. Ionized hypomagnesemia was defined as ionized magnesium level 2 standard deviations below the mean of control subjects in the same age group. Patients were analyzed, controlling for cardiopulmonary bypass time.
Results: In the control group ionized magnesium concentrations differed by age. Neonates and adults showed lower ionized magnesium concentrations compared with those of other age groups. Infants exhibited the highest ionized magnesium concentration. In the surgical group patients older than 1 month showed a higher proportion of ionized hypomagnesemia compared with that found in neonates at baseline (P < .001), after bypass (P = .03), and at admission to the intensive care unit (P = .02). Controlling for cardiopulmonary bypass time, patients older than 1 month who were hypomagnesemic during bypass showed longer intubation time (P = .001) and longer intensive care stay (P = .01) and tended to have a higher pediatric severity of illness score on intensive care admission (P = .14) compared with patients without ionized hypomagnesemia.
Conclusions: There are age-related differences in normal ionized magnesium concentrations. Ionized hypomagnesemia is a common and clinically relevant occurrence among patients older than 1 month of age undergoing surgery for congenital heart disease.

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