NONINVASIVE MEASUREMENT OF TISSUE MAGNESIUM AND CORRELATION WITH CARDIAC LEVELS

Background Intracellular magnesium ([Mg](i)) plays an important role i n the regulation of myocardial metabolism, contractility, and the main tenance of transsarcolemmal and intracellular ionic gradients. An unde rstanding of the role of magnesium in the clinical setting, however, i s hampered by the lack of an assay of intracellular tissue magnesium l evels. Methods and Results We used energy-dispersive x-ray analysis to measure [Mg](i) in sublingual epithelial cells and to correlate the l evel with those in atrial biopsy specimens from the same patients duri ng cardiopulmonary bypass. Levels were also measured in acute myocardi al infarction (AMI) patients before and after intravenous magnesium su lfate administration and compared with those from intensive care unit (ICU) patients and healthy individuals. A strong correlation between s ublingual epithelial cell (mean, 32.1+/-0.3 mEq/L) and atrial tissue ( mean, 32.1+/-0.3 mEq/L) [Mg](i) was present in 18 cardiac surgery pati ents (r=.68, P<.002). Epithelial and atrial [Mg](i) levels were lower than in healthy individuals (33.7+/-0.5 mEq/L, P<.01) studied at that time and correlated poorly with serum magnesium. Mean [Mg](i) in 22 AM I patients was 30.7+/-0.4 mEq/L, which was significantly lower than in 21 ICU patients and 15 healthy individuals (35.0+/-0.5 mEq/L and 34.5 +/-0.7 mEq/L, respectively, P<.001). Intravenous magnesium sulfate was administered to most of the AMI patients (mean dose, 36+/-6 mmol). [M g](i) rose significantly in the AMI patients over the first 24 hours, and the magnitude of the increase was greater in those who received hi gher doses of intravenous magnesium sulfate. Conclusions Sublingual ep ithelial cell [Mg](i) correlates well with atrial [Mg](i) but not with serum magnesium. [Mg](i) levels are low in patients undergoing cardia c surgery and those with AMI. Intravenous magnesium sulfate corrects l ow [Mg](i) levels in AMI patients. Energy-dispersive x-ray analysis de termination of sublingual cell [Mg](i) may expedite the investigation of the role of magnesium deficiency in heart disease.