DOSE-RELATED EFFECTS OF MAGNESIUM ON MYOCARDIAL-FUNCTION IN THE NEONATE

The antagonistic effects of magnesium ion as a calcium ion blocker may decrease calcium influx associated with ischemia. However, the effect of magnesium on the preischemic neonataI myocardium has not been inve stigated previously. The purpose of this study was to investigate the effects of the administration of increasing doses of magnesium on left ventricular performance in the neonate. We assessed left ventricular function (pressure-volume data obtained by the conductance catheter/mi cromanometer technique) in three groups (n=6 in each) of newborn pigs (3 to 5 days old) differing with respect to magnesium concentrations. Endsystolic elastance did not change during infusion in group A (magne sium=1.2 mmol/L), whereas in groups B (magnesium=g mmol/L) and C (magn esium=16 mmol/L) it decreased significantly (P<.05) to 67+/-6% and 44/-8% of baseline, respectively. The decrease in end-systolic elastance was associated with a significant reduction in cardiac output (P<.05) and stroke work (P<.05) in group C. After administration of magnesium , end-systolic elastance returned to baseline in group B in contrast t o group C (78+/-3% of baseline value, P<.05). The slope constant of th e end-diastolic pressure-volume relation decreased significantly (P<.0 5) from the preinfusion baseline values of 0.42+/-0.08 mL(-1) in group B and 0.46+/-0.05 mL(-1) in group C to 0.3+/-0.04 and 0.26+/-0.03 mL( -1), respectively, versus no change in group A. At the end of the expe riment, the slope constant returned to baseline value in group B, wher eas in group C it remained significantly lower (P<.05, 78+/-3% of base line). We conclude that administration of 8 to 16 mmol/L magnesium aff ects the systolic function and alters the diastolic properties of the neonatal heart in a dose-response manner.