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J Thorac Cardiovasc Surg 1997;114:1010-1019
© 1997 Mosby, Inc.

SURGERY FOR CONGENITAL HEART DISEASE

THE RELATIONSHIP BETWEEN CALCIUM AND MAGNESIUM IN PEDIATRIC MYOCARDIAL PROTECTION

Sponsor: Reneé S. Hartz, MD

From the Division of Cardiothoracic Surgery at the University of Illinois at Chicago, Chicago, Ill.

*Supported in part by the Pillsbury Fellowship.

Received for publication May 7, 1997 Revisions requested June 25, 1997 Revisions received August 11, 1997 Accepted for publication August 12, 1997 Address for reprints: Bradley S. Allen, MD, Cardiothoracic Surgery Division, Suite 417 CSB (M/C 958), University of Illinois at Chicago, 840 South Wood St., Chicago, IL 60612-7323.

Abstract

Objective: We previously demonstrated that calcium can be harmful to the hypoxic neonatal heart. Despite the fact that magnesium inhibits membrane transport of calcium, few studies have examined whether magnesium can prevent the deleterious effects of calcium in cardioplegic solutions. Methods: Twenty neonatal piglets (5 to 18 days old) underwent 60 minutes of ventilator hypoxia (inspired oxygen fraction 8% to 10%) followed by reoxygenation with the use of cardiopulmonary bypass before cardioplegic arrest to produce a clinically relevant hypoxic "stress" injury. The aorta was then crossclamped for 70 minutes with multidose blood cardioplegia. Ten piglets received a hypocalcemic (0.2 to 0.4 mmol/L) cardioplegic solution without (group 1, n = 5) or with magnesium (10 mEq/L) (group II, n = 5) supplementation. Ten other piglets were protected with a normocalcemic (1.0 to 1.2 mmol/L) cardioplegic solution without (group III, n = 5) or with magnesium (group IV, n = 5). Myocardial function was assessed by means of pressure volume loops and expressed as a percentage of control. Coronary vascular resistance was assessed during each cardioplegic infusion. Results: Adding magnesium to a hypocalcemic cardioplegic solution (groups I and II) had no effect: Both groups had complete preservation of postbypass systolic function (end-systolic elastance 101% vs 104%) and preload recruitable stroke work (101% vs 102%), minimal increase in diastolic stiffness (159% vs 153%), and no difference in myocardial tissue edema (78.8% vs 78.9%) or coronary vascular resistance. Conversely, when a normocalcemic cardioplegic solution was administered without magnesium supplementation (group III), the results were markedly poorer than results obtained with magnesium supplementation (group IV). Without magnesium, there was a marked reduction in postbypass systolic function (end-systolic elastance 49% vs 101%; p < 0.05), increased diastolic stiffness (276% vs 162%; p < 0.05), decreased preload recruitable stroke work (53% vs 102%; p < 0.05), increased myocardial tissue edema (80.0% vs 78.9%; p < 0.05), and a rise in coronary vascular resistance ( p < 0.05). Magnesium supplementation of the normocalcemic cardioplegic solution, by contrast, resulted in complete functional recovery. Conclusions: This study demonstrates that (1) magnesium does not alter the cardioprotective effects of a hypocalcemic cardioplegic solution, (2) a normocalcemic cardioplegic solution is detrimental to neonatal myocardium subjected to a previous hypoxic stress, and (3) magnesium supplementation of normocalcemic cardioplegic solutions prevents the deleterious effects of calcium.

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