Superiority of Magnesium Cardioplegia in Neonatal Myocardial Protection

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1 Superiority of Magnesium Cardioplegia in Neonatal Myocardial Protection Michael T. Kronon, MD, Bradley S. Allen, MD, Janeen Hernan, MS, Ari O. Halldorsson, MD, Shaikh Rahman, PhD, Gerald D. Buckberg, MD, Tingrong Wang, MD, and Michel N. Ilbawi, MD Division of Cardiovascular Surgery, Heart Institute for Children, Hope Children s Hospital, Oak Lawn, Illinois Background. We have shown that magnesium can offset the detrimental effects of normocalcemic cardioplegia in hypoxic neonatal hearts. It is not known, however, whether magnesium offers any additional benefit when used in conjunction with hypocalcemic cardioplegia. Methods. Twenty neonatal piglets underwent 60 minutes of ventilator hypoxia (FiO 2 8% to 10%) followed by 20 minutes of normothermic ischemia on cardiopulmonary bypass (hypoxic-ischemic stress). They then underwent 70 minutes of multidose blood cardioplegic arrest. Five (Group 1), received a hypocalcemic (Ca to 0.4 mm/l) cardiologic solution without magnesium, whereas in 10, magnesium was added at either a low dose (5 to 6 meq/l, Group 2) or high dose (10 to 12 meq/l, Group 3). In the last 5 (Group 4), magnesium (10 to 12 meq/l) was added to a normocalcemic cardioplegic solution. Function was assessed using pressure volume loops and expressed as percentage of control. Results. Compared to hypocalcemia cardioplegic solution without magnesium (Group 1), both high- and low-dose magnesium enrichment (Groups 2 and 3) improved myocardial protection resulting in complete return of systolic (40% vs 101% vs 102%) (p < vs Groups 2 and 3) and global myocardial function (39% vs 102% vs 101%) (p < vs Groups 2 and 3), and reduced diastolic stiffness (267% vs 158% vs 154%) (p < vs Groups 2 and 3). Conversely, even high-dose magnesium supplementation could not offset the detrimental effects of normocalcemic cardioplegia resulting in depressed systolic (End Systolic Elastance [EES] 41% 1%) (p < vs Groups 2 and 3) and global myocardial function (40% 1%) (p < vs Groups 2 and 3), and a marked rise in diastolic stiffness (258% 5%) (p < vs Groups 2 and 3). Hypocalcemic magnesium cardioplegia has now been used successfully in 247 adult and pediatric patients. Conclusions. Magnesium enrichment of hypocalcemic cardioplegic solutions improves myocardial protection resulting in complete functional preservation. However, magnesium cannot prevent the detrimental effects of normocalcemic cardioplegia when the heart is severely stressed. This study, therefore, strongly supports using both a hypocalcemic cardioplegic solution and magnesium supplementation as their benefits are additive. (Ann Thorac Surg 1999;68: ) 1999 by The Society of Thoracic Surgeons Presented at the Thirty-fifth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 25 27, Address reprint requests to Dr Allen, Heart Institute for Children, Hope Children s Hospital, 4440 W 95th St, Oak Lawn, IL 60453; brad@thic.com. The role of magnesium in cardioplegic solutions has predominately been studied experimentally and clinically in the adult heart using crystalloid cardioplegic solutions [1 5]. In the immature heart, our experimental studies show that either magnesium supplementation, or calcium depletion, provides improved protection in blood cardioplegic solutions [6, 7]. Indeed, there appears to be a specific interrelationship between magnesium and calcium that has led to the perception that magnesium may not be necessary when a hypocalcemic cardioplegic solution is used [1, 3, 4, 7]. Whether magnesium enrichment can improve the protection afforded by hypocalcemic blood cardioplegic solution therefore remains unanswered, especially in the immature heart. This study uses hypoxic ischemically damaged neonatal hearts to determine if there is improvement of myocardial protection when magnesium enrichment and hypocalcemia are combined in blood cardioplegic solutions. The data defines the specific benefits and limitations of either hypocalcemia or hypermagnesemia alone, and clarifies the endothelial, metabolic, and myocardial performance consequences of combining hypocalcemia and hypermagnesemia in blood cardioplegia. These results were then the sign post of our initial clinical evaluation of magnesium enrichment of our standard hypocalcemic blood cardioplegic solutions in 247 adult and pediatric patients. Material and Methods Twenty neonatal (5 to 18 day old) piglets (3.5 to 5 kgs) were premedicated with 40 mg/kg ketamine intramuscularly, anesthetized with 30 mg/kg phenobarbital intraperitoneally, followed by 5 mg/kg intravenously each hour, and the lungs ventilated via a tracheotomy using a volume ventilator (Servo 900B, Siemens/Elema, Solna, Sweden). All animals received humane care in compliance with the principles ( Principles of Laboratory Ani by The Society of Thoracic Surgeons /99/$20.00 Published by Elsevier Science Inc PII S (99)01142-X

2 2286 KRONON ET AL Ann Thorac Surg MAGNESIUM CARDIOPLEGIA IN NEONATES 1999;68: Table 1. Warm Blood Cardioplegia Solution Table 2. Cold Blood Cardioplegia Solution Cardioplegia Additive Volume Added (ml) Component Modified Concentration Delivered a Cardioplegia Additive Volume Added (ml) Component Modified Concentration Delivered a KCL (2 meq/ml) 10 K 8 10 meq/l Tham (0.3 mol/l) 225 ph ph CPD b 225 Ca mm/l MgCl 2 (2 meq/ml) c 24 Mg meq/l Aspartate/glutamate 250 Substrate 13 mmol/l each D50W 40 Glucose 400 mg/dl D5W 200 Osmolarity mosm a When mixed in a 4:1 ratio with blood. b For normocalcemic cardioplegia, CaCl 2 was added to make the ionized Ca 2, mol/l when mixed in a 4:1 ratio with blood. c MgCl 2 was added to Groups 2, 3, and 4 (magnesium enrichment). In Group 2, only 12 ml of MgCl 2 was added resulting in a delivered magnesium concentration of 5 6 meq/l. CPD citrate-phosphate-dextrose; D5W 5% dextrose in water; D50W 50% dextrose in water; Tham Tromethamine. mal Care ) formulated by the National Society for Medical Research, and Guide for the Care and Use of Laboratory Animals, prepared by the National Academy of Sciences and published by the National Institutes of Health (NIH), Publication No , Revised The experimental preparation, including cannulation for bypass and blood sample procurement, is comparable to that previously described [6, 7]. Experimental Protocols HYPOXIC-ISCHEMIC INJURY. All piglets underwent 60 minutes of ventilator hypoxia by lowering the fraction of inspired oxygen (FiO 2 ) to 8% to 10% producing an arterial PO 2 of 25 to 35 mm Hg and an oxygen saturation (SaO 2 )of65% to 70%. Before hypoxemia, piglets were transfused as necessary to increase their hematocrit to greater than 35%. This simulates the chronic adaptive change of erythrocytosis and increases oxygen carrying capacity, thereby allowing ischemia to be avoided during hypoxia [6, 7]. At the end of 60 minutes, piglets were placed on cardiopulmonary bypass (CPB) at an FiO 2 of 100% for 5 minutes to produce a reoxygenation injury [6 8]. The aorta was then clamped for 20 minutes at 37 C to add a normothermic ischemic injury to the hypoxic stress. Piglets then underwent 70 minutes of cardioplegic arrest using the protocol described below. CARDIOPLEGIA ADMINISTRATION. Cardioplegia solutions (CAPS Service, Research Medical Inc, Salt Lake City, UT) are shown in Tables 1 and 2. Following the 20 minute normothermic-ischemic insult (see above), piglets underwent 70 minutes of cardioplegic arrest using a protocol consisting of 5 minutes of warm (37 C) induction (Table 1) followed by 4 minutes of cold multidose cardioplegia (Table 2), a 2 minute cold multidose infusion every 20 minutes, and a 4-minute warm (37 C) cardioplegic reperfusate ( hot shot ) before aortic unclamping. Cardioplegia was always infused at a continuously measured aortic root pressure of 40 to 50 mm Hg. Immediately after cross clamping the aorta, all piglets were cooled to a systemic temperature of 26 C, and warming to 37 C was KCI (2 meq/ml) 5 K 8 10 meq/l Tham (0.3 mol/l) 200 ph ph CPD b 50 Ca mm/l MgCl 2 (2 meq/ml) c 18 Mg mg/l D5W 550 Osmolarity mosm a When mixed in a 4:1 ratio with blood. b For normocalcemic cardioplegia, CaCl 2 was added to make the ionized Ca 2, mol/l when mixed in a 4:1 ratio with blood. c MgCl 2 was added to Groups 2, 3, and 4 (magnesium enrichment). In Group 2, only 9 ml of MgCl 2 was added resulting in a delivered magnesium concentration of 5 6 meq/l. CPD citrate-phosphate-dextrose; D5W 5% dextrose in water; Tham Tromethamine. begun 16 minutes before aortic unclamping. All piglets were weaned from CPB with no inotropic support 30 minutes after aortic unclamping. After arterial blood gases, Ca 2, and K were normalized, final functional and biochemical measurements were made 30 minutes later. Experimental Groups Piglets were broken into 4 groups based on the composition of the cardioplegic solution. Low Ca 2 /No Mg 2 (Group 1): Five piglets received a hypocalcemic blood cardioplegic solution without magnesium. Low Ca 2 /Low Mg 2 (Group 2): Five piglets received a hypocalcemic cardioplegic solution enriched with 5 to 6 meq/l of magnesium (low dose). Low Ca 2 /High Mg 2 (Group 3): Five piglets received a hypocalcemic cardioplegic solution enriched with 10 to 12 meq/l of magnesium (high dose). High Ca 2 /High Mg 2 (Group 4): The final 5 piglets received a normocalcemic cardioplegic solution enriched with 10 to 12 meq/l of magnesium (high dose). Myocardial Oxygen Consumption After cardioplegic arrest, blood was obtained at 1 minute intervals from the cardioplegic line and coronary sinus over the 5 minutes of warm cardioplegic induction (Groups 1 to 3) and myocardial oxygen consumption (MVO 2 ) was determined as previously described [9]. The cumulative 5-minute MVO 2 was determined by the sum of the individual 1-minute values and expressed per 100 grams of heart tissue, which was determined by weighing the left ventricle at the conclusion of the experiment. Myocardial Performance Left ventricular (LV) pressure and conductance catheter signals were amplified and digitized to inscribe LV pressure volume loops after first correcting for parallel conductance (myocardial tissue and blood viscosity) using hypertonic saline. A series of pressure volume loops was

3 Ann Thorac Surg KRONON ET AL 1999;68: MAGNESIUM CARDIOPLEGIA IN NEONATES 2287 generated by transient occlusion of the inferior vena cava during an 8-second period of apnea. Measurements were made before hypoxia (baseline) and 30 minutes after cardiopulmonary bypass was discontinued. The end systolic and end diastolic pressure volume relationship and the preload recruitable stroke relationship were analyzed as previously described [6, 7]. Functional measurements are expressed as percent recovery of baseline values with each piglet acting as its own control. After final hemodynamic measurements, piglets were placed back on bypass and transmural LV biopsies obtained. Endocardial and epicardial portions were separated, frozen quickly in liquid nitrogen, and stored for biochemical analysis. A separate sample was obtained for myocardial water. Physiologic Measurements Coronary vascular resistance (CVR) was determined as previously described during each cardioplegic infusion by measuring coronary sinus pressure and cardioplegic flow once a constant infusion rate with an aortic root pressure between 40 and 50 mm Hg was achieved [6, 7]. Biochemical Analysis ADENOSINE POOL. Myocardial samples were crushed in a liquid nitrogen cooled mortar and pestle and lyophilized. The adenosine pool was determined as described previously [6, 7]. Adenosine triphosphate (ATP) levels are expressed as g/g dry tissue. MYOCARDIAL WATER. Ventricular samples were placed in preweighed vials and dried to a constant weight at a temperature of 85 C. The percent myocardial water was then calculated as previously described [6, 7]. Clinical Studies The charts of 247 consecutive patients receiving magnesium enriched hypocalcemic blood cardioplegic solution were retrospectively reviewed to determine the clinical safety and efficacy of this solution. There were 179 adult and 68 pediatric patients, and the preoperative demographics and operations performed are depicted in Table 3. The cardioplegic solutions are the same as those used experimentally, except that the warm solution contained a delivered concentration of 10 to 12 meq/l of magnesium, whereas the cold multidose solution only contained 5 to 6 meq/l of magnesium. An integrated cardioplegia strategy that has been described previously was used in all adult patients [10, 11]. This strategy incorporates warm and cold cardioplegia, antegrade and retrograde delivery, and continuous and intermittent infusions. Pediatric patients received cold multidose blood cardioplegia delivered every 10 to 15 minutes and a terminal warm substrate enriched reperfusate. Retrograde delivery was given in any pediatric patient where antegrade infusions were not possible every 10 to 15 minutes (ie, arterial switch procedures). Statistics Data were analyzed using JMP V2.0 (SAS Institute, Inc, Cary, NC) on a Macintosh IIVX computer (Apple Inc, Table 3. Adult and Pediatric Patient Status Adult patients (179) Age 64 2y Preoperative ejection fraction 47 2% Parsonnet (severity) score 7 1 Operation CABG 131 CABG and valve 6 CABG and left ventricular aneurysm 3 Mitral valve replacement or repair 13 Aortic valve replacement 10 Atrial septal defect 9 Ross procedure 7 Pediatric patients (68) Age 17 2mo Operation Ventricular septal defect 20 Right ventricular conduit or pulmonary 10 valve replacement Atrial septal defect 9 Fontan operation 8 Repair tetralogy of Fallot 6 Atrial and ventricular canal repair 5 Aortic valve repair 2 Mitral valve repair 2 Repair truncus arteriosis 2 Atrial switch procedure 1 Kono procedure 1 Norwood procedure 1 Repair total anomalous pulmonary 1 venous return CABG coronary artery bypass graft. Cupertino, CA). Paired Student s t test and one-way analysis of variance was used for comparison of variables among experimental groups. If the analysis of variance revealed a significant interaction, pairwise tests of individual group means were compared by means of multiple comparisons (Tukey s test). Group data are expressed as mean standard error of the mean. Results There was no difference between groups for prehypoxic (baseline) values of left ventricular contractility, (35 2) diastolic compliance, ( ), or preload recruitable stroke work (71 3). All piglets remained stable during the 60 minutes of hypoxia. Hemodynamic and Physiologic Measurements Results are depicted in Figures 1 through 4. There was no change or difference in the X-axis intercept point (V 0 ) for end systolic elastance (pre , post ) or preload recruitable stroke work (pre , post ) between pre (baseline) and postbypass values in any experimental group. Therefore, the change in slope of end systolic elastance and preload recruitable stroke work can be interpreted to express variability in the contractile state of the myocardium compared to

4 2288 KRONON ET AL Ann Thorac Surg MAGNESIUM CARDIOPLEGIA IN NEONATES 1999;68: Fig 1. Left ventricular systolic function as measured by the end systolic elastance (EES) and expressed as percent of recovery of baseline values. Hearts protected with a hypocalcemic cardioplegic solution alone exhibited marked loss of systolic function. In contrast, there is complete preservation of systolic function when magnesium is added to hypocalcemic cardioplegic solution. However, magnesium enrichment was not able to offset the detrimental effects of a normocalcemic cardioplegic solution, resulting in diminished systolic function. *p baseline values. Hypocalcemic blood cardioplegia without magnesium (Group 1) was unable to resuscitate the severely stressed (hypoxic-ischemic) myocardium resulting in decreased postbypass systolic contractility, markedly increased diastolic stiffness, and reduced preload recruitable stroke work. In contrast, hypocalcemic cardioplegia supplemented with low-dose (5 to 6 meq/l, Group 2), or high-dose (10 to 12 meq/l, Group 3) magnesium resuscitated and protected the severely stressed myocardium resulting in complete return of systolic function, and preloaded recruitable stroke work, and minimally increased diastolic stiffness. These values (Groups 2 and 3) were not statistically different ( p 0.2) Fig 3. Overall left ventricular myocardial function measured by preload recruitable stroke work (PRSW) and expressed as percent recovery compared to baseline values. *p from each other, indicating a similar beneficial effect with either dose. However, magnesium was not able to offset the detrimental effects of normocalcemic cardioplegia (Group 4) resulting in depressed systolic contractility and preload recruitable stroke work, and increased diastolic stiffness. Coronary vascular resistance was lower and similar in piglets receiving hypocalcemic magnesium cardioplegia (Groups 2 and 3), whereas it was increased in piglets receiving hypocalcemic cardioplegia alone (Group 1), or magnesium enriched normocalcemic cardioplegic solution (Group 4). Tissue Studies Results are summarized in Table 4. Although our model of hypoxia does not result in lower energy levels, ATP is depleted during the 20-minute ischemic stress. In hearts receiving hypocalcemia cardioplegia without magnesium (Group 1), ATP remained depressed, and the ATP/adenosine diphosphate (ADP) ratio, which reflects the ability of the mitochondria to phosphorylate ADP to ATP was also reduced. A reduced ratio suggests mitochondrial damage with an impaired capacity to produce ATP, which explains the lower oxygen uptake during cardioplegic induction, as well as the depressed Fig 2. Left ventricular diastolic compliance as measured by the end diastolic pressure volume relationship, and expressed as a percentage of stiffness compared to baseline values. There is a marked increase in diastolic stiffness in hearts protected with a hypocalcemic cardioplegic solution without magnesium. In contrast, there is only a minimal increase in diastolic stiffness in hearts protected with a magnesium enriched hypocalcemic cardioplegic solution. However, this improvement is negated when magnesium is added to a normocalcemic cardioplegic solution. *p Fig 4. Coronary vascular resistance (CVR) measured during each cardioplegic infusion, once the pressure and flow were stable. (See text for details.) *p

5 Ann Thorac Surg KRONON ET AL 1999;68: MAGNESIUM CARDIOPLEGIA IN NEONATES 2289 Table 4. Physiologic and Biochemical Results Group Myocardial Oxygen a Consumption (cc/100 gm/5 min) ATP ( g/g dry wt) ATP/ADP Ratio Myocardial Water (%) Low Ca 2 /No Mg 2 (Group 1) b b b b Low Ca 2 /Low Mg 2 (Group 2) Low Ca 2 /High Mg 2 (Group 3) High Ca 2 /High Mg 2 (Group 4) b b b b F 123 F 87.4 F 43.4 F 21.9 p p p p a During warm cardioplegic induction. b p vs groups 2 and 3. ADP adenosine diphosphate; ATP adenosine triphosphate; Ca 2 calcium; Mg 2 magnesium. postbypass ATP levels. Enriching the hypocalcemic cardioplegia with magnesium (Groups 2 and 3) prevented mitochondrial damage (normal ATP/ADP ratio). This allowed the ATP levels to be restored, resulting in higher oxygen consumption during cardioplegic induction. In contrast, in the presence of normocalcemic cardioplegia, magnesium was unable to prevent a calcium mediated injury resulting in mitochondrial damage (decreased ATP/ADP ratio) with the inability to replenish ATP levels. Myocardial water, which is an indirect measurement of cellular injury, was also higher in Groups 1 and 4, indicating a greater degree of myocardial cell damage. Clinical Studies Results are summarized in Table 5. In adult patients, the mean preoperative ejection fraction was 47% 2%, and 15% (27 of 179) had an ejection fraction less than 30%. The mean number of grafts in patients undergoing coronary artery bypass grafting (CABG) was 4 1. The overall mortality was only 2.2%, despite a mean cross clamp time of almost 2 hours, and a Parsonnet score that predicted a mortality of 7%. In pediatric patients, the mortality was also low, considering the complex nature of many of the operations. Comment The role of magnesium supplementation of hypocalcemic blood cardioplegic solutions was evaluated in hypoxic-ischemic infant hearts, because either hypocalcemia or hypermagnesemia alone confers complete protection in neonatal hearts subjected to a less severe stress [7]. The data demonstrates that (1) magnesium enhances the protective effects of hypocalcemic cardioplegic solutions improving endothelial, metabolic, and myocardial function; (2) this beneficial effect is obtained using either low (5 to 6 meq/l), or high (10 to 12 meq/l) concentrations of magnesium; and (3) even high-dose magnesium (10 to 12 meq/l) cannot prevent the detrimental effects of normocalcemic cardioplegic solution when the heart is severely stressed. Magnesium, the second most abundant intracellular ion, is lost during ischemia, leading to an increase in postoperative arrhythmias and possible impairment of magnesium-dependent cellular reactions [3 5, 7, 12]. Replacing extracellular magnesium by enriching cardiologic solutions has been shown to decrease the incidence of postoperative arrhythmias, as well as improve myocardial protection by a variety of pathways [1, 3 5, 7, 12]. The most important of these is probably magnesium s ability to modulate intracellular calcium levels by inhibiting calcium entry across the cellular membrane, as well as displacing calcium from the binding sites of the sarcolemmal membrane [1, 3, 4, 7]. This prevents mitochondrial calcium uptake, which can lead to uncoupling of oxidative phosphorylation with a decrease in ATP production. Postischemic calcium entry is further limited because magnesium prevents an influx of sodium, which during reperfusion is exchanged for calcium. Reducing calcium entry may be even more important in neonatal hearts because of the decreased ability of the immature heart to sequester excess calcium [13, 14]. Supplemental magnesium can also facilitate asystole at lower potassium concentrations [10]. This is important because high potassium concentrations can damage vascular endothelial cells directly, as well as enhance endothelial and myocyte calcium entry. Although hypocalcemic blood cardioplegia without magnesium completely protects the hypoxic neonatal heart, this same solution was unable to adequately protect hearts subjected to hypoxia and ischemia [6, 7]. However, adding magnesium to the hypocalcemic car- Table 5. Postoperative Clinical Results Adult patients Cross clamp time min Bypass time min ICU length of stay (median) 2 days Hospital length of stay (median) 5 days Mortality 2.2% (4/179) Pediatric patients Cross clamp time 70 5 min Bypass time min ICU length of stay (median) 2 days Hospital length of stay (median) 4 days Mortality 1.5% (1/68) ICU intensive care unit.

6 2290 KRONON ET AL Ann Thorac Surg MAGNESIUM CARDIOPLEGIA IN NEONATES 1999;68: dioplegic solution markedly improved myocardial protection resulting in complete recovery of metabolic and myocardial function despite this severe (hypoxic/ ischemic) stress. This beneficial effect occurred at either low (5 to 6 meq/l) or high (10 to 12 meq/l) magnesium concentrations, and is similar to the improved results obtained when calcium channel blockers were used to retard calcium entry [10, 15]. Calcium channel blockers however, have a prolonged effect, which may depress postoperative function, making them less attractive. Because calcium and magnesium have an interrelationship, similar results might have been obtained in the absence of magnesium by further lowering the cardioplegic calcium concentration. However, myocardial recovery may be reduced when the cardioplegic calcium is less than 100 mol/l, and although unlikely, a calcium paradox can occur if levels are reduced to less than 50 mmol/l [4, 10, 15 17]. The delivered cardioplegic calcium level was achieved by normalizing calcium in the bypass circuit. If a hypocalcemic bypass prime is used, the amount of citrate in the cardioplegic solution should be adjusted to produce the delivered concentrations found in Tables 1 and 2. In contrast to our previous study, magnesium supplementation could not offset the detrimental effects of normocalcemic cardioplegia in severely stressed (hypoxic-ischemic) neonatal hearts [7]. Damage occurred despite using a relatively high concentration of magnesium (10 to 12 meq/l). Because calcium and magnesium, however, do have an interrelationship, it is possible that a higher dose of magnesium would have improved recovery. This may explain why Hearse and colleagues found a magnesium dose of 16 meq/l provided the best protection when used with normocalcemic crystalloid cardioplegia [1]. The optimal dose of magnesium therefore probably depends on the cardioplegic calcium concentration in as much as the beneficial effects of magnesium and hypocalcemia are additive as well as interdependent. Because hypoxia and ischemia change both myocyte as well as endothelial cell function, we measured coronary vascular resistance with each cardioplegic infusion [6 8, 10, 18, 19]. Characteristically, an ischemic stress, or ischemia between cardioplegia doses, causes coronary vasodilation with a decrease in coronary vascular resistance [10]. Despite this, the coronary vascular resistance was increased with either a hypocalcemic blood cardioplegic solution without magnesium (Group 1), or a normocalcemic solution (Group 4). Conversely, the coronary vascular resistance was reduced if a hypocalcemic blood cardioplegic solution was supplemented with magnesium (Groups 2 and 3). These findings imply endothelial cell vascular dysfunction if the magnesium/calcium interaction is absent. Assessing CVR during cardioplegic infusions allows us to determine the response of the coronary vasculature to each cardioplegic solution. Although an elevated CVR is suggestive of a vascular injury, this cannot be proved, because specific tests of endothelial cell function were not done after bypass. However, an altered vascular response during cardioplegic infusions may affect myocardial protection by altering cardioplegic distribution. This, along with a vascular injury, probably explains the direct correlation between an elevated CVR and postbypass myocardial dysfunction. This study uses both an ischemic and hypoxic stress because either hypocalcemic or hypermagnesemic blood cardioplegia alone is able to completely protect the neonatal heart subjected only to acute hypoxia [7]. Furthermore, the combination of hypoxia and ischemia may more closely resemble the cyanotic child because our model of acute hypoxia does not result in ischemia or ATP depletion, whereas the cyanotic infant or chronically hypoxic animal usually has depressed ATP levels [6, 8, 20 22]. This may explain why we saw greater oxygen-free radical production with reoxygenation of cyanotic infants compared to acute hypoxic animals, and why cyanotic infants often have postbypass myocardial dysfunction after apparently successful surgical repair [23, 24]. Even if a hypoxic-ischemic stress does not exactly mimic the cyanotic infant, it allows us to investigate cardioplegic solutions in stressed hearts. This is important as most neonatal hearts are not normal at the time of surgery, but are stressed by hypoxia or pressure volume overload. Based on the experimental infrastructure provided by this as well as other reports, we began using magnesiumenriched hypocalcemic cardioplegic solutions in our adult and pediatric patients with excellent results [1, 3 5, 7]. The object of reporting this data is to document the safety of this solution, not to compare it with a control group. Lack of a control group should not negate our findings, as we believe there is sufficient experimental and clinical infrastructure demonstrating the efficacy of magnesium enrichment of cardioplegic solutions to improve protection and prevent postoperative arrhythmias [3 5, 7, 12]. Although the present experimental study suggests that a magnesium concentration of 5 to 6 meq/l is adequate, we used a higher concentration (10 to 12 meq/l) in our clinical warm cardioplegic solutions for several reasons. Normothermic hearts require higher potassium levels to maintain asystole during cardioplegic induction or terminal reperfusion (hot shot) [10]. Because high potassium levels can directly injure endothelial cells, as well as predispose to calcium influx, a higher concentration of magnesium allows the potassium concentration to be reduced while maintaining myocardial arrest [3, 7, 16, 17, 25]. When the heart is warm, ion fluxes and cellular reactions are faster, and the cell is more susceptible to a reperfusion injury [10]. It is in this setting that we previously demonstrated that lower cardioplegic calcium concentrations, as well as calcium channel blockers, were important [10, 15]. Because magnesium competes with calcium, higher magnesium concentrations should be beneficial for the same reason [3, 4]. Higher magnesium concentrations should also provide greater protection against postoperative arrhythmias, and Hearse and coworkers, and others, have documented that magnesium concentrations as high as 16 meq/l are safe in both adult and pediatric patients [1, 3 5, 12]. In summary, the protective effects of magnesium and

7 Ann Thorac Surg KRONON ET AL 1999;68: MAGNESIUM CARDIOPLEGIA IN NEONATES 2291 hypocalcemic blood cardioplegia are additive resulting in complete myocardial recovery, even in severely stressed hearts. The use of these two modalities is also safe in adult and pediatric patients. Based on this as well as numerous other reports, we believe magnesium should be routinely added to hypocalcemic blood cardioplegic solutions. This study was supported in part by the Pillsbury Fellowship. All experimental and clinical investigations were performed at the Heart Institute for Children and the University of Illinois at Chicago. References 1. Hearse DJ, Stewart DA, Braimbridge MV. Myocardial protection during ischemic cardiac arrest the importance of magnesium in cardioplegic infusates. J Thorac Cardiovasc Surg 1978;75: Tsukube T, McCully JD, Faulk EA, et al. Magnesium cardioplegia reduces cytosolic and nuclear calcium and DNA fragmentation in the senescent myocardium. Ann Thorac Surg 1994;58: McCully JD, Levitsky S. Mechanisms of in vitro cardioprotective action of magnesium on the aging myocardium. Magnes Res 1997;10: Lareau S, Boyle AJ, Stewart LC, et al. The role of magnesium in myocardial preservation. Magnes Res 1995;8: Sha Kerinia T, Ali IM, Sullivan JA. Magnesium in cardioplegia: is it necessary? Can J Surg 1996;39: Bolling KS, Kronon M, Allen BS, et al. Myocardial protection in normal and hypoxically stressed neonatal hearts: the superiority of hypocalcemic versus normocalcemic blood cardioplegia. J Thorac Cardiovasc Surg 1996;112: Kronon M, Bolling KS, Allen BS, Rahman SK, Wang T, Feinberg H. The relationship between calcium and magnesium in pediatric myocardial protection. J Thorac Cardiovasc Surg 1997;114: Bolling KS, Halldorsson A, Allen BS, et al. Prevention of the hypoxic/reoxygenation injury using a leukocyte depleting filter. J Thorac Cardiovasc Surg 1997;113: Hanafy HM, Allen BS, Winkelmann BS, Ham JW, Osimani D, Hartz RS. Warm blood cardioplegic induction: an underused modality. Ann Thorac Surg 1994;58: Buckberg GD, Allen BS. Myocardial protection management during adult cardiac operations. In: Baue AE, Geha AS, Hammond GL, Laks H, Naunheim KS, eds. Glenn s thoracic and cardiovascular surgery, 6th ed. Stamford, CT: Appleton & Lange, 1995: Buckberg GD, Beyersdorf F, Allen BS, Robertson JR. Integrated myocardial management: background and initial application. J Cardiac Surg 1995;10: Caspi J, Rudis E, Bar I, Safadi T, Saute M. Effects of magnesium on myocardial function after coronary artery bypass grafting. Ann Thorac Surg 1995;59: Boland R, Martonosi A, Tillack TW. Developmental changes in the composition and function of sarcoplasmic reticulum. J Biol Chem 1974;249: Jarmakani JM, Nagatomo T, Langer GA. The effect of calcium and high phosphate compounds on myocardial contracture in the newborn and adult rabbit. J Mol Cell Cardiol 1978;10: Allen BS, Okamoto F, Buckberg GD, Bugyi HI, Leaf J. Studies of controlled reperfusion after ischemia IX. Reperfusate composition: benefits of marked hypocalcemia and diltiazem on regional recovery. J Thorac Cardiovasc Surg 1986;92 (Suppl 3): Hearse D, Humphrey S, Bullock G. The oxygen paradox and the calcium paradox: two facets of the same problem? J Mol Cell Cardiol 1978;10: Tsukube T, McCully JD, Federman M, Krukenkamp IB, Levitsky S. Developmental differences in cytosolic calcium accumulation associated with surgically induced global ischemia: optimization of cardioplegic protection and mechanism of action. J Thorac Cardiovasc Surg 1996;112: Ihnken K, Morita K, Buckberg GD, et al. Studies of hypoxemic/reoxygenation injury: without aortic clamping II. Evidence for reoxygenation damage. J Thorac Cardiovasc Surg 1995;110: Urthaler F, Walker AA, Reeves RC, Hefner LL. Effects of hypoxia on intracellular calcium and contractility [Letter]. J Thorac Cardiovasc Surg 1993;105: Fujiwara T, Kurtts T, Anderson W, Heinle J, Mayer JE Jr. Myocardial protection in cyanotic neonatal limbs. J Thorac Cardiovasc Surg 1988;96: Hammon JW Jr, Graham TP Jr, Boucek RJ, Parrish MD, Merrill WH, Bender HW Jr. Myocardial adenosine triphosphate content as a measure of metabolic and functional myocardial protection in children undergoing cardiac operation. Ann Thorac Surg 1987;44: Silverman N, Kohler J, Levitsky S, Pavel D, Fang R, Feinberg H. Chronic hypoxemia depresses global ventricular function and predisposes to the depletion of high-energy phosphates during cardioplegic arrest: implications for surgical repair of cyanotic congenital heart defects. Ann Thorac Surg 1984;37: Rocchini AP, Keane JF, Cantaneda AR, Nadas AS. Left ventricular function following attempted surgical repair of tetralogy of Fallot. Circulation 1978;57: Allen BS, Rahman SK, Ilbawi M, Feinberg H, Bolling KS, Kronon M. The detrimental effects of cardiopulmonary bypass in cyanotic infants: preventing the reoxygenation injury. Ann Thorac Surg 1997;64: Lansman JB, Hess P, Tsien RW. Blockade of current through single calcium channels by Cd 2,Mg 2, and Ca 2 : voltage and concentration dependence of calcium entry into the pore. J Gen Physiol 1986;88: DISCUSSION DR WILLIAM I. DOUGLAS (Chicago, IL): Why did you use the hypoxia model to provide myocardial stress instead of merely putting the hearts under 60, 90, or 120 minutes of ischemic stress? DR ALLEN: We wanted to try to mimic as best we could clinical conditions in pediatric patients. We realize that an acute hypoxic ischemic stress is not exactly like a chronic cyanotic infant. However, it probably simulates the pediatric patient better than pure ischemic stress, because most infants are primarily hypoxic, and then become ischemic as a result of increased demands in the face of low oxygen. This explains why there is greater oxygen radical formation during reoxygenation in cyanotic infants, compared to animals subjected to either acute hypoxia or ischemia alone. The type of stress is, however, probably not as important as making sure to investigate cardioplegic solutions using stressed hearts, because most pediatric hearts are subjected to either hypoxia or pressure volume overload before surgery. Stressed hearts are also more difficult to protect and have an increased incidence of postoperative