Myocardial preservation in neonatal lambs

Na/H exchange inhibition protects newborn heart from ischemia/reperfusion injury by limiting Na+-dependent Ca2+ overload

The results of the Guardian/Expedition trials demonstrate the need for more precisely controlled studies to inhibit Na/H exchange (NHE1) during ischemia/reperfusion. This is because overwhelming evidence is consistent with the hypothesis that myocardial ischemic injury results in part from increases in intracellular Na (Nai) mediated by NHE1 that in turn promote Na/Ca exchanger-mediated increases in intracellular Ca ([Ca]i) and Ca-dependent cell damage. We used a more potent and specific NHE1 inhibitor HOE 694 (HOE) to test whether inhibition of NHE1 during ischemia limits increases in Nai and [Ca]i in newborns. NMR was used to measure pHi, Nai, [Ca]i, and ATP in isolated newborn rabbit hearts. Perfusion pressure, left ventricular developed pressure, and creatine kinase were measured. HOE was added before global ischemia. Results are reported as mean +/- SE. Nai (mEq/kg dry weight) rose from 11.6 +/- 0.9 before ischemia to 114.0 +/- 16.1 at the end of ischemia and recovered to 55.2 +/- 11.8 in the control group. During ischemia and reperfusion, the corresponding values for Nai in the HOE group (63.1 +/- 8.4 and 15.9 +/- 2.5, respectively, P < 0.05) were lower than control. In the control group [Ca]i (nM/L) rose from 331 +/- 41 to 1069 +/- 71 and recovered to 814 +/- 51, whereas in the HOE group [Ca]i rose less (P < 0.05): 359 +/- 50, 607 +/- 85, and 413 +/- 40, respectively. Total creatine kinase release was significantly reduced in the HOE group. Perfusion pressure and left ventricular developed pressure also recovered significantly better in the HOE group than in the control. In conclusion, NHE1 inhibition diminishes ischemia-induced increases in Nai and therefore [Ca], and thus diminishes myocardial injury in neonatal hearts.

Pediatric myocardial protection: a cardioplegic strategy is the "solution"

This article describes the experimental infrastructure and subsequent successful clinical application of a comprehensive cardioplegic strategy that limits intraoperative injury and improves postoperative outcomes in pediatric patients. The infant heart is at high risk of damage from poor protection as a result of preoperative hypertrophy, cyanosis, and ischemia. These factors may also make the immature (pediatric) heart more sensitive to cardioplegic arrest compared with the mature (adult) heart. The preoperative factors of cyanosis and pressure volume overload are discussed, followed by the infrastructure of the strategies of warm induction and reperfusion with substrate enhancements, multidose cardioplegia, and a "modified" integrated approach to allow ischemia only when visualization is needed in pediatric surgeries. The importance of using a blood cardioplegia solution, with reduced calcium, increased magnesium, and low perfusion pressure are also shown. A practical clinical framework based on these experimentally proven principles is then presented to allow the surgeon to apply these strategies clinically. The results of using these principles are depicted in a series of 567 patients, including 93 patients with hypoplastic left heart syndrome. Applications of these concepts should improve the safety of protection of the infant heart and reduce postoperative morbidity and mortality.

Myocardial metabolic changes during pediatric cardiac surgery: a randomized study of 3 cardioplegic techniques

BACKGROUND

Blood cardioplegia and terminal warm blood cardioplegic reperfusion ("hot shot") reduce myocardial injury and improve metabolic recovery in hypoxic but not normoxic experimental models. However, there is little evidence of a benefit of either technique in pediatric clinical practice compared with crystalloid cardioplegia.

METHODS

Pediatric patients undergoing cardiac surgery were randomized to receive intermittent antegrade cold crystalloid cardioplegia, cold blood cardioplegia, or cold blood cardioplegia with a hot shot. Right ventricular biopsy specimens were collected before ischemia, at the end of ischemia, and 20 minutes after reperfusion. Cellular metabolites were analyzed. In acyanotic patients postoperative serum troponin I levels were also measured at 1, 4, 12, 24, and 48 hours.

RESULTS

Of 103 patients recruited, 32 (22 acyanotic and 10 cyanotic), 36 (24 acyanotic and 12 cyanotic), and 35 (25 acyanotic and 10 cyanotic), respectively, were allocated to the groups receiving cold crystalloid cardioplegia, cold blood cardioplegia, and cold blood cardioplegia with a hot shot. Cyanotic patients were younger, with longer crossclamp times. There were no significant differences in clinical outcomes between cardioplegic methods. The cardioplegic method had no overall effect in terms of adenosine triphosphate, ln(adenosine triphosphate/adenosine diphosphate), or ln(glutamate) in acyanotic patients (P =.11, P =.66, and P =.30, respectively). Also, there was no significant difference between groups in troponin I release. However, in cyanotic patients cold blood cardioplegia with a hot shot significantly reduced the decrease in adenosine triphosphate, ln(adenosine triphosphate/adenosine diphosphate), and glutamate observed at the end of ischemia and after reperfusion compared with the decrease seen in those receiving cold crystalloid cardioplegia (P =.002, P =.003, and P =.008, respectively), with cold blood cardioplegia representing an intermediate.

CONCLUSIONS

For cyanotic patients (younger, with longer crossclamp times), cold blood cardioplegia with a hot shot is the best method of myocardial protection. For acyanotic patients (older, with shorter crossclamp times), cardioplegic technique is not critical.

Cardioplegia in pediatric cardiac surgery: do we believe in magic?

Cardioplegia has become the gold standard of myocardial protection for practically every type of heart surgery during which the ascending aorta must be clamped. Although there is little doubt about the efficacy of cardioplegia in the adult heart, there are few studies on the pediatric heart and their results are contradictory. The physiology of pediatric heart muscle differs considerably from that of the adult myocardium. The pediatric heart distinguishes itself from that of the adult most impressively in its greater tolerance for ischemia. This ischemia tolerance is enhanced by the use of hypothermia. Considering that hypothermia is a powerful tool to prolong ischemia tolerance and that most pediatric cardiac surgeons report similar results using different types of cardioplegia, some surgeons are tempted to suspect that the contribution of the cardioplegia composition to protecting the pediatric heart may be overestimated. This provocative statement is critically discussed in this article. We examine the protective potential of cardioplegia (in various compositions), or of hypothermia, or of both in pediatric cardiac surgery. We pay special attention to several key differences between the physiologies of the pediatric myocardium and the adult myocardium and attempt to relate them to the available surgical methods of myocardial protection. We conclude that the composition of cardioplegia indeed is an important component of successful operative management in pediatric heart surgery. We provide evidence that the benefit of cardioplegia over hypothermia alone is minor at low temperatures (below 15 degrees C), but becomes substantial when the temperature increases.

Cold blood versus cold crystalloid cardioplegia for repair of ventricular septal defects in pediatric heart surgery: a randomized controlled trial

BACKGROUND

There is little evidence in the literature on the benefits of cold blood cardioplegia in pediatric cardiac surgery. This study investigates the effects of cold crystalloid versus cold blood cardioplegia on myocardial metabolism, reperfusion injury, and clinical outcomes in patients undergoing ventricular septal defect (VSD) repair.

METHODS

Patients were randomly assigned to receive antegrade cold (4 to 6 degrees C) St Thomas's I crystalloid (CCC) or blood (CBC) cardioplegia. Changes in myocardial adenine nucleotides and purine levels were monitored in right ventricular biopsies and postoperative serum troponin I (TnI) and lactate release were measured.

RESULTS

Forty patients were randomly assigned to CCC (n = 21; age 21.1 +/- 40.8 months) or to CBC (n = 19; age 27.4 +/- 39.3 months). Patient characteristics were similar in the two groups and there was no mortality. After the ischemic period there was a significant drop in adenosine triphosphate levels compared with control values in the CCC (40.4 +/- 9.9 versus 27.5 +/- 12.5 nmol/mg protein, p = 0.01) but not in the CBC group (40.3 +/- 23.2 versus 37.3 +/- 18.9 nmol/mg protein). The fall was more marked in infants compared with children (40% versus 10%, respectively, p = 0.01). Mean total TnI release was 42% lower in the CBC than the CBC group (95% confidence interval 10% to 62%, p = 0.015). Total TnI release was significantly associated with age (p < 0.001) but as levels in infants in the CBC group were the same as for children, the reduction with age was seen almost entirely in the CCC group. There were no differences in the duration of inotropic support, ventilation time, intensive care unit, or hospital stay in the two groups.

CONCLUSION

The use of CBC is associated with less metabolic myocardial ischemic stress and reperfusion injury when compared with CCC in pediatric patients undergoing VSD repair.

Pediatric myocardial protection: an overview

This article describes the experimental infrastructure and subsequent successful clinical application of a comprehensive bypass and cardioplegic strategy that limits intraoperative injury and improves postoperative outcomes in pediatric patients. The infant heart is at high risk of damage from poor protection because of preoperative hypertrophy, cyanosis, and ischemia. The background factors of vulnerability to damage caused by cyanosis and ischemia are discussed, together with studies of the infrastructure of strategies to use normoxia versus hyperoxia as bypass starts, white blood cell filtration, warm induction and reperfusion with substrate enhancements, multidose blood cardioplegia, and an integrated approach to allow ischemia only when vision is needed in pediatric surgeries. Data on cardioplegic management, including reducing calcium, increasing magnesium, and reducing perfusion pressure are shown, as used during this technique. These principles were applied to a consecutive series of 567 patients at the Heart Institute for Children and University of Illinois hospital over a 2-year period. Included also were 72 patients with hypoplastic left heart over a 4-year period with this myocardial management strategy. Application of these concepts may improve the safety of protection in infant hearts.

Effects of retrograde cardioplegia on myocardial perfusion and energy metabolism in immature porcine myocardium

OBJECTIVES

Retrograde cardioplegia has been widely used for the protection of adult hearts during cardiac operations. Its efficacy to protect immature myocardium is still unclear. This study was designed to assess the effects of retrograde cardioplegia on myocardial perfusion and energy metabolism in immature hearts.

METHODS

Piglet hearts were divided into 3 groups. Hearts in group 1 were used to assess myocardial perfusion of retrograde cardioplegia by means of magnetic resonance imaging. Hearts in groups 2 and 3 were used to assess the effects of retrograde cardioplegia on myocardial energy metabolism by use of phosphorus 31 magnetic resonance spectroscopy.

RESULTS

Magnetic resonance images showed that perfusion with retrograde cardioplegic solution was heterogeneous. A perfusion defect was noted during retrograde cardioplegia in the right ventricular wall and in a portion of the posterior wall of the left ventricle in 4 of 6 hearts. Phosphorus 31 spectra showed that at the end of 45-minute retrograde cardioplegia, myocardial intracellular pH was 6.83 +/- 0.17 and phosphocreatine was 53.5% +/- 27% of its prearrest value. The adenosine triphosphate level, however, remained normal throughout the retrograde cardioplegia period. Last, the hearts subjected to retrograde cardioplegia or antegrade cardioplegia showed similar and complete metabolic and functional recovery during reperfusion.

CONCLUSIONS

Retrograde cardioplegia provides heterogeneous perfusion. Its ability to protect the right ventricular myocardium is poor and varies between individuals. Myocardial perfusion provided by retrograde cardioplegia is slightly less than that needed to sustain normal myocardial energy metabolism under normothermic conditions.

Myocardial protection in normal and hypoxically stressed neonatal hearts: the superiority of blood versus crystalloid cardioplegia

OBJECTIVES

Blood cardioplegia predominates in the adult because it provides superior myocardial protection, especially in the ischemically stressed heart. However, the superiority of blood over crystalloid cardioplegia in the pediatric population is unproved. Furthermore, because many pediatric hearts undergo a preoperative stress such as hypoxia, it is important to compare the different methods of protection in both normal and hypoxic hearts.

METHODS

Twenty neonatal piglets were supported by cardiopulmonary bypass and subjected to 70 minutes of cardioplegic arrest. Of 10 nonhypoxic hearts, five (group 1) were protected with blood cardioplegia and five (group 2) with crystalloid cardioplegia (St. Thomas' Hospital solution). Ten other piglets underwent 60 minutes of ventilator hypoxia (inspired oxygen concentration 8% to 10%) before cardioplegic arrest. Five (group 3) were then protected with blood cardioplegia and the other five (group 4) with crystalloid cardioplegia. Myocardial function was assessed by means of pressure volume loops and expressed as a percentage of control. Coronary vascular resistance was measured with each infusion of cardioplegic solution.

RESULTS

No difference was noted between blood (group 1) or crystalloid cardioplegia (group 2) in nonhypoxic hearts regarding systolic function (end-systolic elastance 104% vs 103%), diastolic stiffness (156% vs 159%), preload recruitable stroke work (102% vs 101%), or myocardial tissue edema (78.9% vs 78.9%). Conversely, in hearts subjected to a hypoxic stress, blood cardioplegia (group 3) provided better protection than crystalloid cardioplegia (group 4) by preserving systolic function (end-systolic elastance 106% vs 40%; p < 0.05) and preload recruitable stroke work (103% vs 40%; p < 0.05); reducing diastolic stiffness (153% vs 240%; p < 0.05) and myocardial tissue edema (79.6% vs 80.1%); and preserving vascular function, as evidenced by unaltered coronary vascular resistance (p < 0.05).

CONCLUSION

This study demonstrates that (1) blood or crystalloid cardioplegia is cardioprotective in hearts not compromised by preoperative hypoxia and (2) blood cardioplegia is superior to crystalloid cardioplegia in hearts subjected to the preoperative stress of acute hypoxia.

Depression of cardiac function after deep hypothermic circulatory arrest in deeply anesthetized neonatal lambs

Cardiac dysfunction is common after neonatal cardiac operations. Previous in vivo studies in neonatal animal models however, have failed to demonstrate decreased left ventricular function after ischemia and reperfusion. Cardiac dysfunction may have been masked in these studies by increased endogenous catecholamine levels associated with the use of light halothane anesthesia. Currently, neonatal cardiac operations are often performed with deep opiate anesthesia, which suppresses catecholamine surges and may affect functional recovery. We therefore examined the recovery of left ventricular function after ischemia and reperfusion in neonatal lambs anesthetized with high-dose fentanyl citrate (450 micrograms/kg administered intravenously). Seven intact neonatal lambs with open-chest preparation were instrumented with left atrial and left ventricular pressure transducers, left ventricular dimension crystals, and a flow transducer. The lambs were cooled (< 18 degrees C) on cardiopulmonary bypass (22 +/- 6 minutes), exposed to deep hypothermic circulatory arrest (46 +/- 1 minutes), and rewarmed on cardiopulmonary bypass (30 +/- 10 minutes). Catecholamine levels and indexes of left ventricular function were determined before (baseline) and 30, 60, 120, 180, and 240 minutes after termination of cardiopulmonary bypass. Levels of epinephrine, norepinephrine, and dopamine were unchanged from baseline values. Left ventricular contractility (slope of end-systolic pressure-volume relationship) was depressed from baseline value (31.7 +/- 9.3 mm Hg/ml) at 30 minutes (15.7 +/- 6.4 mm Hg/ml) and 240 minutes (22.7 +/- 6.4 mm Hg/ml) but unchanged between 60 and 180 minutes. Left ventricular relaxation (time constant of isovolumic relaxation) was prolonged from baseline value (19.0 +/- 3.0 msec) at 30 minutes (31.4 +/- 10.0 msec) and 240 minutes (22.1 +/- 2.8 msec) but unchanged between 60 and 180 minutes. Afterload (left ventricular end-systolic meridional wall stress) was decreased at 30, 60, and 240 minutes. Indexes of global cardiac function (cardiac output, stroke volume), preload (end-diastolic volume), and left ventricular compliance (elastic constant of end-diastolic pressure-volume relationship) were unchanged from baseline values. In deeply anesthetized neonatal lambs exposed to ischemia and reperfusion, left ventricular contractility, relaxation, and afterload are markedly but transiently depressed early after reperfusion and mildly depressed late after reperfusion.

Effects of endothelin-1 and L-arginine after cold ischemia in lamb hearts

BACKGROUND

Prior studies from our laboratory have suggested an important role for the coronary endothelium in the injury resulting from hypothermic ischemia and reperfusion. A decreased endothelial response to intraarterial acetylcholine occurs after ischemia/reperfusion, implying a reduced release of the vasodilator nitric oxide by endothelial cells, but the role of endothelial-derived vasoconstrictor endothelin-1 in ischemia/reperfusion and interactions between endothelin-1 and nitric oxide in ischemia/reperfusion are still unclear.

METHODS

We examined the effects of endothelin-1 and L-arginine, the precursor for nitric oxide, on functional recovery of isolated, blood-perfused neonatal lamb hearts undergoing 2 hours of ischemia at 10 degrees C. One group (n = 8) received 10 pmol/L endothelin-1 before reperfusion, and a second group (n = 8) received a continuous infusion of 3 mmol/L L-arginine during the initial 20 minutes of reperfusion. The third group (n = 8) received both endothelin-1 and L-arginine in the same way as in the endothelin-1 and L-arginine groups. The fourth group underwent the same period of hypothermic ischemia without interventions during reperfusion.

RESULTS

After 30 minutes of reperfusion, the endothelin-1-treated hearts showed significantly reduced recovery of left ventricular systolic function (positive maximum dP/dt and volume normalized [V10] dP/dt) and diastolic function (negative maximum dP/dt), coronary blood flow, and myocardial oxygen consumption compared with the control group (p < 0.05). These effects of endothelin-1 were offset to equal the values observed in controls having unmodified reperfusion by adding L-arginine. The L-arginine group had significantly greater recovery of left ventricular systolic function (positive maximum dP/dt, maximum developed pressure, dP/dt at V10, and developed pressure at V10) and diastolic function (negative maximum dP/dt), coronary blood flow, and myocardial oxygen consumption compared with the control group (p < 0.05).

CONCLUSION

These results, combined with our previous observations that endothelin-1 levels are unchanged by hypothermic ischemia and reperfusion, suggest that there is an imbalance between the endothelial production of endothelin-1 and nitric oxide, which affects postischemic coronary blood flow and the recovery of ventricular function. Interventions that modify this imbalance of endothelially derived substances could favorably influence the outcome after a period of hypothermic ischemia and reperfusion.

Age differences in effects of hypothermic ischemia on endothelial and ventricular function

BACKGROUND

Prior studies indicate that immature myocardium has a greater tolerance to ischemia. Prior studies from our laboratory have shown that impaired postischemic endothelial function was correlated with reduced ventricular contractility, and that coronary endothelium has an important role in ischemia and reperfusion injury in neonatal hearts.

METHODS

We examined the differences of endothelial function as well as ventricular function between immature and mature hearts in isolated blood-perfused lamb and sheep hearts after 2 hours of 15 degrees C cardioplegic ischemia. Three groups were defined according to age: neonatal ( < 1 week) hearts (n = 8), infant (1 month) hearts (n = 8), and adult (1 year) hearts (n = 6). Each of the three groups underwent a similar protocol including ischemic time, myocardial temperature, and cardioplegic solution. Based on earlier work, all had low perfusion pressures during the first 10 minutes of reperfusion. Thereafter the perfusion pressure was constant at 60 mm Hg in the neonatal hearts, 80 mm Hg in the infant hearts, and 100 mm Hg in the adult hearts to match the mean arterial pressure at each age in this species.

RESULTS

At 30 minutes of reperfusion, the neonatal and infant hearts achieved significantly improved recovery of left ventricular systolic (maximum developed pressure and positive first time derivative of pressure, and volume normalized developed pressure and first time derivative of pressure) and diastolic (negative maximum first time derivative of pressure) functions and coronary blood flow. The postischemic endothelial function determined by the coronary vasodilator response to acetytlcholine was better in the neonatal and infant hearts compared with the adult hearts (p < 0.05).

CONCLUSIONS

These results show that the immature hearts had better recovery of endothelial function and coronary blood flow as well as ventricular function compared with adult hearts after hypothermic ischemia and reperfusion. These results combined with previous studies add further support to the concept that events in the coronary vascular bed play an important role in reperfusion injury in both immature and mature hearts.

Anti-CD11b monoclonal antibody improves myocardial function after six hours of hypothermic storage

BACKGROUND

The shortage of pediatric heart donors often necessitates considerable travel time and, as a result, prolonged donor heart ischemia. This excessive hypothermic storage may contribute markedly to myocardial dysfunction in the recipient.

METHODS

We investigated the role of leukocyte-endothelial interactions in this dysfunction in an isolated, immature (mean age, 11.8 +/- 1.6 days) swine heart model using a monoclonal antibody against a leukocyte adhesion molecule. We studied a total of 20 hearts subjected to 6 hours of cardioplegic arrest at 4 degrees C. Group M1/70 (n = 6) received at reperfusion 15 micrograms/mL of a monoclonal antibody F(ab')2 fragment to CD11b, the alpha-subunit of the leukocyte adhesion molecule Mac-1. Group MB10.6 (n = 8) received 15 micrograms/mL of the swine unreactive F(ab')2 MB10.6, and the third group received saline vehicle.

RESULTS

Administration of M1/70 resulted in improved postischemic recovery of ventricular function compared with the two control groups (p < 0.05).

CONCLUSIONS

These data implicate leukocyte-endothelial interactions mediated by the leukocyte adhesion molecule CD11b in myocardial dysfunction after long-term hypothermic ischemia. Specific antiadhesion strategies such as this may safely extend storage time for pediatric donor hearts.

Surgical management of congenital heart defects: current trends

Surgical treatment for congenital heart disease has become available over the last five decades. Palliative procedures have been designed to improve physiologic abnormalities, for example systemic artery (or venous) to pulmonary artery shunts of various types to increase the pulmonary blood flow, pulmonary artery constriction (banding) to decrease the pulmonary blood flow, and surgical or transcatheter atrial septostomy to augment intracardiac mixing. These can be performed with a low mortality. The majority of congenital heart defects can be corrected by open heart surgical techniques; some require prior palliation and others can be operated without prior palliative surgery. Recent surgical advances include early total surgical correction for tetralogy of Fallot, arterial switch procedure for transposition of the great arteries, Fontan operation and its modifications for tricuspid atresia and single ventricle, new operations for hypoplastic left heart syndrome, newer prosthetic valves, myocardial preservation and cardiac transplantation.