Studies of hypoxemic/reoxygenation injury: without aortic clamping. II. Evidence for reoxygenation damage

From Other Journals: A Review of Recent Articles by Our Editorial Team

In this review, we provide a brief description of recently published articles addressing topics relevant to pediatric cardiologists. Our hope is to provide a summary of the latest articles published recently in other journals in our field. The articles reviewed in this manuscript address (1) long-term fate of the truncal valve, (2) comparison of combined heart‒liver vs heart-only transplantation in pediatric and young adult Fontan recipients showing non-inferior survival of heart-liver transplant in a small sample, (3) impact of palliation strategy on interstage feeding and somatic growth for infants with ductal-dependent pulmonary blood flow showing no difference in growth between ductal stenting and Blalock-Taussig shunt, (4) biventricular repair in interrupted aortic arch and ventricular septal defect with a small left ventricular outflow tract, (5) a randomized controlled trial comparing controlled reoxygenation and standard cardiopulmonary bypass in pediatric cardiac surgery, and (6) tricuspid valve and right ventricular function throughout the hybrid palliation strategy for hypoplastic left heart syndrome and variants.

Normoxic management of cardiopulmonary bypass reduces myocardial oxidative stress in adult patients undergoing coronary artery bypass graft surgery

INTRODUCTION

We aimed to investigate whether normoxic cardiopulmonary bypass would limit myocardial oxidative stress in adults undergoing coronary artery bypass grafting.

METHODS

Patients scheduled to undergo elective isolated on-pump coronary artery bypass grafting were randomized to normoxia and hyperoxia groups. The normoxia group received 35% oxygen during anesthetic induction, 35% during hypothermic bypass, and 45% during rewarming. The hyperoxia group received 70%, 50%, and 70% oxygen, respectively. Coronary sinus blood samples were taken prior to initiation of cardiopulmonary bypass and after reperfusion for myocardial total oxidant and antioxidant status measurements. The primary endpoint was myocardial total oxidant status. Secondary endpoints were myocardial total antioxidant status and length of intensive care unit and hospital stay.

RESULTS

Forty-eight patients were included. Twenty-two received normoxic management. Mean ± standard deviation of age was 58 ± 9.07 years. Groups were balanced in terms of demographics, risk factors, and operative data. Myocardial total oxidant status was significantly lower in the normoxia group following reperfusion (p = 0.03). There was no statistically significant difference regarding myocardial total antioxidant status and length of intensive care unit and hospital stay (p = 0.08, p = 0.82, and p = 0.54, respectively).

CONCLUSIONS

Normoxic cardiopulmonary bypass is associated with reduced myocardial oxidative stress compared to hyperoxic cardiopulmonary bypass in adult coronary artery bypass patients.

Myocardial Damage Induced by Uncontrolled Reoxygenation

To evaluate myocardial impairment induced by uncontrolled reoxygenation, the effects of hypoxia-reoxygenation were compared with ischemia-reperfusion in isolated rat hearts. After stabilization, 2 groups (n = 8) of Langendorff-perfused rat hearts were exposed to 40 minutes of ischemia (10% of baseline flow) or hypoxia (10% of baseline oxygen content) followed by a sudden return to baseline conditions (reperfusion or reoxygenation). The O2 content was identical for the two groups during baseline conditions, O2 shortage, and O2 readmission. Metabolic (lactate production) and functional parameters (heart rate, peak systolic pressure, left ventricular developed pressure, maximal contraction and relaxation rates, end-diastolic pressure, coronary perfusion pressure) were recorded at the end of stabilization, after O2 deficiency, and after 2 minutes of reoxygenation. Systolic function was significantly depressed after ischemia (p < 0.0001) but completely recovered to baseline values after 2 minutes of reperfusion. In contrast, systolic function was less severely depressed after hypoxia but failed to return to baseline after 2 minutes of reoxygenation. Diastolic function, unchanged during ischemia-reperfusion, remained significantly impaired during hypoxia-reoxygenation.

Oxidative Stress after Surgery on the Immature Heart

Paediatric heart surgery is associated with increased inflammation and the production of reactive oxygen species. Use of the extracorporeal cardiopulmonary bypass during correction of congenital heart defects generates reactive oxygen species by various mechanisms: haemolysis, neutrophil activation, ischaemia reperfusion injury, reoxygenation injury, or depletion of the endogenous antioxidants. The immature myocardium is more vulnerable to reactive oxygen species because of developmental differences compared to the adult heart but also because of associated congenital heart diseases that can deplete its antioxidant reserve. Oxidative stress can be manipulated by various interventions: exogenous antioxidants, use of steroids, cardioplegia, blood prime strategies, or miniaturisation of the cardiopulmonary bypass circuit. However, it is unclear if modulation of the redox pathways can alter clinical outcomes. Further studies powered to look at clinical outcomes are needed to define the role of oxidative stress in paediatric patients.

Effect of reoxygenation on the electrical stability of the rat heart in vivo: a chronobiological study

Reoxygenation following hypoxic episodes can increase the risk for the development of ventricular arrhythmias, which, in addition to circadian aspects of reoxygenation arrhythmias has not been studied extensively. The aim of the present study was to evaluate circadian changes in the electrical stability of the rat heart during reoxygenation following a hypoventilatory episode. The electrical stability of the heart, defined in the present study as the ventricular arrhythmia threshold (VAT), was measured at 3 h intervals at clock times 09:00, 12:00, 15:00, 18:00, 21:00, 24:00, 03:00, 06:00 and 09:00 during 20 min hypoventilation (20 breaths/min, tidal volume = 0.5 ml/100 g body weight [n=17]) and subsequent 20 min reoxygenation (50 breaths/min, tidal volume = 1 ml/100 g body weight [n=4]) intervals. The experiments were performed using pentobarbital-anesthetized (40 mg/kg intraperitoneally) female Wistar rats that first underwent a four-week adaptation to a 12 h light:12 h dark regimen. Detailed analysis showed that circadian VATs changed to biphasic rhythms at 10 min of hypoventilation. The VAT circadian rhythms were observed immediately following the commencement of reoxygenation, with the highest values measured between 12:00 and 15:00, and the lowest values between 24:00 and 03:00. These results suggest that myocardial vulnerability is dependent on the light:dark cycle and characteristics of pulmonary ventilation.

Controlling oxygenation during initiation of cardiopulmonary bypass: can it improve immediate postoperative outcomes in cyanotic children undergoing cardiac surgery? A prospective randomized study

OBJECTIVE

Cardiopulmonary bypass (CPB) initiated with high oxygen levels may expose cyanotic children to reoxygenation injury. The ideal method of initiation of bypass to prevent this phenomenon still remains largely unproven. This study tested the hypothesis that controlling oxygenation during initiation of CPB improves early postoperative outcomes.

METHODS

Thirty-one cyanotic children were randomized to two treatment arms of the study. In group A (intervention), CPB was initiated with fraction of inspired oxygen (Fio 2) 0.21, and after one minute of full bypass, Fio 2 was increased at increments of 0.1 per minute to reach 0.6. In group B (hyperoxemic), CPB was initiated using Fio 2 >0.6. Aortic cross clamp time (minutes), CPB time (minutes), creatine phosphokinase-MB (CPK-MB) levels (U/L), lactate levels (mmol/L), duration of ventilator support (hours), inotropic support (hours), and intensive care unit (ICU) stay (hours) as well as hospital mortality were measured.

RESULTS

Levels of CPK-MB (group A mean = 59.6 U/L, 95% confidence interval [CI]: 45.9-73.3; group B mean = 82.6 U/L, 95% CI: 66.1-99.1, P = .016) and ventilation time (group A median = 16.5 hours; interquartile range [IQR] = 11.25-23; group B median = 27.5 hours; IQR = 17-54, P = .045) were significantly lower in the intervention group. Other parameters showed no significant differences: CPB time (group A median = 71.5 minutes, IQR = 64-100; group B median = 95.5 minutes, IQR = 58-145, P = .71), cross clamp time (group A mean = 59.2 minutes, 95% CI: 47.6-70.8; group B mean = 66.57 minutes, 95% CI: 47.6-88.5, P =.57), lactate levels (mmol/L; group A median = 1.8, IQR = 1.48-2.59; group B median = 2.1, IQR = 1.29-2.62, P = 1), inotropic support (group A median = 47.5 hours, IQR = 36-73.75; group B median = 59.5 hours, IQR = 41.75-92.5, P = .27), ICU stay (group A median = 59.5 hours, IQR = 48.25-118.5; group B median = 85 hours, IQR = 47.75-137.50, P = .21), and mortality (group A n = 2, group B n = 2).

CONCLUSION

A controlled oxygenation protocol was associated with significantly lower postoperative CPK-MB levels. Evaluation of other end points including ventilation times requires a study with larger sample size for validation.

Controlled reoxygenation cardiopulmonary bypass is associated with reduced transcriptomic changes in cyanotic tetralogy of Fallot patients undergoing surgery

In cyanotic patients undergoing repair of heart defects, high level of oxygen during cardiopulmonary bypass (CPB) leads to greater susceptibility to myocardial ischemia and reoxygenation injury. This study investigates the effects of controlled reoxygenation CPB on gene expression changes in cyanotic hearts of patients undergoing surgical correction of tetralogy of Fallot (TOF). We randomized 49 cyanotic TOF patients undergoing corrective cardiac surgery to receive either controlled reoxygenation or hyperoxic/standard CPB. Ventricular myocardium biopsies were obtained immediately after starting and before discontinuing CPB. Microarray analyses were performed on samples, and array results validated with real-time PCR. Gene expression profiles before and after hyperoxic/standard CPB revealed 35 differentially expressed genes with three upregulated and 32 downregulated. Upregulated genes included two E3 Ubiquitin ligases. The products of downregulated genes included intracellular signaling kinases, metabolic process proteins, and transport factors. In contrast, gene expression profiles before and after controlled reoxygenation CPB revealed only 11 differentially expressed genes with 10 upregulated including extracellular matrix proteins, transport factors, and one downregulated. The comparison of gene expression following hyperoxic/standard vs. controlled reoxygenation CPB revealed 59 differentially expressed genes, with six upregulated and 53 downregulated. Upregulated genes included PDE1A, MOSC1, and CRIP3. Downregulated genes functionally clustered into four major classes: extracellular matrix/cell adhesion, transcription, transport, and cellular metabolic process. This study provides direct evidence that hyperoxic CPB decreases the adaptation and remodeling capacity in cyanotic patients undergoing TOF repair. This simple CPB strategy of controlled reoxygenation reduced the number of genes whose expression was altered following hyperoxic/standard CPB.

Oral administration of L-arginine in patients with angina or following myocardial infarction may be protective by increasing plasma superoxide dismutase and total thiols with reduction in serum cholesterol and xanthine oxidase

Administration of L-arginine has been shown to control ischemic injury by producing nitric oxide which dilates the vessels and thus maintains proper blood flow to the myocardium. In the present study attempt has been made to determine whether oral administration of L-arginine has any effect on oxidant/antioxidant homeostasis in ischemic myocardial patients [represented by the patients of acute angina (AA) and acute myocardial infarction (MI)]. L-arginine has antioxidant and antiapoptotic properties, decreases endothelin-1 expression and improves endothelial function, thereby controlling oxidative injury caused during myocardial ischemic syndrome. Effect of L-arginine administration on the status of free radical scavenging enzymes, pro-oxidant enzyme and antioxidants viz. total thiols, carbonyl content and plasma ascorbic acid levels in the patients has been evaluated. We have observed that L-arginine administration (three grams per day for 15 days) resulted in increased activity of free radical scavenging enzyme superoxide dismutase (SOD) and increase in the levels of total thiols (T-SH) and ascorbic acid with concomitant decrease in lipid per-oxidation, carbonyl content, serum cholesterol and the activity of proxidant enzyme, xanthine oxidase (XO). These findings suggest that the supplementation of L-arginine along with regular therapy may be beneficial to the patients of ischemic myocardial syndromes.

Transcriptomic analysis of patients with tetralogy of Fallot reveals the effect of chronic hypoxia on myocardial gene expression

OBJECTIVES

In cyanotic patients undergoing repair of heart defects, chronic hypoxia is thought to lead to greater susceptibility to ischemia and reoxygenation injury. We sought to find an explanation to such a hypothesis by investigating the cardiac gene expression in patients with tetralogy of Fallot undergoing cardiac surgery.

METHODS

The myocardial gene profile was investigated in right ventricular biopsy specimens obtained from 20 patients with a diagnosis of cyanotic (n = 11) or acyanotic (n = 9) tetralogy of Fallot undergoing surgical repair. Oligonucleotide microarray analyses were performed on the samples, and the array results were validated with Western blotting and enzyme-linked immunosorbent assay.

RESULTS

Data revealed 795 differentially expressed genes in cyanotic versus acyanotic hearts, with 198 upregulated and 597 downregulated. Growth/morphogenesis, remodeling, and apoptosis emerged as dominant functional themes for the upregulated genes and included the apoptotic gene TRAIL (tumor necrosis factor-related apoptosis-inducing ligand), the remodeling factor OPN (osteopontin), and the mitochondrial function gene COX11 (cytochrome-c oxidase 11). In contrast, transcription, mitogen-activated protein kinase signaling, and contractile machinery were the dominant functional classes for the downregulated genes, which included the calcium-handling gene NCX1 (sodium-calcium exchanger). Protein levels of COX11, NCX1, OPN, and LYZ (lysozyme) in the myocardium followed the same pattern obtained by means of transcriptomics. The TRAIL level did not change in myocardium but increased in circulating blood of cyanotic patients, suggesting the myocardium as a possible source. Additionally, our data showed increased protein expression of apoptosis markers in cyanotic myocardium.

CONCLUSIONS

Chronic hypoxia in cyanotic children with tetralogy of Fallot induced the expression of genes associated with apoptosis and remodeling and reduced the expression of genes associated with myocardium contractility and function.

The effects of normoxic versus hyperoxic cardiopulmonary bypass on oxidative stress and inflammatory response in cyanotic pediatric patients undergoing open cardiac surgery: a randomized controlled trial

OBJECTIVES

This study investigates the effects of controlled reoxygenation cardiopulmonary bypass on oxidative stress, inflammatory response, and organ function in children undergoing repair of cyanotic congenital heart defects.

METHODS

Sixty-seven cyanotic patients (median age 15 months, interquartile range 6-49 months) undergoing corrective cardiac surgery were randomized to receive either controlled normoxic (50-0 mm Hg; n = 35) or hyperoxic (150-180 mm Hg; n = 32) cardiopulmonary bypass. Troponin I and 8-isoprostane, C3a, interleukins 6, 8, and 10, cortisol, protein S100, and alpha-glutamate transferase were measured preoperatively and 10 and 30 minutes after starting bypass, on removal of the aortic crossclamp, and 12 and 24 hours thereafter.

RESULTS

Overall, troponin I and 8-isoprostane levels were lower in the controlled normoxic group (-29%, 95% CI -48% to -3%, P = .03, and -26%, 95% CI -44% to -2%, P = .03, respectively). Protein S100 release was also lower in the normoxic group 10 minutes after starting bypass (-26%, 95% CI -40% to -9%, P = .005) and 10 minutes after aortic crossclamp removal (-23%, 95% CI -38% to -3%, P = .02, respectively), but similar at other time points in the two groups (P >or= .17). The alpha-glutamate transferase release was significantly lower in the normoxic group 10 minutes after aortic crossclamp removal (-28%, 95% CI -44% to -9%, P = .006, respectively) but was similar at other times (P >or= .11). Release of C3a, interleukins 6, 8, and 10, and cortisol was similar in the two groups throughout (P >or= .15).

CONCLUSION

Controlled reoxygenation on starting cardiopulmonary bypass is associated with reduced myocardial damage, oxidative stress, and cerebral and hepatic injury compared with hyperoxic bypass and similar whole body inflammatory and stress response in cyanotic children undergoing open cardiac surgery.

Change in the membranous lipid composition accelerates lipid peroxidation in young rat hearts subjected to 2 weeks of hypoxia followed by hyperoxia

BACKGROUND

The effects of chronic hypoxia on cardiac membrane fatty acids and on lipid peroxidation were examined, as well as the effect of l-carnitine (LCAR), which suppresses lipid peroxidation, on this process.

METHODS AND RESULTS

Four-week-old Sprague-Dawley rats were exposed to 10% oxygen for 14 days ("Hypoxia"), and then to 100% oxygen for 12 h (O2). LCAR (200 mg/kg) was administered by intraperitoneal injection daily for 2 weeks. Fatty acid composition, malondialdehyde (MDA) as a lipid peroxidation product, and antioxidants (superoxide dismutase (SOD), glutathione peroxidase and catalase) were measured. The concentration of linoleic acid was lower, and that of docosahexaenoic acid, which has more double bonds than linoleic acid, was increased in hypoxic hearts. SOD activity decreased in hypoxia, whereas MDA was unchanged, but significantly increased in "Hypoxia"+O2. LCAR reduced the increase in MDA, and had no effect on SOD activity or fatty acid composition. The administration of LCAR caused an increase in the ventricular levels of acetylcarnitine.

CONCLUSIONS

These results suggest that chronic hypoxia changes the cardiac fatty acid composition of juvenile rats to fatty acids that contain more double-bonds and reduce SOD activity, and that lipid peroxidation was augmented by exposure to oxygen.

Does normoxemic cardiopulmonary bypass prevent myocardial reoxygenation injury in cyanotic children?

OBJECTIVE

To evaluate whether the deleterious effect of cardiopulmonary bypass (CPB) can be prevented by controlling PaO(2) in cyanotic children.

DESIGN

Prospective, randomized, clinical study.

SETTING

Single university hospital.

PARTICIPANTS

Pediatric patients undergoing cardiac surgery for repair of congenital heart disease (n = 24).

INTERVENTIONS

Patients were randomly allocated into 3 groups. Patients in the acyanotic group (group I, n = 10) had CPB initiated at a fraction of inspired oxygen (F(I)O(2)) of 1.0 (PO(2), 300 to 350 mmHg). Cyanotic patients were subdivided as follows: Group II (n = 7) had CPB initiated at an F(I)O(2) of 1.0, and group III (n = 7) had CPB initiated at an F(I)O(2) of 0.21 (PO(2), 90 to 110 mmHg). A biopsy specimen of right atrial tissue was removed during venous cannulation, and another sample was removed after CPB before aortic cross-clamping. The tissue was incubated in 4 mmol/L of t-butylhydroperoxide, and the malondialdehyde (MDA) level was measured to determine the antioxidant reserve capacity. Blood samples for cytokine levels, tumor necrosis factor (TNF)-alpha, and interleukin (IL)-6 response to CPB were collected after induction of anesthesia and at the end of CPB before protamine administration.

MEASUREMENTS AND MAIN RESULTS

After initiation of CPB, MDA level rose markedly in the cyanotic groups compared with the acyanotic group (210 +/- 118% v 52 +/- 34%, p < 0.05), which indicated the depletion of antioxidants. After initiation of CPB, TNF-alpha and IL-6 levels of the cyanotic groups were higher than for the acyanotic group (168 +/- 77 v 85 +/- 57, p < 0.001; 249 +/- 131 v 52 +/- 40; p < 0.001). When a comparison between the cyanotic groups was performed, group II (initiating CPB at an F(I)O(2) of 1.0) had significantly increased MDA production compared with group III (initiating CPB at an F(I)O(2) of 0.21) (302 +/- 134% v 133 +/- 74%, p < 0.05). Group II had higher TNF-alpha and IL-6 levels than group III (204 +/- 81 v 131 +/- 52, p < 0.001; 308 +/- 147 v 191 +/- 81, p < 0.01).

CONCLUSION

Conventional clinical methods of initiating CPB at a hyperoxemic PO(2) may increase the possibility of myocardial reoxygenation injury in cyanotic children. This deleterious effect of reoxygenation can be modified by initiating CPB at a lower level of oxygen concentration. Subsequent long-term studies are needed to determine the best method of decreasing the oxygen concentration of the CPB circuit.

Hypoxia, reoxygenation and the role of systemic leukodepletion in pediatric heart surgery

As cardiopulmonary bypass (CPB) in infants and neonates is becoming more frequent, the technical performance of these operations for congenital heart disease has made significant progress. However, little research has been carried out into the conduct of CBP in producing myocardial and pulmonary dysfunction. Postoperative organ dysfunction is a problem, particularly for cyanotic infants. This paper examines the experimental and clinical experience of injury brought about by abrupt reoxygenation of the hypoxic, or cyanotic, heart. The modalities of gradual reoxygenation and leukodepletion in limiting this injury are examined, leading to the conclusion that injury can be reduced and possibly ameliorated by changes in intraoperative management during CBP in children with cyanotic disease.

Postischemic reperfusion injury can be attenuated by oxygen tension control

Oxygen-derived free radicals cause cytotoxic damage during reperfusion after a period of ischemia and the production of these free radicals may be proportionate to oxygen tension (PO2). The present study tested the hypothesis that oxidative damage may be limited by maintaining a more physiologic PO2 following ischemia. An experimental study in Wistar rats were mounted on a Langendorff apparatus was conducted to estimate baseline aortic flow (AF), coronary flow (CF), cardiac output (CO), systolic pressure (SP), heart rate (HR), and the rate-pressure product (RPP: HRxSP). The hearts were divided into 3 groups (n=7, hearts/group): group 1, hypoxic (PO2=300+/-50 mmHg) reperfusion; group 2, middleoxic (PO2=500+/-50 mmHg) reperfusion; and group 3, hyperoxic (PO2=700+/-50 mmHg) reperfusion. Following 30 min of warm ischemia, hearts in all groups were reperfused at each oxygen pressure. The recovery of cardiac function of each heart was measured at the end of reperfusion. Concentrations of lactate (LAC), lactate dehydrogenase (LDH), and creatine kinase (CK) in the coronary perfusate during reperfusion were measured. The recovery rate of CO, SP, and RPP in group 2 were all significantly better than in the other 2 groups. CK leakage in group 2 was significantly lower than in group 3. A clinical study was also conducted during elective coronary artery bypass grafts in 16 consecutive patients who underwent either hyperoxic (n=8, PO2=450-550 mmHg) or more physiologic (n=8, PO2=200-250 mmHg) cardiopulmonary bypass after aortic unclamping. The clinical study assessed CK-MB, LDH, LAC, and malondialdehyde (MDA) in patient blood prior to starting the surgical procedure and at 30 min and 3, 9, and 21 h after unclamping. Cardiac index (CI), central venous pressure, pulmonary capillary wedge pressure, systolic arterial pressure, and the dose of cathecholamines were also measured. Although no significant differences were present in the dose of cathecholamines, the CI in the more physiologic oxygen tension group was significantly higher than in the hyperoxic group at 3 and 6 h after unclamping. The levels of MDA in the more physiologic PO2 group was significantly lower at 30 min after aortic unclamping than in the hyperoxic group. The present results suggest that in the experimental as well as in the clinical study, high PO2 leads to myocardial reperfusion damage; however, maintaining a more physiologic PO2 during reperfusion following ischemia may attenuate reperfusion injury.

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.

Cardiac function after eight hour storage by using polyethylene glycol hemoglobin versus crystalloid perfusion

Efforts to extend myocardial preservation for transplantation by crystalloid perfusion have been limited by edema and compromised function. We hypothesized that hypothermic perfusion preservation with a polyethylene glycol (PEG) conjugated hemoglobin solution may extend preservation times. The purpose of this study was to compare cardiac function after continuous perfusion by using a hypocalcemic, normokalemic crystalloid perfusate with and without the addition of PEG-hemoglobin (Hb). The hearts of 20 anesthetized and ventilated New Zealand White rabbits were harvested after cold cardioplegic arrest. Group I (n = 10) hearts were continuously perfused with a hypocalcemic, normokalemic 3% bovine PEG-Hb solution at 20 degrees C and 30 mm Hg for 8 hours. Group II (n = 10) hearts were continuously perfused with an identical crystalloid solution without PEG-Hb for 8 hours under the same conditions as group I hearts. Cardiac function was measured with a left ventricular force transducer after transfer to a standard crystalloid Langendorff circuit at 37 degrees C and an aortic root pressure of 59 mm Hg. After 8 hours of perfusion preservation, heart rate was similar for groups I and II (p = not significant [NS]). Coronary blood flow after and during preservation was similar between PEG-Hb and crystalloid preserved hearts (p = NS). Left ventricular developed pressure, peak dP/dt, and peak -dP/dt were superior in hearts preserved with PEG-Hb. Percent water of total ventricular weight was 82.0% for group I and 81.6% for group II (p = NS). Continuous perfusion preservation of rabbit hearts for 8 hours with a hypocalcemic normokalemic PEG-Hb based solution at 30 mm Hg and 20 degrees C yields left ventricular function that is superior to perfusion with a similar crystalloid solution without PEG-Hb, despite similar myocardial edema and coronary flow. Extended cardiac perfusion preservation with this PEG-Hb based solution deserves further study, including comparison with traditional cardioplegic preservation solutions.

Cardiac performance after deep hypothermic circulatory arrest in chronically cyanotic neonatal lambs

OBJECTIVES

It is controversial whether immature cyanotic hearts are more susceptible to ischemic injury than normoxemic hearts. Acutely induced alveolar hypoxic stress before cardiopulmonary bypass has been used as a model of cyanosis and is reported to worsen recovery of immature hearts after subsequent ischemic insult by means of a free radical injury mechanism. Because of concerns about the relevance of acute alveolar repair to the chronic cyanosis encountered clinically, we assessed the effects of chronic cyanosis without alveolar hypoxia, acute alveolar hypoxia, and normoxemia on recovery of cardiac function after deep hypothermic circulatory arrest.

METHODS

A chronic cyanosis model was created in 8 lambs by an anastomosis between the pulmonary artery and the left atrium (cyanosis group). Eight lambs underwent sham operation (control). One week later, the animals underwent cardiopulmonary bypass with 90 minutes of deep hypothermic circulatory arrest at 18 degrees C. Another 8 lambs underwent 45 minutes of hypoxic ventilation before bypass, with arterial oxygen tension being maintained at 30 mm Hg (acute hypoxia group). Cardiac index, preload recruitable stroke work, and tau were measured. Malondialdehyde and nitrate-nitrite, nitric oxide metabolites, were also measured in the coronary sinus. Myocardial antioxidant reserve capacity at 2 hours of reperfusion was assessed by measuring lipid peroxidation in left ventricular tissue samples incubated with t-butylhydroperoxide at 37 degrees C.

RESULTS

Oxygen tension was 35 +/- 3 mm Hg in the acute hypoxia group versus 93 +/- 7 mm Hg in the control group. In the acute hypoxia group the recovery of cardiac index, preload recruitable stroke work, and tau were significantly worse than that found in both the control and cyanosis groups. Preload recruitable stroke work at 2 hours of reperfusion was slightly but significantly lower in the cyanosis group than in the control group. The postischemic level of nitric oxide metabolites was significantly lower in the acute hypoxia group than in the cyanosis and control groups. However, malondialdehyde levels in the coronary sinus and myocardial antioxidant reserve capacity were not significantly different among the groups.

CONCLUSION

Recovery of left ventricular function after deep hypothermic circulatory arrest in neonatal lambs with chronic cyanosis was slightly worse than that found in acyanotic animals. Acute hypoxia before bypass was associated with significantly worse recovery of left ventricular function, and the mechanism of injury may be related to an impairment of nitric oxide production. Free radical injury does not appear to explain any differences among cyanotic, acyanotic, and acutely hypoxic animals in recovery of left ventricular function after ischemia.

Superiority of magnesium cardioplegia in neonatal myocardial protection

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 (FiO2 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+2 0.2 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 < 0.001 vs Groups 2 and 3) and global myocardial function (39% vs 102% vs 101%) (p < 0.001 vs Groups 2 and 3), and reduced diastolic stiffness (267% vs 158% vs 154%) (p < 0.001 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 < 0.001 vs Groups 2 and 3) and global myocardial function (40%+/-1%) (p < 0.001 vs Groups 2 and 3), and a marked rise in diastolic stiffness (258%+/-5%) (p < 0.001 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.

Bosentan prevents hypoxia-reoxygenation-induced pulmonary hypertension and improves pulmonary function

BACKGROUND

Acute hypoxia results in increased pulmonary vascular resistance. Despite reoxygenation, pulmonary vascular resistance remains elevated and pulmonary function is altered. Endothelin-1 might contribute to hypoxia-reoxygenation-induced pulmonary hypertension and to reoxygenation injury by stimulating leukocytes. This study was carried out using an established model of hypoxia and reoxygenation to determine whether endothelin-1 blockade with Bosentan could prevent hypoxia-reoxygenation-induced pulmonary hypertension and reoxygenation injury.

METHODS

Twenty neonatal piglets underwent 90 minutes of hypoxia, 60 minutes of reoxygenation on cardiopulmonary bypass, and 2 hours of recovery. Control animals (n = 12) received no drug treatment, whereas the treatment group (n = 8) received the endothelin-1 receptor antagonist, Bosentan, throughout hypoxia.

RESULTS

In controls, pulmonary vascular resistance increased during hypoxia to 491% of baseline and remained elevated after reoxygenation; however in the Bosentan group, it increased to only 160% of baseline by end-hypoxia, then decreased to 76% at end-recovery. Arterial endothelin-1 levels in controls increased to 591% of baseline after reoxygenation. Arterial nitrite levels decreased during hypoxia in controls but were maintained in the Bosentan group. Consequently, animals in the Bosentan group had better postreoxygenation pulmonary vascular resistance, A-a gradient, and airway resistance along with lower myeloperoxidase levels than controls.

CONCLUSIONS

Acute hypoxia and postreoxygenation pulmonary hypertension was attenuated by Bosentan, which maintained nitric oxide levels during hypoxia, decreased leukocyte-mediated injury, and improved pulmonary function.

Dose dependency of L-arginine in neonatal myocardial protection: the nitric oxide paradox

OBJECTIVES

Recent experimental studies have suggested that enriching cardioplegic solution with L-arginine improves myocardial protection by increasing nitric oxide production. Nitric oxide, however, also generates the toxic oxygen-derived free radical peroxynitrite; thus these beneficial effects may be dose dependent, especially in vulnerable (stressed) hearts.

METHODS

Fifteen neonatal piglets underwent 60 minutes of ventilator hypoxia (inspired oxygen fraction 8%-10%) followed by 20 minutes of normothermic ischemia on cardiopulmonary bypass (stress). They were then protected for 70 minutes with multiple doses of blood cardioplegic solution. In 5 (group 1), the cardioplegic solution contained no L-arginine, in 5 (group 2), it was enriched with a 4 mmol/L concentration of L-arginine, and in 5 (group 3), a 10 mmol/L concentration of L-arginine. Myocardial function was assessed by means of pressure volume loops and expressed as a percentage of control, and coronary vascular resistance and conjugated diene production were measured during infusions of cardioplegic solution.

RESULTS

Compared with the protection afforded by blood cardioplegic solution without L-arginine (group 1), the addition of a 4 mmol/L concentration of L-arginine (group 2) significantly improved myocardial protection, resulting in complete return of systolic function (end-systolic elastance 38% vs 100%; P <.001 vs 4 mmol/L L-arginine) and preload recruitable stroke work (40% vs 100%; P <. 001 vs 4 mmol/L L-arginine); minimal increase in diastolic stiffness (239% vs 158%; P <.001 vs 4 mmol/L L-arginine); and lower coronary vascular resistance, conjugated diene production, and myeloperoxidase activity (P <.001 vs 4 mmol/L L-arginine in each case). Conversely, supplementing the cardioplegic solution with a 10 mmol/L dose of L-arginine (group 3) negated these beneficial effects, resulting in depressed systolic function (end-systolic elastance 41% +/- 2%; P <.001 vs 4 mmol/L L-arginine) and preload recruitable stroke work (40% +/- 2%; P <.001 vs 4 mmol/L L-arginine); increased diastolic stiffness (246% +/- 7%; P <.001 vs 4 mmol/L L-arginine); and higher conjugated diene production, myeloperoxidase activity, and coronary vascular resistance (P <.001 vs 4 mmol/L L-arginine in each case).

CONCLUSIONS

Enriching cardioplegic solution with a 4 mmol/L concentration of L-arginine significantly improves myocardial protection by reducing oxygen-derived free radical formation by white blood cells, thus preserving vascular and myocardial function. However, these beneficial effects are dose dependent because 10 mmol/L concentrations of L-arginine increase oxygen-derived free radical production, resulting in vascular and myocardial dysfunction.

Optimizing liquid ventilation as a lung protection strategy for neonatal cardiopulmonary bypass: full functional residual capacity dosing is more effective than half functional residual capacity dosing

OBJECTIVE

To evaluate and compare the protective effects of two different perflubron doses on hemodynamics and lung function in a neonatal animal model of cardiopulmonary bypass-induced lung injury.

DESIGN

Prospective, randomized, controlled study.

SETTING

Animal laboratory of the Department of Surgery, Duke University Medical Center.

SUBJECTS

Twenty-one neonatal swine.

INTERVENTIONS

One-wk-old swine (2.2-3.2 kg) were randomized to receive cardiopulmonary bypass with full functional residual capacity perflubron (n = 7), cardiopulmonary bypass with half functional residual capacity perflubron (n = 7), or cardiopulmonary bypass alone (n = 7). This last group served as control animals, receiving cardiopulmonary bypass with conventional ventilation. Liquid lung ventilation animals received perflubron via the endotracheal tube at either full functional residual capacity (16-20 mL/kg) or half functional residual capacity (10 mL/kg) before the initiation of cardiopulmonary bypass. Each animal was placed on nonpulsatile cardiopulmonary bypass and cooled to a nasopharyngeal temperature of 18 degrees C (64.4 degrees F). Low-flow cardiopulmonary bypass (35 mL/kg/min) was instituted for 90 mins. The blood flow rate was then returned to 100 mL/kg/min. The animals were warmed to 36 degrees C (96.8 degrees F) and separated from cardiopulmonary bypass. Data were obtained at 30, 60, and 90 mins after separation from cardiopulmonary bypass.

MEASUREMENTS AND MAIN RESULTS

Cardiopulmonary bypass without liquid lung ventilation resulted in a significant decrease in cardiac output and oxygen delivery and a significant increase in pulmonary vascular resistance in the post-bypass period. Full functional residual capacity liquid lung ventilation administered before bypass resulted in no change in cardiac output and oxygen delivery after bypass. Full functional residual capacity liquid lung ventilation resulted in lower pulmonary vascular resistance after bypass compared with both control and half functional residual capacity liquid lung ventilation animals.

CONCLUSIONS

These data suggest that liquid lung ventilation dosing at full functional residual capacity before bypass is more effective than half functional residual capacity in minimizing the lung injury associated with neonatal cardiopulmonary bypass. Full functional residual capacity dosing may optimize alveolar distention and lung volume, as well as improve oxygen delivery compared with half functional residual capacity dosing.

The importance of cardioplegic infusion pressure in neonatal myocardial protection

BACKGROUND

Cardioplegia infusion pressure is usually not directly monitored during neonatal heart operations. We hypothesize that the immature newborn heart may be damaged by even moderate elevation of cardioplegic infusion pressure, which in the absence of direct aortic monitoring may occur without the surgeon's knowledge.

METHODS

Twenty neonatal piglets received cardiopulmonary bypass and the heart was protected for 70 minutes with multidose blood cardioplegia infused at an aortic root pressure of 30 to 50 mm Hg (low pressure) or 80 to 100 mm Hg (high pressure). Group 1 (n = 5, low pressure), and group 2 (n = 5, high pressure) were uninjured (nonhypoxic) hearts. Group 3 (n = 5, low pressure) and group 4 (n = 5, high pressure) first underwent 60 minutes of ventilator hypoxia (FiO2 8% to 10%) before initiating cardiopulmonary bypass to produce a clinically relevant hypoxic stress before cardiac arrest. Function was assessed using pressure volume loops (expressed as a percentage of control), and coronary vascular resistance was measured with each cardioplegic infusion.

RESULTS

In nonhypoxic (uninjured) hearts (groups 1 and 2) cardioplegic infusion pressure did not significantly affect systolic function (end systolic elastance, 104% versus 96%), preload recruitable stroke work (102% versus 96%) diastolic compliance (152% versus 156%), or coronary vascular resistance but did raise myocardial water (78.9% versus 80.1%; p < 0.01). Conversely, if the cardioplegic solution was infused at even a slightly higher pressure in hypoxic hearts (group 4), there was deterioration of systolic function (end systolic elastance, 28% versus 106%) (p < 0.001) and preload recruitable stroke work (31% versus 103%; p < 0.001), rise in diastolic stiffness (274% versus 153%; p < 0.001), greater myocardial edema (80.5% versus 79.6%), and marked increase in coronary vascular resistance (p < 0.001) compared to hypoxic hearts given cardioplegia at low infusion pressures (group 3), which preserved function.

CONCLUSIONS

Hypoxic neonatal hearts are very sensitive to cardioplegic infusion pressures, such that even moderate elevations cause significant damage resulting in myocardial depression and vascular dysfunction. This damage is avoided by using low infusion pressures. Because small differences in infusion pressure may be difficult to determine without a direct aortic measurement, we believe it is imperative that surgeons directly monitor cardioplegia infusion pressure, especially in cyanotic patients.

Normoxic cardiopulmonary bypass reduces oxidative myocardial damage and nitric oxide during cardiac operations in the adult

OBJECTIVE

Hyperoxic cardiopulmonary bypass is widely used during cardiac operations in the adult. This management may cause oxygenation injury induced by oxygen-derived free radicals and nitric oxide. Oxidative damage may be significantly limited by maintaining a more physiologic oxygen tension strategy (normoxic cardiopulmonary bypass).

METHODS

During elective coronary artery bypass grafting, 40 consecutive patients underwent either hyperoxic (oxygen tension = 400 mm Hg) or normoxic (oxygen tension = 140 mm Hg) cardiopulmonary bypass. At the beginning and the end of bypass this study assessed polymorphonuclear leukocyte elastase, nitrate, creatine kinase, and lactic dehydrogenase, antioxidant levels, and malondialdehyde in coronary sinus blood. Cardiac index was measured before and after cardiopulmonary bypass.

RESULTS

There was no difference between groups with regard to age, sex, severity of disease, ejection fraction, number of grafts, duration of cardiopulmonary bypass, or ischemic time. Hyperoxic bypass resulted in higher levels of polymorphonuclear leukocyte elastase (377 +/- 34 vs 171 +/- 32 ng/ml, p = 0.0001), creatine kinase 672 +/- 130 vs 293 +/- 21 U/L, p = 0.002), lactic dehydrogenase (553 +/- 48 vs 301 +/- 12 U/L, p = 0.003), antioxidants (1.97 +/- 0.10 vs 1.41 +/- 0.11 mmol/L, p = 0.01), malondialdehyde (1.36 +/- 0.1 micromol/L,p = 0.005), and nitrate (19.3 +/- 2.9 vs 10.1 +/- 2.1 micromol/L, p = 0.002), as well as reduction in lung vital capacity (66% +/- 2% vs 81% +/- 1%,p = 0.01) and forced 1-second expiratory volume (63% +/- 10% vs 93% +/- 4%, p = 0.005) compared with normoxic management. Cardiac index after cardiopulmonary bypass at low filling pressure was similar between groups (3.1 +/- 0.2 vs 3.3 +/- 0.3 L/min per square meter). [Data are mean +/- standard error (analysis of variance), with p values compared with an oxygen tension of 400 mm Hg.]

CONCLUSIONS

Hyperoxic cardiopulmonary bypass during cardiac operations in adults results in oxidative myocardial damage related to oxygen-derived free radicals and nitric oxide. These adverse effects can be markedly limited by reduced oxygen tension management. The concept of normoxic cardiopulmonary bypass may be applied to surgical advantage during cardiac operations.

Delayed cardioplegic reoxygenation reduces reoxygenation injury in cyanotic immature hearts

BACKGROUND

Hypoxemic developing hearts are susceptible to oxygen-mediated damage that occurs after reintroduction of molecular oxygen. This unintended hypoxemic/reoxygenation injury leads to lipid peroxidation and membrane damage and may contribute to postoperative cardiac dysfunction. Biochemical and functional status are improved by delaying reoxygenation on cardiopulmonary bypass (CPB) until cardioplegic arrest.

METHODS

Six immature piglets (3 to 5 kg) without hypoxemia underwent 30 minutes of cardioplegic arrest during 1 hour of CPB. Fourteen others underwent 2 hours of hypoxemia on ventilator before reoxygenation on CPB. Reflecting our clinical routine, 9 were reoxygenated on CPB for 5 minutes followed by 30 minutes of cardioplegic arrest and 25 minutes of reperfusion. The other 5 were put on hypoxemic CPB for 5 minutes, before being reoxygenated during cardioplegic arrest for 30 minutes followed by 25 minutes of reperfusion.

RESULTS

Cardioplegic arrest (no hypoxemia group) caused no functional or biochemical changes. In contrast, by preceding hypoxemia with subsequent reoxygenation on CPB (no treatment group) we found 39.5% decrease in antioxidant reserve capacity, 1,212% increase in myocardial conjugated diene production, significant increase in coronary sinus blood conjugated dienes, and an 81% reduction of left ventricular contractility, all of which were statistically significant (p < 0.05) when compared with the no hypoxemia group. Conversely, delaying reoxygenation until cardioplegic arrest (treatment group) resulted in 33.1% improvement in antioxidant reserve capacity, 91.7% less conjugated diene production, lower coronary sinus blood conjugated diene levels, and a 95% improved contractility, all of which were significant (p < 0.05) when compared with the no treatment group.

CONCLUSIONS

A reoxygenation injury associated with lipid peroxidation and decreased postbypass contractility occurs in cyanotic immature hearts when reoxygenated on CPB. Delaying reoxygenation until cardioplegic arrest by starting CPB with ambient partial pressure of oxygen results in significantly improved myocardial status.

Liquid ventilation improves pulmonary function and cardiac output in a neonatal swine model of cardiopulmonary bypass

OBJECTIVE

Neonatal and infant cardiopulmonary bypass results in multiorgan system dysfunction. Organ protective strategies have traditionally been directed at the myocardium and brain while neglecting the sometimes severe injury to the lungs. We hypothesized that liquid ventilation would improve pulmonary function and cardiac output in neonates after cardiopulmonary bypass.

METHODS

Twenty neonatal swine were randomized to receive cardiopulmonary bypass with or without liquid ventilation. In the liquid-ventilated group, a single dose of perflubron was administered before bypass. The control group was conventionally ventilated. Each animal was placed on nonpulsatile, hypothermic bypass. Low-flow cardiopulmonary bypass was performed for 60 minutes. The flow rate was returned to 125 ml/kg per minute, and after warming to 37 degrees C, the animals were removed from bypass. Hemodynamic and ventilatory data were obtained after bypass to assess the effects of liquid ventilation.

RESULTS

Without liquid ventilation, cardiopulmonary bypass resulted in a significant decrease in cardiac output, oxygen delivery, and static pulmonary compliance compared with prebypass values. Input pulmonary resistance and characteristic impedance increased in these control animals. At 30, 60, and 90 minutes after bypass, the animals receiving liquid ventilation showed significantly increased cardiac output and static compliance and significantly decreased input pulmonary resistance and characteristic impedance compared with control animals not receiving liquid ventilation.

CONCLUSIONS

Liquid ventilation improved pulmonary function after neonatal cardiopulmonary bypass while increasing cardiac output. The morbidity associated with cardiopulmonary bypass may be significantly reduced if the adverse pulmonary sequelae of bypass can be diminished. Liquid ventilation may become an important technique to protect the lungs from the deleterious effects of cardiopulmonary bypass.

The relationship between calcium and magnesium in pediatric myocardial protection

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.

Ischemic and reperfusion injury of cyanotic myocardium in chronic hypoxic rat model: changes in cyanotic myocardial antioxidant system

OBJECTIVE

The objective was to evaluate the effect of left ventricular function on cyanotic myocardium after ischemia-reperfusion and to determine the effect of cyanosis on the myocardial antioxidant system.

METHODS

Cyanotic hearts (cyanotic group) were obtained from rats housed in a hypoxic chamber (10% oxygen) for 2 weeks and control hearts (control group) from rats maintained in ambient air. Isolated, crystalloid perfused working hearts were subjected to 15 minutes of global normothermic ischemia and 20 minutes of reperfusion, and functional recovery was evaluated in the two groups. Myocardial superoxide dismutase, glutathione peroxidase, glutathione reductase activity, and reduced glutathione content were measured separately in the cytoplasm and mitochondria at the end of the preischemic, ischemic, and reperfusion periods.

RESULTS

Mean cardiac output/left ventricular weight was not significantly different between the two groups. Percent recovery of cardiac output was significantly lower in the cyanotic group than in the control group (56.1% +/- 5.7% vs 73.0% +/- 3.1%, p = 0.001). Mitochondrial superoxide dismutase, mitochondrial and cytosolic glutathione reductase activity, and cytosolic reduced glutathione were significantly lower in the cyanotic group than in the control group at end-ischemia (superoxide dismutase, 3.7 +/- 1.3 vs 5.9 +/- 1.5 units/mg protein, p = 0.012; mitochondrial glutathione reductase, 43.7 +/- 14.0 vs 71.0 +/- 30.3 munits/mg protein, p = 0.039; cytosolic glutathione reductase, 13.7 +/- 2.0 vs 23.2 +/- 4.2 munits/mg protein, p < 0.001; and reduced glutathione, 0.69 +/- 0.10 vs 0.91 +/- 0.24 microgram/mg protein, p = 0.037).

CONCLUSIONS

Cyanosis impairs postischemic functional recovery and depresses myocardial antioxidant reserve during ischemia. Reduced antioxidant reserve at end-ischemia may result in impaired postischemic functional recovery of cyanotic myocardium.

Detrimental effects of cardiopulmonary bypass in cyanotic infants: preventing the reoxygenation injury

BACKGROUND

Recent experimental studies have shown that acute hypoxia followed by abrupt reoxygenation using cardiopulmonary bypass (CPB) results in an unintended injury mediated by oxygen free radicals, which can be modified by initiating CPB at a lower fraction of inspired oxygen (FiO2) or by leukocyte filtration. However, the clinical relevance of these experimental studies has been questioned because chronic hypoxia may allow compensatory changes to occur.

METHODS

Seven acyanotic infants had CPB initiated at an FiO2 of 1.0. Of 21 cyanotic infants, 7 (group 1) had CPB initiated at an FiO2 of 1.0, 6 (group 2) at an FiO2 of 0.21, and 8 (group 3) underwent CPB using leukocyte filtration. Biopsy of right atrial tissue was performed before and 10 to 20 minutes after the initiation of CPB. The tissue was incubated in 4-mmol/L t-butylhydroperoxide (a strong oxidant), and the malondialdehyde (MDA) level was measured to determine the antioxidant reserve capacity. The more MDA produced, the greater was the depletion of tissue antioxidants secondary to oxygen free radical formation during reoxygenation.

RESULTS

There was no difference in the prebypass antioxidant reserve capacity between cyanotic and acyanotic hearts (492 +/- 72 versus 439 +/- 44 nmol MDA/g protein). However, after the initiation of CPB without leukocyte filtration, MDA production rose markedly in the cyanotic (groups 1 and 2) as compared with the acyanotic hearts (322% versus 40%; p < 0.05), indicating a depletion of antioxidants. In cyanotic hearts, initiating CPB at an FiO2 of 1.0 (group 1) resulted in increased MDA production (407% versus 227%) as compared with hearts in which CPB was initiated at an FiO2 of 0.21 (group 2), indicating a greater generation of oxygen free radicals in group 1. Conversely, there was only a minimal increase in MDA production in 8 of the 21 infants (group 3) in whom white blood cells were effectively filtered (19% versus 322%; p < 0.05).

CONCLUSIONS

First, increased amounts of oxygen free radicals are generated in cyanotic infants with the initiation of CPB. Second, this production is reduced by initiating CPB at an FiO2 of 0.21 or by effectively filtering white blood cells. Third, these changes parallel those seen in the acute experimental model, validating its use for future study.

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.

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

OBJECTIVES

The ideal cardioplegic calcium (Ca+2) concentration in newborns continues to be debated. Most studies examining cardioplegia calcium concentrations have been done with a nonclinical model (i.e., isolated heart preparation), the results of which may not be clinically applicable, and they have not examined the effect of calcium concentration in a clinically relevant stressed (hypoxic) heart.

METHODS

Twenty neonatal piglets 5 to 18 days old were placed on cardiopulmonary bypass, and their aortas were crossclamped for 70 minutes with hypocalcemic or normocalcemic multidose blood cardioplegic infusions. Group 1 (n = 5; low Ca+2, 0.2 to 0.4 mmol/L) and group 2 (n = 5; normal Ca+2, 1.0 to 1.3 mmol/L) were nonhypoxic (uninjured) hearts. Ten other piglets were first ventilated at an FiO2 of 8% to 10% (O2 saturation 65% to 70%) for 60 minutes (i.e., causing hypoxia) and then reoxygenated at an FiO2 of 100% with cardiopulmonary bypass, which produces a clinically relevant stress injury. They then underwent cardioplegic arrest (as described above) with a hypocalcemic (n = 5, group 3) or normocalcemic (n = 5, group 4) blood cardioplegic solution. Myocardial function was assessed with pressure volume loops and expressed as a percentage of control values. Coronary vascular resistance was measured during each cardioplegic infusion. All values were reported as the mean +/- standard error.

RESULTS

In nonhypoxic hearts (groups 1 and 2), good myocardial protection was achieved at either concentration of cardioplegia calcium, as demonstrated by preservation of postbypass systolic function (104% vs 99% end-systolic elastance), minimally increased diastolic stiffness (152% vs 162%), no difference in myocardial water (78.9% vs 78.9%), and no change in adenosine triphosphate levels or coronary vascular resistance. Low-calcium blood cardioplegia solution repaired the hypoxic reoxygenation injury in stressed hearts (group 3), resulting in no statistical difference in myocardial function, coronary vascular resistance, or adenosine triphosphate levels compared with nonhypoxic hearts (groups 1 and 2). Conversely, when a normocalcemic cardioplegia solution was used in hypoxic hearts (group 4), there was marked reduction in postbypass systolic function (49% +/- 4% end-systolic elastance; p < 0.05), increased diastolic stiffness (276% +/- 9%; p < 0.05), increased myocardial water (80.1% +/- 0.2%; p < 0.05), rise in coronary vascular resistance (p < 0.05), and lower adenosine triphosphate levels compared with groups 1, 2, and 3.

CONCLUSIONS

This study demonstrates that, in the clinically relevant, intact animal model, good myocardial protection is independent of cardioplegia calcium concentration in nonhypoxic (noninjured) hearts; hypoxic (stressed) hearts are extremely sensitive to the cardioplegic calcium concentration; and normocalcemic cardioplegia is detrimental to neonatal myocardium subjected to a preoperative hypoxic stress.

Studies of hypoxemic/reoxygenation injury: without aortic clamping. VI. Counteraction of oxidant damage by exogenous antioxidants: N-(2-mercaptopropionyl)-glycine and catalase

This study tests the hypothesis that antioxidants administered before reoxygenation can reduce oxygen-mediated damage and improve myocardial performance. Of 25 Duroc-Yorkshire piglets (2 to 3 weeks, 3 to 5 kg) five underwent 60 minutes of cardiopulmonary bypass without hypoxemia (control group), and five others underwent 30 minutes of hypoxemia on cardiopulmonary bypass with a circuit primed with oxygen tension about 25 mm Hg blood followed by reoxygenation on cardiopulmonary bypass (no treatment). In vitro studies were performed to obtain the optimal dosage of the antioxidants N-(2-mercaptopropionyl)-glycine and and catalase to be used in subsequent in vivo experimental studies; cardiac homogenates were incubated in 0 to 5 mmol/L concentrations of the oxidant t-butylhydroperoxide and malondialdehyde production was measured. Fifteen piglets were made hypoxemic on cardiopulmonary bypass for 30 minutes, and the antioxidants N-(2-mercaptopropionyl)-glycine at either 30 or 80 mg/kg body weight or N-(2-mercaptopropionyl)-glycine, 30 mg/kg body weight, and catalase, 50,000 U/kg body weight, were added to the cardiopulmonary bypass circuit 15 minutes before reoxygenation. Left ventricular contractility, which was expressed as end-systolic elastance, was measured by conductance catheter before hypoxemia and after reoxygenation. Myocardial antioxidant reserve capacity was determined after reoxygenation by incubating cardiac homogenates in the oxidant t-butylhydroperoxide and measuring subsequent malondialdehyde elution. The in vitro bioassay studies showed a dose-dependent reduction of lipid peroxidation with N-(2-mercaptopropionyl)-glycine, with maximal benefits of a 40% decrease and malondialdehyde elaboration occurring with N-(2-mercaptopropionyl)-glycine and catalase compared with untreated cardiac homogenates. Cardiopulmonary bypass (no hypoxemia) caused no oxidant damage or changes in contractile function after cardiopulmonary bypass. Reoxygenation without treatment raised conjugated diene levels 57%,* lowered antioxidant reserve capacity 51%,* and was associated with only 38%* recovery of contractile function (p < 0.05 vs control). In contrast, treatment with antioxidants avoided lipid peroxidation, maintained antioxidant reserve capacity, and resulted in a dose-dependent improvement in left ventricular contractility with complete recovery occurring in N-(2-mercaptopropionyl)-glycine and catalase-treated piglets (*p < 0.05 vs no treatment). This study confirms the occurrence of hypoxemic/reoxygenation injury in immature hearts placed on cardiopulmonary bypass and shows that biochemical and functional damage can be counteracted by adding antioxidants to the cardiopulmonary bypass priming fluid. Contractile function improved in a dose-dependent manner, and oxygen-mediated damage could be avoided by mercaptopropionyl glycine/catalase treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies of hypoxemic/reoxygenation injury: without aortic clamping. IX. Importance of avoiding perioperative hyperoxemia in the setting of previous cyanosis

This study of an in vivo infantile piglet model of compensated hypoxemia tests the hypothesis that reoxygenation on hyperoxemic cardiopulmonary bypass produces oxygen-mediated myocardial injury that can be limited by normoxemic management of cardiopulmonary bypass and the interval after cardiopulmonary bypass. Twenty-five immature piglets (< 3 weeks old) were placed on 120 minutes of cardiopulmonary bypass and five piglets served as a biochemical control group without cardiopulmonary bypass. Five piglets underwent cardiopulmonary bypass without hypoxemia (cardiopulmonary bypass control). Twenty others became hypoxemic on cardiopulmonary bypass for 60 minutes by lowering oxygen tension to about 25 mm Hg. The study was terminated in five piglets at the end of hypoxemia, whereas 15 others were reoxygenated at an oxygen tension about 400 mm Hg or about 100 mm Hg for 60 minutes. Oxygen delivery was maintained during hypoxemia by increasing cardiopulmonary bypass flow and hematocrit level to avoid metabolic acidosis and lactate production. Myocardial function after cardiopulmonary bypass was evaluated from end-systolic elastance (conductance catheter) and Starling curve analysis. Myocardial conjugated diene production and creatine kinase leakage were assessed as biochemical markers of injury, and antioxidant reserve capacity was determined by measuring malondialdehyde after cardiopulmonary bypass in myocardium incubated in the oxidant, t-butylhydroperoxide. Cardiopulmonary bypass without hypoxemia caused no oxidant or functional damage. Conversely, reoxygenation at an oxygen tension about 400 mm Hg raised myocardial conjugated diene level and creatine kinase production (CD: 3.5 +/- 0.7 A233 nm/min/100 g, creatine kinase: 8.5 +/- 1.5 U/min/100 g, p < 0.05 vs cardiopulmonary bypass control), reduced antioxidant reserve capacity (malondialdehyde: 1115 +/- 60 nmol/g protein at 4.0 mmol t-butylhydroperoxide, p < 0.05 vs control), and produced severe postbypass dysfunction (end-systolic elastance recovered only 39% +/- 7%, p < 0.05 vs cardiopulmonary bypass control). Lowering oxygen tension to about 100 mm Hg during reoxygenation avoided conjugated diene production and creatine kinase release, retained normal antioxidant reserve, and improved functional recovery (80% +/- 11%, p < 0.05 vs oxygen tension about 400 mm Hg). These findings show that conventional hyperoxemic cardiopulmonary bypass causes unintended reoxygenation injury in hypoxemic immature hearts that may contribute to myocardial dysfunction after cardiopulmonary bypass and that normoxemic management may be used to surgical advantage.

Studies of hypoxemic/reoxygenation injury: with aortic clamping. XIII. Interaction between oxygen tension and cardioplegic composition in limiting nitric oxide production and oxidant damage

This study tests the interaction between oxygen tension and cardioplegic composition on nitric oxide production and oxidant damage during reoxygenation of previously cyanotic hearts. Of 35 Duroc-Yorkshire piglets (2 to 3 weeks, 3 to 5 kg), six underwent 30 minutes of blood cardioplegic arrest with hyperoxemic (oxygen tension about 400 mm Hg), hypocalcemic, alkalotic, glutamate/aspartate blood cardioplegic solution during 1 hour of cardiopulmonary bypass without hypoxemia (control). Twenty-nine others were subjected to up to 120 minutes of ventilator hypoxemia (oxygen tension about 25 mm Hg) before reoxygenation on CPB. To simulate routine clinical management, nine piglets underwent uncontrolled cardiac reoxygenation, whereby cardiopulmonary bypass was started at oxygen tension of about 400 mm Hg followed by the aforementioned blood cardioplegic protocol 5 minutes later. All 20 other piglets underwent controlled cardiac reoxygenation, whereby cardiopulmonary bypass was started at the ambient oxygen tension (about 25 mm Hg), and reoxygenation was delayed until blood cardioplegia was given. The blood cardioplegia solution was kept normoxemic (oxygen tension about 100 mm Hg) in 10 piglets and made hyperoxemic (oxygen tension about 400 mm Hg) in 10 others. The cardioplegic composition was also varied so that the cardioplegic solution in each subgroup contained either KCl only (30 mEq/L) or components that theoretically inhibit nitric oxide synthase by including hypocalcemia, alkalosis, and glutamate/aspartate. Function (end-systolic elastance) and myocardial nitric oxide production, conjugated diene production, and antioxidant reserve capacity were measured. Blood cardioplegic arrest without hypoxemia did not cause myocardial nitric oxide or conjugated diene production, reduce antioxidant reserve capacity, or change left ventricular functional recovery. In contrast, uncontrolled cardiac reoxygenation raised nitric oxide and conjugated diene production 19- and 13-fold, respectively (p < 0.05 vs control), reduced antioxidant reserve capacity 40%, and contractility recovered only 21% of control levels. After controlled cardiac reoxygenation at oxygen tension about 400 mm Hg with cardioplegic solution containing KCl only, nitric oxide and conjugated diene production rose 16- and 12-fold, respectively (p < 0.05 vs control), and contractility recovered only 43% +/- 5%. Normoxemic (oxygen tension of about 100 mm Hg) controlled cardiac reoxygenation with the same solution reduced nitric oxide and conjugated diene production 85% and 71%, and contractile recovery rose to 55% +/- 7% (p < 0.05 vs uncontrolled reoxygenation). In comparison, controlled cardiac reoxygenation with an oxygen tension of about 400 mm Hg hypocalcemic, alkalotic, glutamate/aspartate blood cardioplegic solution reduced nitric oxide and conjugated diene production 85% and 62%, respectively, and contractility recovered 63% +/- 4% (p < 0.05 vs KCl only).(ABSTRACT TRUNCATED AT 400 WORDS)

Studies of hypoxemic/reoxygenation injury with aortic clamping: XI. Cardiac advantages of normoxemic versus hyperoxemic management during qardiopulmonary bypass

The conventional way to start cardiopulmonary bypass is to prime the cardiopulmonary bypass circuit with hyperoxemic blood (oxygen tension about 400 mm Hg) and deliver cardioplegic solutions at similar oxygen tension levels. This study tests the hypothesis that an initial normoxemic oxygen tension strategy to decrease the oxygen tension-dependent rate of oxygen free radical production will, in concert with normoxemic blood cardioplegia, limit reoxygenation damage and make subsequent hyperoxemia (oxygen tension about 400 mm Hg) safer. Thirty-five immature (3 to 5 kg, 2 to 3 week old) piglets underwent 60 minutes of cardiopulmonary bypass. Eleven control studies at conventional hyperoxemic oxygen tension (about 400 mm Hg) included six piglets that also underwent 30 minutes of blood cardioplegic arrest. Of 25 studies in which piglets were subjected to up to 120 minutes of ventilator hypoxemia (reducing fraction of inspired oxygen to 5% to 7%; oxygen tension about 25 mm Hg), 11 underwent either abrupt (oxygen tension about 400 mm Hg, n = 6) or gradual (increasing oxygen tension from 100 to 400 mm Hg over a 1-hour period, n = 5) reoxygenation without blood cardioplegia. Fourteen others underwent 30 minutes of blood cardioplegic arrest during cardiopulmonary bypass. Of these, nine were reoxygenated at oxygen tension about 400 mm Hg, and five others underwent normoxemic cardiopulmonary bypass and blood cardioplegia (oxygen tension about 100 mm Hg) with systemic oxygen tension raised to 400 mm Hg after aortic unclamping. Measurements of lipid peroxidation (conjugated dienes and antioxidant reserve capacity) and contractile function (pressure-volume loops, conductance catheter, end-systolic elastance) were made before and during hypoxemia and 30 minutes after reoxygenation. Hyperoxemic cardiopulmonary bypass did not produce oxidant damage or reduce functional recovery after cardiopulmonary bypass in nonhypoxemic controls. In contrast, abrupt and gradual reoxygenation without blood cardioplegia produced significant lipid peroxidation (84% increase in conjungated dienes), lowered antioxidant reserve capacity 68% +/- 5%, 44% +/- 8%, respectively, and decreased functional recovery 75% +/- 6% (p < 0.05), 66% +/- 4% (p < 0.05). Similar impairment followed abrupt reoxygenation before blood cardioplegic myocardial management, because conjungated diene production increased 13-fold, antioxidant reserve capacity fell 40%, and contractility recovered only 21% +/- 2% (p < 0.05). Conversely, normoxemic induction of cardiopulmonary bypass and blood cardioplegic myocardial management reduced conjungated diene production 73%, avoided impairment of antioxidant reserve capacity, and resulted in 58% +/- 11% recovery of contractile function.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies of hypoxemic/reoxygenation injury: without aortic clamping. IV. Role of the iron-catalyzed pathway: deferoxamine

This study tests the hypothesis that an iron chelator, deferoxamine, can reduce oxygen-mediated myocardial injury and avoid myocardial dysfunction after cardiopulmonary bypass by its action on the iron-catalyzed Haber-Weiss pathway. Twenty-one immature 2- to 3-week-old piglets were placed on cardiopulmonary bypass for 120 minutes, and five piglets served as biochemical controls without cardiopulmonary bypass. Five piglets underwent cardiopulmonary bypass without hypoxemia (cardiopulmonary bypass control). Sixteen others became hypoxemic while undergoing cardiopulmonary bypass for 60 minutes by lowering oxygen tension to about 25 mm Hg, followed by reoxygenation at oxygen tension about 400 mm Hg for 60 minutes. Oxygen delivery was maintained during hypoxemia by increasing cardiopulmonary bypass flow and hematocrit level. In seven piglets deferoxamine (50 mg/kg total dose) was given both intravenously just before reoxygenation and by a bolus injection (5 mg/kg) into the cardiopulmonary bypass circuit; nine others were not treated (no therapy). Myocardial function after cardiopulmonary bypass was evaluated form end-systolic elastance (conductance catheter) and Starling curve analysis. Myocardial conjugated diene production and creatine kinase leakage were assessed as biochemical markers of injury, and antioxidant reserve capacity was determined by measuring malondialdehyde in postcardiopulmonary bypass myocardium incubated in the oxidant, t-butylhydroperoxide. Cardiopulmonary bypass without hypoxemia caused no oxidant or functional damage. Conversely, reoxygenation (no therapy) raised myocardial conjugated diene levels and creatine kinase production (conjugated diene: 3.5 +/- 0.7 absorbance 233 nm/min/100 g, creatine kinase: 8.5 +/- 1.5 U/min/100 g; p < 0.05 versus cardiopulmonary bypass control), reduced antioxidant reserve capacity (malondialdehyde: 1115 +/- 60 nmol/g protein at 4 mmol/L t-butylhydroperoxide; p < 0.05 versus control), and produced severe post-bypass dysfunction (end-systolic elastance recovered only 39% +/- 7%, p < 0.05 versus cardiopulmonary bypass control). Deferoxamine avoided conjugated diene production and creatine kinase release and retained normal antioxidant reserve, and functional recovery was complete (95% +/- 11%, p < 0.05 versus no treatment). These findings show that iron-catalyzed oxidants may contribute to a reoxygenation injury and imply that deferoxamine may be used to surgical advantage.

Studies of hypoxemic/reoxygenation injury: without aortic clamping. VII. Counteraction of oxidant damage by exogenous antioxidants: coenzyme Q10

Coenzyme Q10 (CoQ10) is a natural mitochondrial respiratory chain constituent with antioxidant properties. This study tests the hypothesis that CoQ10 administered before the onset of reoxygenation on cardiopulmonary bypass, can reduce oxygen-mediated myocardial injury and avoid myocardial dysfunction after cardiopulmonary bypass. The antioxidant properties of CoQ10 were confirmed by an in vitro study in which normal myocardial homogenates were incubated with the oxidant, t-butylhydroperoxide. Fifteen immature piglets (< 3 weeks old) were placed on 60 minutes of cardiopulmonary bypass. Five piglets underwent cardiopulmonary bypass without hypoxemia (oxygen tension about 400 mm Hg). Ten others became hypoxemic on cardiopulmonary bypass for 30 minutes by lowering oxygen tension to approximately 25 mm Hg, followed by reoxygenation at oxygen tension about 400 mm Hg for 30 minutes. In five piglets, CoQ10 (45 mg/kg) was added to the cardiopulmonary bypass circuit 15 minutes before reoxygenation, and five others were not treated (no treatment). Myocardial function after cardiopulmonary bypass was evaluated from end-systolic elastance (conductance catheter), oxidant damage (lipid peroxidation) was assessed by measuring conjugated diene levels in coronary sinus blood, and antioxidant reserve capacity was determined by measuring malondialdehyde in myocardium after cardiopulmonary bypass incubated in the oxidant, t-butylhydroperoxide. Cardiopulmonary bypass without hypoxemia caused no oxidant damage and allowed complete functional recovery. Reoxygenated hearts (no treatment) showed a progressive increase in conjugated diene levels in coronary sinus blood after reoxygenation (2.3 +/- 0.6 A233 nm/0.5 ml plasma at 30 minutes after reoxygenation) and reduced antioxidant reserve capacity (malondialdehyde: 1219 +/- 157 nmol/g protein at 4.0 mmol/L t-butylhydroperoxide), resulting in severe postbypass dysfunction (percent end-systolic elastance = 38 +/- 6). Conversely, CoQ10 treatment avoided the increase in conjugated diene levels (2.1 +/- 0.6 vs 1.1 +/- 0.3, p < 0.05 vs no treatment), retained normal antioxidant reserve (896 +/- 76 nmol/g protein, p < 0.05 vs no treatment), and allowed nearly complete recovery of function (94% +/- 7%, p < 0.05 vs no treatment). We conclude that reoxygenation of the hypoxemic immature heart on cardiopulmonary bypass causes oxygen-mediated myocardial injury, which can be limited by CoQ10 treatment before reoxygenation. These findings imply that coenzyme Q10 can be used to surgical advantage in cyanotic patients, because therapeutic blood levels can be achieved by preoperative oral administration of this approved drug.

Studies of hypoxemic/reoxygenation injury: with aortic clamping. X. Exogenous antioxidants to avoid nullification of the cardioprotective effects of blood cardioplegia

This study tests the hypothesis that reoxygenation of cyanotic immature hearts when starting cardiopulmonary bypass produces an "unintended" reoxygenation injury that (1) nullifies the cardioprotective effects of blood cardioplegia and (2) is avoidable by adding antioxidants N-(2-mercaptopropionyl)-glycine plus catalase to the cardiopulmonary bypass prime. Twenty immature piglets (2 to 3 weeks) underwent 30 minutes of aortic clamping with a blood cardioplegic solution that was hypocalcemic, alkalotic, hyperosmolar, and enriched with glutamate and aspartate during 1 hour of cardiopulmonary bypass. Of these, six piglets did not undergo hypoxemia (blood cardioplegic control) and 14 others remained hypoxemic (oxygen tension about 25 mm Hg) for up to 2 hours by lowering ventilator fraction of inspired oxygen before reoxygenation on cardiopulmonary bypass. The primary solution of the cardiopulmonary bypass circuit was unchanged in eight piglets (no treatment) and supplemented with the antioxidants N-(2-mercaptopropionyl)-glycine (80 mg/kg) and catalase (5 mg/kg) in six others (N-(2-mercaptopropionyl)-glycine and catalase). Myocardial function (end-systolic elastance), lipid peroxidation (myocardial conjugated diene production), and antioxidant reserve capacity were evaluated. Blood cardioplegic arrest produced no biochemical or functional changes in nonhypoxemic control piglets. Reoxygenation caused an approximate 10-fold increase in conjugated production that persisted throughout cardiopulmonary bypass, lowered antioxidant reserve capacity 86% +/- 12%, and produced profound myocardial dysfunction, because end-systolic elastance recovered only 21% +/- 2%. Supplementation of the cardiopulmonary bypass prime with N-(2-mercaptopropionyl)-glycine and catalase reduced lipid peroxidation, restored antioxidant reserve capacity, and allowed near complete functional recovery (80% +/- 8%).** Lipid peroxidation (conjugated diene) production was lower during warm blood cardioplegic reperfusion than during induction in all reoxygenated hearts, which suggests that blood cardioplegia did not injure reoxygenated myocardium. We conclude that reoxygenation of the hypoxemic immature heart causes cardiac functional and antioxidant damage that nullifies the cardioprotective effects of blood cardioplegia that can be avoided by supplementation of the cardiopulmonary bypass prime with antioxidants (*p < 0.05 vs blood cardioplegic control; **p < 0.05 vs reoxygenation).