Substrate enhancement of cardioplegic solution: experimental studies and clinical evaluation

Myocardial protection: a forgotten modality

The goals of a cardiac surgical procedure are both technical excellence and complete protection of cardiac function. Cardioplegia is used almost universally to protect the heart and provide a quiet bloodless field for surgical accuracy. Yet, despite the importance of myocardial protection in cardiac surgery, manuscripts or dedicated sessions at major meetings on this subject have become relatively rare, as though contemporary techniques now make them unnecessary. Nevertheless, septal dysfunction and haemodynamic support (inotropes, intra-aortic balloon pump, assist devices) are common in postoperative patients, indicating that myocardial damage following cardiac surgery is still prevalent with current cardioplegic techniques and solutions. This article first describes why cardiac enzymes and septal function are the ideal markers for determining the adequacy of myocardial protection. It also describes the underappreciated consequences of postoperative cardiac enzyme release or septal dysfunction (which currently occurs in 40-80% of patients) from inadequate protection, and how they directly correlate with early and especially late mortality. Finally, it reviews the various myocardial protection techniques available to provide a detailed understanding of the cardioplegic methods that can be utilized to protect the heart. This will allow surgeons to critically assess their current method of protection and, if needed, make necessary changes to provide their patients with optimal protection.

Untargeted high-resolution plasma metabolomic profiling predicts outcomes in patients with coronary artery disease

Objective

Patients with CAD have substantial residual risk of mortality, and whether hitherto unknown small-molecule metabolites and metabolic pathways contribute to this risk is unclear. We sought to determine the predictive value of plasma metabolomic profiling in patients with CAD.

Approach and results

Untargeted high-resolution plasma metabolomic profiling of subjects undergoing coronary angiography was performed using liquid chromatography/mass spectrometry. Metabolic features and pathways associated with mortality were identified in 454 subjects using metabolome-wide association studies and Mummichog, respectively, and validated in 322 subjects. A metabolomic risk score comprising of log-transformed HR estimates of metabolites that associated with mortality and passed LASSO regression was created and its performance validated. In 776 subjects (66.8 years, 64% male, 17% Black), 433 and 357 features associated with mortality (FDR-adjusted q<0.20); and clustered into 21 and 9 metabolic pathways in first and second cohorts, respectively. Six pathways (urea cycle/amino group, tryptophan, aspartate/asparagine, lysine, tyrosine, and carnitine shuttle) were common. A metabolomic risk score comprising of 7 metabolites independently predicted mortality in the second cohort (HR per 1-unit increase 2.14, 95%CI 1.62, 2.83). Adding the score to a model of clinical predictors improved risk discrimination (delta C-statistic 0.039, 95%CI -0.006, 0.086; and Integrated Discrimination Index 0.084, 95%CI 0.030, 0.151) and reclassification (continuous Net Reclassification Index 23.3%, 95%CI 7.9%, 38.2%).

Conclusions

Differential regulation of six metabolic pathways involved in myocardial energetics and systemic inflammation is independently associated with mortality in patients with CAD. A novel risk score consisting of representative metabolites is highly predictive of mortality.

Oxidative Stress-Responsive MicroRNAs in Heart Injury

Reactive oxygen species (ROS) are important molecules in the living organisms as a part of many signaling pathways. However, if overproduced, they also play a significant role in the development of cardiovascular diseases, such as arrhythmia, cardiomyopathy, ischemia/reperfusion injury (e.g., myocardial infarction and heart transplantation), and heart failure. As a result of oxidative stress action, apoptosis, hypertrophy, and fibrosis may occur. MicroRNAs (miRNAs) represent important endogenous nucleotides that regulate many biological processes, including those involved in heart damage caused by oxidative stress. Oxidative stress can alter the expression level of many miRNAs. These changes in miRNA expression occur mainly via modulation of nuclear factor erythroid 2-related factor 2 (Nrf2), sirtuins, calcineurin/nuclear factor of activated T cell (NFAT), or nuclear factor kappa B (NF-κB) pathways. Up until now, several circulating miRNAs have been reported to be potential biomarkers of ROS-related cardiac diseases, including myocardial infarction, hypertrophy, ischemia/reperfusion, and heart failure, such as miRNA-499, miRNA-199, miRNA-21, miRNA-144, miRNA-208a, miRNA-34a, etc. On the other hand, a lot of studies are aimed at using miRNAs for therapeutic purposes. This review points to the need for studying the role of redox-sensitive miRNAs, to identify more effective biomarkers and develop better therapeutic targets for oxidative-stress-related heart diseases.

Mitochondrial Dysfunction in Cardiac Surgery

Mitochondria are key to the cellular response to energetic demand, but are also vital to reactive oxygen species signaling, calcium hemostasis, and regulation of cell death. Cardiac surgical patients with diabetes, heart failure, advanced age, or cardiomyopathies may have underlying mitochondrial dysfunction or be more sensitive to perioperative mitochondrial injury. Mitochondrial dysfunction, due to ischemia/reperfusion injury and an increased systemic inflammatory response due to exposure to cardiopulmonary bypass and surgical tissue trauma, impacts myocardial contractility and predisposes to arrhythmias. Strategies for perioperative mitochondrial protection and recovery include both well-established cardioprotective protocols and targeted therapies that remain under investigation.

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.

'Conditioning' the heart during surgery

Coronary heart disease (CHD) is the leading cause of death worldwide. Coronary artery bypass graft (CABG) surgery remains the procedure of choice for coronary artery revascularisation in a large number of patients with severe CHD. However, the profile of patients undergoing CABG surgery is changing with increasingly higher-risk patients being operated upon, resulting in significant morbidity and mortality in this patient group. Myocardial injury sustained during cardiac surgery, most of which can be attributed to acute myocardial ischaemia-reperfusion injury, is associated with worse short-term and long-term clinical outcomes. Clearly, new treatment strategies are required to protect the heart during cardiac surgery in terms of reducing myocardial injury and preserving left ventricular systolic function, such that clinical outcomes can be improved. 'Conditioning' the heart to harness its endogenous cardioprotective capabilities using either brief ischaemia or pharmacological agents, provides a potentially novel approach to myocardial protection during cardiac surgery, and is the subject of this review article.

Effect of cardioplegic and organ preservation solutions and their components on coronary endothelium-derived relaxing factors

Cardioplegic (and organ preservation) solutions were initially designed to protect the myocardium (cardiac myocytes) during cardiac operation (and heart transplantation). Because of differences between cardiac myocytes and vascular (endothelial and smooth muscle) cells in structure and function, the solutions may have an adverse effect on coronary vascular cells. However, such effect is often complicated by many other factors such as ischemia-reperfusion injury, temperature, and perfusion pressure or duration. To evaluate the effect of a solution on the coronary endothelial function, a number of points should be taken into consideration. First, the overall effect on endothelium should be identified. Second, the effect of the solution on the individual endothelium-derived relaxing factors (nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor) must be distinguished. Third, the effect of each major component of the solution should be investigated. Lastly, the effect of a variety of new additives in the solution may be studied. Based on available literature these issues are reviewed to provide information for further development of cardioplegic or organ preservation solutions.

Endothelial Function Related to Vascular Tone in Cardiac Surgery

Vascular endothelium has multiple functions including regulating of vascular tone, preventing platelet aggregation, anti-proliferation, etc. An intact endothelial function is essential to the maintenance of an adequate vascular tone, to prevent platelet aggregation in the intimal surface of blood vessels, to prevent smooth muscle proliferation, and to prevent atherosclerosis. This review focuses on endothelial function related to the vascular tone in cardiac surgery. The review is composed by three sections. In the first section, normal endothelial function related to vascular tone is described. In the second section, coronary endothelial function related to cardiac arrest and cardioplegic exposure is reviewed. In the third section, the endothelial function in the coronary bypass grafts is summarised. It is particularly important to understand that coronary endothelial dysfunction may be one of the major causes of low perfusion of the myocardium after cardiac arrest or donor heart preservation. Further, endothelium plays a major role in the maintenance of vascular tone and in the long-term patency of CABG grafts. The characteristics of endothelium in arterial and venous grafts and the correlation to the long-term patency are now more understood. A number of methods have been suggested to protect endothelial function in either coronary circulation or in coronary artery bypass grafts during cardiac surgery but further investigations in this field are warranted.

Macromolecular oxidation in anisotonic suspensions of mouse spleen cells

Macromolecular oxidative alterations have been analysed in vitro in anisotonic suspensions of mouse splenocytes. Both hypertonicity and hypotonicity induced the generation of thiobarbituric acid reactive species (TBARS) and carbonylation of the proteins, which took place along with cell death. Addition of antioxidants partially inhibited oxidative changes in isotonic and hypotonic suspensions. Anisotonic shock of mouse splenocytes proved to be an inducer of oxidative stress. The oxidative macromolecular alterations might contribute to pathogenesis of cell death caused by osmotic stress.

Microperfusion techniques for long-term hypothermic preservation

BACKGROUND

The aim of this study was to compare several methods of hypothermic heart preservation.

METHODS

We preserved isolated pig hearts for 24 hours in cold cardioplegia (4 degrees C), using either continuous microperfusion (Group I) or simple storage (Group II), and with a new preservative solution (NPS, groups IA and IIA) vs St. Thomas' solution (groups IB and IIB). The main characteristics of the NPS include (1) prevention of cell swelling with polyethelene glycol (PEG), (2) low calcium and magnesium, and (3) presence of metabolic substrates, such as glucose, insulin, pyruvate, aspartate, alanyl-glutamine, and membrane stabilization compounds such as ethanol and chlorpromazine.

RESULTS

The 4 above groups were compared with hearts harvested and immediately reperfused (control group). During preservation, only Group IB showed significant edema (40% +/- 8.4% water gain). Adenylate charge was 25% to 50% higher in microperfused Groups IA and IB (0.678 +/- 0.049 and 0.795 +/- 0.071, respectively) as compared with simple-storage groups IIA and IIB (0.605 +/- 0.048 and 0.524 +/- 0.160, respectively). Ultrastructural analysis showed that tissue injury occurred mainly in Group IIB (altered mitochondria, chromatin clumping). Functional data showed better recovery of NPS groups as compared with St. Thomas groups: coronary flow was identical in Group IB and control (57.8 +/- 22 and 56.6 +/- 14 ml/min/100 g, respectively), and in IA > IB (p < 0.001) and IIA > IIB (p < 0.01); the rate pressure products were higher in NPS groups compared with St. Thomas groups (IA > IB, p < 0.01); IIA > IIB, p < 0.05).

CONCLUSIONS

The microperfusion method associated with the NPS provides excellent protection in long-term hypothermic heart preservation.

Effect of regular training on the myocardial and plasma concentrations of taurine and alpha-amino acids in thoroughbred horses

Exercise induces significant changes in the free intracellular amino acid pool in skeletal muscle but little is known of whether such changes also occur in cardiac muscle. In this study the effect of regular exercise on the size and the constituents of the free amino acid pool in the hearts and in the plasma of thoroughbred horses was investigated. The total free intracellular amino acid pool in the hearts of control horses was 30.9 +/- 1.2 mumol/g wet weight (n = 6). Glutamine but not taurine was present at the highest concentration (13.5 +/- 0.9 and 7.7 +/- 0.69 mumol/g wet weight for glutamine and taurine respectively). As for the rest of the amino acids in the pool, only glutamate and alanine were present at levels greater than 1 mumol/g wet weight (4.6 +/- 0.25 and 1.7 +/- 0.14 for glutamate and alanine respectively). The tissue to plasma ratio was highest for taurine at 155, followed by glutamate at 111, aspartate and glutamine at 37, alanine at 5.8 and ratios of less than 3 for the rest of the amino acids. The total free intracellular amino acid pool in the hearts of exercised horses was slightly but not significantly lower than control (28.1 +/- 1.1 mumol/g wet weight, n = 6). Regular exercise increased the intracellular concentration of threonine, valine, isoleucine, leucine and phenylalanine but was only significant (p < 0.05) for threonine. This work has documented the profile of taurine and protein amino acids in the heart and in the plasma of thoroughbred horses and showed that in contrast to skeletal muscle, heart muscle does not show major changes in amino acids during regular exercise.

Intermittent antegrade hyperkalaemic warm blood cardioplegia supplemented with magnesium prevents myocardial substrate derangement in patients undergoing coronary artery bypass surgery

OBJECTIVE

The influence of the addition of magnesium on myocardial protection with intermittent antegrade warm blood hyperkalaemic cardioplegia in patients undergoing coronary artery surgery was investigated and compared with intermittent antegrade warm blood hyperkalaemic cardioplegia only.

METHODS

Twenty-three patients undergoing primary elective coronary revascularization were randomized to one of two different techniques of myocardial protection. In the first group, myocardial protection was induced using intermittent antegrade warm blood hyperkalaemic cardioplegia. In the second group, the same technique was used except that magnesium was added to the cardioplegia. Intracellular substrates (ATP, lactate and amino acids) were measured in left ventricular biopsies collected 5 min after institution of cardiopulmonary bypass, after 30 min of ischaemic arrest and 20 min after reperfusion.

RESULTS

There were no significant changes in the intracellular concentration of ATP or free amino acid pool in biopsies taken at the end of the period of myocardial ischaemia. However, the addition of magnesium prevented the significant increase in the intracellular concentration of lactate seen with intermittent antegrade warm blood hyperkalaemic cardioplegia. Upon reperfusion there was a significant fall in ATP and amino acid concentration when the technique of intermittent antegrade warm blood hyperkalaemic cardioplegia was used but not when magnesium was added to the cardioplegia.

CONCLUSIONS

This work shows that intermittent antegrade warm blood hyperkalaemic cardioplegia supplemented with magnesium prevents substrate derangement early after reperfusion.

Amino acid release during volume regulation by cardiac cells: cellular mechanisms

Mechanisms of amino acid efflux during volume regulation in hypoosmotically treated isolated rat hearts were studied by collecting the coronary artery perfusate and analysis by high pressure liquid chromatography. Hypoosmotic stress resulted in marked percentage increases in perfusate taurine, aspartate and glutamate levels, smaller increases in phosphoethanolamine, glycine and alanine and non-significant increases in serine and glutamine. Amino acid levels declined during reperfusion with isosmotic perfusate. The anion channel blocker 4-acetamido-4-isothiocyanostilbene-2:2'-disulfonic acid (SITS, 500 microM) significantly reduced hypoosmotic release of taurine, aspartate, glutamate and glycine. Furosemide reduced hypoosmotically-evoked releases of taurine, glycine, alanine and phosphoethanolamine. The polyunsaturated amino acids, arachidonic and linoleic also reduced amino acid efflux. Phospholipase A2 inhibition with 7,7-dimethyleicosadienoic acid (DEDA, 2 microM) reduced osmotically-evoked releases of taurine, aspartate and glutamate. 4-Bromophenacyl bromide (1 microM) inhibited osmotically-evoked release of glutamate and glycine. Combined applications of SITS + DEDA markedly reduced osmotically evoked release of all eight amino acids. Glutamate and aspartate effluxes were not inhibited by the glutamate transport inhibitor dihydrokainic acid (1 mM). These results indicate that the hypoosmotic stress, by inducing cell swelling, can initiate an amino acid efflux as part of a regulatory volume decrease. An opening of anion-permeant channels and phospholipase activation appear to be involved in the regulatory volume decrease phenomenon.

Initial experience using an intraluminal shunt during revascularization of the beating heart

BACKGROUND

For decades, surgeons have relied on extracorporeal circulation and induced cardiac asystole to provide a bloodless, motionless field in which to construct coronary bypass grafts. However, the technique of coronary grafting without heart-lung support is now being revitalized. The current resurgence of off-pump coronary artery bypass grafting and the advent of less invasive incisions make it imperative that technical advances be applied to maximize the safety of these procedures.

METHODS

This report describes an inexpensive intraluminal shunt that maintains coronary perfusion, prevents ischemia, reduces backbleeding, and molds the suture line to prevent accidental missuturing of the posterior coronary wall.

RESULTS

In 63 patients, saphenous grafts were placed to the left anterior descending (49), diagonal (9), and right coronary artery (27) without extracorporeal circulation using an intraluminal shunt. There were no deaths (0% mortality) and one perioperative infarction (1.5%). Complication and graft patency rates were comparable with those obtained by conventional techniques.

CONCLUSIONS

Temporary intraluminal shunting greatly facilitates the surgeons' operative environment by permitting safe and precise construction of coronary artery grafts on the beating heart in a bloodless field. Intraluminal shunting may have future implications on the ability to perform safe and reproducible grafting on the beating heart through minimally invasive or endoscopic approaches.

Myocardial protection for cardiac surgery: the nursing perspective

The advancement of myocardial protection techniques is considered to be the most instrumental in achieving successful cardiac surgical outcomes. Although many issues complicate the efficacy of myocardial protection, warm cardioplegia is instituted more often as a better myocardial protection method for patients undergoing cardiac surgery. Understanding differences in patient response between warm and cold cardioplegia is essential for development of appropriate nursing intervention strategies and prevention of postoperative complications. Advanced practice nurses in cardiac surgical settings must continue to evaluate metabolic, functional, and hemodynamic variations of patients with different cardioplegia for positive patient outcomes.