Reduced myocardial creatine kinase flux in human myocardial infarction: an in vivo phosphorus magnetic resonance spectroscopy study

Regionally resolved cardiac metabolism using a dipole-loop array coil for 7 T 31P-MRSI

PURPOSE

We introduce a novel commercial phosphorus-31 (31P) dipole-loop array coil, describing the coil hardware and testing its performance on phantoms. We used this coil to assess cardiac metabolism per region in healthy volunteers.

METHODS

B1 + field maps were simulated and compared to maps measured with a set of CSI sequences with varying voltages. Seventeen volunteers were scanned with 7 T phosphorus-31 magnetic resonance spectroscopic imaging (31P-MRSI). Reproducibility was assessed in nine of these volunteers. Strain was measured for six of these volunteers at 3 T.

RESULTS

Blood- and saturation-corrected Phosphocreatine/γ-adenosine triphosphate (PCr/ATP) ratios were measured for four regions of the left ventricle: 1.86 in septum, 2.25 in anterior wall, 1.41 in inferior wall, and 1.53 in lateral wall, respectively. These are in the expected range compared to previous studies. B1 + maps show good signal uniformity around the position of the heart (0.13 ± 0.06 μT/sqrt(W)). Intrasession and intersession coefficients of reproducibility were 0.22-0.88 and 0.29-0.79, respectively. Linear modeling shows that regional PCr/γATP correlates with circumferential strain but not radial strain. This requires corroboration by a larger study including patients with impaired function and energetics.

CONCLUSION

Dipole-loop array coils present a promising new approach for human cardiac 31P-MRSI at 7 T. Their favorable B1 + uniformity at depth and specific absorption rate over loop arrays and improved SNR when combined with loops for reception could be beneficial for further clinical studies measuring energetics by 31P-MRSI at 7 T. The new capability to assess PCr/γATP ratios across the whole left ventricle could enable clinical studies to investigate regional changes in cardiac energetics for the first time.

Mitochondrial Dysfunction in Cardiac Disease: The Fort Fell

Myocardial cells and the extracellular matrix achieve their functions through the availability of energy. In fact, the mechanical and electrical properties of the heart are heavily dependent on the balance between energy production and consumption. The energy produced is utilized in various forms, including kinetic, dynamic, and thermal energy. Although total energy remains nearly constant, the contribution of each form changes over time. Thermal energy increases, while dynamic and kinetic energy decrease, ultimately becoming insufficient to adequately support cardiac function. As a result, toxic byproducts, unfolded or misfolded proteins, free radicals, and other harmful substances accumulate within the myocardium. This leads to the failure of crucial processes such as myocardial contraction-relaxation coupling, ion exchange, cell growth, and regulation of apoptosis and necrosis. Consequently, both the micro- and macro-architecture of the heart are altered. Energy production and consumption depend on the heart's metabolic resources and the functional state of the cardiac structure, including cardiomyocytes, non-cardiomyocyte cells, and their metabolic and energetic behavior. Mitochondria, which are intracellular organelles that produce more than 95% of ATP, play a critical role in fulfilling all these requirements. Therefore, it is essential to gain a deeper understanding of their anatomy, function, and homeostatic properties.

Heart Failure: A Deficiency of Energy-A Path Yet to Discover and Walk

Heart failure is a complex syndrome and our understanding and therapeutic approach relies mostly on its phenotypic presentation. Notably, the heart is characterized as the most energy-consuming organ, being both a producer and consumer, in order to satisfy multiple cardiac functions: ion exchange, electromechanical coordination, excitation-contraction coupling, etc. By obtaining further knowledge of the cardiac energy field, we can probably better characterize the basic pathophysiological events occurring in heart disease patients and understand the metabolic substance changes, the relationship between the alteration of energy production/consumption, and hence energetic deficiency not only in the heart as a whole but in every single cardiac territory, which will hopefully provide us with the opportunity to uncover the beginning of the heart failure process. In this respect, using (a) newer imaging techniques, (b) biomedicine, (c) nanotechnology, and (d) artificial intelligence, we can gain a deeper understanding of this complex syndrome. This, in turn, can lead to earlier and more effective therapeutic approaches, ultimately improving human health. To date, the scientific community has not given sufficient attention to the energetic starvation model. In our view, this review aims to encourage scientists and the medical community to conduct studies for a better understanding and treatment of this syndrome.

Human cardiac metabolism

The heart is the most metabolically active organ in the human body, and cardiac metabolism has been studied for decades. However, the bulk of studies have focused on animal models. The objective of this review is to summarize specifically what is known about cardiac metabolism in humans. Techniques available to study human cardiac metabolism are first discussed, followed by a review of human cardiac metabolism in health and in heart failure. Mechanistic insights, where available, are reviewed, and the evidence for the contribution of metabolic insufficiency to heart failure, as well as past and current attempts at metabolism-based therapies, is also discussed.

Association Between Type 2 Diabetes and Changes in Myocardial Structure, Contractile Function, Energetics, and Blood Flow Before and After Aortic Valve Replacement in Patients With Severe Aortic Stenosis

BACKGROUND

Type 2 diabetes (T2D) is associated with an increased risk of left ventricular dysfunction after aortic valve replacement (AVR) in patients with severe aortic stenosis (AS). Persistent impairments in myocardial energetics and myocardial blood flow (MBF) may underpin this observation. Using phosphorus magnetic resonance spectroscopy and cardiovascular magnetic resonance, this study tested the hypothesis that patients with severe AS and T2D (AS-T2D) would have impaired myocardial energetics as reflected by the phosphocreatine to ATP ratio (PCr/ATP) and vasodilator stress MBF compared with patients with AS without T2D (AS-noT2D), and that these differences would persist after AVR.

METHODS

Ninety-five patients with severe AS without coronary artery disease awaiting AVR (30 AS-T2D and 65 AS-noT2D) were recruited (mean, 71 years of age [95% CI, 69, 73]; 34 [37%] women). Thirty demographically matched healthy volunteers (HVs) and 30 patients with T2D without AS (T2D controls) were controls. One month before and 6 months after AVR, cardiac PCr/ATP, adenosine stress MBF, global longitudinal strain, NT-proBNP (N-terminal pro-B-type natriuretic peptide), and 6-minute walk distance were assessed in patients with AS. T2D controls underwent identical assessments at baseline and 6-month follow-up. HVs were assessed once and did not undergo 6-minute walk testing.

RESULTS

Compared with HVs, patients with AS (AS-T2D and AS-noT2D combined) showed impairment in PCr/ATP (mean [95% CI]; HVs, 2.15 [1.89, 2.34]; AS, 1.66 [1.56, 1.75]; P<0.0001) and vasodilator stress MBF (HVs, 2.11 mL min g [1.89, 2.34]; AS, 1.54 mL min g [1.41, 1.66]; P<0.0001) before AVR. Before AVR, within the AS group, patients with AS-T2D had worse PCr/ATP (AS-noT2D, 1.74 [1.62, 1.86]; AS-T2D, 1.44 [1.32, 1.56]; P=0.002) and vasodilator stress MBF (AS-noT2D, 1.67 mL min g [1.5, 1.84]; AS-T2D, 1.25 mL min g [1.22, 1.38]; P=0.001) compared with patients with AS-noT2D. Before AVR, patients with AS-T2D also had worse PCr/ATP (AS-T2D, 1.44 [1.30, 1.60]; T2D controls, 1.66 [1.56, 1.75]; P=0.04) and vasodilator stress MBF (AS-T2D, 1.25 mL min g [1.10, 1.41]; T2D controls, 1.54 mL min g [1.41, 1.66]; P=0.001) compared with T2D controls at baseline. After AVR, PCr/ATP normalized in patients with AS-noT2D, whereas patients with AS-T2D showed no improvements (AS-noT2D, 2.11 [1.79, 2.43]; AS-T2D, 1.30 [1.07, 1.53]; P=0.0006). Vasodilator stress MBF improved in both AS groups after AVR, but this remained lower in patients with AS-T2D (AS-noT2D, 1.80 mL min g [1.59, 2.0]; AS-T2D, 1.48 mL min g [1.29, 1.66]; P=0.03). There were no longer differences in PCr/ATP (AS-T2D, 1.44 [1.30, 1.60]; T2D controls, 1.51 [1.34, 1.53]; P=0.12) or vasodilator stress MBF (AS-T2D, 1.48 mL min g [1.29, 1.66]; T2D controls, 1.60 mL min g [1.34, 1.86]; P=0.82) between patients with AS-T2D after AVR and T2D controls at follow-up. Whereas global longitudinal strain, 6-minute walk distance, and NT-proBNP all improved after AVR in patients with AS-noT2D, no improvement in these assessments was observed in patients with AS-T2D.

CONCLUSIONS

Among patients with severe AS, those with T2D demonstrate persistent abnormalities in myocardial PCr/ATP, vasodilator stress MBF, and cardiac contractile function after AVR; AVR effectively normalizes myocardial PCr/ATP, vasodilator stress MBF, and cardiac contractile function in patients without T2D.

Cardiac 31P MR spectroscopy: development of the past five decades and future vision-will it be of diagnostic use in clinics?

In the past five decades, the use of the magnetic resonance (MR) technique for cardiovascular diseases has engendered much attention and raised the opportunity that the technique could be useful for clinical applications. MR has two arrows in its quiver: One is magnetic resonance imaging (MRI), and the other is magnetic resonance spectroscopy (MRS). Non-invasively, highly advanced MRI provides unique and profound information about the anatomical changes of the heart. Excellently developed MRS provides irreplaceable and insightful evidence of the real-time biochemistry of cardiac metabolism of underpinning diseases. Compared to MRI, which has already been successfully applied in routine clinical practice, MRS still has a long way to travel to be incorporated into routine diagnostics. Considering the exceptional potential of ³¹P MRS to measure the real-time metabolic changes of energetic molecules qualitatively and quantitatively, how far its powerful technique should be waited before a successful transition from "bench-to-bedside" is enticing. The present review highlights the seminal studies on the chronological development of cardiac ³¹P MRS in the past five decades and the future vision and challenges to incorporating it for routine diagnostics of cardiovascular disease.

Phosphorus Magnetic Resonance Spectroscopy (31P MRS) and Cardiovascular Disease: The Importance of Energy

Background and Objectives : The heart is the organ with the highest metabolic demand in the body, and it relies on high ATP turnover and efficient energy substrate utilisation in order to function normally. The derangement of myocardial energetics may lead to abnormalities in cardiac metabolism, which herald the symptoms of heart failure (HF). In addition, phosphorus magnetic resonance spectroscopy (31P MRS) is the only available non-invasive method that allows clinicians and researchers to evaluate the myocardial metabolic state in vivo. This review summarises the importance of myocardial energetics and provides a systematic review of all the available research studies utilising 31P MRS to evaluate patients with a range of cardiac pathologies. Materials and Methods : We have performed a systematic review of all available studies that used 31P MRS for the investigation of myocardial energetics in cardiovascular disease. Results : A systematic search of the Medline database, the Cochrane library, and Web of Science yielded 1092 results, out of which 62 studies were included in the systematic review. The 31P MRS has been used in numerous studies and has demonstrated that impaired myocardial energetics is often the beginning of pathological processes in several cardiac pathologies. Conclusions : The 31P MRS has become a valuable tool in the understanding of myocardial metabolic changes and their impact on the diagnosis, risk stratification, and prognosis of patients with cardiovascular diseases.

Myocardial ATP depletion detected noninvasively predicts sudden cardiac death risk in heart failure patients

BACKGROUND

Sudden cardiac death (SCD) remains a worldwide public health problem in need of better noninvasive predictive tools. Current guidelines for primary preventive SCD therapies, such as implantable cardioverter defibrillators (ICDs), are based on left ventricular ejection fraction (LVEF), but these guidelines are imprecise: fewer than 5% of ICDs deliver lifesaving therapy per year. Impaired cardiac metabolism and ATP depletion cause arrhythmias in experimental models, but to our knowledge a link between arrhythmias and cardiac energetic abnormalities in people has not been explored, nor has the potential for metabolically predicting clinical SCD risk.

METHODS

We prospectively measured myocardial energy metabolism noninvasively with phosphorus magnetic resonance spectroscopy in patients with no history of significant arrhythmias prior to scheduled ICD implantation for primary prevention in the setting of reduced LVEF (≤35%).

RESULTS

By 2 different analyses, low myocardial ATP significantly predicted the composite of subsequent appropriate ICD firings for life-threatening arrhythmias and cardiac death over approximately 10 years. Life-threatening arrhythmia risk was approximately 3-fold higher in patients with low ATP and independent of established risk factors, including LVEF. In patients with normal ATP, rates of appropriate ICD firings were several-fold lower than reported rates of ICD complications and inappropriate firings.

CONCLUSION

To the best of our knowledge, these are the first data linking in vivo myocardial ATP depletion and subsequent significant arrhythmic events in people, suggesting an energetic component to clinical life-threatening ventricular arrhythmogenesis. The findings support investigation of metabolic strategies that limit ATP loss to treat or prevent life-threatening cardiac arrhythmias and herald noninvasive metabolic imaging as a complementary SCD risk stratification tool.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00181233.

FUNDING

This work was supported by the DW Reynolds Foundation, the NIH (grants HL61912, HL056882, HL103812, HL132181, HL140034), and Russell H. Morgan and Clarence Doodeman endowments at Johns Hopkins.

CARDIOKIN1: Computational Assessment of Myocardial Metabolic Capability in Healthy Controls and Patients With Valve Diseases

Supplemental Digital Content is available in the text.

Background:

Many heart diseases can result in reduced pumping capacity of the heart muscle. A mismatch between ATP demand and ATP production of cardiomyocytes is one of the possible causes. Assessment of the relation between myocardial ATP production (MV_ATP) and cardiac workload is important for better understanding disease development and choice of nutritional or pharmacologic treatment strategies. Because there is no method for measuring MV_ATP in vivo, the use of physiology-based metabolic models in conjunction with protein abundance data is an attractive approach.

METHOD:

We developed a comprehensive kinetic model of cardiac energy metabolism (CARDIOKIN1) that recapitulates numerous experimental findings on cardiac metabolism obtained with isolated cardiomyocytes, perfused animal hearts, and in vivo studies with humans. We used the model to assess the energy status of the left ventricle of healthy participants and patients with aortic stenosis and mitral valve insufficiency. Maximal enzyme activities were individually scaled by means of protein abundances in left ventricle tissue samples. The energy status of the left ventricle was quantified by the ATP consumption at rest (MV_ATP[rest]), at maximal workload (MV_ATP[max]), and by the myocardial ATP production reserve, representing the span between MV_ATP(rest) and MV_ATP(max).

Results:

Compared with controls, in both groups of patients, MV_ATP(rest) was increased and MV_ATP(max) was decreased, resulting in a decreased myocardial ATP production reserve, although all patients had preserved ejection fraction. The variance of the energetic status was high, ranging from decreased to normal values. In both patient groups, the energetic status was tightly associated with mechanic energy demand. A decrease of MV_ATP(max) was associated with a decrease of the cardiac output, indicating that cardiac functionality and energetic performance of the ventricle are closely coupled.

Conclusions:

Our analysis suggests that the ATP-producing capacity of the left ventricle of patients with valvular dysfunction is generally diminished and correlates positively with mechanical energy demand and cardiac output. However, large differences exist in the energetic state of the myocardium even in patients with similar clinical or image-based markers of hypertrophy and pump function.

Registration:

URL: https://www.clinicaltrials.gov; Unique identifiers: NCT03172338 and NCT04068740.

Rapid, B 1 -insensitive, dual-band quasi-adiabatic saturation transfer with optimal control for complete quantification of myocardial ATP flux

PURPOSE

Phosphorus saturation-transfer experiments can quantify metabolic fluxes noninvasively. Typically, the forward flux through the creatine kinase reaction is investigated by observing the decrease in phosphocreatine (PCr) after saturation of γ-ATP. The quantification of total ATP utilization is currently underexplored, as it requires simultaneous saturation of inorganic phosphate ( P i ) and PCr. This is challenging, as currently available saturation pulses reduce the already-low γ-ATP signal present.

METHODS

Using a hybrid optimal-control and Shinnar-Le Roux method, a quasi-adiabatic RF pulse was designed for the dual saturation of PCr and P i to enable determination of total ATP utilization. The pulses were evaluated in Bloch equation simulations, compared with a conventional hard-cosine DANTE saturation sequence, before being applied to perfused rat hearts at 11.7 T.

RESULTS

The quasi-adiabatic pulse was insensitive to a >2.5-fold variation in B 1 , producing equivalent saturation with a 53% reduction in delivered pulse power and a 33-fold reduction in spillover at the minimum effective B 1 . This enabled the complete quantification of the synthesis and degradation fluxes for ATP in 30-45 minutes in the perfused rat heart. While the net synthesis flux (4.24 ± 0.8 mM/s, SEM) was not significantly different from degradation flux (6.88 ± 2 mM/s, P = .06) and both measures are consistent with prior work, nonlinear error analysis highlights uncertainties in the P_i -to-ATP measurement that may explain a trend suggesting a possible imbalance.

CONCLUSIONS

This work demonstrates a novel quasi-adiabatic dual-saturation RF pulse with significantly improved performance that can be used to measure ATP turnover in the heart in vivo.

Measuring Myocardial Energetics with Cardiovascular Magnetic Resonance Spectroscopy

The heart has the highest energy demands per gram of any organ in the body and energy metabolism fuels normal contractile function. Metabolic inflexibility and impairment of myocardial energetics occur with several common cardiac diseases, including ischemia and heart failure. This review explores several decades of innovation in cardiac magnetic resonance spectroscopy modalities and their use to noninvasively identify and quantify metabolic derangements in the normal, failing, and diseased heart. The implications of this noninvasive modality for predicting significant clinical outcomes and guiding future investigation and therapies to improve patient care are discussed.

Metabolomic Analysis of the Ameliorative Effect of Enhanced Proline Metabolism on Hypoxia-Induced Injury in Cardiomyocytes

Background

Coronary heart disease is currently the leading cause of death in humans. Its poor prognosis and high mortality are associated with myocardial ischemia, which leads to metabolic disorder-related cardiomyocyte apoptosis and reactive oxygen species (ROS) production. Previous cardiovascular metabolomics studies in humans and mice have shown that proline metabolism is severely altered after cardiomyocyte hypoxia. Proline dehydrogenase (PRODH) is located on the inner mitochondrial membrane and is an enzyme that catalyzes the first step of proline catabolism, which plays an important role in improving the cellular redox state. In vitro oxygen-glucose deprivation can mimic in vivo myocardial ischemic injury. This study is aimed at investigating whether enhancing proline metabolism by overexpressing PRODH can ameliorate hypoxia-induced injury in cardiomyocytes and to reveal the related altered metabolites and mechanistic pathway via untargeted metabolomics analysis.

Methods and Results

First, through public database analysis and RT-qPCR and western blot analyses in a cardiomyocyte hypoxia model, we found that the expression of the proline-degrading enzyme PRODH was downregulated after myocardial infarction and hypoxia exposure. Second, LDH assays, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), DHE staining, flow cytometric apoptosis analysis with DCFH and Annexin V-FITC/PI, and western blot analysis were used to assess the injury level in cardiomyocytes. Enhanced proline metabolism induced by PRODH overexpression reduced the levels of reactive oxidative stress and apoptosis, whereas PRODH knockdown had the opposite effects. Third, untargeted metabolomics analysis revealed that the protective effect was associated with significant changes in metabolism linked to sphingolipid signaling pathways, unsaturated fatty acid biosynthesis, phosphocreatine, glutathione disulfide, aminoacyl-tRNA biosynthesis, and ABC transporters.

Conclusions

Our study demonstrated a protective effect of enhanced proline metabolism in cardiomyocytes under hypoxia, providing a novel strategy for exploring new treatments for coronary heart disease.

Non-invasive investigation of myocardial energetics in cardiac disease using 31P magnetic resonance spectroscopy

Cardiac metabolism and function are intrinsically linked. High-energy phosphates occupy a central and obligate position in cardiac metabolism, coupling oxygen and substrate fuel delivery to the myocardium with external work. This insight underlies the widespread clinical use of ischaemia testing. However, other deficits in high-energy phosphate metabolism (not secondary to supply-demand mismatch of oxygen and substrate fuels) may also be documented, and are of particular interest when found in the context of structural heart disease. This review introduces the scope of deficits in high-energy phosphate metabolism that may be observed in the myocardium, how to assess for them, and how they might be interpreted.

Cardiac Energetics in Patients With Aortic Stenosis and Preserved Versus Reduced Ejection Fraction

Supplemental Digital Content is available in the text.

Why some but not all patients with severe aortic stenosis (SevAS) develop otherwise unexplained reduced systolic function is unclear. We investigate the hypothesis that reduced creatine kinase (CK) capacity and flux is associated with this transition.

Creatine kinase rate constant in the human heart at 7T with 1D-ISIS/2D CSI localization

PURPOSE

Creatine Kinase (CK) reaction plays an important role in energy metabolism and estimate of its reaction rate constant in heart provides important insight into cardiac energetics. Fast saturation transfer method ([Formula: see text] nominal) to measure CK reaction rate constant (kf) was previously demonstrated in open chest swine hearts. The goal of this work is to further develop this method for measuring the kf in human myocardium at 7T. [Formula: see text] approach is combined with 1D-ISIS/2D-CSI for in vivo spatial localization and myocardial CK forward rate constant was then measured in 7 volunteers at 7T.

METHODS

[Formula: see text] method uses two partially relaxed saturation transfer (ST) spectra and correction factor to determine CK rate constant. Correction factor is determined by numerical simulation of Bloch McConnell equations using known spin and experimental parameters. Optimal parameters and error estimate in calculation of CK reaction rate constant were determined by simulations. The technique was validated in calf muscles by direct comparison with saturation transfer measurements. [Formula: see text] pulse sequence was incorporated with 1D-image selected in vivo spectroscopy, combined with 2D-chemical shift spectroscopic imaging (1D-ISIS/2D-CSI) for studies in heart. The myocardial CK reaction rate constant was then measured in 7 volunteers.

RESULTS

Skeletal muscle kf determined by conventional approach and [Formula: see text] approach were the same 0.31 ± 0.02 s-1 and 0.30 ± 0.04 s-1 demonstrating the validity of the technique. Results are reported as mean ± SD. Myocardial CK reaction rate constant was 0.29 ± 0.05 s-1, consistent with previously reported studies.

CONCLUSION

[Formula: see text] method enables acquisition of 31P saturation transfer MRS under partially relaxed conditions and enables 2D-CSI of kf in myocardium. This work enables applications for in vivo CSI imaging of energetics in heart and other organs in clinically relevant acquisition time.

A Randomized, Double-Blind, Placebo-Controlled Study to Evaluate the Effectiveness of a Food Supplement Containing Creatine and D-Ribose Combined with a Physical Exercise Program in Increasing Stress Tolerance in Patients with Ischemic Heart Disease

The aim of this study is to establish whether a supplement of creatine and ribose combined with a physical exercise program can improve the total work capacity during exercise in a population of patients with known ischemic heart disease. A double-blind, six-month study was designed in which 53 patients were enrolled and randomized to take either a nutraceutical composition containing creatine, D-ribose, vitamin B₁, and vitamin B₆ (active treatment) or the placebo. Both the nutraceutical supplement and the placebo were supplied by Giellepi S.p.A. Health Science in Lissone, Italy. After six months of study, the cardiac double product at the peak of the load, the delta double product, and the chronotropic index were higher in the active treatment group than in the placebo group. We can conclude that a supplementation with creatine, D-ribose, vitamin B₁, and vitamin B₆, in addition to standard therapy and a physical exercise program, seems to be helpful in improving exercise tolerance compared to the placebo in a population with cardiovascular disease. However, this needs to be further studied, given that there is no clear evidence that the double product can be used as a surrogate measure of exercise tolerance.

Neural-network classification of cardiac disease from 31P cardiovascular magnetic resonance spectroscopy measures of creatine kinase energy metabolism

BACKGROUND

The heart's energy demand per gram of tissue is the body's highest and creatine kinase (CK) metabolism, its primary energy reserve, is compromised in common heart diseases. Here, neural-network analysis is used to test whether noninvasive phosphorus (31P) cardiovascular magnetic resonance spectroscopy (CMRS) measurements of cardiac adenosine triphosphate (ATP) energy, phosphocreatine (PCr), the first-order CK reaction rate kf, and the rate of ATP synthesis through CK (CK flux), can predict specific human heart disease and clinical severity.

METHODS

The data comprised the extant 178 complete sets of PCr and ATP concentrations, kf, and CK flux data from human CMRS studies performed on clinical 1.5 and 3 Tesla scanners. Healthy subjects and patients with nonischemic cardiomyopathy, dilated (DCM) or hypertrophic disease, New York Heart Association (NYHA) class I-IV heart failure (HF), or with anterior myocardial infarction are included. Three-layer neural-networks were created to classify disease and to differentiate DCM, hypertrophy and clinical NYHA class in HF patients using leave-one-out training. Network performance was assessed using 'confusion matrices' and 'area-under-the-curve' (AUC) analyses of 'receiver operating curves'. Possible methodological bias and network imbalance were tested by segregating 1.5 and 3 Tesla data, and by data augmentation by random interpolation of nearest neighbors, respectively.

RESULTS

The network differentiated healthy, HF and non-HF cardiac disease with an overall accuracy of 84% and AUC > 90% for each category using the four CK metabolic parameters, alone. HF patients with DCM, hypertrophy, and different NYHA severity were differentiated with ~ 80% overall accuracy independent of CMRS methodology.

CONCLUSIONS

While sample-size was limited in some sub-classes, a neural network classifier applied to noninvasive cardiac 31P CMRS data, could serve as a metabolic biomarker for common disease types and HF severity with clinically-relevant accuracy. Moreover, the network's ability to individually classify disease and HF severity using CK metabolism alone, implies an intimate relationship between CK metabolism and disease, with subtle underlying phenotypic differences that enable their differentiation.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00181259.

CEST MRI reveals a correlation between visceral fat mass and reduced myocardial creatine in obese individuals despite preserved ventricular structure and function

Systolic cardiac function is typically preserved in obese adults, potentially masking underlying declines in cardiomyocyte metabolism that may contribute to heart failure. We used chemical exchange saturation transfer (CEST) MRI, a sensitive method for measurement of myocardial creatine, to examine whether myocardial creatine levels correlate with cardiac structure, contractile function, or visceral fat mass in obese adults. In this study, obese (body mass index, BMI > 30, n = 20) and healthy (BMI < 25, n = 11) adults were examined with dual-energy x-ray absorptiometry to quantify fat masses. Cine MRI and myocardial tagging were performed at 1.5 T to measure ventricular structure and global function. CEST imaging with offsets in the range of ±10 parts per million (ppm) were performed in one mid-ventricular slice, where creatine CEST contrast was calculated at 1.8 ppm following field homogeneity correction. Ventricular structure, global function (ejection fraction 69.4 ± 4.3% healthy versus 69.6 ± 9.3% obese, NS), and circumferential strain (-17.0 ± 2.3% healthy versus -16.5 ± 1.5% obese, NS) and strain rate were preserved in obese adults. However, creatine CEST contrast was significantly reduced in obese adults (6.8 ± 1.3% healthy versus 4.1 ± 2.7% obese, p = 0.001). Creatine CEST contrast was inversely correlated with total body fat% (ρ = -0.45, p = 0.011), visceral fat mass (ρ = -0.58, p = 0.001), and septal wall thickness (ρ = -0.44, p = 0.013), but uncorrelated to ventricular function or contractile function. In conclusion, creatine CEST-MRI reveals a strong correlation between heightened body and visceral fat masses and reduced myocardial metabolic function that is independent of ventricular structure and global function in obese adults.

Localized rest and stress human cardiac creatine kinase reaction kinetics at 3 T

Changes in the kinetics of the creatine kinase (CK) shuttle are sensitive markers of cardiac energetics but are typically measured at rest and in the prone position. This study aims to measure CK kinetics during pharmacological stress at 3 T, with measurement in the supine position. A shorter "stressed saturation transfer" (StreST) extension to the triple repetition time saturation transfer (TRiST) method is proposed. We assess scanning in a supine position and validate the MR measurement against biopsy assay of CK activity. We report normal ranges of stress CK forward rate (kfCK ) for healthy volunteers and obese patients. TRiST measures kfCK in 40 min at 3 T. StreST extends the previously developed TRiST to also make a further kfCK measurement during <20 min of dobutamine stress. We test our TRiST implementation in skeletal muscle and myocardium in both prone and supine positions. We evaluate StreST in the myocardium of six healthy volunteers and 34 obese subjects. We validated MR-measured kfCK against biopsy assays of CK activity. TRiST kfCK values matched literature values in skeletal muscle (kfCK  = 0.25 ± 0.03 s-1 vs 0.27 ± 0.03 s-1 ) and myocardium when measured in the prone position (0.32 ± 0.15 s-1 ), but a significant difference was found for TRiST kfCK measured supine (0.24 ± 0.12 s-1 ). This difference was because of different respiratory- and cardiac-motion-induced B0 changes in the two positions. Using supine TRiST, cardiac kfCK values for normal-weight subjects were 0.15 ± 0.09 s-1 at rest and 0.17 ± 0.15 s-1 during stress. For obese subjects, kfCK was 0.16 ± 0.07 s-1 at rest and 0.17 ± 0.10 s-1 during stress. Rest myocardial kfCK and CK activity from LV biopsies of the same subjects correlated (R = 0.43, p = 0.03). We present an independent implementation of TRiST on the Siemens platform using a commercially available coil. Our extended StreST protocol enables cardiac kfCK to be measured during dobutamine-induced stress in the supine position.

Cardiac work is related to creatine kinase energy supply in human heart failure: a cardiovascular magnetic resonance spectroscopy study

BACKGROUND

It has been hypothesized that the supply of chemical energy may be insufficient to fuel normal mechanical pump function in heart failure (HF). The creatine kinase (CK) reaction serves as the heart's primary energy reserve, and the supply of adenosine triphosphate (ATP flux) it provides is reduced in human HF. However, the relationship between the CK energy supply and the mechanical energy expended has never been quantified in the human heart. This study tests whether reduced CK energy supply is associated with reduced mechanical work in HF patients.

METHODS

Cardiac mechanical work and CK flux in W/kg, and mechanical efficiency were measured noninvasively at rest using cardiac pressure-volume loops, magnetic resonance imaging and phosphorus spectroscopy in 14 healthy subjects and 27 patients with mild-to-moderate HF.

RESULTS

In HF, the resting CK flux (126 ± 46 vs. 179 ± 50 W/kg, p < 0.002), the average (6.8 ± 3.1 vs. 10.1 ± 1.5 W/kg, p  <0.001) and the peak (32 ± 14 vs. 48 ± 8 W/kg, p < 0.001) cardiac mechanical work-rates, as well as the cardiac mechanical efficiency (53% ± 16 vs. 79% ± 3, p < 0.001), were all reduced by a third compared to healthy subjects. In addition, cardiac CK flux correlated with the resting peak and average mechanical power (p < 0.01), and with mechanical efficiency (p = 0.002).

CONCLUSION

These first noninvasive findings showing that cardiac mechanical work and efficiency in mild-to-moderate human HF decrease proportionately with CK ATP energy supply, are consistent with the energy deprivation hypothesis of HF. CK energy supply exceeds mechanical work at rest but lies within a range that may be limiting with moderate activity, and thus presents a promising target for HF treatment.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00181259 .

Metabolic remodeling in hypertrophied and failing myocardium: a review

The energy starvation hypothesis proposes that maladaptive metabolic remodeling antedates, initiates, and maintains adverse contractile dysfunction in heart failure (HF). Better understanding of the cardiac metabolic phenotype and metabolic signaling could help identify the role metabolic remodeling plays within HF and the conditions known to transition toward HF, including "pathological" hypertrophy. In this review, we discuss metabolic phenotype and metabolic signaling in the contexts of pathological hypertrophy and HF. We discuss the significance of alterations in energy supply (substrate utilization, oxidative capacity, and phosphotransfer) and energy sensing using observations from human and animal disease models and models of manipulated energy supply/sensing. We aim to provide ways of thinking about metabolic remodeling that center around metabolic flexibility, capacity (reserve), and efficiency rather than around particular substrate preferences or transcriptomic profiles. We show that maladaptive metabolic remodeling takes multiple forms across multiple energy-handling domains. We suggest that lack of metabolic flexibility and reserve (substrate, oxidative, and phosphotransfer) represents a final common denominator ultimately compromising efficiency and contractile reserve in stressful contexts.

High-resolution 31 P echo-planar spectroscopic imaging in vivo at 7T

PURPOSE

Conventional ³¹ P chemical shift imaging is time-consuming and yields only limited spatial resolution. The purpose of this study was to demonstrate feasibility of ³¹ P echo-planar spectroscopic imaging (EPSI) in vivo at 7T.

METHODS

A 3D ³¹ P EPSI sequence with trapezoidal-shaped gradient pulses was implemented on a 7T MR scanner. To increase spectral width with reduced demand on gradient performance, a multishot approach was chosen. Acquisition weighting and ³¹ P-{¹ H} double resonance for nuclear Overhauser signal enhancement were applied to increase sensitivity.

RESULTS

3D ³¹ P-{¹ H} EPSI data from model solution and from human calf muscle and brain were obtained from voxels with effective sizes of 4.1 to 16.2 cm³ in measurement times of approximately 10 min. Individual spectra showed well-resolved resonances of endogenous ³¹ P-metabolites without artifacts. Volumetric high-resolution ³¹ P-metabolite maps in vivo showed metabolic heterogeneity of different tissues.

CONCLUSION

In vivo ³¹ P EPSI at 7T yields high-quality metabolic images. The proposed multishot EPSI technique reduces the measurement times for acquisition of volumetric high-resolution maps of ³¹ P-metabolites or intracellular pH in human studies. Magn Reson Med 79:1251-1259, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Cardiac metabolic imaging: current imaging modalities and future perspectives

In this review, current imaging techniques and their future perspectives in the field of cardiac metabolic imaging in humans are discussed. This includes a range of noninvasive imaging techniques, allowing a detailed investigation of cardiac metabolism in health and disease. The main imaging modalities discussed are magnetic resonance spectroscopy techniques for determination of metabolite content (triglycerides, glucose, ATP, phosphocreatine, and so on), MRI for myocardial perfusion, and single-photon emission computed tomography and positron emission tomography for quantitation of perfusion and substrate uptake.

Creatine kinase rate constant in the human heart measured with 3D-localization at 7 tesla

PURPOSE

We present a new Bloch-Siegert four Angle Saturation Transfer (BOAST) method for measuring the creatine kinase (CK) first-order effective rate constant k_f in human myocardium at 7 tesla (T). BOAST combines a variant of the four-angle saturation transfer (FAST) method using amplitude-modulated radiofrequency pulses, phosphorus Bloch-Siegert B1+-mapping to determine the per-voxel flip angles, and nonlinear fitting to Bloch simulations for postprocessing.

METHODS

Optimal flip angles and repetition time parameters were determined from Monte Carlo simulations. BOAST was validated in the calf muscle of two volunteers at 3T and 7T. The myocardial CK forward rate constant was then measured in 10 volunteers at 7T in 82 min (after ¹ H localization).

RESULTS

BOAST kfCK values were 0.281 ± 0.002 s^-1 in the calf and 0.35 ± 0.05 s^-1 in myocardium. These are consistent with literature values from lower fields. Using a literature values for adenosine triphosphate concentration, we computed CK flux values of 4.55 ± 1.52 mmol kg^-1 s^-1 . The sensitive volume for BOAST depends on the B₁ inhomogeneity of the transmit coil.

CONCLUSION

BOAST enables measurement of the CK rate constant in the human heart at 7T, with spatial localization in three dimensions to 5.6 mL voxels, using a 10-cm loop coil. Magn Reson Med 78:20-32, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Creatine kinase in ischemic and inflammatory disorders

The creatine/phosphocreatine pathway plays a conserved and central role in energy metabolism. Compartmentalization of specific creatine kinase enzymes permits buffering of local high energy phosphates in a thermodynamically favorable manner, enabling both rapid energy storage and energy transfer within the cell. Augmentation of this metabolic pathway by nutritional creatine supplementation has been shown to elicit beneficial effects in a number of diverse pathologies, particularly those that incur tissue ischemia, hypoxia or oxidative stress. In these settings, creatine and phosphocreatine prevent depletion of intracellular ATP and internal acidification, enhance post-ischemic recovery of protein synthesis and promote free radical scavenging and stabilization of cellular membranes. The creatine kinase energy system is itself further regulated by hypoxic signaling, highlighting the existence of endogenous mechanisms in mammals that can enhance creatine metabolism during oxygen deprivation to promote tissue resolution and homeostasis. Here, we review recent insights into the creatine kinase pathway, and provide rationale for dietary creatine supplementation in human ischemic and inflammatory pathologies.

Whole-body radiofrequency coil for (31) P MRSI at 7 T

Widespread use of ultrahigh-field (31) P MRSI in clinical studies is hindered by the limited field of view and non-uniform radiofrequency (RF) field obtained from surface transceivers. The non-uniform RF field necessitates the use of high specific absorption rate (SAR)-demanding adiabatic RF pulses, limiting the signal-to-noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body-sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick-up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7-T MRI scanner. The accuracy of power calibration with pick-up probes is analyzed at a clinical 3-T MR system with a close to identical (1) H body coil integrated at the MR system. Finally, we demonstrate high-quality three-dimensional (31) P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.

Myocardial Metabolic Abnormality in a Primary Left Atrial Rhabdomyosarcoma: Localized 1H MR Spectroscopy

We report a 73-year-old woman with primary left atrial rhabdomyosarcoma and its 1H MR spectroscopy (MRS) findings. The tumor showed a 2.8 cm sized lesion in the posteroinferior atrial wall on MRI. 1H MR spectra were acquired using a point-resolved spectroscopy (PRESS) sequence with electrocardiographic (ECG) gating and respiratory motion. The use of 1H-MRS allowed the quantification of triglyceride (TG) peak groups at 0.9 and 1.3 ppm, and unsaturated group of lipids at 2.1 ppm, creatine (Cr) at 3.0 ppm, and choline (Cho) at 3.2 ppm. The percentages of the myocardial metabolites based on water-peak in the interventricular septum were TG 18.4%, Cr 1.6%, Cho 3.3% and unsaturated group 4.0%, whereas the rhabdomyosarcoma showed TG 118.8%, unsaturated group 5.1%, Cr 1.3%, Cho 3.5% and the olefinic components of fatty acid at 5.4 ppm 24.5%. This case demonstrates that 1H-MRS is potentially useful to diagnose the rhabdomyosarcoma by quantifying the myocardial metabolites which are important biomarkers for heart function and diseases.

Cardiovascular imaging: what have we learned from animal models?

Cardiovascular imaging has become an indispensable tool for patient diagnosis and follow up. Probably the wide clinical applications of imaging are due to the possibility of a detailed and high quality description and quantification of cardiovascular system structure and function. Also phenomena that involve complex physiological mechanisms and biochemical pathways, such as inflammation and ischemia, can be visualized in a non-destructive way. The widespread use and evolution of imaging would not have been possible without animal studies. Animal models have allowed for instance, (i) the technical development of different imaging tools, (ii) to test hypothesis generated from human studies and finally, (iii) to evaluate the translational relevance assessment of in vitro and ex-vivo results. In this review, we will critically describe the contribution of animal models to the use of biomedical imaging in cardiovascular medicine. We will discuss the characteristics of the most frequent models used in/for imaging studies. We will cover the major findings of animal studies focused in the cardiovascular use of the repeatedly used imaging techniques in clinical practice and experimental studies. We will also describe the physiological findings and/or learning processes for imaging applications coming from models of the most common cardiovascular diseases. In these diseases, imaging research using animals has allowed the study of aspects such as: ventricular size, shape, global function, and wall thickening, local myocardial function, myocardial perfusion, metabolism and energetic assessment, infarct quantification, vascular lesion characterization, myocardial fiber structure, and myocardial calcium uptake. Finally we will discuss the limitations and future of imaging research with animal models.

Two repetition time saturation transfer (TwiST) with spill-over correction to measure creatine kinase reaction rates in human hearts

BACKGROUND

Phosphorus saturation transfer (ST) magnetic resonance spectroscopy can measure the rate of ATP generated from phosphocreatine (PCr) via creatine kinase (CK) in the human heart. Recently, the triple-repetition time ST (TRiST) method was introduced to measure the CK pseudo-first-order rate constant kf in three acquisitions. In TRiST, the longitudinal relaxation time of PCr while γ-ATP is saturated, T1`, is measured for each subject, but suffers from low SNR because the PCr signal is reduced due to exchange with saturated γ-ATP, and the short repetition time of one of the acquisitions. Here, a two-repetition time ST (TwiST) method is presented. In TwiST, the acquisition with γ-ATP saturation and short repetition time is dropped. Instead of measuring T1`, an intrinsic relaxation time T1 for PCr, T1 (intrinsic), is assumed. The objective was to validate TwiST measurements of CK kinetics in healthy subjects and patients with heart failure (HF).

METHODS

Bloch equation simulations that included the effect of spillover irradiation on PCr were used to derive formulae for T1 (intrinsic) and kf measured by both TRiST and TwiST methods. Spillover was quantified from an unsaturated PCr measurement used in the current protocol for determining PCr and ATP concentrations. Cardiac TRiST and TwiST data were acquired at 3 T from 12 healthy and 17 HF patients.

RESULTS

Simulations showed that both kf measured by TwiST and T1 (intrinsic) require spill-over corrections. In human heart at 3 T, the spill-over corrected T1 (intrinsic) = 8.4 ± 1.4 s (mean ± SD) independent of study group. TwiST and TRiST kf measurements were the same, but TwiST was 9 min faster. Spill-over corrected TwiST kf was 0.33 ± 0.08 s(-1) vs. 0.20 ± 0.06 s(-1) in healthy vs HF hearts, respectively (p < 0.0001).

CONCLUSION

TwiST was validated against TRiST in the human heart at 3 T, generating the same results 9 min faster. TwiST detected significant reductions in CK kf in HF compared to healthy subjects, consistent with prior 1.5 T studies using different methodology.

MRS: a noninvasive window into cardiac metabolism

A well-functioning heart requires a constant supply of a balanced mixture of nutrients to be used for the production of adequate amounts of adenosine triphosphate, which is the main energy source for most cellular functions. Defects in cardiac energy metabolism are linked to several myocardial disorders. MRS can be used to study in vivo changes in cardiac metabolism noninvasively. MR techniques allow repeated measurements, so that disease progression and the response to treatment or to a lifestyle intervention can be monitored. It has also been shown that MRS can predict clinical heart failure and death. This article focuses on in vivo MRS to assess cardiac metabolism in humans and experimental animals, as experimental animals are often used to investigate the mechanisms underlying the development of metabolic diseases. Various MR techniques, such as cardiac (31) P-MRS, (1) H-MRS, hyperpolarized (13) C-MRS and Dixon MRI, are described. A short overview of current and emerging applications is given. Cardiac MRS is a promising technique for the investigation of the relationship between cardiac metabolism and cardiac disease. However, further optimization of scan time and signal-to-noise ratio is required before broad clinical application. In this respect, the ongoing development of advanced shimming algorithms, radiofrequency pulses, pulse sequences, (multichannel) detection coils, the use of hyperpolarized nuclei and scanning at higher magnetic field strengths offer future perspective for clinical applications of MRS.

Suppression of skeletal muscle signal using a crusher coil: A human cardiac (31) p-MR spectroscopy study at 7 tesla

Purpose

The translation of sophisticated phosphorus MR spectroscopy (³¹P‐MRS) protocols to 7 Tesla (T) is particularly challenged by the issue of radiofrequency (RF) heating. Legal limits on RF heating make it hard to reliably suppress signals from skeletal muscle that can contaminate human cardiac ³¹P spectra at 7T. We introduce the first surface‐spoiling crusher coil for human cardiac ³¹P‐MRS at 7T.

Methods

A planar crusher coil design was optimized with simulations and its performance was validated in phantoms. Crusher gradient pulses (100 μs) were then applied during human cardiac ³¹P‐MRS at 7T.

Results

In a phantom, residual signals were 50 ± 10% with BISTRO (B₁‐insensitive train to obliterate signal), and 34 ± 8% with the crusher coil. In vivo, residual signals in skeletal muscle were 49 ± 4% using BISTRO, and 24 ± 5% using the crusher coil. Meanwhile, in the interventricular septum, spectral quality and metabolite quantification did not differ significantly between BISTRO (phosphocreatine/adenosine triphosphate [PCr/ATP] = 2.1 ± 0.4) and the crusher coil (PCr/ATP = 1.8 ± 0.4). However, the specific absorption rate (SAR) decreased from 96 ± 1% of the limit (BISTRO) to 16 ± 1% (crusher coil).

Conclusion

A crusher coil is an SAR‐efficient alternative for selectively suppressing skeletal muscle during cardiac ³¹P‐MRS at 7T. A crusher coil allows the use of sequence modules that would have been SAR‐prohibitive, without compromising skeletal muscle suppression. Magn Reson Med 75:962–972, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Enzymatic activities linked to cardiac energy metabolism of Trypanosoma evansi-infected rats and their possible functional correlations to disease pathogenesis

The aim of this study was to investigate the activities of important enzymes involved in the phosphoryl transfer network (adenylate kinase and creatine kinase (CK)), lactate dehydrogenase (LDH), respiratory chain complexes and biomarkers of cardiac function in rat experimentally infected by Trypanosoma evansi. Rat heart samples were evaluated at 5 and 15 days post-infection (PI). At 5 day PI, there was an increase in LDH and CK activities, and a decrease in respiratory chain complexes II, IV and succinate dehydrogenase activities. In addition, on day 15 PI, a decrease in the respiratory chain complex IV activity was observed. Biomarkers of cardiac function were higher in infected animals on days 5 and 15 PI. Considering the importance of the energy metabolism for heart function, it is possible that the changes in the enzymatic activities involved in the cardiac phosphotransfer network and the decrease in respiratory chain might be involved partially in the role of biomarkers of cardiac function of T. evansi-infected rats.

Effects of acute angiotensin II on ischemia reperfusion injury following myocardial infarction

Myocardial infarction (MI) induces cardiac remodeling. This may increase the susceptibility of the infarcted heart to subsequent ischemic events. While chronic angiotensin II blockade is cardioprotective post-MI, the acute effects of angiotensin II in ischemia-reperfusion injury (IR) remains unclear. In the present study, we tested whether angiotensin II administration altered recovery of left ventricular (LV) function to IR in hearts from sham and MI rats. Echocardiography, LV pressure-volume relationships, and IR performance were established in subsets of sham (N = 27) and MI hearts (N = 41). IR was conducted in red-cell-perfused Langendorff hearts (60 minutes of low-flow ischemia; 30 minutes of reperfusion) during vehicle or angiotensin II infusions (10(-7) M). MI hearts were dilated and had reduced fractional shortening and blunted systolic elastance (p < 0.05). Despite systolic dysfunction in MI, functional recovery to IR was similar to sham. Angiotensin II significantly worsened IR performance in sham (p < 0.05), but not MI. The effect of angiotensin II on in vitro cardiomyocyte survival under various pH conditions was also tested. Acidosis increased cardiomyocyte death and angiotensin II potentiated this effect. We conclude that IR performance is similar between sham and MI hearts and that MI hearts are resistant to angiotensin II-induced cardiac dysfunction in response to IR.

Cardiovascular magnetic resonance in ischemic heart disease

Ischemic heart disease is the major cause of death in developed countries. Recently, cardiovascular magnetic resonance (CMR) has appeared as a powerful technique for diagnosis and prognosis of ischemia, as well as for postischemic therapy follow-up. The objective of this chapter is to provide an overview of the role of CMR in assessing ischemic myocardium. It reviews the most recent studies in this field and includes CMR parameters that are already well established in the clinical setting as well as promising or emerging parameters in clinical use.

Metabolic flux as a predictor of heart failure prognosis

The creatine kinase (CK) system is thought to play an integral role in maintaining levels of chemical energy in the form of ATP, which is essential for normal cardiac function. In the failing heart, it has long been established that multiple components of CK energy metabolism are commonly impaired and that these correlate with disease severity. A recent study published in Science Translational Medicine adds significantly to this body of evidence by demonstrating that the rate of ATP transfer via CK, measured noninvasively by magnetic resonance spectroscopy, is an independent predictor of adverse clinical outcome in patients with nonischemic cardiomyopathy. This finding invites speculation on the future role of metabolic imaging for risk stratification in patients with heart failure. The authors further assert an implied causal role for energetics in disease progression. Although this is not supported by recent findings in loss-of-function mouse models, there is, nonetheless, a strong argument for the development of novel metabolic therapies for the failing heart.

Irisin: a potentially candidate marker for myocardial infarction

Myocardial infarction (MI) causes energy depletion through imbalance between coronary blood supply and myocardial demand. Irisin produced by the heart reduces ATP production by increasing heat generation. Energy depletion affects irisin concentration in circulation and cardiac tissues, suggesting an association with MI. We examined: (1) irisin expression immunohistochemically in rat heart, skeletal muscle, kidney and liver in isoproterenol (ISO)-induced MI, and (2) serum irisin concentration by ELISA. Rats were randomly allocated into 6 groups (n=6), (i) control, (ii) ISO (1h), (iii) ISO (2h), (iv) ISO (4h), (v) ISO (6h), and (vi) ISO (24h), 200mg ISO in each case. Rats were decapitated and the blood and tissues collected for irisin analysis. Blood was centrifuged at 1792 g for 5 min. Tissues were washed with saline and fixed in 10% formalin for histology. Serum irisin levels gradually decreased from 1h to 24h in MI rats compared with controls, the minimum being at 2h, increasing again after 6h. Cardiac muscle cells, glomerular, peritubular renal cortical interstitial cells, hepatocytes and liver sinusoidal cells and perimysium, endomysium and nucleoi of skeletal muscle were irisin positive, but its synthesis decreased 1-4h after MI. At all time-points, irisin increased near myocardial connective tissue, with production in skeletal muscle, liver and kidney recovering after 6h, although slower than controls. Unique insight into the pathogenesis of MI is shown, and the gradually decrease of serum irisin might be a diagnostic marker for MI.

Metabolic rates of ATP transfer through creatine kinase (CK Flux) predict clinical heart failure events and death

Morbidity and mortality from heart failure (HF) are high, and current risk stratification approaches for predicting HF progression are imperfect. Adenosine triphosphate (ATP) is required for normal cardiac contraction, and abnormalities in creatine kinase (CK) energy metabolism, the primary myocardial energy reserve reaction, have been observed in experimental and clinical HF. However, the prognostic value of abnormalities in ATP production rates through CK in human HF has not been investigated. Fifty-eight HF patients with nonischemic cardiomyopathy underwent ³¹P magnetic resonance spectroscopy (MRS) to quantify cardiac high-energy phosphates and the rate of ATP synthesis through CK (CK flux) and were prospectively followed for a median of 4.7 years. Multiple-event analysis (MEA) was performed for HF-related events including all-cause and cardiac death, HF hospitalization, cardiac transplantation, and ventricular-assist device placement. Among baseline demographic, clinical, and metabolic parameters, MEA identified four independent predictors of HF events: New York Heart Association (NYHA) class, left ventricular ejection fraction (LVEF), African-American race, and CK flux. Reduced myocardial CK flux was a significant predictor of HF outcomes, even after correction for NYHA class, LVEF, and race. For each increase in CK flux of 1 μmol g⁻¹ s⁻¹, risk of HF-related composite outcomes decreased by 32 to 39%. These findings suggest that reduced CK flux may be a potential HF treatment target. Newer imaging strategies, including noninvasive ³¹P MRS that detect altered ATP kinetics, could thus complement risk stratification in HF and add value in conditions involving other tissues with high energy demands, including skeletal muscle and brain.

Quantification of human high-energy phosphate metabolite concentrations at 3 T with partial volume and sensitivity corrections

Practical noninvasive methods for the measurement of absolute metabolite concentrations are key to the assessment of the depletion of myocardial metabolite pools which occurs with several cardiac diseases, including infarction and heart failure. Localized MRS offers unique noninvasive access to many metabolites, but is often confounded by nonuniform sensitivity and partial volume effects in the large, poorly defined voxels commonly used for the detection of low-concentration metabolites with surface coils. These problems are exacerbated at higher magnetic field strengths by greater radiofrequency (RF) field inhomogeneity and differences in RF penetration with heteronuclear concentration referencing. An example is the (31)P measurement of cardiac adenosine triphosphate (ATP) and phosphocreatine (PCr) concentrations, which, although central to cardiac energetics, have not been measured at field strengths above 1.5 T. Here, practical acquisition and analysis protocols are presented for the quantification of [PCr] and [ATP] with one-dimensionally resolved surface coil spectra and concentration referencing at 3 T. The effects of nonuniform sensitivity and partial tissue volumes are addressed at 3 T by the application of MRI-based three-dimensional sensitivity weighting and tissue segmentation. The method is validated in phantoms of different sizes and concentrations, and used to measure [PCr] and [ATP] in healthy subjects. In calf muscle (n = 8), [PCr] = 24.7 ± 3.4 and [ATP] = 5.7 ± 1.3 µmol/g wet weight, whereas, in heart (n = 18), [PCr] = 10.4 ± 1.5 and [ATP] = 6.0 ± 1.1 µmol/g wet weight (all mean ± SD), consistent with previous reports at lower fields. The method enables, for the first time, the efficient, semi-automated quantification of high-energy phosphate metabolites in humans at 3 T with nonuniform excitation and detection.

Human cardiac 31P magnetic resonance spectroscopy at 7 Tesla

Purpose

Phosphorus magnetic resonance spectroscopy (³¹P-MRS) affords unique insight into cardiac energetics but has a low intrinsic signal-to-noise ratio (SNR) in humans. Theory predicts an increased ³¹P-MRS SNR at 7T, offering exciting possibilities to better investigate cardiac metabolism. We therefore compare the performance of human cardiac ³¹P-MRS at 7T to 3T, and measure T₁s for ³¹P metabolites at 7T.

Methods

Matched ³¹P-MRS data were acquired at 3T and 7T, on nine normal volunteers. A novel Look-Locker CSI acquisition and fitting approach was used to measure T₁s on six normal volunteers.

Results

T₁s in the heart at 7T were: phosphocreatine (PCr) 3.05 ± 0.41s, γ-ATP 1.82 ± 0.09s, α-ATP 1.39 ± 0.09s, β-ATP 1.02 ± 0.17s and 2,3-DPG (2,3-diphosphoglycerate) 3.05 ± 0.41s (N = 6). In the field comparison (N = 9), PCr SNR increased 2.8× at 7T relative to 3T, the Cramer-Ráo uncertainty (CRLB) in PCr concentration decreased 2.4×, the mean CRLB in PCr/ATP decreased 2.7× and the PCr/ATP SD decreased 2×.

Conclusion

Cardiac ³¹P-MRS at 7T has higher SNR and the spectra can be quantified more precisely than at 3T. Cardiac ³¹P T₁s are shorter at 7T than at 3T. We predict that 7T will become the field strength of choice for cardiac ³¹P-MRS. Magn Reson Med 72:304–315, 2014. © 2013 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited.

Assessing myocardial recovery following ST-segment elevation myocardial infarction: short- and long-term perspectives using cardiovascular magnetic resonance

Myocardial recovery after revascularization for ST-segment elevation myocardial infarction (STEMI) remains a significant diagnostic and, despite novel treatment strategies, a therapeutic challenge. Cardiovascular magnetic resonance (CMR) has emerged as a valuable clinical and research tool after acute STEMI. It represents the gold standard for functional and morphological evaluation of the left ventricle. Gadolinium-based perfusion and late-enhancement viability imaging has expanded our knowledge about the underlying pathologies of inadequate myocardial recovery. T2-weighted imaging of myocardial salvage after early reperfusion of the infarct-related artery underlines the effectiveness of current invasive treatment for STEMI. In the last decade, the number of publications on CMR after acute STEMI continued to rise, with no plateau in sight. Currently, CMR research is gathering robust prognostic data on standardized CMR protocols with the aim to substantially improve patient care and prognosis. Beyond established CMR protocols, more specific methods such as magnetic resonance relaxometry, myocardial tagging, 4D phase-contrast imaging and novel superparamagnetic contrast agents are emerging. This review will discuss the currently available data on the use of CMR after acute STEMI and take a brief look at developing new methods currently under investigation.

Transgenic overexpression of ribonucleotide reductase improves cardiac performance

We previously demonstrated that cardiac myosin can use 2-deoxy-ATP (dATP) as an energy substrate, that it enhances contraction and relaxation with minimal effect on calcium-handling properties in vitro, and that contractile enhancement occurs with only minor elevation of cellular [dATP]. Here, we report the effect of chronically enhanced dATP concentration on cardiac function using a transgenic mouse that overexpresses the enzyme ribonucleotide reductase (TgRR), which catalyzes the rate-limiting step in de novo deoxyribonucleotide biosynthesis. Hearts from TgRR mice had elevated left ventricular systolic function compared with wild-type (WT) mice, both in vivo and in vitro, without signs of hypertrophy or altered diastolic function. Isolated cardiomyocytes from TgRR mice had enhanced contraction and relaxation, with no change in Ca(2+) transients, suggesting targeted improvement of myofilament function. TgRR hearts had normal ATP and only slightly decreased phosphocreatine levels by (31)P NMR spectroscopy, and they maintained rate responsiveness to dobutamine challenge. These data demonstrate long-term (at least 5-mo) elevation of cardiac [dATP] results in sustained elevation of basal left ventricular performance, with maintained β-adrenergic responsiveness and energetic reserves. Combined with results from previous studies, we conclude that this occurs primarily via enhanced myofilament activation and contraction, with similar or faster ability to relax. The data are sufficiently compelling to consider elevated cardiac [dATP] as a therapeutic option to treat systolic dysfunction.

Normalizing the metabolic phenotype after myocardial infarction: impact of subchronic high fat feeding

The normal heart relies primarily on the oxidation of fatty acids (FA) for ATP production, whereas during heart failure (HF) glucose utilization increases, implying pathological changes to cardiac energy metabolism. Despite the noted lipotoxic effects of elevating FA, our work has demonstrated a cardioprotective effect of a high fat diet (SAT) when fed after myocardial infarction (MI), as compared to normal chow (NC) fed cohorts. This data has suggested a mechanistic link to energy metabolism. The goal of this study was to determine the impact of SAT on the metabolic phenotype of the heart after MI. Male Wistar rats underwent coronary ligation surgery (MI) and were evaluated after 8 weeks of SAT. Induction of MI was verified by echocardiography. LV function assessed by in vivo hemodynamic measurements revealed improvements in the MI-SAT group as compared to MI-NC. Perfused working hearts revealed a decrease in cardiac work in MI-NC that was improved in MI-SAT. Glucose oxidation was increased and FA oxidation decreased in MI-NC compared to shams suggesting an alteration in the metabolic profile that was ameliorated by SAT. (31)P NMR analysis of Langendorff perfused hearts revealed no differences in PCr:ATP indicating no overt energy deficit in MI groups. Phospho-PDH and PDK(4) were increased in MI-SAT, consistent with a shift towards fatty acid oxidation (FAO). Overall, these results support the hypothesis that SAT post-infarction promotes a normal metabolic phenotype that may serve a cardioprotective role in the injured heart.

Allopurinol acutely increases adenosine triphospate energy delivery in failing human hearts

OBJECTIVES

This study tested the hypothesis that acute administration of the xanthine oxidase (XO) inhibitor allopurinol improves cardiac high-energy phosphate concentrations in human heart failure (HF) and increases the rate of adenosine triphosphate (ATP) synthesis through creatine kinase (CK), the primary myocardial energy reserve.

BACKGROUND

Studies of patients and animal models implicate impaired myocardial high-energy phosphate availability in HF. The XO reaction is a critical terminal step in ATP and purine degradation and an important source of reactive oxygen species. Thus, XO inhibition is a potentially attractive means to improve energy metabolism in the failing human heart.

METHODS

We randomized 16 patients with nonischemic cardiomyopathy in a double-blind fashion to allopurinol (300 mg intravenously) or placebo infusion, 4-to-1, the latter for purposes of blinding only. The myocardial concentrations of ATP and creatine phosphate (PCr) and the rate of ATP synthesis through CK (CK flux) were determined by (31)P magnetic resonance spectroscopy.

RESULTS

Allopurinol infusion increased mean cardiac PCr/ATP and PCr concentration by ∼11% (p < 0.02), and mean CK flux by 39% (2.07 ± 1.27 μmol/g/s to 2.87 ± 1.82 μmol/g/s, p < 0.007). Calculated cytosolic adenosine diphosphate concentration decreased, whereas the free energy of ATP hydrolysis (ΔG(∼ATP)) increased with allopurinol. The increased CK flux was disproportionate to substrate changes, indicating increased CK enzyme activity.

CONCLUSIONS

Intravenous administration of the XO inhibitor allopurinol acutely improves the relative and absolute concentrations of myocardial high-energy phosphates and ATP flux through CK in the failing human heart, offering direct evidence that myofibrillar CK energy delivery can be pharmaceutically augmented in the failing human heart. (Intravenous Allopurinol in Heart Failure; NCT00181155).

Clinical cardiac magnetic resonance spectroscopy

Cardiac magnetic resonance spectroscopy (MRS) is a noninvasive tool for the assessment of myocardial metabolism, without the use of radiation or intravenous contrast agents. Using the intrinsic magnetic resonance signals from nuclei, including (31)Phosphorus, (1)Hydrogen, (23)Sodium, and (13)Carbon and, more recently, hyperpolarization techniques, MRS provides a comprehensive metabolic assessment of cardiac muscle. This highly versatile technique has provided insights into the pathophysiology of cardiac metabolism in a wide range of conditions, including ischemic heart disease, heart failure, genetic cardiomyopathies, heart transplantation, hypertensive heart disease, valvular heart disease, and diabetes. In addition, MRS has value in the assessment of prognosis and for monitoring therapeutic strategies in heart failure. However, because of the low temporal and spatial resolution of the technique, MRS has so far been limited to research applications. With higher field strength magnets and novel hyperpolarization techniques, the promise of using MRS for clinical applications may eventually be fulfilled.

The PGC-1 cascade as a therapeutic target for heart failure

The PPARγ coactivator-1 (PGC-1) family of transcriptional coactivators, together with estrogen related receptors (ERRs), plays a key role in regulating genes involved in myocardial fuel metabolism and cardiac function. Increasing evidence implicates dysregulation of this transcriptional regulatory circuit in the metabolic and functional disturbances that presage heart failure due to common diseases such as hypertension and diabetes. Accordingly, the PGC-1/ERR axis is a plausible candidate therapeutic target for novel therapeutics aimed at reversing the energy metabolic disturbances that contribute to heart failure. This review describes the biologic actions of the PGC-1 and ERR cascade and summarizes the evidence that dysregulation of this transcriptional regulatory circuit contributes to heart failure. Potential strategies to modulate this target pathway are reviewed. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Rearrangement of energetic and substrate utilization networks compensate for chronic myocardial creatine kinase deficiency

Plasticity of the cellular bioenergetic system is fundamental to every organ function, stress adaptation and disease tolerance. Here, remodelling of phosphotransfer and substrate utilization networks in response to chronic creatine kinase (CK) deficiency, a hallmark of cardiovascular disease, has been revealed in transgenic mouse models lacking either cytosolic M-CK (M-CK(-/-)) or both M-CK and sarcomeric mitochondrial CK (M-CK/ScCKmit(-/-)) isoforms. The dynamic metabolomic signatures of these adaptations have also been defined. Tracking perturbations in metabolic dynamics with (18)O and (13)C isotopes and (31)P NMR and mass spectrometry demonstrate that hearts lacking M-CK have lower phosphocreatine (PCr) turnover but increased glucose-6-phosphate (G-6-P) turnover, glucose utilization and inorganic phosphate compartmentation with normal ATP γ-phosphoryl dynamics. Hearts lacking both M-CK and sarcomeric mitochondrial CK have diminished PCr turnover, total phosphotransfer capacity and intracellular energetic communication but increased dynamics of β-phosphoryls of ADP/ATP, G-6-P and γ-/β-phosphoryls of GTP, indicating redistribution of flux through adenylate kinase (AK), glycolytic and guanine nucleotide phosphotransfer circuits. Higher glycolytic and mitochondrial capacities and increased glucose tolerance contributed to metabolic resilience of M-CK/ScCKmit(-/-) mice. Multivariate analysis revealed unique metabolomic signatures for M-CK(-/-) and M-CK/ScCKmit(-/-) hearts suggesting that rearrangements in phosphotransfer and substrate utilization networks provide compensation for genetic CK deficiency. This new information highlights the significance of integrated CK-, AK-, guanine nucleotide- and glycolytic enzyme-catalysed phosphotransfer networks in supporting the adaptivity and robustness of the cellular energetic system.

Magnetic resonance imaging/magnetic resonance spectroscopy biomarkers evaluation of stunned myocardium in canine model

OBJECTIVES

To evaluate whether dynamic alterations in high-energy phosphate (HEP) occur in postischemic "stunned" myocardium (SM) in canine model and to investigate the correlation between HEP and cardiac function, using cine magnetic resonance imaging (cine-MRI) and phosphorus-31 magnetic resonance spectroscopy (31P-MRS).

MATERIALS AND METHODS

Dogs (n = 13) underwent cine MRI and 31P-MRS at 60 minutes, 8 days after 10 minutes full left anterior descending occlusion followed by reperfusion. The same MRI/MRS experiments were repeated on 5 reference animals (dogs without ischemic reperfusion) at the same time points to serve as internal reference myocardium (RM). After MR data acquisitions, the SM dogs (n = 3 at 60 minutes; n = 10 at 60 minutes and day 8) and RM dogs (n = 5) were euthanized and myocardial tissues were sampled for histologic study by triphenyltetrazolium chloride staining, hematoxylin and eosin staining, and electron microscopic examination.

RESULTS

The myocardial stunning at 60 minutes was confirmed by electron microscopy examinations from the 3 randomly chosen animals with SM. The phosphocreatine (PCr)/β- adenosine triphosphate (ATP) ratio of SM was significantly lower at 60 minutes than that at day 8 (1.07 ± 0.20 vs. 1.97 ± 0.28, P < 0.05). However, no significant difference was found between 60 minutes and day 8 in RM group (1.91 ± 0.14 at 60 minutes vs. 1.89 ± 0.16 at day 8, P > 0.05). At 60 minutes, the PCr/β-ATP ratio has significant difference between SM and RM groups; while at day 8, the ratio shows no significant difference between the 2 groups. The same results were obtained for left ventricle ejection fraction (LVEF). In SM group, LVEF has good correlation with myocardial PCr/β-ATP ratios at 60 minutes (R2 = 0.71, P < 0.05) and at day 8 (R2 = 0.73, P < 0.05), respectively.

CONCLUSIONS

The HEP alterations were confirmed by 31P-MRS in SM and there is a good correlation between PCr/β-ATP ratio and LVEF for SM at 60 minutes and recovered myocardium at day 8. The combined MRS/MRI method offers the potential to systematically assess the cardiac function, morphology, and metabolism of SM. These MRS/MRI biomarker datasets could be used to dynamically monitor therapeutic efficiency and predict cardiac events.

Metabolic manipulation in chronic heart failure: study protocol for a randomised controlled trial

Background

Heart failure is a major cause of morbidity and mortality in society. Current medical therapy centres on neurohormonal modulation with angiotensin converting enzyme inhibitors and β-blockers. There is growing evidence for the use of metabolic manipulating agents as adjunctive therapy in patients with heart failure. We aim to determine the effect of perhexiline on cardiac energetics and alterations in substrate utilisation in patients with non-ischaemic dilated cardiomyopathy.

Methods

A multi-centre, prospective, randomised double-blind, placebo-controlled trial of 50 subjects with non-ischaemic dilated cardiomyopathy recruited from University Hospital Birmingham NHS Foundation Trust and Cardiff and Vale NHS Trust. Baseline investigations include magnetic resonance spectroscopy to assess cardiac energetic status, echocardiography to assess left ventricular function and assessment of symptomatic status. Subjects are then randomised to receive 200 mg perhexiline maleate or placebo daily for 4 weeks with serum drug level monitoring. All baseline investigations will be repeated at the end of the treatment period. A subgroup of patients will undergo invasive investigations with right and left heart catheterisation to calculate respiratory quotient, and mechanical efficiency. The primary endpoint is an improvement in the phosphocreatine to adenosine triphosphate ratio at 4 weeks. Secondary end points are: i) respiratory quotient; ii) mechanical efficiency; iii) change in left ventricular (LV) function.

Trial Registration

ClinicalTrials.gov: NCT00841139

ISRCTN: ISRCTN2887836