[11C]Acetate rest-stress protocol to assess myocardial perfusion and oxygen consumption reserve in a model of congestive heart failure in rats

A ketogenic diet lowers myocardial fatty acid oxidation but does not affect oxygen consumption: a study in overweight humans

OBJECTIVE

A ketogenic diet (KD) characterized by very low carbohydrate intake and high fat consumption may simultaneously induce weight loss and be cardioprotective. The "thrifty substrate hypothesis" posits that ketone bodies are more energy efficient compared with other cardiac oxidative substrates such as fatty acids. This work aimed to study whether a KD with presumed increased myocardial ketone body utilization reduces cardiac fatty acid uptake and oxidation, resulting in decreased myocardial oxygen consumption (MVO2 ).

METHODS

This randomized controlled crossover trial examined 11 individuals with overweight or obesity on two occasions: (1) after a KD and (2) after a standard diet. Myocardial free fatty acid (FFA) oxidation, uptake, and esterification rate were measured using dynamic [11 C]palmitate positron emission tomography (PET)/computed tomography, whereas MVO2 and myocardial external efficiency (MEE) were measured using dynamic [11 C]acetate PET.

RESULTS

The KD increased plasma β-hydroxybutyrate, reduced myocardial FFA oxidation (p < 0.01) and uptake (p = 0.03), and increased FFA esterification (p = 0.03). No changes were observed in MVO2 (p = 0.2) or MEE (p = 0.87).

CONCLUSIONS

A KD significantly reduced myocardial FFA uptake and oxidation, presumably by increasing ketone body oxidation. However, this change in cardiac substrate utilization did not improve MVO2 , speaking against the thrifty substrate hypothesis.

Enhanced Extraction of Blood and Tissue Time-Activity Curves in Cardiac Mouse FDG PET Imaging by Means of Constrained Nonnegative Matrix Factorization

We propose an enhanced method to accurately retrieve time-activity curves (TACs) of blood and tissue from dynamic 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) positron emission tomography (PET) cardiac images of mice. The method is noninvasive and consists of using a constrained nonnegative matrix factorization algorithm (CNMF) applied to the matrix (A) containing the intensity values of the voxels of the left ventricle (LV) PET image. CNMF factorizes A into nonnegative matrices H and W, respectively, representing the physiological factors (blood and tissue) and their associated weights, by minimizing an extended cost function. We verified our method on 32 C57BL/6 mice, 14 of them with acute myocardial infarction (AMI). With CNMF, we could break down the mouse LV into myocardial and blood pool images. Their corresponding TACs were used in kinetic modeling to readily determine the [¹⁸F]FDG influx constant (K_i) required to compute the myocardial metabolic rate of glucose. The calculated K_i values using CNMF for the heart of control mice were in good agreement with those published in the literature. Significant differences in K_i values for the heart of control and AMI mice were found using CNMF. The values of the elements of W agreed well with the LV structural changes induced by ligation of the left coronary artery. CNMF was compared with the recently published method based on robust unmixing of dynamic sequences using regions of interest (RUDUR). A clear improvement of signal separation was observed with CNMF compared to the RUDUR method.

Positron Emission Tomography in Heart Failure: From Pathophysiology to Clinical Application

Imaging modalities are increasingly being used to evaluate the underlying pathophysiology of heart failure. Positron emission tomography (PET) is a non-invasive imaging technique that uses radioactive tracers to visualize and measure biological processes in vivo. PET imaging of the heart uses different radiopharmaceuticals to provide information on myocardial metabolism, perfusion, inflammation, fibrosis, and sympathetic nervous system activity, which are all important contributors to the development and progression of heart failure. This narrative review provides an overview of the use of PET imaging in heart failure, highlighting the different PET tracers and modalities, and discussing fields of present and future clinical application.

A practical fully automated radiosynthesis of [18F]Flurpiridaz on the module modular lab-pharmtracer without external purification

Background

[18F]Flurpiridaz is a promising novel cardiac PET imaging tracer formed by the radiolabeling of pyridaben derivative with fluorine-18. Clinical studies on [18F]Flurpiridaz are currently at the phase III level for the assessment of MPI. Providing high image quality thanks to its relatively long half-life, F-18 is a high-potential radionuclide for the early detection of CAD. In this study, we aimed to develop a fully automated synthesis of [18F]Flurpiridaz without further preparative HPLC purification.

Results

Precursor 6 was obtained by multi-step synthesis starting from mucochloric acid (1) as a sole product with 35% yield and identified by spectroscopic measurement. Manually cold labeling experiments were performed using the stable isotope [19F]F, and TBA-HCO3 PTC provided desirable fluorinated compound with high yield. A fully automated [18F]Flurpiridaz synthesis on the ML-PT device provided 55–65% radiochemical yield with more than 98% radiochemical purity. The final product purification method demonstrated that [18F]Flurpiridaz could be obtained without an external preparative HPLC system as a pharmaceutical quality.

Conclusion

A novel and fascinating strategy was developed for the fully automated synthesis of [18F]Flurpiridaz (7) on ML PT. Organic synthesis of precursor 6 was achieved with a desirable yield and characterized by NMR and HR-MS. A detailed set of cold experiments were completed for optimization conditions before hot trials and TBA-HCO3 increased molar activity with a minimum amount of side products. Radiolabeling showed that our self-designed automated synthesis method enables high radiochemical yield and radiochemical purity for the production of [18F]Flurpiridaz. The desirable radiopharmaceutical quality of the product was obtained without using an additional preparative HPLC system. [18F]Flurpiridaz (7) preserved its stability within 12 h and final specifications were consistent with the acceptance criteria in Ph. Eur. regulations.

Anthracycline-induced cardiotoxicity: From pathobiology to identification of molecular targets for nuclear imaging

Anthracyclines are a widely used class of chemotherapy in pediatric and adult cancers, however, their use is hampered by the development of cardiotoxic side-effects and ensuing complications, primarily heart failure. Clinically used imaging modalities to screen for cardiotoxicity are mostly echocardiography and occasionally cardiac magnetic resonance imaging. However, the assessment of diastolic and global or segmental systolic function may not be sensitive to detect subclinical or early stages of cardiotoxicity. Multiple studies have scrutinized molecular nuclear imaging strategies to improve the detection of anthracycline-induced cardiotoxicity. Anthracyclines can activate all forms of cell death in cardiomyocytes. Injury mechanisms associated with anthracycline usage include apoptosis, necrosis, autophagy, ferroptosis, pyroptosis, reactive oxygen species, mitochondrial dysfunction, as well as cardiac fibrosis and perturbation in sympathetic drive and myocardial blood flow; some of which have been targeted using nuclear probes. This review retraces the pathobiology of anthracycline-induced cardiac injury, details the evidence to date supporting a molecular nuclear imaging strategy, explores disease mechanisms which have not yet been targeted, and proposes a clinical strategy incorporating molecular imaging to improve patient management.

New Insights in Early Detection of Anticancer Drug-Related Cardiotoxicity Using Perfusion and Metabolic Imaging

Cardio-oncology requires a good knowledge of the cardiotoxicity of anticancer drugs, their mechanisms, and their diagnosis for better management. Anthracyclines, anti-vascular endothelial growth factor (VEGF), alkylating agents, antimetabolites, anti-human epidermal growth factor receptor (HER), and receptor tyrosine kinase inhibitors (RTKi) are therapeutics whose cardiotoxicity involves several mechanisms at the cellular and subcellular levels. Current guidelines for anticancer drugs cardiotoxicity are essentially based on monitoring left ventricle ejection fraction (LVEF). However, knowledge of microvascular and metabolic dysfunction allows for better imaging assessment before overt LVEF impairment. Early detection of anticancer drug-related cardiotoxicity would therefore advance the prevention and patient care. In this review, we provide a comprehensive overview of the cardiotoxic effects of anticancer drugs and describe myocardial perfusion, metabolic, and mitochondrial function imaging approaches to detect them before over LVEF impairment.

The ultra high sensitivity blood counter: a compact, MRI-compatible, radioactivity counter for pharmacokinetic studies in µL volumes

Quantification of physiological parameters in preclinical pharmacokinetic studies based on nuclear imaging requires the monitoring of arterial radioactivity over time, known as the arterial input function (AIF). Continuous derivation of the AIF in rodent models is very challenging because of the limited blood volume available for sampling. To address this challenge, an Ultra High Sensitivity Blood Counter (UHS-BC) was developed. The device detects beta particles in real-time using silicon photodiodes, custom low-noise electronics, and 3D-printed plastic cartridges to hold standard catheters. Two prototypes were built and characterized in two facilities. Sensitivities up to 39% for¹⁸F and 58% for¹¹C-based positron emission tomography (PET) tracers were demonstrated.^99mTc and¹²⁵I based Single Photon Emission Computed Tomography (SPECT) tracers were detected with greater than 3% and 10% sensitivity, respectively, opening new applications in nuclear imaging and fundamental biology research. Measured energy spectra show all relevant peaks down to a minimum detectable energy of 20 keV. The UHS-BC was shown to be highly reliable, robust towards parasitic background radiation and electromagnetic interference in the PET or MRI environment. The UHS-BC provides reproducible results under various experimental conditions and was demonstrated to be stable over days of continuous operation. Animal experiments showed that the UHS-BC performs accurate AIF measurements using low detection volumes suitable for small animal models in PET, SPECT and PET/MRI investigations. This tool will help to reduce the time and number of animals required for pharmacokinetic studies, thus increasing the throughput of new drug development.

Autologous skeletal myoblast patch implantation prevents the deterioration of myocardial ischemia and right heart dysfunction in a pressure-overloaded right heart porcine model

Right ventricular dysfunction is a predictor for worse outcomes in patients with congenital heart disease. Myocardial ischemia is primarily associated with right ventricular dysfunction in patients with congenital heart disease and may be a therapeutic target for right ventricular dysfunction. Previously, autologous skeletal myoblast patch therapy showed an angiogenic effect for left ventricular dysfunction through cytokine paracrine effects; however, its efficacy in right ventricular dysfunction has not been evaluated. Thus, this study aimed to evaluate the angiogenic effect of autologous skeletal myoblast patch therapy and amelioration of metabolic and functional dysfunction, in a pressure-overloaded right heart porcine model. Pulmonary artery stenosis was induced by a vascular occluder in minipigs; after two months, autologous skeletal myoblast patch implantation on the right ventricular free wall was performed (n = 6). The control minipigs underwent a sham operation (n = 6). The autologous skeletal myoblast patch therapy alleviated right ventricular dilatation and ameliorated right ventricular systolic and diastolic dysfunction. 11C-acetate kinetic analysis using positron emission tomography showed improvement in myocardial oxidative metabolism and myocardial flow reserve after cell patch implantation. On histopathology, a higher capillary density and vascular maturity with reduction of myocardial ischemia were observed after patch implantation. Furthermore, analysis of mRNA expression revealed that the angiogenic markers were upregulated, and ischemic markers were downregulated after patch implantation. Thus, autologous skeletal myoblast patch therapy ameliorated metabolic and functional dysfunction in a pressure-overloaded right heart porcine model, by alleviating myocardial ischemia through angiogenesis.

68Ga-DOTA chelate, a novel imaging agent for assessment of myocardial perfusion and infarction detection in a rodent model

BACKGROUND

Magnetic resonance imaging (MRI) with Gadolinium 1,4,7,10-tetraazacyclododecane-N',N″,N''',N″″-tetraacetic acid (Gd-DOTA) enables assessment of myocardial perfusion during first-pass of the contrast agent, while increased retention can signify areas of myocardial infarction (MI). We studied whether Gallium-68-labeled analog, 68Ga-DOTA, can be used to assess myocardial perfusion on positron emission tomography/computed tomography (PET/CT) in rats, comparing it with 11C-acetate.

METHODS

Rats were studied with 11C-acetate and 68Ga-DOTA at 24 hours after permanent ligation of the left coronary artery or sham operation. One-tissue compartmental models were used to estimate myocardial perfusion in normal and infarcted myocardium. After the PET scan, hearts were sectioned for autoradiographic detection of 68Ga-DOTA distribution.

RESULTS

11C-acetate PET showed perfusion defects and histology showed myocardial necrosis in all animals after coronary ligation. Kinetic modeling of 68Ga-DOTA showed significantly higher k1 values in normal myocardium than in infarcted areas. There was a significant correlation (r = 0.82, P = 0.001) between k1 values obtained with 68Ga-DOTA and 11C-acetate. After 10 minutes of tracer distribution, the 68Ga-DOTA concentration was significantly higher in the infarcted than normal myocardium on PET imaging and autoradiography.

CONCLUSIONS

Our results indicate that acute MI can be detected as reduced perfusion, as well as increased late retention of 68Ga-DOTA.

Metabolic Imaging in Cardio-oncology

Tremendous progress in cancer detection and therapy has improved survival. However, cardiovascular complications are a major source of morbidity in cancer survivors. Cardiotoxicity is currently defined by structural myocardial changes and cardiac injury biomarkers. In many instances, such changes are late and irreversible. Therefore, diagnostic modalities that can identify early alterations in potentially reversible biochemical and molecular signaling processes are of interest. This review is focused on emerging translational metabolic imaging modalities. We present in context relevant mitochondrial biology aspects that ground the development and application of these technologies for detection of cancer therapy-related cardiac dysfunction (CTRCD). The application of these modalities may improve the assessment of cardiovascular risk when anticancer treatments with a defined cardiometabolic toxic mechanism are to be used. Also, they may serve as screening tools for cardiotoxicity when novel lines of cancer therapies are applied.

Hyperpolarized [1-13C]-acetate Renal Metabolic Clearance Rate Mapping

¹¹C-acetate is a positron emission tomography (PET) tracer of oxidative metabolism, whereas hyperpolarized ¹³C-acetate can be used in magnetic resonance imaging (MRI) for investigating specific metabolic processes. The aims of this study were to examine if the kinetic formalism of ¹¹C-acetate PET in the kidneys is comparable to that of ¹³C-acetate MRI, and to compare the dynamic metabolic information of hyperpolarized ¹³C-acetate MRI with that obtained with ¹¹C-acetate PET. Rats were examined with dynamic hyperpolarized ¹³C-acetate MRI or ¹¹C-acetate PET before and after intravenous injection of furosemide, a loop diuretic known to alter both the hemodynamics and oxygen consumption in the kidney. The metabolic clearance rates (MCR) were estimated and compared between the two modalities experimentally in vivo and in simulations. There was a clear dependency on the mean transit time and MCR for both ¹³C-acetate and ¹¹C-acetate following furosemide administration, while no dependencies on the apparent renal perfusion were observed. This study demonstrated that hyperpolarized ¹³C-acetate MRI is feasible for measurements of the intrarenal energetic demand via the MCR, and that the quantitative measures are correlated with those measured by ¹¹C-acetate PET, even though the temporal window is more than 30 times longer with ¹¹C-acetate.

Transcriptional Changes Associated with Long-Term Left Ventricle Volume Overload in Rats: Impact on Enzymes Related to Myocardial Energy Metabolism

Patients with left ventricle (LV) volume overload (VO) remain in a compensated state for many years although severe dilation is present. The myocardial capacity to fulfill its energetic demand may delay decompensation. We performed a gene expression profile, a model of chronic VO in rat LV with severe aortic valve regurgitation (AR) for 9 months, and focused on the study of genes associated with myocardial energetics. Methods. LV gene expression profile was performed in rats after 9 months of AR and compared to sham-operated controls. LV glucose and fatty acid (FA) uptake was also evaluated in vivo by positron emission tomography in 8-week AR rats treated or not with fenofibrate, an activator of FA oxidation (FAO). Results. Many LV genes associated with mitochondrial function and metabolism were downregulated in AR rats. FA β-oxidation capacity was significantly impaired as early as two weeks after AR. Treatment with fenofibrate, a PPARα agonist, normalized both FA and glucose uptake while reducing LV dilation caused by AR. Conclusion. Myocardial energy substrate preference is affected early in the evolution of LV-VO cardiomyopathy. Maintaining a relatively normal FA utilization in the myocardium could translate into less glucose uptake and possibly lesser LV remodeling.

Investigation of metabolic changes in STZ-induced diabetic rats with hyperpolarized [1-13C]acetate

In the metabolism of acetate several enzymes are involved, which play an important role in free fatty acid oxidation. Fatty acid metabolism is altered in diabetes patients and therefore acetate might serve as a marker for pathological changes in the fuel selection of cells, as these changes occur in diabetes patients. Acetylcarnitine is a metabolic product of acetate, which enables its transport into the mitochondria for energy production. This study investigates whether the ratio of acetylcarnitine to acetate, measured by noninvasive hyperpolarized [1-¹³C]acetate magnetic resonance spectroscopy, could serve as a marker for myocardial, hepatic, and renal metabolic changes in rats with Streptozotocin (STZ)-induced diabetes in vivo. We demonstrate that the conversion of acetate to acetylcarnitine could be detected and quantified in all three organs of interest. More interestingly, we found that the hyperpolarized acetylcarnitine to acetate ratio was independent of blood glucose levels and prolonged hyperglycemia following diabetes induction in a type-1 diabetes model.

Endurance training or beta-blockade can partially block the energy metabolism remodeling taking place in experimental chronic left ventricle volume overload

Background

Patients with chronic aortic valve regurgitation (AR) causing left ventricular (LV) volume overload can remain asymptomatic for many years despite having a severely dilated heart. The sudden development of heart failure is not well understood but alterations of myocardial energy metabolism may be contributive. We studied the evolution of LV energy metabolism in experimental AR.

Methods

LV glucose utilization was evaluated in vivo by positron emission tomography (microPET) scanning of 6-month AR rats. Sham-operated or AR rats (n = 10-30 animals/group) were evaluated 3, 6 or 9 months post-surgery. We also tested treatment intervention in order to evaluate their impact on metabolism. AR rats (20 animals) were trained on a treadmill 5 times a week for 9 months and another group of rats received a beta-blockade treatment (carvedilol) for 6 months.

Results

MicroPET revealed an abnormal increase in glucose consumption in the LV free wall of AR rats at 6 months. On the other hand, fatty acid beta-oxidation was significantly reduced compared to sham control rats 6 months post AR induction. A significant decrease in citrate synthase and complex 1 activity suggested that mitochondrial oxidative phosphorylation was also affected maybe as soon as 3 months post-AR.

Moderate intensity endurance training starting 2 weeks post-AR was able to partially normalize the activity of various myocardial enzymes implicated in energy metabolism. The same was true for the AR rats treated with carvedilol (30 mg/kg/d). Responses to these interventions were different at the level of gene expression. We measured mRNA levels of a number of genes implicated in the transport of energy substrates and we observed that training did not reverse the general down-regulation of these genes in AR rats whereas carvedilol normalized the expression of most of them.

Conclusion

This study shows that myocardial energy metabolism remodeling taking place in the dilated left ventricle submitted to severe volume overload from AR can be partially avoided by exercise or beta-blockade in rats.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2261-14-190) contains supplementary material, which is available to authorized users.

Metabolic imaging of hyperpolarized [1-(13) C]acetate and [1-(13) C]acetylcarnitine - investigation of the influence of dobutamine induced stress

PURPOSE

The metabolism of acetate in the heart resembles fatty acid metabolism, which is altered in several diseases like ischemia, diabetes mellitus, and heart failure. A signal-to-noise ratio (SNR) optimized imaging framework for in vivo measurements of hyperpolarized [1-(13) C]acetate and its metabolic product [1-(13) C]acetylcarnitine (ALCAR) in rats at 3 Tesla (T) is presented in this work.

METHODS

A spectrospatial pulse was combined with IDEAL encoding to acquire well separated metabolic maps. The influence of dobutamine induced stress onto this metabolic system was investigated in spectra and in an imaging study.

RESULTS

An increase of the ALCAR to acetate ratio with dobutamine induced stress was shown in slice selective spectra containing the rat hearts and skeletal muscles. Metabolic maps of acetate and ALCAR were acquired with an acceptable SNR. Quantification of the apparent conversion rate showed stable results in the heart in a time-window of 30 s. The effect of dobutamine on the signal intensities was shown to originate mainly from skeletal than cardiac muscles.

CONCLUSION

The acetate activation was mapped with hyperpolarized [1-(13) C]acetate in a clinical 3T system. Quantitative measurement of the activity was possible in the heart, indicating that dobutamine induced stress does not improve the ALCAR SNR in the heart.

[(11)C]-Acetoacetate PET imaging: a potential early marker for cardiac heart failure

The ketone body acetoacetate could be used as an alternate nutrient for the heart, and it also has the potential to improve cardiac function in an ischemic-reperfusion model or reduce the mitochondrial production of oxidative stress involved in cardiotoxicity. In this study, [(11)C]-acetoacetate was investigated as an early marker of intracellular damage in heart failure.

METHODS

A rat cardiotoxicity heart failure model was induced by doxorubicin, Dox(+). [(14)C]-Acetoacetate, a non-positron (β-) emitting radiotracer, was used to characterize the arterial blood input function and myocardial mitochondrial uptake. Afterward, [(11)C]-acetoacetate (β+) myocardial PET images were obtained for kinetic analysis and heart function assessment in control Dox(-) (n=15) and treated Dox(+) (n=6) rats. The uptake rate (K1) and myocardial clearance rate (k2or kmono) were extracted.

RESULTS

[(14)C]-Acetoacetate in the blood was increased in Dox(+), from 2 min post-injection until the last withdrawal point when the heart was harvested, as well as the uptake in the heart and myocardial mitochondria (unpaired t-test, p <0.05). PET kinetic analysis of [(11)C]-acetoacetate showed that rate constants K1, k2 and kmono were decreased in Dox(+) (p <0.05) combined with a reduction of 24% of the left ventricular ejection fraction (p <0.001).

CONCLUSION

Radioactive acetoacetate ex vivo analysis [(14)C], and in vivo kinetic [(11)C] studies provided evidence that [(11)C]-acetoacetate can assess heart failure Dox(+). Contrary to myocardial flow reserve (rest-stress protocol), [(11)C]-acetoacetate can be used to assess reduced kinetic rate constants without requirement of hyperemic stress response. The proposed [(11)C]-acetoacetate cardiac radiotracer in the investigation of heart disease is novel and paves the way to a potential role for [(11)C]-acetoacetate in cardiac pathophysiology.

Angiotensin II-converting enzyme inhibition improves survival, ventricular remodeling, and myocardial energetics in experimental aortic regurgitation

BACKGROUND

Aortic valve regurgitation (AR) is a volume-overload disease causing severe eccentric left ventricular (LV) hypertrophy and eventually heart failure. There is currently no approved drug to treat patients with AR. Many vasodilators including angiotensin-converting enzyme inhibitors have been evaluated in clinical trials, but although some results were promising, others were inconclusive. Overall, no drug has yet been able to improve clinical outcome in AR and the controversy remains. We have previously shown in an animal model that captopril (Cpt) reduced LV hypertrophy and protected LV systolic function, but we had not evaluated the clinical outcome. This protocol was designed to evaluate the effects of a long-term Cpt treatment on survival in the same animal model of severe aortic valve regurgitation.

METHODS AND RESULTS

Forty Wistar rats with AR were treated or untreated with Cpt (1 g/L in drinking water) for a period of 7 months to evaluate survival, myocardial remodeling, and function by echocardiography as well as myocardial metabolism by µ positron emission tomography scan. Survival was significantly improved in Cpt-treated animals with a survival benefit visible as soon as after 4 months of treatment. Cpt reduced LV dilatation and LV hypertrophy. It also significantly improved the myocardial metabolic profile by restoring the level of fatty acids metabolic enzymes and use.

CONCLUSIONS

In a controlled animal model of pure severe aortic valve regurgitation, Cpt treatment reduced LV remodeling and LV hypertrophy and improved myocardial metabolic profile and survival. These results support the need to reevaluate the role of angiotensin-converting enzyme inhibitors in humans with AR in a large, carefully designed prospective clinical trial.

High-resolution positron emission tomography/computed tomography imaging of the mouse heart

Different animal models have been used to reproduce coronary heart disease, but in recent years mice have become the animals of choice, because of their short life cycle and the possibility of genetic manipulation. Various techniques are currently used for cardiovascular imaging in mice, including high-resolution ultrasound, X-ray computed tomography (CT), magnetic resonance imaging and nuclear medicine procedures. In particular, molecular imaging with cardiac positron emission tomography (PET) allows non-invasive evaluation of changes in myocardial perfusion, metabolism, apoptosis, inflammation and gene expression or measurement of changes in left ventricular functional parameters. With technological advances, dedicated small laboratory PET/CT imaging has emerged in cardiovascular research, providing in vivo a non-invasive, serial and quantitative assessment of left ventricular function, myocardial perfusion and metabolism at a molecular level. This non-invasive methodology might be useful in longitudinal studies to monitor cardiac biochemical parameters and might facilitate studies to assess the effect of different interventions after acute myocardial ischaemia.