Exacerbation of cardiac energetic impairment during exercise in hypertrophic cardiomyopathy: a potential mechanism for diastolic dysfunction

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.

2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline for the Management of Hypertrophic Cardiomyopathy: A Report of the American Heart Association/American College of Cardiology Joint Committee on Clinical Practice Guidelines

AIM

The "2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline for the Management of Hypertrophic Cardiomyopathy" provides recommendations to guide clinicians in the management of patients with hypertrophic cardiomyopathy.

METHODS

A comprehensive literature search was conducted from September 14, 2022, to November 22, 2022, encompassing studies, reviews, and other evidence on human subjects that were published in English from PubMed, EMBASE, the Cochrane Library, the Agency for Healthcare Research and Quality, and other selected databases relevant to this guideline. Additional relevant studies, published through May 23, 2023, during the guideline writing process, were also considered by the writing committee and added to the evidence tables, where appropriate.

STRUCTURE

Hypertrophic cardiomyopathy remains a common genetic heart disease reported in populations globally. Recommendations from the "2020 AHA/ACC Guideline for the Diagnosis and Treatment of Patients With Hypertrophic Cardiomyopathy" have been updated with new evidence to guide clinicians.

2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline for the Management of Hypertrophic Cardiomyopathy: A Report of the American Heart Association/American College of Cardiology Joint Committee on Clinical Practice Guidelines

AIM

The "2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline for the Management of Hypertrophic Cardiomyopathy" provides recommendations to guide clinicians in the management of patients with hypertrophic cardiomyopathy.

METHODS

A comprehensive literature search was conducted from September 14, 2022, to November 22, 2022, encompassing studies, reviews, and other evidence on human subjects that were published in English from PubMed, EMBASE, the Cochrane Library, the Agency for Healthcare Research and Quality, and other selected databases relevant to this guideline. Additional relevant studies, published through May 23, 2023, during the guideline writing process, were also considered by the writing committee and added to the evidence tables, where appropriate.

STRUCTURE

Hypertrophic cardiomyopathy remains a common genetic heart disease reported in populations globally. Recommendations from the "2020 AHA/ACC Guideline for the Diagnosis and Treatment of Patients With Hypertrophic Cardiomyopathy" have been updated with new evidence to guide clinicians.

Maintaining energy provision in the heart: the creatine kinase system in ischaemia-reperfusion injury and chronic heart failure

The non-stop provision of chemical energy is of critical importance to normal cardiac function, requiring the rapid turnover of ATP to power both relaxation and contraction. Central to this is the creatine kinase (CK) phosphagen system, which buffers local ATP levels to optimise the energy available from ATP hydrolysis, to stimulate energy production via the mitochondria and to smooth out mismatches between energy supply and demand. In this review, we discuss the changes that occur in high-energy phosphate metabolism (i.e., in ATP and phosphocreatine) during ischaemia and reperfusion, which represents an acute crisis of energy provision. Evidence is presented from preclinical models that augmentation of the CK system can reduce ischaemia-reperfusion injury and improve functional recovery. Energetic impairment is also a hallmark of chronic heart failure, in particular, down-regulation of the CK system and loss of adenine nucleotides, which may contribute to pathophysiology by limiting ATP supply. Herein, we discuss the evidence for this hypothesis based on preclinical studies and in patients using magnetic resonance spectroscopy. We conclude that the correlative evidence linking impaired energetics to cardiac dysfunction is compelling; however, causal evidence from loss-of-function models remains equivocal. Nevertheless, proof-of-principle studies suggest that augmentation of CK activity is a therapeutic target to improve cardiac function and remodelling in the failing heart. Further work is necessary to translate these findings to the clinic, in particular, a better understanding of the mechanisms by which the CK system is regulated in disease.

Integrated Functions of Cardiac Energetics, Mechanics, and Purine Nucleotide Metabolism

Purine nucleotides play central roles in energy metabolism in the heart. Most fundamentally, the free energy of hydrolysis of the adenine nucleotide adenosine triphosphate (ATP) provides the thermodynamic driving force for numerous cellular processes including the actin-myosin crossbridge cycle. Perturbations to ATP supply and/or demand in the myocardium lead to changes in the homeostatic balance between purine nucleotide synthesis, degradation, and salvage, potentially affecting myocardial energetics and, consequently, myocardial mechanics. Indeed, both acute myocardial ischemia and decompensatory remodeling of the myocardium in heart failure are associated with depletion of myocardial adenine nucleotides and with impaired myocardial mechanical function. Yet there remain gaps in the understanding of mechanistic links between adenine nucleotide degradation and contractile dysfunction in heart disease. The scope of this article is to: (i) review current knowledge of the pathways of purine nucleotide depletion and salvage in acute ischemia and in chronic heart disease; (ii) review hypothesized mechanisms linking myocardial mechanics and energetics with myocardial adenine nucleotide regulation; and (iii) highlight potential targets for treating myocardial metabolic and mechanical dysfunction associated with these pathways. It is hypothesized that an imbalance in the degradation, salvage, and synthesis of adenine nucleotides leads to a net loss of adenine nucleotides in both acute ischemia and under chronic high-demand conditions associated with the development of heart failure. This reduction in adenine nucleotide levels results in reduced myocardial ATP and increased myocardial inorganic phosphate. Both of these changes have the potential to directly impact tension development and mechanical work at the cellular level. © 2024 American Physiological Society. Compr Physiol 14:5345-5369, 2024.

A high-throughput drug screening identifies luteolin as a therapeutic candidate for pathological cardiac hypertrophy and heart failure

Background

Pathological cardiac hypertrophy is commonly resulted from sustained pressure overload and/or metabolic disorder and eventually leads to heart failure, lacking specific drugs in clinic. Here, we aimed to identify promising anti-hypertrophic drug(s) for heart failure and related metabolic disorders by using a luciferase reporter-based high-throughput screening.

Methods

A screen of the FDA-approved compounds based on luciferase reporter was performed, with identified luteolin as a promising anti-hypertrophic drug. We systematically examined the therapeutic efficacy of luteolin on cardiac hypertrophy and heart failure in vitro and in vivo models. Transcriptome examination was performed to probe the molecular mechanisms of luteolin.

Results

Among 2,570 compounds in the library, luteolin emerged as the most robust candidate against cardiomyocyte hypertrophy. Luteolin dose-dependently blocked phenylephrine-induced cardiomyocyte hypertrophy and showed extensive cardioprotective roles in cardiomyocytes as evidenced by transcriptomics. More importantly, gastric administration of luteolin effectively ameliorated pathological cardiac hypertrophy, fibrosis, metabolic disorder, and heart failure in mice. Cross analysis of large-scale transcriptomics and drug-target interacting investigations indicated that peroxisome proliferator activated receptor γ (PPARγ) was the direct target of luteolin in the setting of pathological cardiac hypertrophy and metabolic disorders. Luteolin can directly interact with PPARγ to inhibit its ubiquitination and subsequent proteasomal degradation. Furthermore, PPARγ inhibitor and PPARγ knockdown both prevented the protective effect of luteolin against phenylephrine-induced cardiomyocyte hypertrophy in vitro.

Conclusion

Our data clearly supported that luteolin is a promising therapeutic compound for pathological cardiac hypertrophy and heart failure by directly targeting ubiquitin-proteasomal degradation of PPARγ and the related metabolic homeostasis.

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.

Left Atrial Myopathy is Associated With Exercise Incapacity and Ventilatory Inefficiency in Hypertrophic Cardiomyopathy

BACKGROUND

Left atrial (LA) myopathy is an established component of hypertrophic cardiomyopathy (HCM); however, the data about its association with exercise incapacity or ventilatory inefficiency that may be seen in HCM patients are limited. This study aimed to explore the association between LA myopathy, evaluated by echocardiography LA strain, and exercise capacity and ventilatory efficiency, evaluated by cardiopulmonary exercise testing (CPET), in HCM patients.

METHODS

This study included 241 consecutive HCM patients (aged 51.2±15.7 years 67.2% male) in sinus rhythm who underwent CPET and transthoracic echocardiography at the same visit. Exercise incapacity (maximal/predicted oxygen consumption [%peakVO₂] <80%) and ventilatory inefficiency (ventilation/carbon dioxide output [VE/VCO₂] slope >34) were assessed by CPET. Left atrial myopathy was examined by speckle-tracking myocardial deformation parameters: LA reservoir, conduit and booster strain.

RESULTS

All three LA strain values were univariate predictors of exercise capacity and ventilatory efficiency. Among them, LA reservoir strain had the higher r correlation coefficient for predicting both %peakVO₂ and VE/VCO₂ slope. Left atrial reservoir strain, presence of angina and family history of HCM were independent predictors of exercise capacity. Left atrial reservoir strain, male gender and non-sustained ventricular tachycardia were independent predictors of ventilatory efficiency. Left atrial reservoir strain was a significant predictor of %peakVO₂<80% with an optimal cut-off value of 27% (sensitivity 87% and specificity 31%) and VE/VCO₂>34 with an optimal cut-off value of 18% (sensitivity 71% and specificity 83%).

CONCLUSION

Left atrial myopathy, as reflected by the LA strain values, was associated with exercise incapacity and ventilatory inefficiency in HCM individuals. Left atrial reservoir strain was the only common independent predictor of %peakVO₂ and VE/VCO₂ slope.

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.

Imaging Methods: Magnetic Resonance Imaging

Myocardial inflammation occurs following activation of the cardiac immune system, producing characteristic changes in the myocardial tissue. Cardiovascular magnetic resonance is the non-invasive imaging gold standard for myocardial tissue characterization, and is able to detect image signal changes that may occur resulting from inflammation, including edema, hyperemia, capillary leak, necrosis, and fibrosis. Conventional cardiovascular magnetic resonance for the detection of myocardial inflammation and its sequela include T2-weighted imaging, parametric T1- and T2-mapping, and gadolinium-based contrast-enhanced imaging. Emerging techniques seek to image several parameters simultaneously for myocardial tissue characterization, and to depict subtle immune-mediated changes, such as immune cell activity in the myocardium and cardiac cell metabolism. This review article outlines the underlying principles of current and emerging cardiovascular magnetic resonance methods for imaging myocardial inflammation.

Magnetic resonance imaging of cardiac metabolism in heart failure: how far have we come?

As one of the highest energy consumer organs in the body, the heart requires tremendous amount of adenosine triphosphate (ATP) to maintain its continuous mechanical work. Fatty acids, glucose, and ketone bodies are the primary fuel source of the heart to generate ATP with perturbations in ATP generation possibly leading to contractile dysfunction. Cardiac metabolic imaging with magnetic resonance imaging (MRI) plays a crucial role in understanding the dynamic metabolic changes occurring in the failing heart, where the cardiac metabolism is deranged. Also, targeting and quantifying metabolic changes in vivo noninvasively is a promising approach to facilitate diagnosis, determine prognosis, and evaluate therapeutic response. Here, we summarize novel MRI techniques used for detailed investigation of cardiac metabolism in heart failure including magnetic resonance spectroscopy (MRS), hyperpolarized MRS, and chemical exchange saturation transfer based on evidence from preclinical and clinical studies and to discuss the potential clinical application in heart failure.

Coexistent Diabetes Is Associated With the Presence of Adverse Phenotypic Features in Patients With Hypertrophic Cardiomyopathy

OBJECTIVE

Type 2 diabetes mellitus (T2DM) is associated with worsened clinical outcomes in hypertrophic cardiomyopathy (HCM) patients. We sought to investigate whether HCM patients with T2DM comorbidity exhibit adverse cardiac alterations in myocardial energetics, function, perfusion, or tissue characteristics.

RESEARCH DESIGN AND METHODS

A total of 55 participants with concomitant HCM and T2DM (HCM-DM) (n = 20) or isolated HCM (n = 20) and healthy volunteers (HV) (n = 15) underwent 31P-MRS and cardiovascular MRI. The HCM groups were matched for HCM phenotype.

RESULTS

Mean ± SD European Society of Cardiology sudden cardiac death risk scores were comparable between the HCM groups (HCM 2.2 ± 1.5%, HCM-DM 1.9 ± 1.2%; P = not significant), and sarcomeric mutations were equally common. HCM-DM patients had the highest median NT-proBNP levels (HV 42 ng/L [interquartile range 35-66], HCM 298 ng/L [157-837], HCM-DM 726 ng/L [213-8,695]; P < 0.0001). Left ventricular (LV) ejection fraction, mass, and wall thickness were similar between the HCM groups. HCM-DM patients displayed a greater degree of fibrosis burden with higher scar percentage and lower global longitudinal strain compared with HCM patients. PCr/ATP (the relative concentrations of phosphocreatine and ATP) was significantly lower in the HCM-DM group than in both HCM and HV (HV 2.17 ± 0.49, HCM 1.93 ± 0.38, HCM-DM 1.54 ± 0.27; P = 0.002). In a similar pattern, stress myocardial blood flow was significantly lower in the HCM-DM group than in both HCM and HV (HV 2.06 ± 0.42 mL/min/g, HCM 1.74 ± 0.44 mL/min/g, HCM-DM 1.39 ± 0.42 mL/min/g; P = 0.002).

CONCLUSIONS

We show for the first time that HCM-DM patients display greater reductions in myocardial energetics, perfusion, and contractile function and higher myocardial scar burden and serum NT-proBNP levels compared with patients with isolated HCM despite similar LV mass and wall thickness and presence of sarcomeric mutations. These adverse phenotypic features may be important components of the adverse clinical manifestation attributable to a combined presence of HCM and T2DM.

Efficacy and Safety of Hybrid Cardiac Telerehabilitation in Patients with Hypertrophic Cardiomyopathy without Left Ventricular Outflow Tract Obstruction and Preserved Ejection Fraction—A Study Design

Hypertrophic cardiomyopathy (HCM) is the most common congenital disease increasing the risk of sudden cardiac death. For many years, HCM patients were excluded from exercise training. However, there are data showing that patients with HCM undergoing supervised exercise training could improve physical performance without serious adverse events. A project was designed as a randomized clinical trial to assess the effectiveness and safety of hybrid cardiac rehabilitation (HCR)—a combination of hospital-based cardiac rehabilitation (1 month) with a new form of home-based telemonitored cardiac rehabilitation (2 months) in HCM patients without left ventricular (LV) outflow tract obstruction and preserved systolic function. Sixty patients who fulfil the inclusion criteria have been randomly assigned (1:1) to either HCR plus usual care (training group) or usual care only (control group). The primary endpoint is a functional capacity evaluated by peak oxygen uptake (pVO2). Secondary endpoints include workload time during the cardiopulmonary exercise testing, a six-minute walk test distance, NT-pro BNP level, echocardiographic parameters of the left ventricular diastolic function (E/A, E/e’, myocardial strain rate), right ventricular systolic pressure, a gradient in the LV outflow tract, and quality of life. The tertiary analysis includes safety, acceptance and adherence to the HCR program. Our research will provide innovative data on the effectiveness and safety of hybrid cardiac rehabilitation in HCM patients without LV outflow tract obstruction and preserved systolic function. Clinical trials registry: ClinicalTrials.gov Identifier NCT03178357.

Exercise oxygen pulse kinetics in patients with hypertrophic cardiomyopathy

OBJECTIVES

Reduced cardiac output (CO) has been considered crucial in symptoms' genesis in hypertrophic cardiomyopathy (HCM). Absolute value and temporal behaviour of O₂-pulse (oxygen uptake/heart rate (VO₂/HR)), and the VO₂/work relationship during exercise reflect closely stroke volume (SV) and CO changes, respectively. We hypothesise that adding O₂-pulse absolute value and kinetics, and VO₂/work relationship to standard cardiopulmonary exercise testing (CPET) could help identify more exercise-limited patients with HCM.

METHODS

CPETs were performed in 3 HCM dedicated clinical units. We retrospectively enrolled non-end-stage consecutive patients with HCM, grouped according to left ventricle outflow tract obstruction (LVOTO) at rest or during Valsalva manoeuvre (72% of patients with LVOTO <30; 10% between 30 and 49 and 18% ≥50 mm Hg). We evaluated the CPET response in HCM focusing on parameters strongly associated with SV and CO, such as O₂-pulse and VO₂, respectively, considering their absolute values and temporal behaviour during exercise.

RESULTS

We included 312 patients (70% males, age 49±18 years). Peak VO₂ (percentage of predicted), O₂-pulse and ventilation to carbon dioxide production (VE/VCO₂) slope did not change across LVOTO groups. Ninety-six (31%) patients with HCM presented an abnormal O₂-pulse temporal behaviour, irrespective of LVOTO values. These patients showed lower peak systolic pressure, workload (106±45 vs 130±49 W), VO₂ (21.3±6.6 vs 24.1±7.7 mL/min/kg; 74%±17% vs 80%±20%) and O₂-pulse (12 (9-14) vs 14 (11-17) mL/beat), with higher VE/VCO₂ slope (28 (25-31) vs 27 (24-31)) (p<0.005 for all). Only 2 patients had an abnormal VO₂/work slope.

CONCLUSION

None of the frequently used CPET parameters, either as absolute values or dynamic relationships, were associated with LVOTO. Differently, an abnormal temporal behaviour of O₂-pulse during exercise, which is strongly related to inadequate SV increase, correlates with reduced functional capacity (peak and anaerobic threshold VO₂ and workload) and increased VE/VCO₂ slope, identifying more advanced disease irrespectively of LVOTO.

Critical Evaluation of Current Hypotheses for the Pathogenesis of Hypertrophic Cardiomyopathy

Hereditary hypertrophic cardiomyopathy (HCM), due to mutations in sarcomere proteins, occurs in more than 1/500 individuals and is the leading cause of sudden cardiac death in young people. The clinical course exhibits appreciable variability. However, typically, heart morphology and function are normal at birth, with pathological remodeling developing over years to decades, leading to a phenotype characterized by asymmetric ventricular hypertrophy, scattered fibrosis and myofibrillar/cellular disarray with ultimate mechanical heart failure and/or severe arrhythmias. The identity of the primary mutation-induced changes in sarcomere function and how they trigger debilitating remodeling are poorly understood. Support for the importance of mutation-induced hypercontractility, e.g., increased calcium sensitivity and/or increased power output, has been strengthened in recent years. However, other ideas that mutation-induced hypocontractility or non-uniformities with contractile instabilities, instead, constitute primary triggers cannot yet be discarded. Here, we review evidence for and criticism against the mentioned hypotheses. In this process, we find support for previous ideas that inefficient energy usage and a blunted Frank–Starling mechanism have central roles in pathogenesis, although presumably representing effects secondary to the primary mutation-induced changes. While first trying to reconcile apparently diverging evidence for the different hypotheses in one unified model, we also identify key remaining questions and suggest how experimental systems that are built around isolated primarily expressed proteins could be useful.

Major Clinical Issues in Hypertrophic Cardiomyopathy

By actively implementing contemporary management strategies in hypertrophic cardiomyopathy, morbidity and mortality can be substantially reduced. In this review, we discuss the pathophysiology and management of the major clinical issues in hypertrophic cardiomyopathy, including sudden cardiac death, atrial fibrillation and thromboembolism, dynamic left ventricular outflow tract obstruction, and heart failure progression. Although echocardiography and cardiac magnetic resonance imaging currently play an essential and complementary role in the management of hypertrophic cardiomyopathy, further studies are needed to establish how developing techniques such as myocardial deformation and late gadolinium enhancement can provide better risk stratification and guide treatment.

Hypertrophic cardiomyopathy (HCM) is one of the most common inheritable cardiomyopathies. Contemporary management strategies, including the advent of implantable cardioverter-defibrillators and effective anticoagulation, have substantially improved the clinical course of HCM patients; however, the disease burden of HCM is still high in Korea. Sudden cardiac death (SCD), atrial fibrillation and thromboembolic risk, dynamic left ventricular outflow tract (LVOT) obstruction, and heart failure (HF) progression remain important issues in HCM. SCD in HCM can be effectively prevented with implantable cardioverter-defibrillators. However, appropriate patient selection is important for primary prevention, and the 5-year SCD risk score and the presence of major SCD risk factors should be considered. Anticoagulation should be initiated in all HCM patients with atrial fibrillation regardless of the CHA₂DS₂-VASc score, and non-vitamin K antagonist oral anticoagulants are the first option. Symptomatic dynamic LVOT obstruction is first treated medically with negative inotropes, and if symptoms persist, septal reduction therapy is considered. The recently approved myosin inhibitor mavacamten is promising. HF in HCM is usually related to diastolic dysfunction, while about 5% of HCM patients show reduced left ventricular ejection fraction <50%, also referred to as “end-stage” HCM. Myocardial fibrosis plays an important role in the progression to advanced HF in patients with HCM. Patients who do not respond to guideline-directed medical therapy can be considered for heart transplantation. The development of imaging techniques, such as myocardial deformation on echocardiography and late gadolinium enhancement on cardiac magnetic resonance, can provide better risk evaluation and decision-making for management strategies in HCM.

PPAR control of metabolism and cardiovascular functions

Peroxisome proliferator-activated receptor-α (PPARα), PPARδ and PPARγ are transcription factors that regulate gene expression following ligand activation. PPARα increases cellular fatty acid uptake, esterification and trafficking, and regulates lipoprotein metabolism genes. PPARδ stimulates lipid and glucose utilization by increasing mitochondrial function and fatty acid desaturation pathways. By contrast, PPARγ promotes fatty acid uptake, triglyceride formation and storage in lipid droplets, thereby increasing insulin sensitivity and glucose metabolism. PPARs also exert antiatherogenic and anti-inflammatory effects on the vascular wall and immune cells. Clinically, PPARγ activation by glitazones and PPARα activation by fibrates reduce insulin resistance and dyslipidaemia, respectively. PPARs are also physiological master switches in the heart, steering cardiac energy metabolism in cardiomyocytes, thereby affecting pathological heart failure and diabetic cardiomyopathy. Novel PPAR agonists in clinical development are providing new opportunities in the management of metabolic and cardiovascular diseases.

Myocardial inflammation and energetics by cardiac MRI: a review of emerging techniques

The role of inflammation in cardiovascular pathophysiology has gained a lot of research interest in recent years. Cardiovascular Magnetic Resonance has been a powerful tool in the non-invasive assessment of inflammation in several conditions. More recently, Ultrasmall superparamagnetic particles of iron oxide have been successfully used to evaluate macrophage activity and subsequently inflammation on a cellular level. Current evidence from research studies provides encouraging data and confirms that this evolving method can potentially have a huge impact on clinical practice as it can be used in the diagnosis and management of very common conditions such as coronary artery disease, ischaemic and non-ischaemic cardiomyopathy, myocarditis and atherosclerosis. Another important emerging concept is that of myocardial energetics. With the use of phosphorus magnetic resonance spectroscopy, myocardial energetic compromise has been proved to be an important feature in the pathophysiological process of several conditions including diabetic cardiomyopathy, inherited cardiomyopathies, valvular heart disease and cardiac transplant rejection. This unique tool is therefore being utilized to assess metabolic alterations in a wide range of cardiovascular diseases. This review systematically examines these state-of-the-art methods in detail and provides an insight into the mechanisms of action and the clinical implications of their use.

Investigating the effect of trigger delay on cardiac 31P MRS signals

The heart’s geometry and its metabolic activity vary over the cardiac cycle. The effect of these fluctuations on phosphorus (³¹P) magnetic resonance spectroscopy (MRS) data quality and metabolite ratios was investigated. 12 healthy volunteers were measured using a 7 T MR scanner and a cardiac ³¹P-¹H loop coil. ³¹P chemical shift imaging data were acquired untriggered and at four different times during the cardiac cycle using acoustic triggering. Signals of adenosine-triphosphate (ATP), phosphocreatine (PCr), inorganic phosphate (Pi) and 2,3-diphosphoglycerate (2,3-DPG) and their fit quality as Cramér-Rao lower bounds (CRLB) were quantified including corrections for contamination by ³¹P signals from blood, flip angle, saturation and total acquisition time. The myocardial filling factor was estimated from cine short axis views. The corrected signals of PCr and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma$$\end{document}γ-ATP were higher during end-systole and lower during diastasis than in untriggered acquisitions (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$P<0.05$$\end{document}P<0.05). Signal intensities of untriggered scans were between those with triggering to end-systole and diastasis. Fit quality of PCr and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma$$\end{document}γ-ATP peaks was best during end-systole when blood contamination of ATP and Pi signals was lowest. While metabolite ratios and pH remained stable over the cardiac cycle, signal amplitudes correlated strongly with myocardial voxel filling. Triggering of cardiac ³¹P MRS acquisitions improves signal amplitudes and fit quality if the trigger delay is set to end-systole. We conclude that triggering to end-systole is superior to triggering to diastasis.

Exercise Cardiovascular Magnetic Resonance: A Review

While pharmacologic stress cardiovascular magnetic resonance imaging (MRI) is a robust noninvasive tool in the diagnosis and prognostication of epicardial coronary artery disease, clinical guidelines recommend exercise-based testing in those patients who can exercise. This review describes the development of exercise cardiovascular MRI protocols, summarizes the insights across various patient populations, and highlights future research initiatives. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 2.

Cardiovascular Magnetic Resonance for the Differentiation of Left Ventricular Hypertrophy

PURPOSE OF REVIEW

Left ventricular hypertrophy (LVH) is a common presentation encountered in clinical practice with a diverse range of potential aetiologies. Differentiation of pathological from physiological hypertrophy can be challenging but is crucial for further management and prognostication. Cardiovascular magnetic resonance (CMR) with advanced myocardial tissue characterisation is a powerful tool that may help to differentiate these aetiologies in the assessment of LVH.

RECENT FINDINGS

The use of CMR for detailed morphological assessment of LVH is well described. More recently, advanced CMR techniques (late gadolinium enhancement, parametric mapping, diffusion tensor imaging, and myocardial strain) have been used. These techniques are highly promising in helping to differentiate key aetiologies of LVH and provide valuable prognostic information. Recent advancements in CMR tissue characterisation, such as parametric mapping, in combination with detailed morphological assessment and late gadolinium enhancement, provide a powerful resource that may help assess and differentiate important causes of LVH.

Quantifying the effect of dobutamine stress on myocardial Pi and pH in healthy volunteers: A 31 P MRS study at 7T

Purpose

Phosphorus spectroscopy (³¹P‐MRS) is a proven method to probe cardiac energetics. Studies typically report the phosphocreatine (PCr) to adenosine triphosphate (ATP) ratio. We focus on another ³¹P signal: inorganic phosphate (Pi), whose chemical shift allows computation of myocardial pH, with Pi/PCr providing additional insight into cardiac energetics. Pi is often obscured by signals from blood 2,3‐diphosphoglycerate (2,3‐DPG). We introduce a method to quantify Pi in 14 min without hindrance from 2,3‐DPG.

Methods

Using a ³¹P stimulated echo acquisition mode (STEAM) sequence at 7 Tesla that inherently suppresses signal from 2,3‐DPG, the Pi peak was cleanly resolved. Resting state UTE‐chemical shift imaging (PCr/ATP) and STEAM ³¹P‐MRS (Pi/PCr, pH) were undertaken in 23 healthy controls; pH and Pi/PCr were subsequently recorded during dobutamine infusion.

Results

We achieved a clean Pi signal both at rest and stress with good 2,3‐DPG suppression. Repeatability coefficient (8 subjects) for Pi/PCr was 0.036 and 0.12 for pH. We report myocardial Pi/PCr and pH at rest and during catecholamine stress in healthy controls. Pi/PCr was maintained during stress (0.098 ± 0.031 [rest] vs. 0.098 ± 0.031 [stress] P = .95); similarly, pH did not change (7.09 ± 0.07 [rest] vs. 7.08 ± 0.11 [stress] P = .81). Feasibility for patient studies was subsequently successfully demonstrated in a patient with cardiomyopathy.

Conclusion

We introduced a method that can resolve Pi using 7 Tesla STEAM ³¹P‐MRS. We demonstrate the stability of Pi/PCr and myocardial pH in volunteers at rest and during catecholamine stress. This protocol is feasible in patients and potentially of use for studying pathological myocardial energetics.

Exercise cardiovascular magnetic resonance: development, current utility and future applications

Stress cardiac imaging is the current first line investigation for coronary artery disease diagnosis and decision making and an adjunctive tool in a range of non-ischaemic cardiovascular diseases. Exercise cardiovascular magnetic resonance (Ex-CMR) has developed over the past 25 years to combine the superior image qualities of CMR with the preferred method of exercise stress. Presently, numerous exercise methods exist, from performing stress on an adjacent CMR compatible treadmill to in-scanner exercise, most commonly on a supine cycle ergometer. Cardiac conditions studied by Ex-CMR are broad, commonly investigating ischaemic heart disease and congenital heart disease but extending to pulmonary hypertension and diabetic heart disease. This review presents an in-depth assessment of the various Ex-CMR stress methods and the varied pulse sequence approaches, including those specially designed for Ex-CMR. Current and future developments in image acquisition are highlighted, and will likely lead to a much greater clinical use of Ex-CMR across a range of cardiovascular conditions.

Optimal cutting temperature medium embedding and cryostat sectioning are valid for cardiac myofilament function assessment

To maximize data obtainment from valuable cardiac tissue, we hypothesized that myocardium fixed in optimal cutting temperature (OCT) medium for histology could also be used to investigate the function of myofilament proteins in situ. We compared tissue prepared via conventional liquid nitrogen (LN) snap freezing with tissue fixed in OCT and then sectioned in fiber-parallel orientation. We found that actin-myosin Ca²⁺ sensitivity, activation rate by Ca²⁺, cooperativity along the thin filament, as well as cross-bridge cycling rate were unaffected by OCT storage and could reliably be interpreted after sectioning. Absolute values in maximum force generation per cross-sectional area, as well as passive strain, are difficult to investigate after sectioning, as myofibrillar continuity along the preparation cannot be guaranteed. We have shown that myocardial tissue stored in OCT and sectioned before analysis is available for functional analysis, a valuable means of maximizing usage of precious cardiac biopsies.NEW & NOTEWORTHY Myocardial tissue in optimal cutting temperature (OCT) fixation and cryostat sectioning was tested as a means of storing and preparing tissue for myofilament function analysis in relation to conventional liquid nitrogen freezing and dissection. Actomyosin interaction, Ca²⁺ force activation, and passive compliance were tested. The study concluded that OCT storage and cryostat sectioning do not interfere with the actomyosin cross-bridge dynamics or Ca²⁺ activation but that absolute tension values suffer and may not be investigated by this method.

Mitochondrial Energetics and Ca2+-Activated ATPase in Obstructive Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common genetic disease of the myocardium associated to mutations in sarcomeric genes, but the link between genotype and phenotype remains poorly understood. Magnetic resonance spectroscopy studies have demonstrated impaired cardiac energetics in patients with HCM, and altered mitochondria were described in biopsies, but little is known about possible perturbations of mitochondrial function and adenosine triphosphate (ATP) production/consumption. The aim of this study was to investigate possible abnormalities in mitochondrial enzymes generating/scavenging reactive oxygen species, and changes in the Ca²⁺-activated ATPases in myocardial tissue from patients with obstructive HCM undergoing surgical myectomy compared to unused donor hearts (CTRL). Methods and Results: Both the amount and activity of mitochondrial Complex I (nicotinamide adenine dinucleotide -reduced form, NADH, dehydrogenase) were upregulated in HCM vs. CTRL, whilst the activity of Complex V (ATP synthase) was not reduced and ATP levels were significantly higher in HCM vs. CTRL. Antioxidant Mn-activated superoxide dismutase (SOD2) and (m)-aconitase activities were increased in HCM vs. CTRL. The Cu/Zn-activated superoxide dismutase (SOD1) amount and mtDNA copy number were unaltered in HCM. Total Ca²⁺-activated ATPase activity and absolute amount were not different HCM vs. CTRL, but the ratio between ATPase sarcoplasmic/endoplasmic reticulum Ca²⁺ transporting type 2 (ATP2A2) and type 1 (ATP2A1), ATP2A2/ATP2A1, was increased in HCM in favor of the slow isoform (ATP2A2). Conclusion: HCM is characterized by mitochondrial Complex I hyperactivity and preserved Ca²⁺-activated ATPase activity with a partial switch towards slow ATP2A2. This data may give insight into the abnormal cellular energetics observed in HCM cardiomyopathy but other studies would need to be performed to confirm the observations described here.

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.

Differential contributions of sarcomere and mitochondria-related multigene variants to the endophenotype of hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a multigenic disease that occurs due to various genetic modifiers. We investigated phenotype-based clinical and genetic characteristics of HCM patients using comprehensive genetic tests and rare variant association analysis.

METHODS

A comprehensive HCM-specific panel, consisting of 82 nuclear DNAs (nDNAs: 33 sarcomere-associated genes, 5 phenocopy genes, and 44 nuclear genes linked to mitochondrial cardiomyopathy) and 37 mitochondrial DNAs (mtDNAs), was analyzed. Rare variant analysis was performed to determine the association of specific genes with different phenotypes.

RESULTS

Among the 212 patients, pathogenic variants in sarcomere-associated genes were more prevalent in non-apical HCM (41.4%, 46/111; P = 0.001) than apical HCM (20.8%, 21/101). Apical HCM exhibits mild phenotypes than non-apical HCM, and it showed fewer numbers of sarcomere mutations than non-apical HCM. Interestingly, inverted mutation frequency of TNNI3 (35%) and MYH7 (9%) was observed in apical HCM. In a rare variant analysis, MT-RNR2 positively correlated with apical HCM (OR: 1.37, P = 0.025). And, MYBPC3 (sarcomere gene) negatively contributed to apical HCM (OR: 0.54, P = 0.027). On the other hand, both pathogenic mutation (P < 0.05) and rare variants in sarcomere-associated genes (OR: 2.78-3.47, P < 0.05) were related to diastolic dysfunction and left atrium remodeling, which correlated with poor prognosis in HCM patients.

CONCLUSIONS

Our results provide a clue towards explaining the difference between the prevalence and phenotype of apical HCM in Asian populations, and a foundation for genetics-based approaches that may enable individualized risk stratification for HCM patients.

Exercise and myocardial injury in hypertrophic cardiomyopathy

Objective

Troponin and high signal intensity on T2-weighted (HighT2) cardiovascular magnetic resonance imaging (CMRi) are both markers of myocardial injury in hypertrophic cardiomyopathy (HCM). The interplay between exercise and disease development remains uncertain in HCM. We sought to assess the occurrence of postexercise troponin rises and its determinants.

Methods

Multicentre project on patients with HCM and mutation carriers without hypertrophy (controls). Participants performed a symptom limited bicycle test with hs-cTnT assessment pre-exercise and 6 hours postexercise. Pre-exercise CMRi was performed in patients with HCM to assess measures of hypertrophy and myocardial injury. Depending on baseline troponin (< or >13 ng/L), a rise was defined as a >50% or >20% increase, respectively.

Results

Troponin rises occurred in 18% (23/127) of patients with HCM and 4% (2/53) in mutation carriers (p=0.01). Comparing patients with HCM with and without a postexercise troponin rise, maximum heart rates (157±19 vs 143±23, p=0.004) and maximal wall thickness (20 mm vs 17 mm, p=0.023) were higher in the former, as was the presence of late gadolinium enhancement (85% vs 57%, p=0.02). HighT2 was seen in 65% (13/20) and 19% (15/79), respectively (p<0.001). HighT2 was the only independent predictor of troponin rise (adjusted odds ratio 7.9; 95% CI 2.7 to 23.3; p<0.001).

Conclusions

Postexercise troponin rises were seen in about 20% of patients with HCM, almost five times more frequent than in mutation carriers. HighT2 on CMRi may identify a group of particularly vulnerable patients, supporting the concept that HighT2 reflects an active disease state, prone to additional injury after a short episode of high oxygen demand.

Progression of myocardial fibrosis in hypertrophic cardiomyopathy: mechanisms and clinical implications

Aims

Myocardial fibrosis as detected by late gadolinium enhancement (LGE) on cardiac magnetic resonance (CMR) is a powerful prognostic marker in hypertrophic cardiomyopathy (HCM) and may be progressive. The precise mechanisms underlying fibrosis progression are unclear. We sought to assess the extent of LGE progression in HCM and explore potential causal mechanisms and clinical implications.

Methods and results

Seventy-two HCM patients had two CMR (CMR1-CMR2) at an interval of 5.7 ± 2.8 years with annual clinical follow-up for 6.3 ± 3.6 years from CMR1. A combined endpoint of heart failure progression, cardiac hospitalization, and new onset ventricular tachycardia was assessed. Cine and LGE imaging were performed to assess left ventricular (LV) mass, function, and fibrosis on serial CMR. Stress perfusion imaging and cardiac energetics were undertaken in 38 patients on baseline CMR (CMR1). LGE mass increased from median 4.98 g [interquartile range (IQR) 0.97–13.48 g] to 6.30 g (IQR 1.38–17.51 g) from CMR1 to CMR2. Substantial LGE progression (ΔLGE ≥ 4.75 g) occurred in 26% of patients. LGE increment was significantly higher in those with impaired myocardial perfusion reserve (<MPRI 1.40) and energetics (phosphocreatine/adenosine triphosphate <1.44) on baseline CMR (P ≤ 0.01 for both). Substantial LGE progression was associated with LV thinning, increased cavity size and reduced systolic function, and conferred a five-fold increased risk of subsequent clinical events (hazard ratio 5.04, 95% confidence interval 1.85–13.79; P = 0.002).

Conclusion

Myocardial fibrosis is progressive in some HCM patients. Impaired energetics and perfusion abnormalities are possible mechanistic drivers of the fibrotic process. Fibrosis progression is associated with adverse cardiac remodelling and predicts an increased risk of subsequent clinical events in HCM.

MR compatible ergometers for dynamic 31P MRS

Magnetic Resonance (MR) compatible ergometers are specialized ergometers used inside the MR scanners for the characterization of tissue metabolism changes during physical stress. They are most commonly used for dynamic phosphorous magnetic resonance spectroscopy (31P MRS), but can also be used for lactate production measurements, perfusion studies using arterial spin labelling or muscle oxygenation measurements by blood oxygen dependent contrast sequences. We will primarily discuss the importance of ergometers in the context of dynamic 31P MRS. Dynamic 31P MRS can monitor muscle fatigue and energy reserve during muscle contractions as well as the dynamics of recuperation of skeletal muscle tissue during the following recovery through signal changes of phosphocreatine (PCr), inorganic phosphate and adenosine triphosphate (ATP). Based on the measured data it is possible to calculate intracellular pH, metabolic flux of ATP through creatine-kinase reaction, anaerobic glycolysis and oxidative phosphorylation and other metabolic parameters as mitochondrial capacity. This review primarily focuses on describing various technical designs of MR compatible ergometers for dynamic 31P MRS that must be constructed with respect to the presence of magnetic field. It is also expected that the construction of ergometers will be easy for the handling and well accepted by examined subjects.

Association between Lifelong Physical Activity and Disease Characteristics in HCM

PURPOSE

Hypertrophic cardiomyopathy (HCM) is characterized by inappropriate left ventricular (LV) wall thickness. Adaptations to exercise can occasionally mimic certain HCM characteristics. However, it is unclear whether physical activity affects HCM genotype expression and disease characteristics. Consequently, we compared lifelong physical activity volumes between HCM gene carriers with and without HCM phenotype, and compared disease characteristics among tertiles of physical activity in phenotypic HCM patients.

METHODS

We enrolled n = 22 genotype positive/phenotype negative (G+/P-) HCM gene carriers, n = 44 genotype positive/phenotype positive (G+/P+) HCM patients, and n = 36 genotype negative/phenotype positive (G-/P+) HCM patients. Lifelong physical activity was recorded using a questionnaire and quantified as metabolic equivalent of task hours per week.

RESULTS

We included 102 participants (51 ± 16 yr, 49% male). Lifelong physical activity volumes were not different between G+/P+ and G+/P- subjects (16 [10-29] vs 14 [6-26] metabolic equivalent of task-hours per week, P = 0.33). Among phenotypic HCM patients, there was no difference in LV wall thickness, mass, and late gadolinium enhancement across physical activity tertiles. Patients with the highest reported physical activity volumes were younger at the time of diagnosis (tertile 1: 52 ± 14 yr, tertile 2: 49 ± 15 yr, tertile 3: 41 ± 18 yr; P = 0.03), and more often had a history of nonsustained ventricular tachycardia (4% vs 30% vs 30%, P = 0.03).

CONCLUSIONS

Lifelong physical activity volumes are not associated with genotype-to-phenotype transition in HCM gene carriers. We also found no difference in LV wall thickness across physical activity tertiles. However, the most active HCM patients were younger at the time of diagnosis and had a higher arrhythmic burden. These observations warrant further exploration of the role of exercise in HCM disease development.

Cardiac Disorders and Pathophysiology of Sarcomeric Proteins

The sarcomeric proteins represent the structural building blocks of heart muscle, which are essential for contraction and relaxation. During recent years, it has become evident that posttranslational modifications of sarcomeric proteins, in particular phosphorylation, tune cardiac pump function at rest and during exercise. This delicate, orchestrated interaction is also influenced by mutations, predominantly in sarcomeric proteins, which cause hypertrophic or dilated cardiomyopathy. In this review, we follow a bottom-up approach starting from a description of the basic components of cardiac muscle at the molecular level up to the various forms of cardiac disorders at the organ level. An overview is given of sarcomere changes in acquired and inherited forms of cardiac disease and the underlying disease mechanisms with particular reference to human tissue. A distinction will be made between the primary defect and maladaptive/adaptive secondary changes. Techniques used to unravel functional consequences of disease-induced protein changes are described, and an overview of current and future treatments targeted at sarcomeric proteins is given. The current evidence presented suggests that sarcomeres not only form the basis of cardiac muscle function but also represent a therapeutic target to combat cardiac disease.

The HCM-causing Y235S cMyBPC mutation accelerates contractile function by altering C1 domain structure

Mutations in cardiac myosin binding protein C (cMyBPC) are a major cause of hypertrophic cardiomyopathy (HCM). In particular, a single amino acid substitution of tyrosine to serine at residue 237 in humans (residue 235 in mice) has been linked to HCM with strong disease association. Although cMyBPC truncations, deletions and insertions, and frame shift mutations have been studied, relatively little is known about the functional consequences of missense mutations in cMyBPC. In this study, we characterized the functional and structural effects of the HCM-causing Y235S mutation by performing mechanical experiments and molecular dynamics simulations (MDS). cMyBPC null mouse myocardium was virally transfected with wild-type (WT) or Y235S cMyBPC (KO^Y235S). We found that Y235S cMyBPC was properly expressed and incorporated into the cardiac sarcomere, suggesting that the mechanism of disease of the Y235S mutation is not haploinsufficiency or poison peptides. Mechanical experiments in detergent-skinned myocardium isolated from KO^Y235S hearts revealed hypercontractile behavior compared to KO^WT hearts, evidenced by accelerated cross-bridge kinetics and increased Ca²⁺ sensitivity of force generation. In addition, MDS revealed that the Y235S mutation causes alterations in important intramolecular interactions, surface conformations, and electrostatic potential of the C1 domain of cMyBPC. Our combined in vitro and in silico data suggest that the Y235S mutation directly disrupts internal and surface properties of the C1 domain of cMyBPC, which potentially alters its ligand-binding interactions. These molecular changes may underlie the mechanism for hypercontractile cross-bridge behavior, which ultimately results in the development of cardiac hypertrophy and in vivo cardiac dysfunction.

The interplay between metabolic alterations, diastolic strain rate and exercise capacity in mild heart failure with preserved ejection fraction: a cardiovascular magnetic resonance study

BACKGROUND

Heart failure (HF) is characterized by altered myocardial substrate metabolism which can lead to myocardial triglyceride accumulation (steatosis) and lipotoxicity. However its role in mild HF with preserved ejection fraction (HFpEF) is uncertain. We measured myocardial triglyceride content (MTG) in HFpEF and assessed its relationships with diastolic function and exercise capacity.

METHODS

Twenty seven HFpEF (clinical features of HF, left ventricular EF >50%, evidence of mild diastolic dysfunction and evidence of exercise limitation as assessed by cardiopulmonary exercise test) and 14 controls underwent 1H-cardiovascular magnetic resonance spectroscopy (1H-CMRS) to measure MTG (lipid/water, %), 31P-CMRS to measure myocardial energetics (phosphocreatine-to-adenosine triphosphate - PCr/ATP) and feature-tracking cardiovascular magnetic resonance (CMR) imaging for diastolic strain rate.

RESULTS

When compared to controls, HFpEF had 2.3 fold higher in MTG (1.45 ± 0.25% vs. 0.64 ± 0.16%, p = 0.009) and reduced PCr/ATP (1.60 ± 0.09 vs. 2.00 ± 0.10, p = 0.005). HFpEF had significantly reduced diastolic strain rate and maximal oxygen consumption (VO2 max), which both correlated significantly with elevated MTG and reduced PCr/ATP. On multivariate analyses, MTG was independently associated with diastolic strain rate while diastolic strain rate was independently associated with VO2 max.

CONCLUSIONS

Myocardial steatosis is pronounced in mild HFpEF, and is independently associated with impaired diastolic strain rate which is itself related to exercise capacity. Steatosis may adversely affect exercise capacity by indirect effect occurring via impairment in diastolic function. As such, myocardial triglyceride may become a potential therapeutic target to treat the increasing number of patients with HFpEF.

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 .

Coronary arterial vasculature in the pathophysiology of hypertrophic cardiomyopathy

Alterations in the coronary vascular system are likely associated with a mismatch between energy demand and energy supply and critical in triggering the cascade of events that leads to symptomatic hypertrophic cardiomyopathy. Targeting the early events, particularly vascular remodeling, may be a key approach to developing effective treatments. Improvement in our understanding of hypertrophic cardiomyopathy began with the results of early biophysical studies, proceeded to genetic analyses pinpointing the mutational origin, and now pertains to imaging of the metabolic and flow-related consequences of such mutations. Microvascular dysfunction has been an ongoing hot topic in the imaging of genetic cardiomyopathies marked by its histologically significant remodeling and has proven to be a powerful asset in determining prognosis for these patients as well as enlightening scientists on a potential pathophysiological cascade that may begin early during the developmental process. Here, we discuss questions that continue to remain on the mechanistic processes leading to microvascular dysfunction, its correlation to the morphological changes in the vessels, and its contribution to disease progression.

Cardiopulmonary Exercise Test in Hypertrophic Cardiomyopathy

Understanding the functional limitation in hypertrophic cardiomyopathy, the most common inherited heart disease, is challenging. In addition to the occurrence of disease-related complications, several factors are potential determinants of exercise limitation, including left ventricular hypertrophy, myocardial fiber disarray, left ventricular outflow tract obstruction, microvascular ischemia, and interstitial fibrosis. Furthermore, drugs commonly used in the daily management of these patients may interfere with exercise capacity, especially those with a negative chronotropic effect. Cardiopulmonary exercise testing can safely and objectively evaluate the functional capacity of these patients and help the physician in understanding the mechanisms that underlie this limitation. Features that reduce exercise capacity may predict progression to heart failure in these patients and even the risk of sudden cardiac death.

Stress Myocardial Blood Flow Heterogeneity Is a Positron Emission Tomography Biomarker of Ventricular Arrhythmias in Patients With Hypertrophic Cardiomyopathy

Patients with hypertrophic cardiomyopathy (HC) are at increased risk of sudden cardiac death. Abnormalities in myocardial blood flow (MBF) detected by positron emission tomography (PET) are common in HC, but a PET marker that identifies patients at risk of sudden cardiac death is lacking. We hypothesized that disparities in regional myocardial perfusion detected by PET would identify patients with HC at risk of ventricular arrhythmias. To test this hypothesis, we quantified global and regional MBFs by ¹³NH₃-PET at rest and at stress, and developed a heterogeneity index to assess MBF heterogeneity in 133 symptomatic patients with HC. The MBF heterogeneity index was computed by dividing the highest by the lowest regional MBF value, at rest and after vasodilator stress, in each patient. High stress MBF heterogeneity was defined as an index of ≧1.85. Patients with HC were stratified by the presence or the absence of ventricular arrhythmias, defined as sustained ventricular tachycardia (VT) and/or nonsustained VT, during follow-up. We found that global and regional MBFs at rest and stress were similar in patients with HC with or without ventricular arrhythmias. Variability in regional stress MBF was observed in both groups, but the stress MBF heterogeneity index was significantly higher in patients with HC who developed ventricular arrhythmias (1.82 ± 0.77 vs 1.49 ± 0.25, p <0.001). A stress MBF heterogeneity index of ≧1.85 was an independent predictor of both sustained VT (hazard ratio 16.1, 95% confidence interval 3.2 to 80.3) and all-VT (sustained-VT + nonsustained VT: hazard ratio 3.7, 95% confidence interval 1.4 to 9.7). High heterogeneity of stress MBF, reflected by an MBF heterogeneity index of ≥1.85, is a PET biomarker for ventricular arrhythmias in symptomatic patients with HC.

Human Cardiac 31P-MR Spectroscopy at 3 Tesla Cannot Detect Failing Myocardial Energy Homeostasis during Exercise

Phosphorus-31 magnetic resonance spectroscopy (³¹P-MRS) is a unique non-invasive imaging modality for probing in vivo high-energy phosphate metabolism in the human heart. We investigated whether current ³¹P-MRS methodology would allow for clinical applications to detect exercise-induced changes in (patho-)physiological myocardial energy metabolism. Hereto, measurement variability and repeatability of three commonly used localized ³¹P-MRS methods [3D image-selected in vivo spectroscopy (ISIS) and 1D ISIS with 1D chemical shift imaging (CSI) oriented either perpendicular or parallel to the surface coil] to quantify the myocardial phosphocreatine (PCr) to adenosine triphosphate (ATP) ratio in healthy humans (n = 8) at rest were determined on a clinical 3 Tesla MR system. Numerical simulations of myocardial energy homeostasis in response to increased cardiac work rates were performed using a biophysical model of myocardial oxidative metabolism. Hypertrophic cardiomyopathy was modeled by either inefficient sarcomere ATP utilization or decreased mitochondrial ATP synthesis. The effect of creatine depletion on myocardial energy homeostasis was explored for both conditions. The mean in vivo myocardial PCr/ATP ratio measured with 3D ISIS was 1.57 ± 0.17 with a large repeatability coefficient of 40.4%. For 1D CSI in a 1D ISIS-selected slice perpendicular to the surface coil, the PCr/ATP ratio was 2.78 ± 0.50 (repeatability 42.5%). With 1D CSI in a 1D ISIS-selected slice parallel to the surface coil, the PCr/ATP ratio was 1.70 ± 0.56 (repeatability 43.7%). The model predicted a PCr/ATP ratio reduction of only 10% at the maximal cardiac work rate in normal myocardium. Hypertrophic cardiomyopathy led to lower PCr/ATP ratios for high cardiac work rates, which was exacerbated by creatine depletion. Simulations illustrated that when conducting cardiac ³¹P-MRS exercise stress testing with large measurement error margins, results obtained under pathophysiologic conditions may still lie well within the 95% confidence interval of normal myocardial PCr/ATP dynamics. Current measurement precision of localized ³¹P-MRS for quantification of the myocardial PCr/ATP ratio precludes the detection of the changes predicted by computational modeling. This hampers clinical employment of ³¹P-MRS for diagnostic testing and risk stratification, and warrants developments in cardiac ³¹P-MRS exercise stress testing methodology.

Evaluation of cardiac energetics by non-invasive 31P magnetic resonance spectroscopy

Alterations in myocardial energy metabolism have been implicated in the pathophysiology of cardiac diseases such as heart failure and diabetic cardiomyopathy. ³¹P magnetic resonance spectroscopy (MRS) is a powerful tool to investigate cardiac energetics non-invasively in vivo, by detecting phosphorus (³¹P)-containing metabolites involved in energy supply and buffering. In this article, we review the historical development of cardiac ³¹P MRS, the readouts used to assess cardiac energetics from ³¹P MRS, and how ³¹P MRS studies have contributed to the understanding of cardiac energy metabolism in heart failure and diabetes. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Cardiovascular homeostasis dependence on MICU2, a regulatory subunit of the mitochondrial calcium uniporter

Hypertension increases the risk for development of abdominal aortic aneurysms, a silent pathology that is prone to rupture and cause sudden cardiac death. Male gender, smoking, and hypertension appear to increase risk for development of abdominal aortic aneurysms by provoking oxidative stress responses in cardiovascular tissues. Here we uncovered unexpected linkages between the calcium-sensing regulatory subunit MICU2 of the mitochondrial calcium uniporter and stress responses. We show that naive Micu2^−/− mice had abnormalities of cardiac relaxation but, with modest blood pressure elevation, developed abdominal aortic aneurysms with spontaneous rupture. These findings implicate mitochondrial calcium homeostasis as a critical pathway involved in protecting cardiovascular tissues from oxidative stress.

Comparative analyses of transcriptional profiles from humans and mice with cardiovascular pathologies revealed consistently elevated expression of MICU2, a regulatory subunit of the mitochondrial calcium uniporter complex. To determine if MICU2 expression was cardioprotective, we produced and characterized Micu2^−/− mice. Mutant mice had left atrial enlargement and Micu2^−/− cardiomyocytes had delayed sarcomere relaxation and cytosolic calcium reuptake kinetics, indicating diastolic dysfunction. RNA sequencing (RNA-seq) of Micu2^−/− ventricular tissues revealed markedly reduced transcripts encoding the apelin receptor (Micu2^−/− vs. wild type, P = 7.8 × 10⁻⁴⁰), which suppresses angiotensin II receptor signaling via allosteric transinhibition. We found that Micu2^−/− and wild-type mice had comparable basal blood pressures and elevated responses to angiotensin II infusion, but that Micu2^−/− mice exhibited systolic dysfunction and 30% lethality from abdominal aortic rupture. Aneurysms and rupture did not occur with norepinephrine-induced hypertension. Aortic tissue from Micu2^−/− mice had increased expression of extracellular matrix remodeling genes, while single-cell RNA-seq analyses showed increased expression of genes related to reactive oxygen species, inflammation, and proliferation in fibroblast and smooth muscle cells. We concluded that Micu2^−/− mice recapitulate features of diastolic heart disease and define previously unappreciated roles for Micu2 in regulating angiotensin II-mediated hypertensive responses that are critical in protecting the abdominal aorta from injury.

Linking myofilaments to sudden cardiac death: recent advances

The major goal of this focused review is to highlight some of the recent advances and remaining open questions about how a mutation in a myofilament protein leads to an increased risk for sudden cardiac death (SCD). The link between myofilaments and SCD has been known for over 25 years, but identifying mutation carriers at risk for SCD is still a challenge and currently the only effective prevention is implantation of a defibrillator (ICD). In addition to recognized risk factors, other contributing factors need to be considered and assessed, e.g. 'microvascular dysfunction', to calibrate individual risk more accurately. Similarly, improving our understanding about the underlying mechanisms of SCD in patients with sarcomeric mutations will also allow us to design new and less invasive treatment options that will minimize risk and hopefully make implantation of an ICD unnecessary.