Investigating myocardial energetic deficit across the spectrum of cardiac disease

Introduction

The phosphocreatine-to-adenosine triphosphate ratio (PCr/ATP) is a sensitive marker of the energetic state of the heart and can be reliably measured non-invasively using 31Phosphorus magnetic resonance spectroscopy (31P-MRS). Derangements in cardiac energetics are a distinctive feature in the pathophysiology of several cardiac diseases, and thus potential therapeutic targets.

Purpose

We sought to compare cardiac PCr/ATP across a range of cardiac pathologies.

Methods

Using a 3D chemical shift 31P spectral acquisition we recorded PCr/ATP in 515 participants: athletes (n=17), healthy controls with normal weight (n=148), overweight (n=67) and with obesity (n=73), diabetes (n=23), heart failure with preserved ejection fraction (HFpEF) (n=33), heart failure with reduced ejection fraction (HFrEF) (n=63), amyloid (n=9), severe aortic stenosis (AS) (n=29), severe mitral regurgitation (MR) (n=18), and hypertrophic cardiomyopathy (HCM) (n=35).

Results

A spectrum of myocardial PCr/ATP exists ranging from normal in athletes (2.23±0.28) and those with normal weight (2.05±0.38) to severely impaired in severe MR (1.56±0.32) and cardiac amyloid (1.34±0.19, Figure 1). Despite normal systolic function (all LVEF >57%) those living with obesity and diabetes have lower PCr/ATP than normal (all p<0.001). In all groups with HF, regardless of aetiology, myocardial energetics were impaired (all p<0.001). Across the whole cohort PCr/ATP was negatively correlated with body mass index (r −0.28, p<0.001), age (r −0.34, p<0.001) and LV mass (r −0.1, p<0.001). PCr/ATP was not related to systolic or diastolic blood pressure in these cohorts.

Conclusions

We demonstrate a spectrum of energetic deficit in cardiac disease and this is affected by not only myocardial pathology but also by obesity and age. Derangements in myocardial energetics are present in myocardial pathologies independent of underlying aetiology.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): We acknowledge support from the British Heart Foundation Oxford Center of Research Excellence.

Adverse right ventricular remodelling, function, and stress responses in obesity: insights from cardiovascular magnetic resonance

AIMS

We aimed to determine the effect of increasing body weight upon right ventricular (RV) volumes, energetics, systolic function, and stress responses using cardiovascular magnetic resonance (CMR).

METHODS AND RESULTS

We first determined the effects of World Health Organization class III obesity [body mass index (BMI) > 40 kg/m2, n = 54] vs. healthy weight (BMI < 25 kg/m2, n = 49) upon RV volumes, energetics and systolic function using CMR. In less severe obesity (BMI 35 ± 5 kg/m2, n = 18) and healthy weight controls (BMI 21 ± 1 kg/m2, n = 9), we next performed CMR before and during dobutamine to evaluate RV stress response. A subgroup undergoing bariatric surgery (n = 37) were rescanned at median 1 year to determine the effects of weight loss. When compared with healthy weight, class III obesity was associated with adverse RV remodelling (17% RV end-diastolic volume increase, P < 0.0001), impaired cardiac energetics (19% phosphocreatine to adenosine triphosphate ratio reduction, P < 0.001), and reduction in RV ejection fraction (by 3%, P = 0.01), which was related to impaired energetics (R = 0.3, P = 0.04). Participants with less severe obesity had impaired RV diastolic filling at rest and blunted RV systolic and diastolic responses to dobutamine compared with healthy weight. Surgical weight loss (34 ± 15 kg weight loss) was associated with improvement in RV end-diastolic volume (by 8%, P = 0.006) and systolic function (by 2%, P = 0.03).

CONCLUSION

Increasing body weight is associated with significant alterations in RV volumes, energetic, systolic function, and stress responses. Adverse RV modelling is mitigated with weight loss. Randomized trials are needed to determine whether intentional weight loss improves symptoms and outcomes in patients with obesity and heart failure.

Neutrophils incite and macrophages avert electrical storm after myocardial infarction

Sudden cardiac death, arising from abnormal electrical conduction, occurs frequently in patients with coronary heart disease. Myocardial ischemia simultaneously induces arrhythmia and massive myocardial leukocyte changes. In this study, we optimized a mouse model in which hypokalemia combined with myocardial infarction triggered spontaneous ventricular tachycardia in ambulatory mice, and we showed that major leukocyte subsets have opposing effects on cardiac conduction. Neutrophils increased ventricular tachycardia via lipocalin-2 in mice, whereas neutrophilia associated with ventricular tachycardia in patients. In contrast, macrophages protected against arrhythmia. Depleting recruited macrophages in Ccr2 -/- mice or all macrophage subsets with Csf1 receptor inhibition increased both ventricular tachycardia and fibrillation. Higher arrhythmia burden and mortality in Cd36 -/- and Mertk -/- mice, viewed together with reduced mitochondrial integrity and accelerated cardiomyocyte death in the absence of macrophages, indicated that receptor-mediated phagocytosis protects against lethal electrical storm. Thus, modulation of leukocyte function provides a potential therapeutic pathway for reducing the risk of sudden cardiac death.

Cardiovascular magnetic resonance facilitates entirely contrast-free transcatheter aortic valve implantation: case report

Background

Transcatheter aortic valve implantation (TAVI) is usually planned using contrast-enhanced computed tomography (CT) to determine the suitability of cardiovascular anatomy. Computed tomography for TAVI planning requires the administration of intravenous contrast, which may not be desirable in patients with severely reduced renal function.

Case summary

We present an unusual case of an 89-year-old patient with an urgent need for treatment of critical, symptomatic aortic stenosis who also had severe chronic kidney disease. We judged that this posed a relative contraindication to the use of intravenous contrast. We designed and implemented a novel, contrast-free cardiovascular magnetic resonance (CMR) protocol and used this to plan all aspects of the procedure. Transcatheter aortic valve implantation was conducted successfully with zero contrast medium administration leading to an excellent clinical result and recovery of renal function.

Conclusion

Contrast-free CMR appears to be a viable alternative to CT for planning structural aortic valve intervention in the rare cases where intravenous contrast is relatively contraindicated.

Obesity in the absence of comorbidities is not related to clinically meaningful left ventricular hypertrophy

Obesity is associated with the development of left ventricular (LV) hypertrophy. Whether obesity in in the absence of comorbidities can cause LV hypertrophy to an extent which could create diagnostic uncertainty with pathological states (such as hypertrophic cardiomyopathy) is unknown. We used cine cardiovascular magnetic resonance imaging to precisely measure LV wall thickness in the septum and lateral wall in 764 people with body mass indices ranging from 18.5 kg/m2 to 59.2 kg/m2 in the absence of major comorbidities. Obesity was related to LV wall thickness across the cohort (basal septum r 0.30, P < 0.001 and basal lateral wall r 0.18, P < 0.001). Although no participant had hypertension, these associations remained highly significant after controlling for systolic blood pressure (all P < 0.01). Each 10 kg/m2 increase in BMI was associated with an increase in basal septal wall thickness of 1.0 mm males and 0.8 mm in females, with no statistically significant difference between genders (P = 0.1). Even in class 3 obesity (BMI > 40 kg/m2), no LV wall thickness > 13.4 mm in males or > 12.7 mm in females was observed in this cohort. We confirm that obesity in the absence of comorbidities is associated with LV hypertrophy, and establish that the magnitude of this change is modest even in severe obesity. LV hypertrophy > 14 mm cannot safely be attributed to obesity alone and alternative diagnoses should be considered.

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.

Cardiovascular magnetic resonance imaging for inflammatory heart diseases

Inflammatory myocardial diseases represent a diverse group of conditions in which abnormal inflammation within the myocardium is the primary driver of cardiac dysfunction. Broad causes of myocarditis include infection by cardiotropic viruses or other infectious agents, to systemic autoimmune disease, or to toxins. Myocarditis due to viral aetiologies is a relatively common cause of acute chest pain syndromes in younger and middle-aged patients and often has a benign prognosis, though this and other forms of myocarditis also cause serious sequelae, including heart failure, arrhythmia and death. Endomyocardial biopsy remains the gold standard tool for tissue diagnosis of myocarditis in living individuals, although new imaging technologies have a crucial and complementary role. This review outlines the current state-of-the-art and future experimental cardiovascular magnetic resonance (CMR) imaging approaches for the detection of inflammation and immune cell activity in the heart. Multiparametric CMR, particularly with novel quantitative T1- and T2-mapping, is a valuable and widely-available tool for the non-invasive assessment of inflammatory heart diseases. Novel CMR molecular contrast agents will enable a more targeted assessment of immune cell activity and may be useful in guiding the development of novel therapeutics for myocarditis.

The use of cardiovascular magnetic resonance for the assessment of left ventricular hypertrophy

Cardiovascular magnetic resonance (CMR) is a powerful tool to assess and diagnose the cause of left ventricular hypertrophy (LVH). This review provides an overview of the typical CMR findings in the various causes of LVH, focusing mainly on late gadolinium enhancement (LGE) imaging. It will also cover the more novel techniques of T1 mapping, extracellular volume (ECV) fraction estimation and diffusion tensor imaging (DTI) and their role in the imaging assessment of LVH.

Noninvasive In Vivo Assessment of Cardiac Metabolism in the Healthy and Diabetic Human Heart Using Hyperpolarized 13C MRI

RATIONALE

The recent development of hyperpolarized 13C magnetic resonance spectroscopy has made it possible to measure cellular metabolism in vivo, in real time.

OBJECTIVE

By comparing participants with and without type 2 diabetes mellitus (T2DM), we report the first case-control study to use this technique to record changes in cardiac metabolism in the healthy and diseased human heart.

METHODS AND RESULTS

Thirteen people with T2DM (glycated hemoglobin, 6.9±1.0%) and 12 age-matched healthy controls underwent assessment of cardiac systolic and diastolic function, myocardial energetics (31P-magnetic resonance spectroscopy), and lipid content (1H-magnetic resonance spectroscopy) in the fasted state. In a subset (5 T2DM, 5 control), hyperpolarized [1-13C]pyruvate magnetic resonance spectra were also acquired and in 5 of these participants (3 T2DM, 2 controls), this was successfully repeated 45 minutes after a 75 g oral glucose challenge. Downstream metabolism of [1-13C]pyruvate via PDH (pyruvate dehydrogenase, [13C]bicarbonate), lactate dehydrogenase ([1-13C]lactate), and alanine transaminase ([1-13C]alanine) was assessed. Metabolic flux through cardiac PDH was significantly reduced in the people with T2DM (Fasted: 0.0084±0.0067 [Control] versus 0.0016±0.0014 [T2DM], Fed: 0.0184±0.0109 versus 0.0053±0.0041; P=0.013). In addition, a significant increase in metabolic flux through PDH was observed after the oral glucose challenge (P<0.001). As is characteristic of diabetes mellitus, impaired myocardial energetics, myocardial lipid content, and diastolic function were also demonstrated in the wider study cohort.

CONCLUSIONS

This work represents the first demonstration of the ability of hyperpolarized 13C magnetic resonance spectroscopy to noninvasively assess physiological and pathological changes in cardiac metabolism in the human heart. In doing so, we highlight the potential of the technique to detect and quantify metabolic alterations in the setting of cardiovascular disease.

Noninvasive Immunometabolic Cardiac Inflammation Imaging Using Hyperpolarized Magnetic Resonance

RATIONALE

Current cardiovascular clinical imaging techniques offer only limited assessment of innate immune cell-driven inflammation, which is a potential therapeutic target in myocardial infarction (MI) and other diseases. Hyperpolarized magnetic resonance (MR) is an emerging imaging technology that generates contrast agents with 10- to 20 000-fold improvements in MR signal, enabling cardiac metabolite mapping.

OBJECTIVE

To determine whether hyperpolarized MR using [1-¹³C]pyruvate can assess the local cardiac inflammatory response after MI.

METHODS AND RESULTS

We performed hyperpolarized [1-¹³C]pyruvate MR studies in small and large animal models of MI and in macrophage-like cell lines and measured the resulting [1-¹³C]lactate signals. MI caused intense [1-¹³C]lactate signal in healing myocardial segments at both day 3 and 7 after rodent MI, which was normalized at both time points after monocyte/macrophage depletion. A near-identical [1-¹³C]lactate signature was also seen at day 7 after experimental MI in pigs. Hyperpolarized [1-¹³C]pyruvate MR spectroscopy in macrophage-like cell suspensions demonstrated that macrophage activation and polarization with lipopolysaccharide almost doubled hyperpolarized lactate label flux rates in vitro; blockade of glycolysis with 2-deoxyglucose in activated cells normalized lactate label flux rates and markedly inhibited the production of key proinflammatory cytokines. Systemic administration of 2-deoxyglucose after rodent MI normalized the hyperpolarized [1-¹³C]lactate signal in healing myocardial segments at day 3 and also caused dose-dependent improvement in IL (interleukin)-1β expression in infarct tissue without impairing the production of key reparative cytokines. Cine MRI demonstrated improvements in systolic function in 2-DG (2-deoxyglucose)-treated rats at 3 months.

CONCLUSIONS

Hyperpolarized MR using [1-¹³C]pyruvate provides a novel method for the assessment of innate immune cell-driven inflammation in the heart after MI, with broad potential applicability across other cardiovascular disease states and suitability for early clinical translation.

Assessment of Metformin-Induced Changes in Cardiac and Hepatic Redox State Using Hyperpolarized[1-13C]Pyruvate

Metformin improves cardiovascular outcomes in type 2 diabetes, but its exact mechanisms of action remain controversial. We used hyperpolarized [1-¹³C]pyruvate magnetic resonance spectroscopy to determine the effects of metformin treatment on heart and liver pyruvate metabolism in rats in vivo. Both oral treatment for 4 weeks and a single intravenous metformin infusion significantly increased the cardiac [1-¹³C]lactate:[1-¹³C]pyruvate ratio but had no effect on the [1-¹³C]bicarbonate + ¹³CO₂:[1-¹³C]pyruvate ratio, an index of pyruvate dehydrogenase flux. These changes were paralleled by a significant increase in the heart and liver cytosolic redox state, estimated from the [lactate]:[pyruvate] ratio but not the whole-cell [NAD⁺]/[NADH] ratio. Hyperpolarized MRI localized the increase in cardiac lactate to the left ventricular myocardium, implying a direct myocardial effect, though metformin had no effect on systolic or diastolic cardiac function. These findings demonstrate the ability of hyperpolarized pyruvate magnetic resonance spectroscopy to detect metformin-induced changes in cytosolic redox biology, suggest that metformin has a previously unrecognized effect on cardiac redox state, and help to refine the design of impending hyperpolarized magnetic resonance studies in humans.

Aortic 4D flow: quantification of signal-to-noise ratio as a function of field strength and contrast enhancement for 1.5T, 3T, and 7T

PURPOSE

To investigate for the first time the feasibility of aortic four-dimensional (4D) flow at 7T, both contrast enhanced (CE) and non-CE. To quantify the signal-to-noise ratio (SNR) in aortic 4D flow as a function of field strength and CE with gadobenate dimeglumine (MultiHance).

METHODS

Six healthy male volunteers were scanned at 1.5T, 3T, and 7T with both non-CE and CE acquisitions. Temporal SNR was calculated. Flip angle optimization for CE 4D flow was carried out using Bloch simulations that were validated against in vivo measurements.

RESULTS

The 7T provided 2.2 times the SNR of 3T while 3T provided 1.7 times the SNR of 1.5T in non-CE acquisitions in the descending aorta. The SNR gains achieved by CE were 1.8-fold at 1.5T, 1.7-fold at 3T, and 1.4-fold at 7T, respectively.

CONCLUSION

The 7T provides a new tool to explore aortic 4D flow, yielding higher SNR that can be used to push the boundaries of acceleration and resolution. Field strength and contrast enhancement at all fields provide significant improvements in SNR.

Norepinephrine and arginine vasopressin increase hepatic but not renal inflammatory activation during hemodynamic resuscitation in a rodent model of brain-dead donors

OBJECTIVES

Hypotension that occurs after brain death causes a deterioration in organ function, which in turn restricts the number of organs that can be retrieved and leads to graft dysfunction. The correction of hypotension by the administration of norepinephrine increases the number of organs suitable for retrieval but is associated with cardiac allograft failure. Arginine vasopressin is relatively less cardiotoxic; however, the effect of that drug on intra-abdominal organs is unknown. We used a rodent model and real-time reverse transcription polymerase chain reaction to assess changes in the expression of inflammatory mediators in livers and kidneys that occurred in response to resuscitation with those drugs.

MATERIALS AND METHODS

Fifty outbred male Wistar rats were anesthetized, and an intracranial balloon was inserted. In 35 rats, the balloon was inflated to induce brain death and subsequent hypotension. In 20 of those rats, hypotension was corrected with either norepinephrine (n = 10) or vasopressin (n = 10), while the remaining 15 rats received no resuscitation. Brain death was not induced in 15 rats that did not become hypotensive or receive resuscitation. Organs were retrieved 30 minutes, 2 hours, and 5 hours after balloon insertion, and inflammatory activation was assessed via real-time reverse transcription polymerase chain reaction.

RESULTS

Significant time-dependent up-regulation of CXC motif chemokine ligand 1, interleukin-1beta, and heme oxygenase 1 occurred after brain death. Significantly greater up-regulation of CXC motif chemokine ligand and interleukin-1beta occurred in the livers of rats that received norepinephrine and vasopressin than in those that received no resuscitation. No increase in the expression of those mediators was noted in the kidneys.

CONCLUSIONS

This study showed that both norepinephrine and vasopressin amplified the inflammatory response that followed brain death in the livers, but not the kidneys, of rats in this model.