Introduction of new tests: low are the mountains, high the expectations

The Cardiorenal Axis: Myocardial Perfusion, Metabolism, and Innervation

PURPOSE OF THE REVIEW

Cardiorenal syndrome (CRS), defined as concomitant heart and kidney disease, has been a focus of attention for nearly a decade. As more patients survive severe acute and chronic heart and kidney diseases, CRS has emerged as an "epidemic" of modern medicine. Significant advances have been made in unraveling the complex mechanisms that underlie CRS based on classification of the condition into five pathophysiologic subtypes. In types 1 and 2, acute or chronic heart disease results in renal dysfunction, while in types 3 and 4, acute or chronic kidney diseases are the inciting factors for heart disease. Type 5 CRS is defined as concomitant heart and kidney dysfunction as part of a systemic condition such as sepsis or autoimmune disease.

RECENT FINDINGS

There are ongoing efforts to better define subtypes of CRS based on historical information, clinical manifestations, laboratory data (including biomarkers), and imaging characteristics. Systematic evaluation of CRS by advanced cardiac imaging, however, has been limited in scope and mostly focused on type 4 CRS. This is in part related to lack of clinical trials applying advanced cardiac imaging in the acute setting and exclusion of patients with significant renal disease from studies of such techniques in chronic HF. Advanced cardiac nuclear imaging is well poised for assessment of the pathophysiology of CRS by offering a myriad of molecular probes without the need for nephrotoxic contrast agents. In this review, we examine the current or potential future application of advanced cardiac imaging to evaluation of myocardial perfusion, metabolism, and innervation in patients with CRS.

The Potential of Clinical Phenotyping of Heart Failure With Imaging Biomarkers for Guiding Therapies: A Focused Update

The need for noninvasive assessment of cardiac volumes and ejection fraction (EF) ushered in the use of cardiac imaging techniques in heart failure (HF) trials that investigated the roles of pharmacological and device-based therapies. However, in contrast to HF with reduced EF (HFrEF), modern HF pharmacotherapy has not improved outcomes in HF with preserved EF (HFpEF), largely attributed to patient heterogeneity and incomplete understanding of pathophysiological insights underlying the clinical presentations of HFpEF. Modern cardiac imaging methods offer insights into many sets of changes in cardiac tissue structure and function that can precisely link cause with cardiac remodeling at organ and tissue levels to clinical presentations in HF. This has inspired investigators to seek a more comprehensive understanding of HF presentations using imaging techniques. This article summarizes the available evidence regarding the role of cardiac imaging in HF. Furthermore, we discuss the value of cardiac imaging techniques in identifying HF patient subtypes who share similar causes and mechanistic pathways that can be targeted using specific HF therapies.

Cardiac molecular imaging to track left ventricular remodeling in heart failure

Cardiac left ventricular (LV) remodeling is the final common pathway of most primary cardiovascular diseases that manifest clinically as heart failure (HF). The more advanced the systolic HF and LV dysfunction, the worse the prognosis. The knowledge of the molecular, cellular, and neurohormonal mechanisms that lead to myocardial dysfunction and symptomatic HF has expanded rapidly and has allowed sophisticated approaches to understanding and management of the disease. New therapeutic targets for pharmacologic intervention in HF have also been identified through discovery of novel cellular and molecular components of membrane-bound receptor-mediated intracellular signal transduction cascades. Despite all advances, however, the prognosis of systolic HF has remained poor in general. This is, at least in part, related to the (1) relatively late institution of treatment due to reliance on gross functional and structural abnormalities that define the "heart failure phenotype" clinically; (2) remarkable genetic-based interindividual variations in the contribution of each of the many molecular components of cardiac remodeling; and (3) inability to monitor the activity of individual pathways to cardiac remodeling in order to estimate the potential benefits of pharmacologic agents, monitor the need for dose titration, and minimize side effects. Imaging of the recognized ultrastructural components of cardiac remodeling can allow redefinition of heart failure based on its "molecular phenotype," and provide a guide to implementation of "personalized" and "evidence-based" evaluation, treatment, and longitudinal monitoring of the disease beyond what is currently available through randomized controlled clinical trials.

Radionuclide imaging of cardiac sympathetic innervation in heart failure: unlocking untapped potential

Heart failure (HF) is associated with sympathetic overactivity, which contributes to disease progression and arrhythmia development. Cardiac sympathetic innervation imaging can be performed using radiotracers that are taken up in the presynaptic nerve terminal of sympathetic nerves. The commonly used radiotracers are (123)I-metaiodobenzylguanidine ((123)I-mIBG) for planar and single-photon emission computed tomography imaging, and (11)C-hydroxyephedrine for positron emission tomography imaging. Sympathetic innervation imaging has been used in assessing prognosis, response to treatment, risk of ventricular arrhythmias and sudden death and prediction of response to cardiac resynchronization therapy in patients with HF. Other potential applications of these techniques are in patients with chemotherapy-induced cardiomyopathy, predicting myocardial recovery in patients with left ventricular assist devices, and assessing reinnervation following cardiac transplantation. There is a lack of standardization with respect to technique of (123)I-mIBG imaging that needs to be overcome for the imaging modality to gain popularity in clinical practice.

Chemistry and biology of radiotracers that target changes in sympathetic and parasympathetic nervous systems in heart disease

Following the discovery of the sympathetic and parasympathetic nervous system, numerous adrenoceptor drugs were radiolabeled and potent radioligands were prepared in order to image the β-adrenergic and the muscarinic systems. But the greatest effort has been in preparing noradrenaline analogs, such as norepinephrine, (11)C-metahydroxyephedrine, and (123)I-metaiodobenzylguanidine that measure cardiac sympathetic nerve varicosities. Given the technical and clinical challenges in designing and validating targeted adrenoceptor-binding radiotracers, namely the heavily weighted flow dependence and relatively low target-to-background ratio, both requiring complicated mathematic analysis, and the inability of targeted adrenoceptor radioligands to have an impact on clinical care of heart disease, the emphasis has been on radioligands monitoring the norepinephrine pathway. The chemistry and biology of such radiotracers, and the clinical and prognostic impact of these innervation imaging studies in patients with heart disease, are examined.

The potential role of iodine-123 metaiodobenzylguanidine imaging for identifying sustained ventricular tachycardia in patients with cardiomyopathy

Implantable cardioverter-defibrillators (ICDs) significantly reduce mortality in patients with depressed left ventricular ejection fraction (LVEF) and heart failure (HF). However, shortcomings of LVEF to accurately identify those at greatest risk of ventricular tachyarrhythmias have led to the pursuit of alternative means to refine qualification criteria for ICD implantation. It is well established that imaging the cardiac nervous system with¹²³I meta-iodobenzylguanidine (¹²³I-mIBG) provides incremental prognostic value in patients with HF beyond LVEF. Whether ¹²³I-mIBG will also play an important role for identifying and/or predicting sustained ventricular tachyarrhythmias in patients with cardiomyopathy and determining those who may benefit from ICD implantation is currently under investigation. Novel imaging approaches that pinpoint the site of ventricular arrhythmias and guide ventricular tachycardia ablation are presented.

Variation of heart-to-mediastinal ratio in (123)I-mIBG cardiac sympathetic imaging: its affecting factors and potential corrections

Studies have repeatedly shown the utility of (123)I-mIBG cardiac sympathetic imaging for identifying symptomatic heart failure patients most likely to experience adverse cardiac events. Delayed heart-to-mediastinal ratio (H/M) and washout rate derived from meta-iodobenzylguanidine (mIBG) scintigraphy have been used to monitor response to medical treatment. However, there is great variation of H/M ratio among publications from various institutions. The current article systemically reviews factors that can potentially affect H/M ratio, particularly the acquisition parameters, and proposes new approaches and/or procedures to standardize the imaging procedure and minimize the variation of H/M among institutions.