Cardiac resynchronization therapy and cardiac sympathetic function

Neurohumoral Activation in Heart Failure

In patients with heart failure (HF), the neuroendocrine systems of the sympathetic nervous system (SNS), the renin-angiotensin-aldosterone system (RAAS) and the arginine vasopressin (AVP) system, are activated to various degrees producing often-observed tachycardia and concomitant increased systemic vascular resistance. Furthermore, sustained neurohormonal activation plays a key role in the progression of HF and may be responsible for the pathogenetic mechanisms leading to the perpetuation of the pathophysiology and worsening of the HF signs and symptoms. There are biomarkers of activation of these neurohormonal pathways, such as the natriuretic peptides, catecholamine levels and neprilysin and various newer ones, which may be employed to better understand the mechanisms of HF drugs and also aid in defining the subgroups of patients who might benefit from specific therapies, irrespective of the degree of left ventricular dysfunction. These therapies are directed against these neurohumoral systems (neurohumoral antagonists) and classically comprise beta blockers, angiotensin-converting enzyme (ACE) inhibitors/angiotensin receptor blockers and vaptans. Recently, the RAAS blockade has been refined by the introduction of the angiotensin receptor-neprilysin inhibitor (ARNI) sacubitril/valsartan, which combines the RAAS inhibition and neprilysin blocking, enhancing the actions of natriuretic peptides. All these issues relating to the neurohumoral activation in HF are herein reviewed, and the underlying mechanisms are pictorially illustrated.

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

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

Effect of cardiac resynchronization therapy on ambulatory blood pressure monitoring indices in patients with heart failure and hypertension

OBJECTIVE

Blood pressure (BP) variability is associated with increased mortality in patients with hypertension and heart failure. The aim of our study was to evaluate the effect of cardiac resynchronization therapy (CRT) on different parameters of 24-h ambulatory blood pressure monitoring (ABPM) in patients with hypertension and heart failure.

METHODS

Patients with heart failure and hypertension who were candidate for CRT implantation were enrolled in our study. Twenty-four hour ABPM was performed before, and 6 months after CRT implantation. BP variables and average real variability (ARV) were compared in all patients.

RESULT

Sixteen consecutive patients entered our study. There was a significant increase in ARV following CRT implantation (P value = 0.009).

CONCLUSION

CRT implantation is associated with increased ARV, but the effect of this improvement on cardiovascular outcome needs more investigation.

Effects of Cardiac Resynchronization Therapy on Cardio-Respiratory Coupling

In this study, the effect of cardiac resynchronization therapy (CRT) on the relationship between the cardiovascular and respiratory systems in heart failure subjects was examined for the first time. We hypothesized that alterations in cardio-respiratory interactions, after CRT implantation, quantified by signal complexity, could be a marker of a favorable CRT response. Sample entropy and scaling exponents were calculated from synchronously recorded cardiac and respiratory signals 20 min in duration, collected in 47 heart failure patients at rest, before and 9 months after CRT implantation. Further, cross-sample entropy between these signals was calculated. After CRT, all patients had lower heart rate and CRT responders had reduced breathing frequency. Results revealed that higher cardiac rhythm complexity in CRT non-responders was associated with weak correlations of cardiac rhythm at baseline measurement over long scales and over short scales at follow-up recording. Unlike CRT responders, in non-responders, a significant difference in respiratory rhythm complexity between measurements could be consequence of divergent changes in correlation properties of the respiratory signal over short and long scales. Asynchrony between cardiac and respiratory rhythm increased significantly in CRT non-responders during follow-up. Quantification of complexity and synchrony between cardiac and respiratory signals shows significant associations between CRT success and stability of cardio-respiratory coupling.

Quantifying cardiac sympathetic denervation: first studies of 18F-fluorohydroxyphenethylguanidines in cardiomyopathy patients

PURPOSE

4-¹⁸F-Fluoro-m-hydroxyphenethylguanidine (¹⁸F-4F-MHPG) and 3-¹⁸F-fluoro-p-hydroxyphenethylguanidine (¹⁸F-3F-PHPG) were developed for quantifying regional cardiac sympathetic nerve density using tracer kinetic analysis. The aim of this study was to evaluate their performance in cardiomyopathy patients.

METHODS

Eight cardiomyopathy patients were scanned with ¹⁸F-4F-MHPG and ¹⁸F-3F-PHPG. Also, regional resting perfusion was assessed with ¹³N-ammonia. ¹⁸F-4F-MHPG and ¹⁸F-3F-PHPG kinetics were analyzed using the Patlak graphical method to obtain Patlak slopes K_p (mL/min/g) as measures of regional nerve density. Patlak slope polar maps were used to evaluate the pattern and extent of cardiac denervation. For comparison, "retention index" (RI) values (mL blood/min/mL tissue) were also calculated and used to assess denervation. Perfusion polar maps were used to estimate the extent of hypoperfusion.

RESULTS

Patlak analysis of ¹⁸F-4F-MHPG and ¹⁸F-3F-PHPG kinetics was successful in all subjects, demonstrating the robustness of this approach in cardiomyopathy patients. Substantial regional denervation was observed in all subjects, ranging from 25 to 74% of the left ventricle. Denervation zones were equal to or larger than the size of corresponding areas of hypoperfusion. The two tracers provided comparable metrics of regional nerve density and the extent of left ventricular denervation. ¹⁸F-4F-MHPG exhibited faster liver clearance than ¹⁸F-3F-PHPG, reducing spillover from the liver into the inferior wall. ¹⁸F-4F-MHPG was also metabolized more consistently in plasma, which may allow application of population-averaged metabolite corrections.

CONCLUSION

The advantages of ¹⁸F-4F-MHPG (more rapid liver clearance, more consistent metabolism in plasma) make it the better imaging agent to carry forward into future clinical studies in patients with cardiomyopathy.

TRIAL REGISTRATION

Registered at the ClinicalTrials.gov website (NCT02669563). URL: https://clinicaltrials.gov/ct2/show/NCT02669563.

Cardiac Sympathetic Innervation Imaging with PET Radiotracers

PURPOSE OF REVIEW

The present article reviews the pathophysiology of cardiac sympathetic denervation, the principles of positron emission tomography (PET) imaging of the sympathetic innervation of the heart and its potential clinical role, based on current and expected future evidence.

RECENT FINDINGS

Imaging of cardiac sympathetic denervation can be performed with radiolabeled noradrenaline analogues, e.g., ¹¹C-hydroxyephedrine. A greater burden of sympathetic denervation carries prognostic significance, e.g., in patients with ischemic cardiomyopathy and a left ventricular ejection fraction ≤ 35%, who are more likely to experience sudden cardiac death. Abnormalities of sympathetic cardiac innervation have been demonstrated in hypertrophic, dilated, and arrhythmic right ventricular cardiomyopathies, and may be helpful in better phenotyping patients who will benefit from device therapy, e.g., cardiac resynchronization and implantable cardioverter-defibrillator implantation. The results of future trials, e.g., the Prediction of Arrhythmic Events with Positron Emission Tomography (PAREPET) II study, are awaited to inform on the role of PET cardiac sympathetic imaging in the selection of device therapy. PET cardiac sympathetic innervation imaging allows visualization and quantification of autonomic denervation secondary to various cardiac diseases, and has significant potential to influence clinical decision-making, e.g., the titration of pharmacotherapy and more directed selection of candidates for device implantation.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

Cardiac Innervation and the Autonomic Nervous System in Sudden Cardiac Death

Neural remodeling in the autonomic nervous system contributes to sudden cardiac death. The fabric of cardiac excitability and propagation is controlled by autonomic innervation. Heart disease predisposes to malignant ventricular arrhythmias by causing neural remodeling at the level of the myocardium, the intrinsic cardiac ganglia, extracardiac intrathoracic sympathetic ganglia, extrathoracic ganglia, spinal cord, and the brainstem, as well as the higher centers and the cortex. Therapeutic strategies at each of these levels aim to restore the balance between the sympathetic and parasympathetic branches. Understanding this complex neural network will provide important therapeutic insights into the treatment of sudden cardiac death.

Recent Advances and Clinical Applications of PET Cardiac Autonomic Nervous System Imaging

PURPOSE OF REVIEW

The purpose of this review was to summarize current advances in positron emission tomography (PET) cardiac autonomic nervous system (ANS) imaging, with a specific focus on clinical applications of novel and established tracers.

RECENT FINDINGS

[11C]-Meta-hydroxyephedrine (HED) has provided useful information in evaluation of normal and pathological cardiovascular function. Recently, [11C]-HED PET imaging was able to predict lethal arrhythmias, sudden cardiac death (SCD), and all-cause mortality in heart failure patients with reduced ejection fraction (HFrEF). In addition, initial [11C]-HED PET imaging studies have shown the potential of this agent in elucidating the relationship between impaired cardiac sympathetic nervous system (SNS) innervation and the severity of diastolic dysfunction in HF patients with preserved ejection fraction (HFpEF) and in predicting the response to cardiac resynchronization therapy (CRT) in HFrEF patients. Longer half-life 18F-labeled presynaptic SNS tracers (e.g., [18F]-LMI1195) have been developed to facilitate clinical imaging, although no PET radiotracers that target the ANS have gained wide clinical use in the cardiovascular system. Although the use of parasympathetic nervous system radiotracers in cardiac imaging is limited, the novel tracer, [11C]-donepezil, has shown potential utility in initial studies. Many ANS radioligands have been synthesized for PET cardiac imaging, but to date, the most clinically relevant PET tracer has been [11C]-HED. Recent studies have shown the utility of [11C]-HED in relevant clinical issues, such as in the elusive clinical syndrome of HFpEF. Conversely, tracers that target cardiac PNS innervation have been used less clinically, but novel tracers show potential utility for future work. The future application of [11C]-HED and newly designed 18F-labeled tracers for targeting the ANS hold promise for the evaluation and management of a wide range of cardiovascular diseases, including the prognostication of patients with HFpEF.

Diabetes Mellitus and Outcomes of Cardiac Resynchronization With Implantable Cardioverter-Defibrillator Therapy in Older Patients With Heart Failure

Background—

Large-scale data on outcomes with cardiac resynchronization therapy with defibrillator in patients with diabetes mellitus are limited. We compared outcomes after cardiac resynchronization therapy with defibrillator implantation among patients with heart failure who have diabetes mellitus versus those without diabetes mellitus.

Methods and Results—

Survival curves and covariate adjusted hazard ratio (HR) or odds ratio were used to assess the risks for death, readmission, and device-related complications by diabetes mellitus status among 18 428 patients at least 65 years old receiving cardiac resynchronization therapy with defibrillator from the National Cardiovascular Data Registry, implantable cardioverter-defibrillator registry between 2006 and 2009, with up to 3 years of follow-up. Accounting for differences between groups, compared with those without diabetes mellitus (n=11 345), patients with diabetes mellitus (n=7083) had a higher risk of death both at 1 year (HR, 1.16 [95% confidence interval (CI), 1.05–1.29]; P =0.0037) and 3 years (HR, 1.21 [1.14–1.29]; P <0.001) after device implantation and higher risks of all-cause readmission (sub-HR, 1.16 [1.11–1.21] at 1 year; P <0.0001; sub-HR, 1.15 [1.11–1.19] at 3 years; P <0.0001) and heart failure–related readmission (sub-HR, 1.18 [1.09–1.28] at 1 year; P <0.0001; and sub-HR, 1.22 [1.15–1.30] at 3 years; P <0.0001). Device-related complications within 90 days did not differ between those with and without diabetes mellitus (odds ratio: 0.90 [0.77–1.06]; P =0.37). Interactions of age, sex, ischemic cardiomyopathy, renal failure, or QRS duration were not significant.

Conclusions—

In older patients with heart failure receiving cardiac resynchronization therapy with defibrillator, diabetes mellitus was independently associated with greater risks of death and rehospitalization, but similar risks of procedural complications.

Pathophysiology of dyssynchrony: of squirrels and broken bones

The genesis of cardiac resynchronisation therapy (CRT) consists of 'bedside' research and 'bench' studies that are performed in series with each other. In this field, the bench studies are crucial for understanding the pathophysiology of dyssynchrony and resynchronisation. In a way, CRT started with the insight that abnormal ventricular conduction, as caused by right ventricular pacing, has adverse effects. Out of this research came the ground-breaking insight that 'simple' disturbances in impulse conduction, which were initially considered innocent, proved to result in a host of molecular and cellular derangements that lead to a vicious circle of remodelling processes that facilitate the development of heart failure. As a consequence, CRT does not only correct conduction abnormalities, but also improves myocardial properties at many levels. Interestingly, corrections by CRT do not exactly reverse the derangements, induced by dyssynchrony, but also activate novel pathways, a property that may open new avenues for the treatment of heart failure.