Sympathetic reinnervation of sinus node and left ventricle after heart transplantation in humans: regional differences assessed by heart rate variability and positron emission tomography

Decoding cardiac reinnervation from cardiac autonomic markers: A mathematical model approach

BACKGROUND

Although cardiac autonomic markers (CAMs) are commonly used to assess cardiac reinnervation in heart-transplant patients, their relationship to the degree of sympathetic and vagal cardiac reinnervation is not well understood yet. To study this relationship, we applied a mathematical model of the cardiovascular system and its autonomic control.

METHODS

By simulating varying levels of sympathetic and vagal efferent sinoatrial reinnervation, we analyzed the induced changes in CAMs including resting heart rate (HR), bradycardic and tachycardic HR response to Valsalva maneuver, root mean square of successive differences between normal heartbeats (RMSSD), low-frequency (LF), high-frequency (HF), and total spectral power (TSP).

RESULTS

For assessment of vagal cardiac reinnervation levels >20%, resting HR (ρ = 0.99, p < 0.05), RMSSD (ρ = 0.97, p < 0.05), and TSP (ρ = 0.96, p < 0.05) may be equally suitable as HF-power (ρ = 0.97, p < 0.05). To assess sympathetic reinnervation, LF/HF ratio (ρ = 0.87, p < 0.05) and tachycardic response to Valsalva maneuver (ρ = 0.9, p < 0.05) may be more suitable than LF-power (ρ = 0.77, p < 0.05).

CONCLUSIONS

Our model reports mechanistic relationships between CAMs and levels of efferent autonomic sinoatrial reinnervation. The results indicate differences in the suitability of these markers to assess vagal and sympathetic reinnervation. Although our analysis is purely conceptual, the developed model can help to gain important insights into the genesis of CAMs and their relationship to efferent sinoatrial reinnervation and, thus, provide indications for clinical study evaluation.

Factors Associated With Sinoatrial Reinnervation After Heart Transplantation

Background

Factors associated with sympathetic and parasympathetic sinoatrial reinnervation after heart transplantation (HTx) are inadequately studied.

Methods

Fifty transplant recipients were examined at 7 to 12 wk (index visit), 6, 12, 24, and 36 mo after HTx. Supine rest heart rate variability in the low-frequency (LF) domain (sympathetic and parasympathetic sinoatrial reinnervation) and the high-frequency (HF) domain (parasympathetic sinoatrial reinnervation) were measured repeatedly and related to selected recipient, donor, and perisurgical characteristics. We primarily aimed to identify index visit factors that affect the sinoatrial reinnervation process. Secondarily, we examined overall associations between indices of reinnervation and repeatedly measured recipient characteristics to generate new hypotheses regarding the consequences of reinnervation.

Results

LF and HF variability increased time dependently. In multivariate modeling, a pretransplant diagnosis of nonischemic cardiomyopathy (P = 0.038) and higher index visit handgrip strength (P = 0.028) predicted improved LF variability. Recipient age, early episodes of rejection, and duration of extracorporeal circulation were not associated with indices of reinnervation. Study average handgrip strength was positively associated with LF and HF variability (respectively, P = 0.005 and P = 0.029), whereas study average C-reactive protein was negatively associated (respectively, P = 0.015 and P = 0.008).

Conclusions

Indices of both sympathetic and parasympathetic sinoatrial reinnervation increased with time after HTx. A pretransplant diagnosis of nonischemic cardiomyopathy and higher index visit handgrip strength predicted higher indices of mainly sympathetic reinnervation, whereas age, rejection episodes, and duration of extracorporeal circulation had no association. HTx recipients with higher indices of reinnervation had higher average handgrip strength, suggesting a link between reinnervation and improved frailty. The more reinnervated participants had lower average C-reactive protein, suggesting an inhibitory effect of reinnervation on inflammation, possibly through enhanced function of the inflammatory reflex. These potential effects of reinnervation may affect long-term morbidity in HTx patients and should be scrutinized in future research.

Effect of Physical Training on the Morphology of Parasympathetic Atrial Ganglia after Unilateral Vagotomy in Rats

Cardiac denervation is a serious problem in a number of patients, including patients after heart transplantation. The status of the parasympathetic ganglia after crossing the preganglionic fibers of the vagus nerve has not been enough studied. The aim of our study was to assess the effect of physical training on the morphological parameters of the parasympathetic atrial ganglia and autonomic regulation of heart rate after right- and left-sided vagotomy in rats. Morphometric characteristics of the right atrial ganglia were evaluated using an immunohistochemical method after a study that included a three-time assessment of heart rate variability. It was found that right-sided vagotomy leads to both an increase in the volume of ganglion and autonomic dysfunction. No significant change in the number of nerve cells was found in animals with false and left-sided vagotomy while maintaining preganglionic innervation after the physical training, whereas exercises led to a decrease in the volume of nerve tissue of rats with right-sided denervation. It was also found that in animals with preserved vagal innervation, the volume of atrial ganglion tissue correlates with overall heart rate variability and a normalized parasympathetic component. Therefore, a positive effect from regular physical activity on parasympathetic regulation can be expected only if preganglionic vagal influence is preserved.

Nerve regeneration in transplanted organs and tracer imaging studies: A review

The technique of organ transplantation is well established and after transplantation the patient might be faced with the problem of nerve regeneration of the transplanted organ. Transplanted organs are innervated by the sympathetic, parasympathetic, and visceral sensory plexuses, but there is a lack of clarity regarding the neural influences on the heart, liver and kidneys and the mechanisms of their innervation. Although there has been considerable recent work exploring the potential mechanisms of nerve regeneration in organ transplantation, there remains much that is unknown about the heterogeneity and individual variability in the reinnervation of organ transplantation. The widespread availability of radioactive nerve tracers has also made a significant contribution to organ transplantation and has helped to investigate nerve recovery after transplantation, as well as providing a direction for future organ transplantation research. In this review we focused on neural tracer imaging techniques in humans and provide some conceptual insights into theories that can effectively support our choice of radionuclide tracers. This also facilitates the development of nuclear medicine techniques and promotes the development of modern medical technologies and computer tools. We described the knowledge of neural regeneration after heart transplantation, liver transplantation and kidney transplantation and apply them to various imaging techniques to quantify the uptake of radionuclide tracers to assess the prognosis of organ transplantation. We noted that the aim of this review is both to provide clinicians and nuclear medicine researchers with theories and insights into nerve regeneration in organ transplantation and to advance imaging techniques and radiotracers as a major step forward in clinical research. Moreover, we aimed to further promote the clinical and research applications of imaging techniques and provide clinicians and research technology developers with the theory and knowledge of the nerve.

Early Signs of Sinoatrial Reinnervation in the Transplanted Heart

BACKGROUND

Heart transplantation (HTx) surgically transects all connections to the heart, including the autonomic nerves. We prospectively examined signs, timing and consequences of early sympathetic and parasympathetic sinoatrial reinnervation, as well as explored indirect evidence of afferent cardiopulmonary reinnervation.

METHODS

Fifty HTx recipients were assessed at 2.5, 6, and 12 mo after HTx. For comparison, 50 healthy controls were examined once. Continuous, noninvasive recordings of hemodynamic variables and heart rate variability indices were performed at supine rest, 0.2 Hz controlled breathing, 60° head-up-tilt, during the Valsalva maneuver and during handgrip isometric exercise.

RESULTS

In HTx recipients, supine low-frequency heart rate variability gradually increased; supine high-frequency variability did not change; heart rate variability indices during controlled breathing remained unaltered; heart rate responses during tilt and isometric exercise gradually increased; the tachycardia response during Valsalva maneuver increased, while the bradycardia response remained unchanged; and indices of baroreflex sensitivity improved. Responses remained low compared to healthy controls. A negative correlation between indices of preload and heart rate response during head-up tilt emerged at 12 mo.

CONCLUSIONS

Results suggest that sympathetic reinnervation of the sinoatrial node starts within 6 mo after HTx and strengthens during the first year. No evidence of early parasympathetic reinnervation was found. Indirect signs of afferent reinnervation of cardiopulmonary low-pressure baroreceptors emerged at 12 mo. Better sympathetic sinoatrial control improved heart rate responsiveness to orthostatic challenge and isometric exercise, as well as heart rate buffering of blood pressure fluctuations.

Management of Arrhythmias After Heart Transplant: Current State and Considerations for Future Research

Orthotropic heart transplantation remains the most effective therapy for patients with end-stage heart failure, with a median survival of ≈13 years. Yet, a number of complications are observed after orthotropic heart transplantation, including atrial and ventricular arrhythmias. Several factors contribute to arrhythmias, such as autonomic denervation, effect of the surgical technique, acute and chronic rejection, and transplant vasculopathy among others. To minimize risk of future arrhythmias, the bicaval technique and minimizing ischemic time are current surgical standards. Sinus node dysfunction is the most common indication for early (within 30 days) pacemaker implantation, whereas atrioventricular block incidence increases as time from transplant increases. Atrial fibrillation can occur in the first few weeks following transplantation but is uncommon in the long term unless secondary to a precipitant such as acute rejection. The most common atrial arrhythmias are atrial flutters, which are mainly typical, but atypical circuits can be observed such as those that involve the remnant donor atrium in regions immediately adjacent to the atrioatrial anastomosis suture line. Choosing the appropriate pharmacological therapy requires careful consideration due to the potential interaction with immunosuppressive agents. Despite historical concerns, adenosine is effective and safe at reduced doses if administered under cardiac monitoring. Catheter ablation has emerged as an effective treatment strategy for symptomatic supraventricular tachycardias, including ablation of atypical flutter circuits. Cardiac allograft vasculopathy is an important risk factor for sudden cardiac death, yet the role of prophylactic implantable cardioverter-defibrillator implant for sudden death prevention is unclear. Current indications for implantable cardioverter-defibrillator implantation are as in the nontransplant population. A number of questions for future research are posed.

Functional evidence of low-pressure cardiopulmonary baroreceptor reinnervation 1 year after heart transplantation

PURPOSE

Heart transplantation (HTx) implies denervation of afferent neural connections. Reinnervation of low-pressure cardiopulmonary baroreceptors might impact the development and treatment of hypertension, but little is known of its occurrence. The present prospective study investigated possible afferent reinnervation of low-pressure cardiopulmonary baroreceptors during the first year after heart transplantation.

METHODS

A total of 50 heart transplant recipients (HTxRs) were included and were evaluated 7-12 weeks after transplant surgery, with follow-up 6 and 12 months later. In addition, a reference group of 50 healthy control subjects was examined once. Continuous, non-invasive recordings of cardiovascular variables were carried out at supine rest, during 15 min of 20° head-up tilt, during Valsalva maneuver and during 1 min of 30% maximal voluntary handgrip. In addition, routine clinical data including invasive measurements were used in the analyses.

RESULTS

During the first year after HTx, the heart rate (HR) response to 20° head-up tilt partly normalized, a negative relationship between resting mean right atrial pressure and HR tilt response developed, low-frequency variability of the RR interval and systolic blood pressure at supine rest increased, and the total peripheral resistance response to Valsalva maneuver became stronger.

CONCLUSION

Functional assessments suggest that afferent reinnervation of low-pressure cardiopulmonary receptors occurs during the first year after heart transplantation, partially restoring reflex-mediated responses to altered cardiac filling.

Takotsubo syndrome in a heart transplant recipient with poor cardiac sympathetic reinnervation

Takotsubo syndrome (TTS), also referred to as stress cardiomyopathy, is characterized by transient left ventricular apical ballooning in the absence of obstructive coronary artery disease. Catecholamine-induced cardiac injury or vasospasm has been implicated in this pathophysiology. We present a case of a 67-year-old man 10 years after heart transplantation diagnosed with TTS. Sympathetic reinnervation could not be detected by iodine-123 meta iodobenzylguanidine uptake, suggesting that TTS can occur in the absence of functional sympathetic nerve systems reconstruction.

Reinnervation post-heart transplantation

Heart transplantation results in complete denervation of the donor heart with loss of afferent and efferent nerve connections. The majority of patients remain completely denervated during the first 6-12 months following transplantation. Evidence of reinnervation is usually found during the second year after transplantation and involve the myocardial muscle, sinoatrial node, and coronary vessels, but remains incomplete and regionally limited many years post-transplant. Restoration of cardiac innervation can improve exercise capacity as well as blood flow regulation in the coronary arteries, and hence improve quality of life. As yet, there is no evidence that the reinnervation process is associated with the occurrence of allograft-related events or survival.

Ageing, the autonomic nervous system and arrhythmia: From brain to heart

An ageing myocardium possesses significant electrophysiological alterations that predisposes the elderly patient to arrhythmic risk. Whilst these alterations are intrinsic to the cardiac myocytes, they are modulated by the cardiac autonomic nervous system (ANS) and consequently, ageing of the cardiac ANS is fundamental to the development of arrhythmias. A systems-based approach that incorporates the influence of the cardiac ANS could lead to better mechanistic understanding of how arrhythmogenic triggers and substrates interact spatially and temporally to produce sustained arrhythmia and why its incidence increases with age. Despite the existence of physiological oscillations of ANS activity on the heart, pathological oscillations can lead to defective activation and recovery properties of the myocardium. Such changes can be attributable to the decrease in functionality and structural alterations to ANS specific receptors in the myocardium with age. These altered ANS adaptive responses can occur either as a normal ageing process or accelerated in the presence of specific cardiac pathologies, such as genetic mutations or neurodegenerative conditions. Targeted intervention that seek to manipulate the ageing ANS influence on the myocardium may prove to be an efficacious approach for the management of arrhythmia in the ageing population.

Electrocardiographic alterations associated with heart transplantation. Triggers, mechanisms and meaning

Introducción. Las alteraciones del ritmo cardíaco están asociadas con un aumento en la morbimortalidad; sin embargo, en pacientes con trasplante cardíaco no son claros sus desencadenantes ni implicaciones.Objetivos. Realizar una búsqueda en la literatura para identificar y explicar los determinantes en la generación de alteraciones de la conducción eléctrica en pacientes con trasplante cardíaco, así como describir las principales arritmias que pueden presentarse, explicando sus implicaciones patológicas.Materiales y métodos. Se realizó una búsqueda en la base de datos PubMed que arrojó un total de 411 resultados. Además, se buscaron las guías de práctica clínica sobre trasplante cardíaco, electrofisiología cardiovascular y endocarditis infecciosa. Se eligieron 60 artículos que lograban responder a los objetivos de este estudio.Resultados. La técnica quirúrgica, la denervación cardíaca, las lesiones del nodo sinusal, el rechazo del injerto, las biopsias endomiocárdicas y las infecciones son los principales factores que comprometen la viabilidad del órgano y la vida del paciente trasplantado, manifestándose como alteraciones del ritmo sinusal.Conclusiones. Ante la detección de alguna arritmia cardíaca, el equipo médico debe proporcionar un manejo que no se limite al control sintomático y del ritmo sinusal, sino que se debe iniciar una búsqueda activa de su etiología, ya que esta puede ser la manifestación de un proceso patológico subyacente.

Measures of and changes in heart rate variability in pediatric heart transplant recipients

Heart rate variability is primarily regulated by the autonomic nervous system. Heart transplant recipients undergo surgical denervation of the graft, which results in interruption of autonomic innervation with resultant diminished heart rate variability although some degree of autonomic control may return. This study aimed to characterize heart rate variability in this population. We report a retrospective review of Holter monitor data from transplanted patients between 2005 and 2013. Studies with significant atrial or ventricular arrhythmias were excluded. We evaluated changes over time and compared standard time domain measures to published pediatric normal values. Data were reviewed from 582 monitors in 152 patients. We found that pediatric heart transplant recipients have lower heart rate variability than age-matched controls and higher average heart rate in recipients older than 3 years. There is an increase in measures of variability through the first 3 years post-transplant with plateau after that time. Surgical technique in regard to interruption of the vagus nerve does not affect variability, nor does underlying congenital vs acquired heart disease.

Hypertension-induced remodelling: on the interactions of cardiac risk factors

Hypertension induces considerable cardiac remodelling, such as hypertrophy, interstitial fibrosis, and abnormal activity of the cardiac sympathetic nervous system, which are established risk factors in several highly dangerous heart diseases, such as ventricular fibrillation and congestive heart failure. All these risk factors and heart diseases are studied extensively in isolation, but to our knowledge, there is no comprehensive review of their interactions. At the same time, there is growing evidence suggesting that such interactions are numerous and that a successful therapy against a particular condition may have unexpectedly weak effects on mortality, as treated patients may die of a different cause exacerbated by the therapy. In this article, we present a multiscale review of the literature focusing on the relationships between the above‐mentioned risk factors and heart diseases, and introduce a framework that gives insight into their possible interactions. We use this framework to demonstrate that conditions such as fibrosis and elevated activity of the sympathetic nervous system may be compensatory, rather than purely pathological, mechanisms in certain contexts. Finally, we show why the described mechanisms are relevant not only in hypertension, but also in the case of healed myocardial infarction.

Ischemic Stroke after Heart Transplantation

Cerebrovascular complications after orthotopic heart transplantation (OHT) are more common in comparison with neurological sequelae subsequent to routine cardiac surgery. Ischemic stroke and transient ischemic attack (TIA) are more common (with an incidence of up to 13%) than intracranial hemorrhage (2.5%). Clinically, ischemic stroke is manifested by the appearance of focal neurologic deficits, although sometimes a stroke may be silent or manifests itself by the appearance of encephalopathy, reflecting a diffuse brain disorder. Ischemic stroke subtypes distribution in perioperative and postoperative period after OHT is very different from classical distribution, with different pathogenic mechanisms. Infact, ischemic stroke may be caused by less common and unusual mechanisms, linked to surgical procedures and to postoperative inflammation, peculiar to this group of patients. However, many strokes (40%) occur without a well-defined etiology (cryptogenic strokes). A silent atrial fibrillation (AF) may play a role in pathogenesis of these strokes and P wave dispersion may represent a predictor of AF. In OHT patients, P wave dispersion correlates with homocysteine plasma levels and hyperhomocysteinemia could play a role in the pathogenesis of these strokes with multiple mechanisms increasing the risk of AF. In conclusion, stroke after heart transplantation represents a complication with considerable impact not only on mortality but also on subsequent poor functional outcome.

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

Cardiac autonomic function in adolescents operated by arterial switch surgery

BACKGROUND

Children with transposition of the great arteries, in whom an arterial switch operation (ASO) is performed, have been shown to have an increased incidence of sudden death, which may be due to cardiac autonomic imbalance and repolarisation instability. We hypothesised that i) cardiac norepinephrine (NE) kinetics and ii) arterial baroreflex sensitivity (BRS), reflecting sympathetic activity and vagal function respectively, are altered in this group.

METHODS AND RESULTS

17 children (15.8 ± 1.5 years of age) with ASO-surgery in the neonatal period were studied. 17 had cardiac BRS assessed by spontaneous fluctuations of systolic blood pressure and RR-interval, and repolarisation was measured as QT variability index. Matched healthy subjects were controls. Cardiac vagal function and repolarisation pattern were unchanged following ASO-surgery. At cardiac catheterisation, we infused tritiated NE in 8 of these children to examine total body and cardiac sympathetic function at baseline and following 5 min of adenosine infusion to induce reflex sympathetic activation. Blood was sampled simultaneously from the aorta and coronary sinus. Cardiac fractional extraction of ([3H])NE was substantially lower in operated children, being 56 ± 10 vs. 82 ± 9% (p=0.0001). Following i.v. adenosine in the operated group, NE total body spillover doubled vs. baseline (p<0.002) and the coronary venous-arterial concentration gradient of ([3H])dihydroxyphenylglycol increased 4-fold (p=0.04).

CONCLUSIONS

Arterial switch operation performed neonatally appears to leave cardiac vagal function intact and, although cardiac sympathetic activation in response to adenosine occurs, cardiac neuronal NE reuptake is impaired. This may be pro-arrhythmic by reducing removal capacity of NE from the cardiac synaptic cleft.

Enhanced exercise performance and survival associated with evidence of autonomic reinnervation in pediatric heart transplant recipients

Following heart transplantation (HTx), loss of autonomic input to the allograft results in elevated resting heart rate (HR) and decreased chronotropic reserve. As enhanced exercise capacity and HR recovery post exercise are suggestive of reinnervation in pediatric cohorts, we used heart rate variability (HRV) analysis to assess autonomic reinnervation in pediatric HTx recipients. Pediatric patients transplanted between 1996 and 2010 and with serial 24-hour Holter recordings post-HTx were analyzed for HRV using time and frequency domain measures. Of 112 patients, 68 (57%) showed evidence of autonomic reinnervation that was not associated with age at HTx. Evidence of reinnervation was associated with a significant increase in low-frequency power spectrum (p<0.001), suggesting sympathetic reinnervation. Patients with evidence of reinnervation showed higher percent-predicted maxVO(2) on performing an exercise test (+10.2 ± 3.6%, p = 0.006) and improved HR recovery at 3 minutes (-11.4 ± 3.9 bpm, p = 0.004), but no difference in percent-predicted maximal HR. Cox hazards modeling using presumed sinus reinnervation criteria at last Holter recording as a time-dependent covariate was associated with decreased hazard of mortality and/or retransplantation (HR: 0.2, 95% CI 0.04-1.0, p = 0.05). In conclusion, a majority of pediatric HTx recipients demonstrate evidence of reinnervation that is associated with functional outcomes. Studies to assess graft reinnervation as a marker of long-term prognosis are warranted.

Remote noninvasive allograft rejection monitoring for heart transplant recipients: study protocol for the novel evaluation with home electrocardiogram and remote transmission (NEW HEART) study

Background

Acute allograft rejection is a major cause of early mortality in the first year after heart transplantation in adults. Although endomyocardial biopsy (EMB) is not a perfect "gold standard" for a correct diagnosis of acute allograft rejection, it is considered the best available test and thus, is the current standard practice. Unfortunately, EMB is an invasive and costly procedure that is not without risk. Recent evidence suggests that acute allograft rejection causes delays in ventricular repolarization and thereby increases the cellular action potential duration resulting in a longer QT interval on the electrocardiogram (ECG). No prospective study to date has investigated whether such increases in the QT interval could provide early detection of acute allograft rejection. Therefore, in the Novel Evaluation With Home Electrocardiogram And Remote Transmission (NEW HEART) study, we plan to investigate the potential benefit of daily home QT interval monitoring to predict acute allograft rejection.

Methods/design

The NEW HEART study is a prospective, double-blind, multi-center descriptive research study. A sample of 325 adult heart transplant recipients will be recruited within six weeks of transplant from three sites in the United States. Subjects will receive the HeartView™ ECG recorder and its companion Internet Transmitter, which will transmit the subject's ECG to a Core Laboratory. Subjects will be instructed to record and transmit an ECG recording daily for 6 months. An increase in the QT_C interval from the previous day of at least 25 ms that persists for 3 consecutive days will be considered abnormal. The number and grade of acute allograft rejection episodes, as well as all-cause mortality, will be collected for one year following transplant surgery.

Discussion

This study will provide "real world" prospective data to determine the sensitivity and specificity of QT_C as an early non invasive marker of cellular rejection in transplant recipients during the first post-transplant year. A non-invasive indicator of early allograft rejection in heart transplant recipients has the potential to limit the number and severity of rejection episodes by reducing the time and cost of rejection surveillance and by shortening the time to recognition of rejection.

Trial Registration

ClinicalTrials.gov: NCT01365806

Heart rate variability after heart transplantation: a 10-year longitudinal follow-up study

BACKGROUND

Cross-sectional studies suggest, by use of heart rate variability (HRV), that partial re-innervation of the sinus node may occur after heart transplantation (HTx). Our aim was to test this hypothesis by examining HRV in long-term longitudinal follow-up study of HTx recipients.

METHODS AND RESULTS

14 HTx recipients (11 men) were studied 1-48 (median 13) months (baseline) and 119-172 (median 141) months after HTx (follow-up). At baseline and follow-up, electrocardiographic RR interval was continuously recorded in the supine position for 20 min. The signals were analyzed in the time domain and in the frequency domain by means of power spectral analysis. RR-interval decreased significantly from baseline to follow-up (p<0.05). This was associated with an increase of total power (p<0.001), absolute low frequency (p<0.001), and high frequency power (p<0.001), but unchanged relative low frequency and high frequency power.

CONCLUSIONS

The observed changes in HRV during long-term follow-up after HTx are compatible with partial re-innervation of the cardiac sinus node, as has been suggested by cross-sectional studies.

Hypertension after heart and heart/lung transplantation in childhood--study on the evolution of short-term blood pressure regulation

Arterial hypertension complicates the follow-up of heart- and heart/lung-transplanted children. We investigated the evolution of BRS as short-time BP regulation mechanism and BP after heart and heart/lung transplantation. Twenty patients (15 males; mean age 15.1 ± 4.3 yr) were studied twice at intervals of 2.96 ± 0.87 yr. BRS was calculated using non-invasive beat-to-beat BP measurement system. HRV was calculated (LF, sympathetic influence; HF, parasympathetic influence). BRS increased in 10 patients (3.67 ± 1.43 ms/mmHg vs. 7.59 ± 3.40 mmHg, p = 0.005) (group 1). Six of 10 patients received antihypertensive medication. BRS decreased or remained unchanged in 10 patients (8.93 ± 7.9 ms/mmHg vs. 5.32 ± 6.6 ms/mmHg, p = 0.008) (group 2) with 9/10 patients necessitating antihypertensive medication. Group 1 showed LF/HF increase (LF/HF 1.03 ± 0.9 vs. 4.36 ± 2.32, p = 0.03); group 2 showed LF/HF decrease (LF/HF 3.7 ± 2.1 vs. 1.84 ± 1.1, p = 0.023). Evolution of BRS after heart and heart/lung transplantation in childhood seems to influence the necessity of antihypertensive medication. With time, increasing short-time BP regulation involving sympathetic reinnervation may improve BP.

Short-term testing of heart rate variability in heart-transplanted children: equal to 24-h ECG recordings?

INTRODUCTION

Heart rate variability (HRV) is reduced in adults and children after cardiac transplantation. Testing of HRV has been used to assess re-innervation of the cardiac graft; its reliability in ruling out acute graft rejection is still under investigation. This study used a short-term test on HRV in 23 heart and heart-lung transplanted children and adolescents and compared the results with 24-h ECG recordings.

PATIENTS AND METHODS

Twenty-three subjects (16.3+/-4.2 yr; 10 females) underwent a 10-min HRV test at two occasions and one 24-h ECG. HRV was calculated according to the time domain method (RR interval, standard deviation of RR interval) and the frequency domain method (total power, LF and HF for assessment of sympathovagal modulation of heart rate).

RESULTS

Correlation between the short-term tests and 24-h ECG was high with regard to the frequency domain analysis of HRV. Correlation was less pronounced in the time domain method.

CONCLUSIONS

In heart and heart-lung-transplanted children and adolescents, due to reduced overall HRV short-term testing may give as reliable data as 24-h ECG. Therefore, especially when power spectral analysis has to be performed as a longitudinal assessment of re-innervation of the cardiac graft, short-term testing may offer an easily applicable and non-invasive diagnostic tool. Further studies are warranted to investigate whether HRV testing may contribute to rule out acute graft rejection.

Reinnervation after heart transplantation in children: results of short-time heart rate variability testing

AIMS

To detect impairment in short-term heart rate variability (HRV) in children after heart and heart-lung transplantation (TX) as reported in adults. To assess vagal and sympathetic influence on the donor heart rate using frequency domain analysis of HRV.

METHODS AND RESULTS

Measurement of short-term HRV was performed in 17 patients (age 16.9+/-3.6, 6.1+/-3.7 yr after TX) and 12 healthy controls (age 14.8+/-3.0 yr). Testing consisted of a resting phase of 15 minutes followed by a tilt phase of 45 min. All HRV parameters were significantly impaired in transplanted patients: RR interval (RRI) 717.2+/-122.5 m/s (controls 827+/-139.7, p<0.05), standard deviation of RR interval (RRI-SD) 20.1+/-15.5 (89.9+/-38.4, p<0.001), RRI at tilt 607.9+/-79.7 (654.0+/-104.7, NS), RRI-SD at tilt 21.1+/-20.0 (60.4+/-31.4, p<0.001). Low-frequency (LF)/High-frequency (HF) ratio of HRV showed prominent sympathetic influence in TX-patients (3.38+/-5.60 vs. 1.18+/-0.86, NS) increasing during tilting (5.91+/-8.36 vs. 4.74+/-5.27, NS). In subgroup analysis, 4 yr after TX an increasing sympathetic control of heart rate was observed.

CONCLUSION

Short-term HRV is severely impaired in children after TX. If changes are observed, they are time-related and show increasing sympathetic influence starting from 4 yr after TX.

Hypertension in Heart and Heart-Lung Transplanted Children: Does Impaired Baroreceptor Function Play a Role?

BACKGROUND

Baroreceptor control of beat-to-beat blood pressure in heart and heart-lung-transplanted children is impaired. Time-related trends of baroreceptor function recovery are studied and a possible correlation of baroreflex impairment and systolic hypertension may give evidence for supplemental medical treatment of hypertension.

METHODS

Seventeen patients (six female) 6.1 +/- 3.7 years (range 0.8-13.0 years) after heart (n = 14) and heart-lung (n = 3) transplantation (TX) were studied. Twelve healthy children and 10 children after liver and bone marrow TX taking cyclosporine A (CyA) served as control groups 1 and 2, respectively. Baroreceptor sensitivity (BRS) was calculated from noninvasive systolic beat-to-beat blood pressure (sBP) measurement during a resting phase and a tilt-table test.

RESULTS

BRS was significantly impaired in the study group at rest and during tilting; mean sBP was slightly elevated. Significant difference between patients on CyA and healthy controls was not observed. Discrete recovery of BRS occurred after 4 years postTX with decreased sBP (n = 12 pts, BRS 6.78 +/- 7.44 msec/mmHg, sBP 116.2 +/- 12.4 mmHg) when compared to a postTX time course of less than 4 years (n = 5 pts, BRS 4.02 +/- 4.21 msec/mmHg, sBP 122.0 +/- 6.7 mmHg, P = NS).

CONCLUSION

BRS is disturbed after TX in children; four years postTX, a minimal recovery of BRS and a discrete reduction of sBP seem to occur. Those patients with a persistent low BRS and elevated sBP may profit from pharmacological influence in sympathovagal imbalance.

Different evolutions in heart rate variability after heart transplantation: 10-year follow-up

BACKGROUND

After heart transplantation, the donor heart is extrinsically denervated. No input of sympathetic or vagal nerves can influence the heart rate, resulting in a flat power spectrum of the beat-to-beat variability. The occurrence and the significance of reinnervation remain controversial.

METHODS AND RESULTS

We monitored the evolution of heart rate variability (HRV) after heart transplantation, starting from a few weeks postoperatively up to 10 years after surgery. Twenty-four-hour Holter recordings of 216 heart-transplant patients were analyzed using time and frequency domain analysis of HRV. Analysis of all data revealed an increase in 24-hour and night-time total power starting from 2 years after transplantation. Low-frequency oscillations calculated over the total 24 hours, day- and nighttime increased significantly starting from year 4 and onward (year 4-8: P < 0.005). No evolution was found in high-frequency power. Subgroup analysis revealed a group with a clear spectral component (n = 16), a group with a small component (n = 124), and a group with a flat spectrum (n = 76). Only the first group revealed an evolution in both high- and low-frequency power.

CONCLUSION

These results indicate three different types of evolution in HRV, with reinnervating patterns present in only a minority of the patients. The vast majority of the patients show no signs of reinnervation.

Re-innervation after heart transplantation: a multidisciplinary study

BACKGROUND

Several authors have demonstrated the existence and implications of re-innervation in the transplanted heart. Our aim was to study this phenomenon using 3 different techniques and to analyze the correlation among them.

METHODS

The study population consisted of 55 patients who had undergone heart transplantation 0.5 to 160 months earlier. We used a control group of 10 healthy individuals for comparison. To detect re-innervation, we used 1) planar and single photon emission computed tomography (SPECT) scintigraphic imaging of cardiac sympathetic activity with 123I-metaiodobenzylguanidine (MIBG), 2) analysis of heart-rate variability based on 24-hour Holter recordings; and 3) immunohistochemical study of endomyocardial biopsy specimens with anti-S100 antibody.

RESULTS

The SPECT images showed evidence of sympathetic re-innervation in 17 patients (31%), predominantly in the anterior and in the septal regions of the left ventricle. Sympathetic activity increased during the post-transplant time course (r = 0.32; p = 0.017), although it did not reach normal values. We found a correlation between the low-frequency component of heart-rate variability (a marker of sympathetic activity) and the degree of MIBG uptake according to scintigraphy (r = 0.32; p = 0.015). Immunostaining study demonstrated the existence of nerve fibers in 36 patients (65%) who had greater values of heart-rate variability parameters reflecting parasympathetic activity.

CONCLUSIONS

The 3 techniques evidenced re-innervation after heart transplantation. A correlation exists between sympathetic activity detected using MIBG scintigraphy and analysis of heart-rate variability. Patients in whom endomyocardial biopsy specimen reveals the presence of nerve fibers show more parasympathetic activity in the heart-rate variability analysis.

Degeneration and regeneration of perivascular innervation in arterial grafts

Because the understanding of postoperative changes in arterial graft innervation is limited, this study was performed to characterize neuronal degeneration and regeneration events immunohistochemically in femoral arterial grafts transplanted to carotid arteries in rats. Specimens taken 1 day, 3 days, 7 days, 1 month, 3 months, and 5 months after surgery were assessed for vasoactive intestinal peptide, neurofilaments, growth-associated protein 43, tyrosine hydroxylase, and nitric oxide synthase isoenzymes. During neuronal degeneration, vasoactive intestinal peptide disappeared within 1 day, transmitter-synthesizing enzymes (nitric oxide synthase and tyrosine hydroxylase) had vanished by day 7, and neurofilaments (cytoskeletal markers) had essentially disappeared after 1 week. In the regeneration phase, the most robust axonal growth, as visualized by growth-associated protein 43, was observed at 1 month, followed by a gradual increase in neurotransmitter markers at 1 and 3 months, whereas the neurofilaments increased gradually up to the end of the 5-month observation period. Reinnervation proceeded from the proximal carotid (host) trunk distally to the graft. Axonal re-growth occurred mainly in arterial adventitia. Innervation density, as visually assessed, was denser in the graft than in the host. These findings suggest that 1) the main sequence of degeneration and regeneration follows that reported in other models of neuronal degeneration; 2) reinnervation of the arterial grafts comes mainly from the host arteries; and 3) the innervation density in the graft may differ from that in the host, which may suggest target-derived regulation of innervation. The latter finding may have clinical implications. It suggests that for a good outcome it would be beneficial to choose a sparsely innervated graft rather than a densely innervated one.

Assessment of cardiac sympathetic neuronal function using PET imaging

The autonomic nervous system plays a key role for regulation of cardiac performance, and the importance of alterations of innervation in the pathophysiology of various heart diseases has been increasingly emphasized. Nuclear imaging techniques have been established that allow for global and regional investigation of the myocardial nervous system. The guanethidine analog iodine 123 metaiodobenzylguanidine (MIBG) has been introduced for scintigraphic mapping of presynaptic sympathetic innervation and is available today for imaging on a broad clinical basis. Not much later than MIBG, positron emission tomography (PET) has also been established for characterizing the cardiac autonomic nervous system. Although PET is methodologically demanding and less widely available, it provides substantial advantages. High spatial and temporal resolution along with routinely available attenuation correction allows for detailed definition of tracer kinetics and makes noninvasive absolute quantification a reality. Furthermore, a series of different radiolabeled catecholamines, catecholamine analogs, and receptor ligands are available. Those are often more physiologic than MIBG and well understood with regard to their tracer physiologic properties. PET imaging of sympathetic neuronal function has been successfully applied to gain mechanistic insights into myocardial biology and pathology. Available tracers allow dissection of processes of presynaptic and postsynaptic innervation contributing to cardiovascular disease. This review summarizes characteristics of currently available PET tracers for cardiac neuroimaging along with the major findings derived from their application in health and disease.

Evolution of heart rate control after transplantation: conduction versus autonomic innervation

In cardiac transplantation, the donor organ is not initially innervated and demonstrates decreased heart rate variability (HRV). However, HRV may improve after several months. The mechanism for HRV improvement has not been elucidated; autonomic "reinnervation" of the donor heart has been proposed. The role of atrioatrial conduction from recipient to donor organ has not been evaluated. We prospectively evaluated cardiac transplant patients with a limited electrophysiology study at the time of their surveillance biopsies. Recordings were made of recipient and donor signals, observing conduction properties between recipient and donor atria. Holter recordings were analyzed and HRV was determined using spectral analysis techniques, recording mean RR interval, low-frequency power (LF), high-frequency power (HF), and the LF/HF ratio. These were compared to published norms. From November 1999 to May 2000, 21 patients (6 female) who underwent cardiac transplantation participated at a median age of 101 months (range, 4.1-217 months). Time posttransplant ranged from 26 days to 71 months. Holter data were available for 20 patients and demonstrated dissociated P waves in 13 (65%). The mean heart rate on Holter was 111 beats per minute (bpm) (range, 85-161 bpm). We were able to record distinct recipient atrial signals in 16 of 21 (76%) patients. The average recipient tissue heart rate was 55% that of the donor heart rate. We documented atrioatrial association in only 1 patient. HRV did not reach normal values for most patients and did not increase with time posttransplantation. The LF values were in the normal range for most patients, whereas 3 patients had normal HF values and 2 patients had values just below normal. Recipients of heart transplantation have a predominantly sympathetic influence of HRV. These preliminary data suggest that atrioatrial conduction does not play a role in reestablishing normal heart rate control following pediatric cardiac transplantation.

Impact of heart transplantation on the safety and feasibility of the dobutamine stress test

BACKGROUND

Dobutamine myocardial perfusion imaging is a useful method for evaluation of coronary artery disease. However, this technique does not allow for ischemia monitoring, which may have an impact on the safety of the test in heart transplant recipients due to cardiac sensory denervation. The aim of this study was to assess the impact of heart transplantation on the feasibility and complications of the dobutamine stress test.

METHODS

We studied 225 heart transplant recipients (mean age 57 +/- 7 years) and a control group of 225 patients without previous transplant matched for age and gender by dobutamine (up to 40 microg/kg per minute) stress myocardial perfusion imaging.

RESULTS

During the test, transplant recipients had a lower prevalence of premature ventricular contractions (23% vs. 37%, p < 0.001) and ventricular tachycardia (0.04% vs 7.5%, p < 0.0001) compared with control patients. By multivariate analysis, heart transplantation was a powerful independent variable associated with a reduced risk of ventricular arrhythmias (chi(2) = 20.8, p < 0.0001) and minor side effects (nausea, dizziness, anxiety, flushing, chills) (chi(2) = 20, p < 0.0001) during dobutamine stress. The target heart rate was reached in 82% of transplant recipients and in 77% of the control group. Overall feasibility (achievement of the target heart rate and/or an ischemic end-point) was 87% in the transplant and 86% in the control group.

CONCLUSIONS

Dobutamine stress myocardial perfusion imaging is a safe and feasible method for evaluation of coronary artery disease in heart transplant recipients. The prevalence of arrhythmias and minor complications using the dobutamine stress test is lower in heart transplant recipients compared with control patients. The independent association between heart transplantation and reduced risk of arrhythmias and minor side effects of the dobutamine stress test indicates that cardiac sensory and autonomic nerve function plays a major role in the induction of these complications during the test.

Vagal reinnervation in the long term after orthotopic heart transplantation

BACKGROUND

Sympathetic reinnervation after orthotopic heart transplantation (HTx) has become an accepted phenomenon, particularly in long-term transplanted patients. Parasympathetic reinnervation, however, still remains questionable.

METHODS

In 38 HTx recipients, mean age of 51.6 +/- 9.7 years (range, 29 to 70 years), with a time span after HTx of 4.6 +/- 2.8 years (0.5 to 10.5 years), we stimulated carotid baroreceptors using periodic neck suction at low (0.1 Hz) and high (0.2 Hz) frequencies to test sympathetic and vagal responses, respectively. Respiratory rate was fixed at 0.25 Hz. We simultaneously recorded surface electrocardiogram, finger blood pressure, respiration and neck pressure signals while patients rested in the supine position. Time series of RR intervals, respiration, and neck and blood pressures were generated and subjected to spectral analysis.

RESULTS

All patients demonstrated a 0.25-Hz peak in RR-interval spectrum, caused by non-autonomic respiratory sinus arrhythmia. Thirteen patients (5. 2 +/- 3.5 years after HTx; range, 0.9 to 10.2 years) showed a baroreflex-induced sharp peak at 0.1 Hz in RR-interval power spectrum during 0.1-Hz neck suction, indicating sympathetic reinnervation. However at 0.2-Hz neck suction, 4 of the 13 sympathetically reinnervated patients displayed a baroreflex-induced 0.2-Hz peak, which could be suppressed with atropine administration-strong evidence for vagal reinnervation.

CONCLUSIONS

Non-invasive carotid baroreflex stimulation is an appropriate tool to prove restoration of autonomic control after orthotopic HTx. Sympathetic reinnervation parallels parasympathetic reinnervation in long-term transplanted patients.