Research Opportunities in the Treatment of Mitral Valve Prolapse: JACC Expert Panel.

In light of the adverse prognosis related to severe mitral regurgitation, heart failure, or sudden cardiac death in a subset of patients with mitral valve prolapse (MVP), identifying those at higher risk is key. For the first time in decades, researchers have the means to rapidly advance discovery in the field of MVP thanks to state-of-the-art imaging techniques, novel omics methodologies, and the potential for large-scale collaborations using web-based platforms. The National Heart, Lung, and Blood Institute recently initiated a webinar-based workshop to identify contemporary research opportunities in the treatment of MVP. This report summarizes 3 specific areas in the treatment of MVP that were the focus of the workshop: 1) improving management of degenerative mitral regurgitation and associated left ventricular systolic dysfunction; 2) preventing sudden cardiac death in MVP; and 3) understanding the mechanisms and progression of MVP through genetic studies and small and large animal models, with the potential of developing medical therapies.

Geographical variation in infant mortality due to congenital heart disease in the USA: a population-based cohort study.

BACKGROUND: Little is known about geographical variation in infant mortality due to congenital heart disease (CHD) and the social determinants of health that might mediate such variation. We aimed to examine US county-level estimates of infant mortality due to CHD to understand geographical patterns and factors that might influence variation in mortality. METHODS: This US population-based cohort study used linked livebirth-infant death cohort files from the US National Center for Health Statistics from Jan 1, 2006, to Dec 31, 2015. All deaths attributable to congenital heart disease in infants in a given year were included. We used hierarchical Bayesian models to estimate rates of infant mortality due to congenital heart disease for all US counties. We mapped model-based estimates to explore geographical patterns. Covariates included infant sex, gestational age, maternal race and ethnicity, percentage of the county population below the poverty level, and proximity of the county to a US News & World Report 2015 top-50 ranked paediatric cardiac centre. FINDINGS: From 2006 to 2015, 40 847 089 livebirths occurred, of which there were 13 988 infant deaths attributed to congenital heart disease, with an unadjusted infant mortality rate due to CHD of 0·34 per 1000 livebirths (95% CI 0·34-0·35). Kentucky and Mississippi had the greatest proportions of counties with a predicted rate of infant mortality due to CHD above the 95th percentile. All counties in Connecticut, Massachusetts, and Rhode Island had a predicted rate below the fifth percentile. In the model, lower mortality risk correlated with closer proximity to a top-50 ranked paediatric cardiac centre (odds ratio [OR] 0·890, 95% credible interval [CrI] 0·840-0·942), whereas higher mortality risk correlated with higher levels of poverty (OR 1·181, 95% CrI 1·125-1·239). INTERPRETATION: Substantial geographical variation exists in infant mortality due to CHD in the USA, highlighting the potential importance of bolstering care delivery for infants from economically deprived communities and areas remote from top-performing paediatric cardiac centres. FUNDING: None.

Priorities for Patient-Centered Research in Valvular Heart Disease: A Report From the National Heart, Lung, and Blood Institute Working Group.

Over the past decade, the field of valvular heart disease (VHD) has rapidly transformed, largely as a result of the development and improvement of less invasive transcatheter approaches to valve repair or replacement. This transformation has been supported by numerous well-designed randomized trials, but they have centered almost entirely on devices and procedures. Outside this scope of focus, however, myriad aspects of therapy and management for patients with VHD have either no guidelines or recommendations based only on expert opinion and observational studies. Further, research in VHD has often failed to engage patients to inform study design and identify research questions of greatest importance and relevance from a patient perspective. Accordingly, the National Heart, Lung, and Blood Institute convened a Working Group on Patient-Centered Research in Valvular Heart Disease, composed of clinician and research experts and patient advocacy experts to identify gaps and barriers to research in VHD and identify research priorities. While recognizing that important research remains to be done to test the safety and efficacy of devices and procedures to treat VHD, we intentionally focused less attention on these areas of research as they are more commonly pursued and supported by industry. Herein, we present the patient-centered research gaps, barriers, and priorities in VHD and organized our report according to the "patient journey," including access to care, screening and diagnosis, preprocedure therapy and management, decision making when a procedure is contemplated (clinician and patient perspectives), and postprocedure therapy and management. It is hoped that this report will foster collaboration among diverse stakeholders and highlight for funding bodies the pressing patient-centered research gaps, opportunities, and priorities in VHD in order to produce impactful patient-centered research that will inform and improve patient-centered policy and care.

Performance and predictors of recruitment success in National Heart, Lung, and Blood Institute's cardiovascular clinical trials.

Background/Aims Identifying predictors of recruitment success in clinical trials, particularly prior to study launch, could contribute to higher study completion rates and improved scientific return on investment. This article evaluates the performance of clinical trials funded by the National Heart, Lung, and Blood Institute that began recruitment before and after implementation of National Heart, Lung, and Blood Institute's 2009 Accrual Policy and identifies study-related factors that predict recruitment success. Methods A retrospective analysis of National Heart, Lung, and Blood Institute's cardiovascular clinical trials with initial funding from 1996 to 2012 was performed to assess recruitment success. Success was defined as ≥100% enrollment of the proposed sample size within the duration initially proposed by investigators. Trials were assigned to categories (pre-policy vs post-policy) based on whether the first patient was enrolled before or after the 2009 Accrual Policy implementation. Potential determinants of successful recruitment were evaluated using multivariable logistic regression. Results Of 167 trials analyzed, 26.3% met the definition of success. Twenty-four trials (14.4%) were terminated early and 15 (62.5%) for insufficient recruitment. Trials failed due to <100% enrollment (22.8%), longer duration (19.8%), or both (31.1%). Trials testing behavioral interventions, those conducted within a National Heart, Lung, and Blood Institute-funded network, and those with normal controls were predictive of success. The proportion of successful clinical trials increased from 23% in the pre-policy era to 30% post-policy, although the difference was not statistically significant ( p = 0.29). Conclusion Enrollment success rates for National Heart, Lung, and Blood Institute's clinical trials are concerning. The 2009 National Heart, Lung, and Blood Institute Accrual Policy did not significantly improve trial success. Clinical trials testing behavioral interventions, those conducted within networks, and those with normal controls were predictive of recruitment success. Components of networks may provide model practices to help other trials attain success, including close attention to oversight activities such as recruitment plans, real-time enrollment monitoring, corrective action plans to address shortfalls, and close sponsor-investigator collaborations.

Re-evaluating pathogenicity of variants associated with the long QT syndrome.

INTRODUCTION: Genetic testing for congenital long QT syndrome (LQTS) has become common. Recent studies have shown that some variants labelled as pathogenic might be misclassified due to sparse case reports and relatively common allele frequencies (AF) in the general population. This study aims to evaluate the presence of LQTS-associated variants in the Genome Aggregation Database (gnomAD) population, and assess the functional impact of these variants. METHODS AND RESULTS: Variants associated with LQTS from the Human Gene Mutation Database were extracted and matched to the gnomAD to evaluate population-based AF. We used MetaSVM to predict the function of LQTS variants. Allele distribution by protein topology in KCNQ1, KCNH2, and SCN5A was compared between gnomAD (n = 123,136) and a cohort of LQTS patients aggregated from eight published studies (n = 2,683). Among the 1,415 LQTS-associated single nucleotide variants in 30 genes, 347 (25%) are present in gnomAD; 24% of the 347 variants were predicted as functionally tolerated compared with 4% of variants not present in gnomAD (P < 0.001). Of the 347 pathogenic variants in gnomAD, seven (2%) had an AF of ≥ 0.001 and 65 (19%) variants had an AF of ≥ 0.0001. In KCNQ1, KCNH2, and SCN5A, allele distribution by protein functional region was significantly different with gnomAD alleles appearing less frequently in highly pathogenic domains than case alleles. CONCLUSION: A significant number of LQTS variants have insufficient evidence for pathogenicity and relatively common AF in the general population. Caution should be used when ascribing pathogenicity to these variants.

The rationale for simple approaches for sustainability assessment and management in contaminated land practice.

The scale of land-contamination problems, and of the responses to them, makes achieving sustainability in contaminated land remediation an important objective. The Sustainable Remediation Forum in the UK (SuRF-UK) was established in 2007 to support more sustainable remediation practice in the UK. The current international interest in 'sustainable remediation' has achieved a fairly rapid consensus on concepts, descriptions and definitions for sustainable remediation, which are now being incorporated into an ISO standard. However the sustainability assessment methods being used remain diverse with a range of (mainly) semi-quantitative and quantitative approaches and tools developed, or in development. Sustainability assessment is site specific and subjective. It depends on the inclusion of a wide range of considerations across different stakeholder perspectives. Taking a tiered approach to sustainability assessment offers important advantages, starting from a qualitative assessment and moving through to semi-quantitative and quantitative assessments on an 'as required' basis only. It is also clear that there are a number of 'easy wins' that could improve performance against sustainability criteria right across the site management process. SuRF-UK has provided a checklist of 'sustainable management practices' that describes some of these. This paper provides the rationale for, and an outline of, and recently published SuRF-UK guidance on preparing for and framing sustainability assessments; carrying out qualitative sustainability assessment; and simple good management practices to improve sustainability across contaminated land management activities.

Surgical ligation of patent ductus arteriosus in premature infants: trends and practice variation.

OBJECTIVE: We sought to analyse the variation in the incidence of patent ductus arteriosus over three recent time points and characterise ductal ligation practices in preterm infants in the United States, adjusting for demographic and morbidity factors. METHODS: Using the Kids' Inpatient Database from 2003, 2006, and 2009, we identified infants born at ⩽32 weeks of gestation with International Classification of Diseases, Ninth Revision diagnosis of patent ductus arteriosus and ligation code. We examined patient and hospital characteristics and identified patient and hospital variables associated with ligation. RESULTS: Of 182,610 preterm births, 30,714 discharges included a patent ductus arteriosus diagnosis. The rate of patent ductus arteriosus diagnosis increased from 14% in 2003 to 21% in 2009 (p<0.001). A total of 4181 ligations were performed, with an overall ligation rate of 14%. Ligation rate in infants born at ⩽28 weeks of gestation was 20% overall, increasing from 18% in 2003 to 21% in 2009 (p<0.001). The ligation rate varied by state (4-28%), and ligation was associated with earlier gestational age, associated diagnoses, hospital type, teaching hospital status, and region (p<0.001). CONCLUSION: The rates of patent ductus arteriosus diagnosis and ligation have increased in the recent years. Variation exists in the practice of patent ductus arteriosus ligation and is influenced by patient and non-patient factors.

A review of the current state of the art of physiologically-based tests for measuring human dermal in vitro bioavailability of polycyclic aromatic hydrocarbons (PAH) in soil.

Polycyclic Aromatic Hydrocarbons are classed as Persistent Organic Pollutants, a large group of compounds that share similar characteristics. They are lipophilic, resistant to degradation in the environment and harmful to human and environmental health. Soil has been identified as the primary reservoir for Polycyclic Aromatic Hydrocarbons in the United Kingdom. This study reviews the literature associated with, or is relevant to, the measurement and modelling of dermal absorption of Polycyclic Aromatic Hydrocarbons from soils. The literature illustrates the use of in vivo, in vitro and in silico methods from a wide variety of scientific disciplines including occupational and environmental exposure, medical, pharmaceutical and cosmetic research and associated mathematical modelling. The review identifies a number of practical shortcomings which must be addressed if dermal bioavailability tests are to be applied to laboratory analysis of contaminated soils for human health risk assessment.

Prior publication productivity, grant percentile ranking, and topic-normalized citation impact of NHLBI cardiovascular R01 grants.

RATIONALE: We previously demonstrated absence of association between peer-review-derived percentile ranking and raw citation impact in a large cohort of National Heart, Lung, and Blood Institute cardiovascular R01 grants, but we did not consider pregrant investigator publication productivity. We also did not normalize citation counts for scientific field, type of article, and year of publication. OBJECTIVE: To determine whether measures of investigator prior productivity predict a grant's subsequent scientific impact as measured by normalized citation metrics. METHODS AND RESULTS: We identified 1492 investigator-initiated de novo National Heart, Lung, and Blood Institute R01 grant applications funded between 2001 and 2008 and linked the publications from these grants to their InCites (Thompson Reuters) citation record. InCites provides a normalized citation count for each publication stratifying by year of publication, type of publication, and field of science. The coprimary end points for this analysis were the normalized citation impact per million dollars allocated and the number of publications per grant that has normalized citation rate in the top decile per million dollars allocated (top 10% articles). Prior productivity measures included the number of National Heart, Lung, and Blood Institute-supported publications each principal investigator published in the 5 years before grant review and the corresponding prior normalized citation impact score. After accounting for potential confounders, there was no association between peer-review percentile ranking and bibliometric end points (all adjusted P>0.5). However, prior productivity was predictive (P<0.0001). CONCLUSIONS: Even after normalizing citation counts, we confirmed a lack of association between peer-review grant percentile ranking and grant citation impact. However, prior investigator publication productivity was predictive of grant-specific citation impact.

Rising charges and costs for pediatric catheter ablation.

INTRODUCTION: Catheter ablation has been shown to be effective for pediatric tachyarrhythmias, but the associated charges and costs have not been described in the recent era. Understanding such contemporary trends may identify ways to keep an effective therapy affordable while optimizing clinical outcomes. METHODS: We used the 1997-2009 Kids' Inpatient Databases to examine trends in charges and costs for pediatric catheter ablation and identify determinants of temporal changes. RESULTS: There were 7,130 discharges for catheter ablation in the sample. Mean age at ablation was 12.1 ± 0.2 years. Patients with congenital heart disease (CHD) made up 10% of the sample. Complications occurred in 8% of discharges. Mean total charges rose 219% above inflation (from $23,798 ± 1,072 in 1997 to $75,831 ± 2,065 in 2009). From 2003 to 2009, costs rose 25% (from $20,459 ± 780 in 2003 to $25,628 ± 992 in 2009). Charges for ablation increased markedly relative to surgical procedures, but with a similar slope to other catheter-based interventions. Multivariable analysis revealed that year (P < 0.0001), payer (P = 0.0002), CHD (P < 0.0001), valvular heart disease (P = 0.0004), cardiomyopathy (P = 0.0009), hospital region (P < 0.0001), length of stay (P < 0.0001), and complications (P < 0.0001) predicted increased charges. The same factors also predicted increased costs. Charges and costs varied considerably by region, particularly for high-volume centers (P < 0.0001). CONCLUSIONS: Charges and costs for pediatric catheter ablation increased relative to other procedures and significantly outstripped inflation. Further study of complications, length of stay, and regional differences may help control rising costs while maintaining quality of care.

Pediatric ICD utilization in the United States from 1997 to 2006.

BACKGROUND: Indications for implantable cardioverter-defibrillator (ICD) implantation in children have expanded, yet pediatric population-based data on ICD implantation are lacking. OBJECTIVE: We characterized trends in pediatric ICD use in the United States from 1997 to 2006. METHODS: We examined national hospital administrative data from the 1997, 2000, 2003, and 2006 Kids' Inpatient Database (KID) for new ICD implants in patients younger than 18 years of age and characterized patients, hospitals, and hospitalization-related outcomes. RESULTS: The number of pediatric ICD implants per year increased 3-fold (from 130 in 1997 to 396 in 2006, P = .003). Implants with a concomitant diagnosis of life-threatening arrhythmia decreased from 77% to 45% (P = .001). The average age decreased from 13.6 to 12.2 years (P = .01), and the percentage of patients younger than 5 years of age tended to increase (up to 10%, P = .09). In 2006, the number of implants per center ranged from 1 to 24 (median 3). Over time, the complication rate tended to decrease (from 16 to 10%, P = .07). Complication rate was not related to a diagnosis of congenital heart disease, age, or implant volume. CONCLUSION: ICD use increased dramatically in children from 1997 to 2006, although implantation declined in patients with a concomitant diagnosis of life-threatening arrhythmia (those likely to undergo implantation for secondary prevention). The complication rate tended to decrease overall. Each center implants relatively few ICDs per year, which may have implications for competency and training.

Clinical research careers: reports from a NHLBI pediatric heart network clinical research skills development conference.

BACKGROUND: Wyman W. Lai, MD, MPH, and Victoria L. Vetter, MD, MPH. The Pediatric Heart Network (PHN), funded under the U.S. National Institutes of Health-National Heart, Lung, and Blood Institute (NIH-NHLBI), includes two Clinical Research Skills Development (CRSD) Cores, which were awarded to The Children's Hospital of Philadelphia and to the Morgan Stanley Children's Hospital of New York-Presbyterian. To provide information on how to develop a clinical research career to a larger number of potential young investigators in pediatric cardiology, the directors of these two CRSD Cores jointly organized a one-day seminar for fellows and junior faculty from all of the PHN Core sites. The participants included faculty members from the PHN and the NHLBI. The day-long seminar was held on April 29, 2009, at the NHLBI site, immediately preceding the PHN Steering Committee meeting in Bethesda, MD. METHODS: the goals of the seminar were 1) to provide fellows and early investigators with basic skills in clinical research 2) to provide a forum for discussion of important research career choices 3) to introduce attendees to each other and to established clinical researchers in pediatric cardiology, and 4) to publish a commentary on the future of clinical research in pediatric cardiology. RESULTS: the following chapters are compilations of the talks given at the 2009 PHN Clinical Research Skills Development Seminar, published to share the information provided with a broader audience of those interested in learning how to develop a clinical research career in pediatric cardiology. The discussions of types of clinical research, research skills, career development strategies, funding, and career management are applicable to research careers in other areas of clinical medicine as well. CONCLUSIONS: the aim of this compilation is to stimulate those who might be interested in the research career options available to investigators.

Birth weight and prematurity in infants with single ventricle physiology: pediatric heart network infant single ventricle trial screened population.

OBJECTIVES: Although congenital heart disease is associated with low birth weight and prematurity, there is little information about these birth outcomes in infants with single ventricle physiology. We describe the birth outcomes (i.e., gestational age and birth weight) in neonates with single ventricle physiology screened for enrollment in the Pediatric Heart Network's Infant Single Ventricle Trial, compare these outcomes with US norms, and examine the association of birth outcomes with anatomic diagnosis and race. PATIENTS AND METHODS: All neonates with single ventricle physiology presenting to Infant Single Ventricle Trial centers were screened for enrollment. Demographic data and anatomic diagnoses were obtained from medical records. RESULTS: A total of 1245 neonates with single ventricle physiology were screened at 10 centers (63 to 266 per center). Diagnoses included hypoplastic left heart syndrome in 49%, unbalanced atrioventricular septal defect in 12%, and tricuspid atresia in 9%. Preterm birth occurred in 16% of neonates with single ventricle physiology vs. 12% in normal neonates (P < .001), low birth weight (<2.5 kg) in 18% vs. 8% in normals (P < .001), and small for gestational age (<10th percentile by definition) in 22% vs. 10% in normals (P < .001). A genetic syndrome was reported in 8%. The percentage of preterm birth, low birth weight, and small for gestational age was similar between screened neonates with and without hypoplastic left heart syndrome. CONCLUSIONS: In this large, contemporary cohort of neonates with single ventricle physiology, rates of preterm birth, low birth weight, and small for gestational age were higher than in the general population, but similar between screened neonates with and without hypoplastic left heart syndrome.

Accessibility of links between behaviors and outcomes: the case of physical activity.

Expectancy-value models of attitudes include belief statements about associated outcomes, with salient, accessible beliefs most important. This article reports two studies testing automatic accessibility of outcomes for physical activities using response latencies. A behavior presented by computer was followed 500 ms later by an outcome, such as more fit. Participants decided as quickly as possible whether the outcome was likely or unlikely (720 trials). We predicted shorter response latencies for accessible beliefs. Use of positive and negative outcomes (Study 1) produced a paradoxical slowing of response indicating deliberative processing. With only positive poles (Study 2), faster responses occurred for a priori links consistent with enhanced accessibility; some outcomes were accessible for some activities. Comparisons between explicitly reported beliefs and these implicit measures of accessibility revealed differences between the two measures. Discussion focuses on two possible routes to enhanced accessibility of attitudes, namely an explicit, cognitive process and an implicit, experiential process.

Fluorescence absorbance inner-filter decomposition: the role of emission shape on estimates of free Ca(2+) using Rhod-2.

A method for decomposing complex emission spectra by correcting for known inner-filter effects is described. This approach builds on previous work using a linear combination of model emission spectra and combines the known absorption characteristics of the system to fit the composite emission spectrum. Rhod-2, which has a small Stokes shift and significant self-absorption, was used as the model system. By adding the absorption characteristics of Rhod-2 to the model, the degree of fit was significantly improved, thus minimizing residuals, and accurately predicted the spectral shape changes with increasing concentration, [Rhod-2]. More complex studies were conducted with Rhod-2 in isolated cardiac mitochondria with multiple emission and absorption elements. By including known absorbances to the spectral decomposition, the overall precision increased almost four fold. Moreover, this approach eliminated the significant [Rhod-2] dependence on the apparent K(50) and therefore improved the accuracy of free [Ca(2+)] calculations. These data demonstrate that secondary inner-filter correction can significantly improve spectral decomposition of complex emission spectra, which are used in a variety of biological applications.

Endocardial versus epicardial electrical synchrony during LV free-wall pacing.

Cardiac resynchronization therapy has been most typically achieved by biventricular stimulation. However, left ventricular (LV) free-wall pacing appears equally effective in acute and chronic clinical studies. Recent data suggest electrical synchrony measured epicardially is not required to yield effective mechanical synchronization, whereas endocardial mapping data suggest synchrony (fusion with intrinsic conduction) is important. To better understand this disparity, we simultaneously mapped both endocardial and epicardial electrical activation during LV free-wall pacing at varying atrioventricular delays (AV delay 0-150 ms) in six normal dogs with the use of a 64-electrode LV endocardial basket and a 128-electrode epicardial sock. The transition from dyssynchronous LV-paced activation to synchronous RA-paced activation was studied by constructing activation time maps for both endo- and epicardial surfaces as a function of increasing AV delay. The AV delay at the transition from dyssynchronous to synchronous activation was defined as the transition delay (AVt). AVt was variable among experiments, in the range of 44-93 ms on the epicardium and 47-105 ms on the endocardium. Differences in endo- and epicardial AVt were smaller (-17 to +12 ms) and not significant on average (-5.0 +/- 5.2 ms). In no instance was the transition to synchrony complete on one surface without substantial concurrent transition on the other surface. We conclude that both epicardial and endocardial synchrony due to fusion of native with ventricular stimulation occur nearly concurrently. Assessment of electrical epicardial delay, as often used clinically during cardiac resynchronization therapy lead placement, should provide adequate assessment of stimulation delay for inner wall layers as well.

Metabolic network control of oxidative phosphorylation: multiple roles of inorganic phosphate.

Phosphate (Pi) is a putative cytosolic signaling molecule in the regulation of oxidative phosphorylation. Here, by using a multiparameter monitoring system, we show that Pi controls oxidative phosphorylation in a balanced fashion, modulating both the generation of useful potential energy and the formation of ATP by F1F0-ATPase in heart and skeletal muscle mitochondria. In these studies the effect of Pi was determined on the mitochondria [NADH], NADH generating capacity, matrix pH, membrane potential, oxygen consumption, and cytochrome reduction level. Pi enhanced NADH generation and was obligatory for electron flow under uncoupled conditions. Pi oxidized cytochrome b (cyto-b) and reduced cytochrome c (cyto-c), potentially improving the coupling between the NADH free energy and the proton motive force. The apparent limitation in reducing equivalent flow between cyto-b and cyto-c in the absence of Pi was confirmed in the intact heart by using optical spectroscopic techniques under conditions with low cytosolic [Pi]. These results demonstrate that Pi signaling results in the balanced modulation of oxidative phosphorylation, by influencing both deltaGH+ generation and ATP production, which may contribute to the energy metabolism homeostasis observed in intact systems.

Novel technique for cardiac electromechanical mapping with magnetic resonance imaging tagging and an epicardial electrode sock.

Near-simultaneous measurements of electrical and mechanical activation over the entire ventricular surface are now possible using magnetic resonance imaging tagging and a multielectrode epicardial sock. This new electromechanical mapping technique is demonstrated in the ventricularly paced canine heart. A 128-electrode epicardial sock and pacing electrodes were placed on the hearts of four anesthetized dogs. In the magnetic resonance scanner, tagged cine images (8-15 ms/frame) and sock electrode recordings (1000 Hz) were acquired under right-ventricular pacing and temporally referenced to the pacing stimulus. Electrical recordings were obtained during intermittent breaks in image acquisition, so that both data sets represented the same physiologic state. Since the electrodes were not visible in the images, electrode recordings and cine images were spatially registered with Gd-DTPA markers attached to the sock. Circumferential strain was calculated at locations corresponding to electrodes. For each electrode location, electrical and mechanical activation times were calculated and relationships between the two activation patterns were demonstrated. This method holds promise for improving understanding of the relationships between the patterns of electrical activation and contraction in the heart.