Personalized ECG monitoring and adaptive machine learning.

This non-technical review introduces key concepts in personalized ECG monitoring (pECG), which aims to optimize the detection of clinical events and their warning signs as well as the selection of alarm thresholds. We review several pECG methods, including anomaly detection and adaptive machine learning (ML), in which learning is performed sequentially as new data are collected. We describe a distributed-network multiscale pECG system to show how the computational load and time associated with adaptive ML could be optimized. In this architecture, the limited analysis of ECG waveforms is performed locally (e.g., on a smart phone) to determine a small number of clinically important ECG elements, and an adaptive ML engine is located on a remote server (Internet cloud) to determine an individual's "fingerprint" basis patterns and to detect anomalies in those patterns.

Modulation of the cardiac sodium channel NaV1.5 peak and late currents by NAD+ precursors.

RATIONALE: The cardiac sodium channel NaV1.5, encoded by SCN5A, produces the rapidly inactivating depolarizing current INa that is responsible for the initiation and propagation of the cardiac action potential. Acquired and inherited dysfunction of NaV1.5 results in either decreased peak INa or increased residual late INa (INa,L), leading to tachy/bradyarrhythmias and sudden cardiac death. Previous studies have shown that increased cellular NAD+ and NAD+/NADH ratio increase INa through suppression of mitochondrial reactive oxygen species and PKC-mediated NaV1.5 phosphorylation. In addition, NAD+-dependent deacetylation of NaV1.5 at K1479 by Sirtuin 1 increases NaV1.5 membrane trafficking and INa. The role of NAD+ precursors in modulating INa remains unknown. OBJECTIVE: To determine whether and by which mechanisms the NAD+ precursors nicotinamide riboside (NR) and nicotinamide (NAM) affect peak INa and INa,Lin vitro and cardiac electrophysiology in vivo. METHODS AND RESULTS: The effects of NAD+ precursors on the NAD+ metabolome and electrophysiology were studied using HEK293 cells expressing wild-type and mutant NaV1.5, rat neonatal cardiomyocytes (RNCMs), and mice. NR increased INa in HEK293 cells expressing NaV1.5 (500 µM: 51 ± 18%, p = .02, 5 mM: 59 ± 22%, p = .03) and RNCMs (500 µM: 60 ± 26%, p = .02, 5 mM: 74 ± 39%, p = .03) while reducing INa,L at the higher concentration (RNCMs, 5 mM: -45 ± 11%, p = .04). NR (5 mM) decreased NaV1.5 K1479 acetylation but increased INa in HEK293 cells expressing a mutant form of NaV1.5 with disruption of the acetylation site (NaV1.5-K1479A). Disruption of the PKC phosphorylation site abolished the effect of NR on INa. Furthermore, NAM (5 mM) had no effect on INa in RNCMs or in HEK293 cells expressing wild-type NaV1.5, but increased INa in HEK293 cells expressing NaV1.5-K1479A. Dietary supplementation with NR for 10-12 weeks decreased QTc in C57BL/6 J mice (0.35% NR: -4.9 ± 2.0%, p = .14; 1.0% NR: -9.5 ± 2.8%, p = .01). CONCLUSIONS: NAD+ precursors differentially regulate NaV1.5 via multiple mechanisms. NR increases INa, decreases INa,L, and warrants further investigation as a potential therapy for arrhythmic disorders caused by NaV1.5 deficiency and/or dysfunction.

Phenotypic Refinement of Heart Failure in a National Biobank Facilitates Genetic Discovery.

BACKGROUND: Heart failure (HF) is a morbid and heritable disorder for which the biological mechanisms are incompletely understood. We therefore examined genetic associations with HF in a large national biobank, and assessed whether refined phenotypic classification would facilitate genetic discovery. METHODS: We defined all-cause HF among 488 010 participants from the UK Biobank and performed a genome-wide association analysis. We refined the HF phenotype by classifying individuals with left ventricular dysfunction and without coronary artery disease as having nonischemic cardiomyopathy (NICM), and repeated a genetic association analysis. We then pursued replication of lead HF and NICM variants in independent cohorts, and performed adjusted association analyses to assess whether identified genetic associations were mediated through clinical HF risk factors. In addition, we tested rare, loss-of-function mutations in 24 known dilated cardiomyopathy genes for association with HF and NICM. Finally, we examined associations between lead variants and left ventricular structure and function among individuals without HF using cardiac magnetic resonance imaging (n=4158) and echocardiographic data (n=30 201). RESULTS: We identified 7382 participants with all-cause HF in the UK Biobank. Genome-wide association analysis of all-cause HF identified several suggestive loci (P<1×10-6), the majority linked to upstream HF risk factors, ie, coronary artery disease (CDKN2B-AS1 and MAP3K7CL) and atrial fibrillation (PITX2). Refining the HF phenotype yielded a subset of 2038 NICM cases. In contrast to all-cause HF, genetic analysis of NICM revealed suggestive loci that have been implicated in dilated cardiomyopathy (BAG3, CLCNKA-ZBTB17). Dilated cardiomyopathy signals arising from our NICM analysis replicated in independent cohorts, persisted after HF risk factor adjustment, and were associated with indices of left ventricular dysfunction in individuals without clinical HF. In addition, analyses of loss-of-function variants implicated BAG3 as a disease susceptibility gene for NICM (loss-of-function variant carrier frequency=0.01%; odds ratio,12.03; P=3.62×10-5). CONCLUSIONS: We found several distinct genetic mechanisms of all-cause HF in a national biobank that reflect well-known HF risk factors. Phenotypic refinement to a NICM subtype appeared to facilitate the discovery of genetic signals that act independently of clinical HF risk factors and that are associated with subclinical left ventricular dysfunction.

High-energy external defibrillation and transcutaneous pacing during MRI: feasibility and safety.

BACKGROUND: Rapid application of external defibrillation, a crucial first-line therapy for ventricular fibrillation and cardiac arrest, is currently unavailable in the setting of magnetic resonance imaging (MRI), raising concerns about patient safety during MRI tests and MRI-guided procedures, particularly in patients with cardiovascular diseases. The objective of this study was to examine the feasibility and safety of defibrillation/pacing for the entire range of clinically useful shock energies inside the MRI bore and during scans, using defibrillation/pacing outside the magnet as a control. METHODS: Experiments were conducted using a commercial defibrillator (LIFEPAK 20, Physio-Control, Redmond, Washington, USA) with a custom high-voltage, twisted-pair cable with two mounted resonant floating radiofrequency traps to reduce emission from the defibrillator and the MRI scanner. A total of 18 high-energy (200-360 J) defibrillation experiments were conducted in six swine on a 1.5 T MRI scanner outside the magnet bore, inside the bore, and during scanning, using adult and pediatric defibrillation pads. Defibrillation was followed by cardiac pacing (with capture) in a subset of two animals. Monitored signals included: high-fidelity temperature (0.01 °C, 10 samples/sec) under the pads and 12-lead electrocardiogram (ECG) using an MRI-compatible ECG system. RESULTS: Defibrillation/pacing was successful in all experiments. Temperature was higher during defibrillation inside the bore and during scanning compared with outside the bore, but the differences were small (ΔT: 0.5 and 0.7 °C, p = 0.01 and 0.04, respectively). During scans, temperature after defibrillation tended to be higher for pediatric vs. adult pads (p = 0.08). MR-image quality (signal-to-noise ratio) decreased by ~ 10% when the defibrillator was turned on. CONCLUSIONS: Our study demonstrates the feasibility and safety of in-bore defibrillation for the full range of defibrillation energies used in clinical practice, as well as of transcutaneous cardiac pacing inside the MRI bore. Methods for Improving MR-image quality in the presence of a working defibrillator require further study.

Emerging potential benefits of modulating NAD+ metabolism in cardiovascular disease.

Nicotinamide adenine dinucleotide (NAD+) and related metabolites are central mediators of fuel oxidation and bioenergetics within cardiomyocytes. Additionally, NAD+ is required for the activity of multifunctional enzymes, including sirtuins and poly(ADP-ribose) polymerases that regulate posttranslational modifications, DNA damage responses, and Ca2+ signaling. Recent research has indicated that NAD+ participates in a multitude of processes dysregulated in cardiovascular diseases. Therefore, supplementation of NAD+ precursors, including nicotinamide riboside that boosts or repletes the NAD+ metabolome, may be cardioprotective. This review examines the molecular physiology and preclinical data with respect to NAD+ precursors in heart failure-related cardiac remodeling, ischemic-reperfusion injury, and arrhythmias. In addition, alternative NAD+-boosting strategies and potential systemic effects of NAD+ supplementation with implications on cardiovascular health and disease are surveyed.

IDENTIFYING NEW SUDDEN DEATH GENES.

Inherited conditions that lead to cardiac arrhythmias and sudden cardiac death remain an important cause of morbidity and mortality. Identifying the genes responsible for these rare conditions can provide insights into the more common and heritable forms of sudden cardiac death seen in patients with structural heart disease. We and others have used candidate gene approaches and positional cloning in large families to show that mutations in ion channels and ion channel related proteins cause familial arrhythmia syndromes including long QT and Brugada syndromes. The genes responsible for many familial arrhythmia syndromes and the vast majority of the predisposition to common arrhythmias remain unknown. Using whole exome sequencing in families with Brugada syndrome and idiopathic ventricular fibrillation, we now seek to identify mutations in genes previously not thought to play a significant role in the heart.

Serum amine-based metabolites and their association with outcomes in primary prevention implantable cardioverter-defibrillator patients.

AIMS: Heart failure patients are at increased risk of ventricular arrhythmias and all-cause mortality. However, existing clinical and serum markers only modestly predict these adverse events. We sought to use metabolic profiling to identify novel biomarkers in two independent prospective cohorts of patients with implantable cardioverter-defibrillators (ICDs) for primary prevention of sudden cardiac death (SCD). METHODS AND RESULTS: Baseline serum was quantitatively profiled for 42 known biologically relevant amine-based metabolites among 402 patients from the Prospective Observational Study of Implantable Cardioverter-Defibrillators (PROSE-ICD) Study (derivation group) and 240 patients from the Genetic Risk Assessment of Defibrillator Events (GRADE) Study (validation group) for ventricular arrhythmia-induced ICD shocks and all-cause mortality. Three amines, N-methyl-l-histidine, symmetric dimethylarginine (SDMA), and l-kynurenine, were derived and validated to be associated with all-cause mortality. The hazard ratios of mortality in PROSE-ICD and GRADE were 1.48 (95% confidence interval 1.14-1.92) and 1.67 (1.22-2.27) for N-methyl-l-histidine, 1.49 (1.17-1.91) and 1.77 (1.27-2.45) for SDMA, 1.31 (1.06-1.63) and 1.73 (1.32-2.27) for l-kynurenine, respectively. l-Histidine, SDMA, and l-kynurenine were associated with ventricular arrhythmia-induced ICD shocks in PROSE-ICD, but they did not reach statistical significance in the GRADE cohort. CONCLUSION: Utilizing metabolic profiling in two independent prospective cohorts of patients undergoing ICD implantation for primary prevention of SCD, we identified several novel amine markers that were associated with appropriate shock and mortality. These findings shed insight into the potential biologic pathways leading to adverse events in ICD patients. Further studies are needed to confirm the prognostic value of these findings.

Mechanism of automaticity in cardiomyocytes derived from human induced pluripotent stem cells.

BACKGROUND AND OBJECTIVES: The creation of cardiomyocytes derived from human induced pluripotent stem cells (hiPS-CMs) has spawned broad excitement borne out of the prospects to diagnose and treat cardiovascular diseases based on personalized medicine. A common feature of hiPS-CMs is their spontaneous contractions but the mechanism(s) remain uncertain. METHODS: Intrinsic activity was investigated by the voltage-clamp technique, optical mapping of action potentials (APs) and intracellular Ca(2+) (Cai) transients (CaiT) at subcellular-resolution and pharmacological interventions. RESULTS: The frequency of spontaneous CaiT (sCaiT) in monolayers of hiPS-CMs was not altered by ivabradine, an inhibitor of the pacemaker current, If despite high levels of HCN transcripts (1-4). HiPS-CMs had negligible If and IK1 (inwardly-rectifying K(+)-current) and a minimum diastolic potential of -59.1±3.3mV (n=18). APs upstrokes were preceded by a depolarizing-foot coincident with a rise of Cai. Subcellular Cai wavelets varied in amplitude, propagated and died-off; larger Cai-waves triggered cellular sCaTs and APs. SCaiTs increased in frequency with [Ca(2+)]out (0.05-to-1.8mM), isoproterenol (1µM) or caffeine (100µM) (n≥5, p<0.05). HiPS-CMs became quiescent with ryanodine receptor stabilizers (K201=2µM); tetracaine; Na-Ca exchange (NCX) inhibition (SEA0400=2µM); higher [K(+)]out (5→8mM), and thiol-reducing agents but could still be electrically stimulated to elicit CaiTs. Cell-cell coupling of hiPS-CM in monolayers was evident from connexin-43 expression and CaiT propagation. SCaiTs from an ensemble of dispersed hiPS-CMs were out-of-phase but became synchronous through the outgrowth of inter-connecting microtubules. CONCLUSIONS: Automaticity in hiPS-CMs originates from a Ca(2+)-clock mechanism involving Ca(2+) cycling across the sarcoplasmic reticulum linked to NCX to trigger APs.

Genetics of sudden cardiac death.

Sudden cardiac death (SCD) is defined by the World Health Organization (WHO) as death within 1 h of symptom onset (witnessed) or within 24 h of being observed alive and symptom free (unwitnessed). It affects more than 3 million people annually worldwide and affects approximately 1/1000 people each year in the USA. Familial studies of syndromes with Mendelian inheritance, candidate genes analyses, and genome-wide association studies (GWAS) have helped our understanding of the genetics of SCD. We will review the genetics of arrhythmogenic hereditary syndromes with Mendelian inheritance from familial studies with structural heart disease (hypertrophic cardiomyopathy, dilated cardiomyopathy, and arrhythmogenic cardiomyopathy) as well as primary electrical causes (long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, and short QT syndrome). In addition, we will review the genetics of intermediate phenotypes for SCD such as coronary artery disease and electrocardiographic variables (QT interval, QRS duration, and RR interval). Finally, we will review rare and common variants that are associated with SCD in the general population and were identified from candidate gene analyses and GWAS. Our understanding of the genetics of SCD will improve by the use of next-generation sequencing/whole-exome sequencing as well as whole-genome sequencing which have the potential to discover unsuspected common and rare genetic variants that might be associated with SCD.

Nighttime instabilities of neurophysiological, cardiovascular, and respiratory activity: integrative modeling and preliminary results.

Unstable (cyclical alternating pattern, or CAP) sleep is associated with surges of sympathetic nervous system activity, increased blood pressure and vasoconstriction, heightened baroreflex sensitivity, and unstable heart rhythm and breathing. In susceptible persons, CAP sleep provokes clinically significant events, including hypertensive crises, sleep-disordered breathing, and cardiac arrhythmias. Here we explore the neurophysiology of CAP sleep and its impact on cardiovascular and respiratory functions. We show that: (i) an increase in neurophysiological recovery rate can explain the emergence of slow, self-sustained, hypersynchronized A1 CAP-sleep pattern and its transition to the faster A2-A3 CAP-sleep patterns; (ii) in a two-dimensional, continuous model of cardiac tissue with heterogeneous action potential duration (APD) distribution, heart rate accelerations during CAP sleep may encounter incompletely recovered electrical excitability in cell clusters with longer APD. If the interaction between short cycle length and incomplete, spatially heterogeneous repolarization persists over multiple cycles, irregularities and asymmetry of depolarization front may accumulate and ultimately lead to a conduction block, retrograde conduction, breakup of activation waves, reentrant activity, and arrhythmias; and (iii) these modeling results are consistent with the nighttime data obtained from patients with structural heart disease (N=13) that show clusters of atrial and ventricular premature beats occurring during the periods of unstable heart rhythm and respiration that accompany CAP sleep. In these patients, CAP sleep is also accompanied by delayed adaptation of QT intervals and T-wave alternans.

Upstream alternative polyadenylation in SCN5A produces a short transcript isoform encoding a mitochondria-localized NaV1.5 N-terminal fragment that influences cardiomyocyte respiration.

SCN5A encodes the cardiac voltage-gated Na+ channel, NaV1.5, that initiates action potentials. SCN5A gene variants cause arrhythmias and increased heart failure risk. Mechanisms controlling NaV1.5 expression and activity are not fully understood. We recently found a well-conserved alternative polyadenylation (APA) signal downstream of the first SCN5A coding exon. This yields a SCN5A-short transcript isoform expressed in several species (e.g. human, pig, and cat), though rodents lack this upstream APA. Reanalysis of transcriptome-wide cardiac APA-seq and mRNA-seq data shows reductions in both upstream APA usage and short/full-length SCN5A mRNA ratios in failing hearts. Knock-in of the human SCN5A APA sequence into mice is sufficient to enable expression of SCN5A -short transcript, while significantly decreasing expression of full-length SCN5A mRNA. Notably, SCN5A -short transcript encodes a novel protein (NaV1.5-NT), composed of an N-terminus identical to NaV1.5 and a unique C-terminus derived from intronic sequence. AAV9 constructs were able to achieve stable NaV1.5-NT expression in mouse hearts, and western blot of human heart tissues showed bands co-migrating with NaV1.5-NT transgene-derived bands. NaV1.5-NT is predicted to contain a mitochondrial targeting sequence and localizes to mitochondria in cultured cardiomyocytes and in mouse hearts. NaV1.5-NT expression in cardiomyocytes led to elevations in basal oxygen consumption rate, ATP production, and mitochondrial ROS, while depleting NADH supply. Native PAGE analyses of mitochondria lysates revealed that NaV1.5-NT expression resulted in increased levels of disassembled complex V subunits and accumulation of complex I-containing supercomplexes. Overall, we discovered that APA-mediated regulation of SCN5A produces a short transcript encoding NaV1.5-NT. Our data support that NaV1.5-NT plays a multifaceted role in influencing mitochondrial physiology: 1) by increasing basal respiration likely through promoting complex V conformations that enhance proton leak, and 2) by increasing overall respiratory efficiency and NADH consumption by enhancing formation and/or stability of complex I-containing respiratory supercomplexes, though the specific molecular mechanisms underlying each of these remain unresolved.

Identification of In Vivo Internalizing Cardiac-Specific RNA Aptamers.

Background: The pursuit of selective therapeutic delivery to target tissue types represents a key goal in the treatment of a range of adverse health issues, including diseases afflicting the heart. The development of new cardiac-specific ligands is a crucial step towards effectively targeting therapeutics to the heart. Methods: Utilizing an ex vivo and in vivo SELEX approaches, we enriched a library of 2'-fluoro modified aptamers for ventricular cardiomyocyte specificity. Lead candidates were identified from this library, and their binding and internalization into cardiomyocytes was evaluated in both ex vivo and in vivo mouse studies. Results: The ex vivo and in vivo SELEX processes generated an aptamer library with significant cardiac specificity over non-cardiac tissues such as liver and skeletal muscle. Our lead candidate aptamer from this library, CA1, demonstrates selective in vivo targeting and delivery of a fluorophore cargo to ventricular cardiomyocytes within the murine heart, while minimizing off-target localization to non-cardiac tissues, including the liver. By employing a novel RNase-based assay to evaluate aptamer interactions with cardiomyocytes, we discovered that CA1 predominantly internalizes into ventricular cardiomyocytes; conversely, another candidate CA41 primarily binds to the cardiomyocyte cell surface. Conclusions: These findings suggest that CA1 and CA41 have the potential to be promising candidates for targeted drug delivery and imaging applications in cardiac diseases.