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The sinoatrial node (SAN) is the primary pacemaker of the human heart. It is a single, elongated, 3-dimensional (3D) intramural fibrotic structure located at the junction of the superior vena cava intercaval region bordering the crista terminalis (CT). SAN activation originates in the intranodal pacemakers and is conducted to the atria through 1 or more discrete sinoatrial conduction pathways. The complexity of the 3D SAN pacemaker structure and intramural conduction are underappreciated during clinical multielectrode mapping and ablation procedures of SAN and atrial arrhythmias. In fact, defining and targeting SAN is extremely challenging because, even during sinus rhythm, surface-only multielectrode mapping may not define the leading pacemaker sites in intramural SAN but instead misinterpret them as epicardial or endocardial exit sites through sinoatrial conduction pathways. These SAN exit sites may be distributed up to 50 mm along the CT beyond the ∼20-mm-long anatomic SAN structure. Moreover, because SAN reentrant tachycardia beats may exit through the same sinoatrial conduction pathway as during sinus rhythm, many SAN arrhythmias are underdiagnosed. Misinterpretation of arrhythmia sources and/or mechanisms (eg, enhanced automaticity, intranodal vs CT reentry) limits diagnosis and success of catheter ablation treatments for poorly understood SAN arrhythmias. The aim of this review is to provide a state-of-the-art overview of the 3D structure and function of the human SAN complex, mechanisms of SAN arrhythmias and available approaches for electrophysiological mapping, 3D structural imaging, pharmacologic interventions, and ablation to improve diagnosis and mechanistic treatment of SAN and atrial arrhythmias.

Heart failure (HF) is frequently accompanied with the sinoatrial node (SAN) dysfunction, which causes tachy-brady arrhythmias and increased mortality. MicroRNA (miR) alterations are associated with HF progression. However, the transcriptome of HF human SAN, and its role in HF-associated remodeling of ion channels, transporters, and receptors responsible for SAN automaticity and conduction impairments is unknown. We conducted comprehensive high-throughput transcriptomic analysis of pure human SAN primary pacemaker tissue and neighboring right atrial tissue from human transplanted HF hearts (n = 10) and non-failing (nHF) donor hearts (n = 9), using next-generation sequencing. Overall, 47 miRs and 832 mRNAs related to multiple signaling pathways, including cardiac diseases, tachy-brady arrhythmias and fibrosis, were significantly altered in HF SAN. Of the altered miRs, 27 are predicted to regulate mRNAs of major ion channels and neurotransmitter receptors which are involved in SAN automaticity (e.g. HCN1, HCN4, SLC8A1) and intranodal conduction (e.g. SCN5A, SCN8A) or both (e.g. KCNJ3, KCNJ5). Luciferase reporter assays were used to validate interactions of miRs with predicted mRNA targets. In conclusion, our study provides a profile of altered miRs in HF human SAN, and a novel transcriptome blueprint to identify molecular targets for SAN dysfunction and arrhythmia treatments in HF.

Atrial fibrillation (AF) occurrence and maintenance is associated with progressive remodeling of electrophysiological (repolarization and conduction) and 3D structural (fibrosis, fiber orientations, and wall thickness) features of the human atria. Significant diversity in AF etiology leads to heterogeneous arrhythmogenic electrophysiological and structural substrates within the 3D structure of the human atria. Since current clinical methods have yet to fully resolve the patient-specific arrhythmogenic substrates, mechanism-based AF treatments remain underdeveloped. Here, we review current knowledge from in-vivo, ex-vivo, and in-vitro human heart studies, and discuss how these studies may provide new insights on the synergy of atrial electrophysiological and 3D structural features in AF maintenance. In-vitro studies on surgically acquired human atrial samples provide a great opportunity to study a wide spectrum of AF pathology, including functional changes in single-cell action potentials, ion channels, and gene/protein expression. However, limited size of the samples prevents evaluation of heterogeneous AF substrates and reentrant mechanisms. In contrast, coronary-perfused ex-vivo human hearts can be studied with state-of-the-art functional and structural technologies, such as high-resolution near-infrared optical mapping and contrast-enhanced MRI. These imaging modalities can resolve atrial arrhythmogenic substrates and their role in reentrant mechanisms maintaining AF and validate clinical approaches. Nonetheless, longitudinal studies are not feasible in explanted human hearts. As no approach is perfect, we suggest that combining the strengths of direct human atrial studies with the high fidelity approaches available in the laboratory and in realistic patient-specific computer models would elucidate deeper knowledge of AF mechanisms. We propose that a comprehensive translational pipeline from ex-vivo human heart studies to longitudinal clinically relevant AF animal studies and finally to clinical trials is necessary to identify patient-specific arrhythmogenic substrates and develop novel AF treatments.

Introduction: Persistent atrial fibrillation (perAF) may be maintained by localized extra-pulmonary vein (PV) drivers. Most clinical studies have focused on left atrial (LA) sources ablation, but right atrial (RA) arrhythmogenic substrate, due to certain comorbidities, may also maintain perAF. Hypothesis: Patient (Pt)-specific characteristics and comorbidities may predict occurrence of RA AF drivers. Methods: Consecutive Pts (n=105) underwent driver ablation in addition to PV isolation for perAF after panoramic multi-electrode mapping (MEM) at a single medical center. Logistic regression (LR) models were used to characterize the relationship between Pt characteristics and presence of AF drivers in each atrium. LR was used to predict 12-month AF-free survival. Results: Pts were 63.1±9.9 years old, 24.8% female, 21.9% in HF, 76.2% had HTN, 24.8% had DM, 22.9% had CAD and 59.0% had OSA. MEM identified a mean of 1.6±1.2 LA drivers and 1.4±1.2 RA drivers per Pt. Forty two Pts had drivers in both LA and RA, 19 in RA only, and 26 in LA only. Older age was a predictor of RA drivers presence (p=0.06), factors that predicted presence of LA drivers were female gender (p=0.19), HTN (p=0.03), and comorbidity score (p = 0.07). CHADS 2 -VASc score and LVEF predicted both LA and RA drivers. Following ablation, 75% of the Pts sustained sinus rhythm after 6 months, 70% after 12 months and 72% after 18 months. AF recurrence within 12 months of ablation was associated with DM (p=0.03), BMI (p=0.03) and RV systolic pressure (p=0.07). Presence of RA drivers was not associated with the 12 and 18-months freedom from AF. Conclusions: In this cohort of perAF patients, older age and higher CHADS 2 -VASc score predicted RA driver occurrence. These preliminary findings suggest that patient-specific clinical characteristics may uniquely impact LA vs RA myocardium by promoting development of chamber-specific arrhythmogenic structural and functional substrates of AF drivers.

The spontaneous activity of the sinoatrial node initiates the heartbeat. Sino-atrial node dysfunction (SND) and sick sinoatrial (sick sinus) syndrome are caused by the heart's inability to generate a normal sinoatrial node action potential. In clinical practice, SND is generally considered an age-related pathology, secondary to degenerative fibrosis of the heart pacemaker tissue. However, other forms of SND exist, including idiopathic primary SND, which is genetic, and forms that are secondary to cardiovascular or systemic disease. The incidence of SND in the general population is expected to increase over the next half century, boosting the need to implant electronic pacemakers. During the last two decades, our knowledge of sino-atrial node physiology and of the pathophysiological mechanisms underlying SND has advanced considerably. This review summarizes the current knowledge about SND mechanisms and discusses the possibility of introducing new pharmacologic therapies for treating SND.

Background: Multielectrode mapping (MEM) of extra-pulmonary vein (PV) targeted ablation of persistent atrial fibrillation (PsAF) drivers suffers from false positives. Early studies suggest that true AF drivers are anchored to arrhythmogenic fibrosis, which can be visualized with late gadolinium enhanced (LGE) cardiac magnetic resonance (CMR) to distinguish them from false positive drivers on MEM. Hypothesis: Driver regions integral to AF correlate with high atrial fibrosis; therefore, ablation of MEM-defined drivers within fibrotic regions may lead to better outcomes than MEM-defined drivers outside fibrosis. Methods: Pre-ablation, 10 PsAF patients (Pts) (70% male; 65±11 y/o) underwent LGE-CMR at 3T (0.625x0.625x1.25mm 3 , 0.2mmol/kg gadolinium). During ablation, MEM (64-electrode basket catheter) was used to identify Pt-specific extra-PV drivers. Retrospectively, both left (LA) and right atria (RA) were analyzed with atria-specific fibrosis masks (voxels exceeding an intensity of 3-3.5 standard deviations above the mean intensity of nonfibrotic atrial wall). Ablated drivers were classified as fibrotic driver (dense or patchy) or nonfibrotic driver by LGE-CMR and MEM correlation. Results: 30 drivers were ablated in 10 PsAF Pts (2±1 LA drivers/Pt, 1±1 RA drivers/Pt) and were classified if anchored to dense (n=16, 45.4±31.7%, 2.6±1.8cm 2 ), patchy (n=11, 8.9±13.5%, 2.0±2.5cm 2 ), or no (n=3, 2.3±2.0%) fibrosis. At follow-up (13±7 mos), 7/10 Pts remained free from AF and atrial flutter, all of whom had at least one dense fibrosis driver ablated and all but 1 Pt had ablations limited to fibrotic drivers. 2/3 patients with failure at follow-up had a nonfibrotic driver ablated. Conclusion: Our results suggest that limiting ablation of AF drivers to those anchored to dense fibrotic substrate may improve long-term AF-free survival. Identification of Pt-specific fibrotic substrate by LGE-CMR may help specificity of MEM ablation targets for successful PsAF treatment.

Introduction: Dense fibrous connective tissue is inherently found in the human sinoatrial node (hSAN), which further increases in heart failure (HF) leading to sinoatrial node dysfunction (SND). While several factors contribute to cardiac fibrosis, it is unknown if long non-coding RNAs (lncRNA), a novel class of RNA known to affect cardiac fibrosis, are involved in increasing fibrotic content in hSAN in non-failing (nHF) vs HF hearts. Objective: To identify unique lncRNA profiles and pro-fibrotic lncRNAs in isolated SAN and right atrial (RA) fibroblasts (FBs), in HF vs nHF human hearts. Methods: FBs isolated from pure SAN and RA tissues, from HF (n=6; 35-68yo) and nHF (n=4; 26-64yo) cardioplegically arrested human hearts, were cultured with/without transforming growth factor β1 (TGFβ1; 5ng; 48hrs) to activate myofibroblast (myoFB) transition. Frozen FBs and myoFBs were subjected to high throughput Next Generation RNA Sequencing analyses of the whole transcriptome. Results: Averages of total counts across all samples revealed that majority of the genes detected were protein coding 14415(63%), 5876 (26%) lncRNA, 1254 (5%) miscellaneous RNA, 677(3%) miRNA and 577 (3%) other types of RNA (Figure A). Preliminary analyses show that coding mRNA and non-coding lncRNA are differentially expressed in nHF hSAN and RA fibroblasts and TGFβ1 treated myoFBs. Furthermore, these expression patterns were also different in FBs and myoFBs isolated from failing hearts. Conclusions: Our findings show for the first time that lncRNA expression in cultured hSAN Fbs and myoFBs are unique and differentially altered in HF. Ongoing analyses of sequenced transcriptome will identify FB and myoFB lncRNAs associated with intrinsic higher levels of hSAN fibrotic content as well as in HF. We will also determine if they can modify pro-fibrotic activity in HF SAN FBs and myoFBs, which may be beneficial to develop novel molecular approaches to decrease HF-associated SAN fibrosis and associated SND.

Introduction: Atrial fibrillation (AF) is not effectively treated in clinics due to a lack of adequate tools for identifying AF drivers as targets in ablation treatment. Recent studies revealed that 30-50% of patients with persistent AF have drivers not only in the left atrium (LA) but also in the right atrium (RA). However, the precise substrate underlying RA AF drivers remains elusive. Hypothesis: High-resolution analysis of human atrial wall 3D structure can predict the occurrence of right atrial drivers. Methods: Coronary perfused explanted human atria (N=10, 46.2±14.7 y.o.;40% female) were optically mapped with high-resolution CMOS cameras. Sustained AF was induced by fast pacing and perfusion of adenosine and/or isoproterenol. Hearts were imaged at ~170μm 3 resolution by 9.4T gadolinium-enhanced MRI (Fig. A). A coupled PDE approach was utilized to estimate the atrial wall thickness (Fig. B). Fibrosis was identified above a signal intensity threshold validated by histology; and fibrosis density was constructed for each voxel by determining the percent of neighboring fibrotic voxels within a radius of 5 voxels. Results: Optical mapping identified 9 RA drivers in 6 hearts (H1-6) and 7 LA drivers in 5 hearts (H5, 6 and 8-10, Fig. C). All 9 RA AF drivers were anchored in myobundle structure in close proximity to the crista terminalis (CT). Wall thickness distribution in RA excluding the CT was similar to that in the LA (N=10, Fig. D). Structural analysis showed that RA with AF drivers (N=6) was thicker with greater wall variation and higher fibrosis density than RA without AF drivers (N=4, Fig. E). Furthermore, RA driver regions (N=9) were thicker and varied more than other RA regions (4.64±2.86 mm vs 4.56±2.57mm, p Conclusions: For the first time, the 3D structural analysis demonstrates that structural features including atrial wall thickness and its variation near the CT can predict RA driver occurrences in human hearts and be used to improve targeted ablation.

Due to complex 3D human atrial structure, atrial fibrillation (AF) mapping with multielectrode arrays (MEA) mostly represents surface activation. Therefore, MEA may not properly visualize patient specific AF mechanisms, which impairs ablation outcomes. Conversely, near-infrared optical mapping (NIOM) visualizes subsurface intramural activation and can efficiently reveal reentrant drivers responsible for AF maintenance. Delay between surface activation seen by MEA electrograms (EGMs) vs subsurface activation seen by NIOM optical action potentials (OAPs) occurs due to dyssynchrony between myocardium layers, especially during AF. Coronary perfused explanted human atria (n=7) were mapped with NIOM (0.3-0.9mm 2 resolution) and 64-electrode MEA (3mm 2 resolution). Unipolar EGMs were analyzed for the steepest negative deflection. The delay between [-dV/dtmax] of unipolar EGMs and corresponding optical action potentials (OAPs) was compared in 500ms and 300ms pacing and AF. Subsequent structural analysis was done by 9.4T MRI (154-180μm 3 resolution) with gadolinium enhancement and histology. Delay between EGM and OAP local activation times rate-dependently and heterogeneously increased from 6±3 ms and 10±4 ms during 500ms and 300ms pacing to 15±11 ms (with maximum delay 47±18 ms) during pacing induced AF (average cycle length 124±65ms). Large local OAP-EGM delay, seen during AF, correlates with higher fibrosis percentage and fiber twist (p

Background The sinus node (SN) is the primary pacemaker of the heart. SN myocytes possess distinctive action potential morphology with spontaneous diastolic depolarization because of a unique expression of ion channels and Ca 2+ ‐handling proteins. MicroRNAs (miRs) inhibit gene expression. The role of miRs in controlling the expression of genes responsible for human SN pacemaking and conduction has not been explored. The aim of this study was to determine miR expression profile of the human SN as compared with that of non‐pacemaker atrial muscle. Methods and Results SN and atrial muscle biopsies were obtained from donor or post‐mortem hearts (n=10), histology/immunolabeling were used to characterize the tissues, TaqMan Human MicroRNA Arrays were used to measure 754 miRs, Ingenuity Pathway Analysis was used to identify miRs controlling SN pacemaker gene expression. Eighteen miRs were significantly more and 48 significantly less abundant in the SN than atrial muscle. The most interesting miR was miR‐486‐3p predicted to inhibit expression of pacemaking channels: HCN1 (hyperpolarization‐activated cyclic nucleotide‐gated 1), HCN4, voltage‐gated calcium channel (Ca v )1.3, and Ca v 3.1. A luciferase reporter gene assay confirmed that miR‐486‐3p can control HCN4 expression via its 3′ untranslated region. In ex vivo SN preparations, transfection with miR‐486‐3p reduced the beating rate by ≈35±5% ( P P Conclusions The human SN possesses a unique pattern of expression of miRs predicted to target functionally important genes. miR‐486‐3p has an important role in SN pacemaker activity by targeting HCN4, making it a potential target for therapeutic treatment of SN disease such as sinus tachycardia.

Background Atrial fibrillation (AF) driver mechanisms are obscured to clinical multielectrode mapping approaches that provide partial, surface‐only visualization of unstable 3‐dimensional atrial conduction. We hypothesized that transient modulation of refractoriness by pharmacologic challenge during multielectrode mapping improves visualization of hidden paths of reentrant AF drivers for targeted ablation. Methods and Results Pharmacologic challenge with adenosine was tested in ex vivo human hearts with a history of AF and cardiac diseases by multielectrode and high‐resolution subsurface near‐infrared optical mapping, integrated with 3‐dimensional structural imaging and heart‐specific computational simulations. Adenosine challenge was also studied on acutely terminated AF drivers in 10 patients with persistent AF. Ex vivo, adenosine stabilized reentrant driver paths within arrhythmogenic fibrotic hubs and improved visualization of reentrant paths, previously seen as focal or unstable breakthrough activation pattern, for targeted AF ablation. Computational simulations suggested that shortening of atrial refractoriness by adenosine may (1) improve driver stability by annihilating spatially unstable functional blocks and tightening reentrant circuits around fibrotic substrates, thus unmasking the common reentrant path; and (2) destabilize already stable reentrant drivers along fibrotic substrates by accelerating competing fibrillatory wavelets or secondary drivers. In patients with persistent AF, adenosine challenge unmasked hidden common reentry paths (9/15 AF drivers, 41±26% to 68±25% visualization), but worsened visualization of previously visible reentry paths (6/15, 74±14% to 34±12%). AF driver ablation led to acute termination of AF. Conclusions Our ex vivo to in vivo human translational study suggests that transiently altering atrial refractoriness can stabilize reentrant paths and unmask arrhythmogenic hubs to guide targeted AF driver ablation treatment.

Background:Atrial fibrillation (AF) can be maintained by localized intramural reentrant drivers. However, AF driver detection by clinical surface-only multielectrode mapping (MEM) has relied on subjective interpretation of activation maps. We hypothesized that application of machine learning to electrogram frequency spectra may accurately automate driver detection by MEM and add some objectivity to the interpretation of MEM findings.Methods:Temporally and spatially stable single AF drivers were mapped simultaneously in explanted human atria (n=11) by subsurface near-infrared optical mapping (NIOM; 0.3 mm2resolution) and 64-electrode MEM (higher density or lower density with 3 and 9 mm2resolution, respectively). Unipolar MEM and NIOM recordings were processed by Fourier transform analysis into 28 407 total Fourier spectra. Thirty-five features for machine learning were extracted from each Fourier spectrum.Results:Targeted driver ablation and NIOM activation maps efficiently defined the center and periphery of AF driver preferential tracks and provided validated annotations for driver versus nondriver electrodes in MEM arrays. Compared with analysis of single electrogram frequency features, averaging the features from each of the 8 neighboring electrodes, significantly improved classification of AF driver electrograms. The classification metrics increased when less strict annotation, including driver periphery electrodes, were added to driver center annotation. Notably, f1-score for the binary classification of higher-density catheter data set was significantly higher than that of lower-density catheter (0.81±0.02 versus 0.66±0.04,PConclusions:The machine learning model pretrained on Fourier spectrum features allows efficient classification of electrograms recordings as AF driver or nondriver compared with the NIOM gold-standard. Future application of NIOM-validated machine learning approach may improve the accuracy of AF driver detection for targeted ablation treatment in patients.

Bradyarrhythmias are an important cause of mortality in heart failure and previous studies indicate a mechanistic role for electrical remodelling of the key pacemaking ion channel HCN4 in this process. Here we show that, in a mouse model of heart failure in which there is sinus bradycardia, there is upregulation of a microRNA (miR-370-3p), downregulation of the pacemaker ion channel, HCN4, and downregulation of the corresponding ionic current, If, in the sinus node. In vitro, exogenous miR-370-3p inhibits HCN4 mRNA and causes downregulation of HCN4 protein, downregulation of If, and bradycardia in the isolated sinus node. In vivo, intraperitoneal injection of an antimiR to miR-370-3p into heart failure mice silences miR-370-3p and restores HCN4 mRNA and protein and If in the sinus node and blunts the sinus bradycardia. In addition, it partially restores ventricular function and reduces mortality. This represents a novel approach to heart failure treatment.

Delayed afterdepolarizations (DADs) and spontaneous depolarizations (SDs) are typically triggered by spontaneous diastolic Ca 2+ release from the sarcoplasmic reticulum (SR) which is caused by an elevated SR Ca 2+ -ATPase (SERCA) uptake and dysfunctional ryanodine receptors. However, recent studies on the T-box transcription factor gene (TBX5) demonstrated that abnormal depolarizations could occur despite a reduced SERCA uptake. Similar findings have also been reported in experimental or clinical studies of diabetes and heart failure. To investigate the sensitivity of SERCA in the genesis of DADs/SDs as well as its dependence on other Ca 2+ handling channels, we performed systematic analyses using the Maleckar et al . model. Results showed that the modulation of SERCA alone cannot trigger abnormal depolarizations, but can instead affect the interdependency of other Ca 2+ handling channels in triggering DADs/SDs. Furthermore, we discovered the existence of a threshold value for the intracellular concentration of Ca 2+ ([Ca 2+ ] i ) for abnormal depolarizations, which is modulated by the maximum SERCA uptake and the concentration of Ca 2+ in the uptake and release compartments in the SR ([Ca 2+ ] up and [Ca 2+ ] rel ). For the first time, our modelling study reconciles different mechanisms of abnormal depolarizations in the setting of ‘lone’ AF, reduced TBX5, diabetes and heart failure, and may lead to more targeted treatment for these patients. This article is part of the theme issue ‘Uncertainty quantification in cardiac and cardiovascular modelling and simulation’.

Mechanisms for human sinoatrial node (SAN) dysfunction are poorly understood and whether human SAN excitability requires voltage-gated sodium channels (Nav) remains controversial. Here, we report that neuronal (n)Nav blockade and selective nNav1.6 blockade during high-resolution optical mapping in explanted human hearts depress intranodal SAN conduction, which worsens during autonomic stimulation and overdrive suppression to conduction failure. Partial cardiac (c)Nav blockade further impairs automaticity and intranodal conduction, leading to beat-to-beat variability and reentry. Multiple nNav transcripts are higher in SAN vs atria; heterogeneous alterations of several isoforms, specifically nNav1.6, are associated with heart failure and chronic alcohol consumption. In silico simulations of Nav distributions suggest that INa is essential for SAN conduction, especially in fibrotic failing hearts. Our results reveal that not only cNav but nNav are also integral for preventing disease-induced failure in human SAN intranodal conduction. Disease-impaired nNav may underlie patient-specific SAN dysfunctions and should be considered to treat arrhythmias., The role of of voltage-gated sodium channels (Nav) in pacemaking and conduction of the human sinoatrial node is unclear. Here, the authors investigate existence and function of neuronal and cardiac Nav in human sinoatrial nodes, and demonstrate their alterations in explanted human diseased hearts.

Segmentation of the 3D human atria from late gadolinium-enhanced (LGE)-MRIs is crucial for understanding and analyzing the underlying atrial structures that sustain atrial fibrillation (AF), the most common cardiac arrhythmia. However, due to the lack of a large labeled dataset, current automated methods have only been developed for left atrium (LA) segmentation. Since AF is sustained across both the LA and right atrium (RA), an automatic bi-atria segmentation method is of high interest. We have therefore created a 3D LGE-MRI database from AF patients with both LA and RA labels to train a double, sequentially used convolutional neural network (CNN) for automatic LA and RA epicardium and endocardium segmentation. To mitigate issues regarding the severe class imbalance and the complex geometry of the atria, the first CNN accurately detects the region of interest (ROI) containing the atria and the second CNN performs targeted regional segmentation of the ROI. The CNN comprises of a U-Net backbone enhanced with residual blocks, pre-activation normalization, and a Dice loss to improve accuracy and convergence. The receptive field of the CNN was increased by using 5 × 5 kernels to capture large variations in the atrial geometry. Our algorithm segments and reconstructs the LA and RA within 2 s, achieving a Dice accuracy of 94% and a surface-to-surface distance error of approximately 1 pixel. To our knowledge, the proposed approach is the first of its kind, and is currently the most robust automatic bi-atria segmentation method, creating a solid benchmark for future studies.

Cross-bridge attachment allows force generation to occur, and rate of tension redevelopment (ktr) is a commonly used index of cross-bridge cycling rate. Tension overshoots have been observed briefly after a slack-restretch ktr maneuver in various species of animal models and humans. In this study, we set out to determine the properties of these overshoots and their possible underlying mechanism. Utilizing human cardiac trabeculae, we have found that tension overshoots are temperature-dependent and that they do not occur at resting states. In addition, we have found that myosin cross-bridge cycle is vital to these overshoots as inhibition of the cycle results in the blunting of the overshoots and the magnitude of the overshoots are dependent on the level of myofilament activation. Lastly, we show that the number of cross-bridges transiently increase during tension overshoots. These findings lead us to conclude that tension overshoots are likely due to a transient enhancement of the recruitment of myosin heads into the cross-bridge cycling, regulated by the myocardium, and with potential physiological significance in determining cardiac output. News and noteworthy We show that isolated human myocardium is capable of transiently increasing its maximal force generation capability by increasing cross-bridge recruitment following slack-restretch maneuver. This process can potentially have important implications and significance in cardiac contraction in vivo.

Background Pacemakers (PM) are used for managing sick sinus syndrome (SSS). This study evaluates predictors and trends of PM implantation for SSS. Methods Patients were identified from the National Inpatient Sample dataset (2003-13). Included patients were ≥18 years old, had a diagnosis of sinus node dysfunction and atrial arrhythmia (i.e., SSS). Patients who died, transferred out, who had prior device or had a defibrillator or resynchronization therapy device implanted were excluded. Included patients were then stratified by if a PM was implanted. Data regarding SSS, trends of PM utilization and multivariable models of factors associated with PM implantation are presented. Results 328,670 patients satisfied study criteria. This study compared patients who underwent (87.4%) PM implantation to those who did not undergo (12.6%) PM implantation. The annual trends for hospitalization with SSS and PM placement have been decreasing (p < 0.001). Variables associated with lower likelihood for PM implantation include young age, female sex, non-Caucasian race, chronic heart failure, Charlson Comorbidity Score ≥1, emergency room and weekend admission, hospital stay ≤3 days, and high cardiology inpatient volume. Greater likelihood for PM implantation was associated with hyperlipidemia, hypertension, and hospitals that were either private, large, Northeastern location, or with high cardiac procedural volume. Conclusions Analyzing 11-year data from a national inpatient database demonstrate a number of relevant variables that impact PM utilization that include not only clinical but also nonclinical variables such as socioeconomic status, gender, and hospital features. Racial and gender bias toward PM implantation are unchanged and persist through 2013. This article is protected by copyright. All rights reserved

Recent studies strongly suggest that the majority of atrial fibrillation (AF) patients with diagnosed or subclinical cardiac diseases have established or even pre-existing fibrotic structural remodeling, which may lead to conduction abnormalities and reentrant activity that sustain AF. As conventional treatments fail to treat AF in far too many cases, an urgent need exists to identify specific structural arrhythmogenic fibrosis patterns, which may maintain AF, in order to identify effective ablation targets for AF treatment. However, the existing challenge is to define what exact structural remodeling within the complex 3D human atrial wall is arrhythmogenic, as well as linking arrhythmogenic fibrosis to an underlying mechanism of AF maintenance in the clinical setting. This review is focused on the role of 3D fibrosis architecture in the mechanisms of AF maintenance revealed by submillimeter, high-resolution ex-vivo imaging modalities directly of human atria, as well as from in-silico 3D computational techniques that can be able to overcome in-vivo clinical limitations. The systematic integration of functional and structural imaging ex-vivo may inform the necessary integration of electrode and structural mapping in-vivo. A holistic view of AF driver mechanisms may begin to identify the defining characteristics or "fingerprints" of reentrant AF drivers, such as 3D fibrotic architecture, in order to design optimal patient-specific ablation strategies.

Although there have been great technological advances in the treatment of atrial fibrillation (AF), current therapies remain limited due to a narrow understanding of AF mechanisms in the human heart. This review will highlight our recent studies on explanted human hearts where we developed and employed a novel functional-structural mapping approach by integrating high-resolution simultaneous endo-epicardial and panoramic optical mapping with 3D gadolinium-enhanced MRI to define the spatiotemporal characteristics of AF drivers and their structural substrates. The results allow us to postulate that the primary mechanism of AF maintenance in human hearts is a limited number of localized intramural microanatomic reentrant AF drivers anchored to heart-specific 3D fibrotically insulated myobundle tracks, which may remain hidden to clinical single-surface electrode mapping. We suggest that ex vivo human heart studies, by using an integrated 3D functional and structural mapping approach, will help to reveal defining features of AF drivers as well as validate and improve clinical approaches to detect and target these AF drivers in patients with cardiac diseases.

Increased fibrosis is a characteristic remodeling response to biomechanical and neurohumoral stress and a determinant of cardiac mechanical and electrical dysfunction in disease. Stress-induced activation of cardiac fibroblasts (CFs) is a critical step in the fibrotic response, although the precise sequence of events underlying activation of these critical cells in vivo remain unclear. Here, we tested the hypothesis that a β(IV)-spectrin/STAT3 complex is essential for maintenance of a quiescent phenotype (basal nonactivated state) in CFs. We reported increased fibrosis, decreased cardiac function, and electrical impulse conduction defects in genetic and acquired mouse models of β(IV)-spectrin deficiency. Loss of β(IV)-spectrin function promoted STAT3 nuclear accumulation and transcriptional activity, and it altered gene expression and CF activation. Furthermore, we demonstrate that a quiescent phenotype may be restored in β(IV)-spectrin–deficient fibroblasts by expressing a β(IV)-spectrin fragment including the STAT3-binding domain or through pharmacological STAT3 inhibition. We found that in vivo STAT3 inhibition abrogates fibrosis and cardiac dysfunction in the setting of global β(IV)-spectrin deficiency. Finally, we demonstrate that fibroblast-specific deletion of β(IV)-spectrin is sufficient to induce fibrosis and decreased cardiac function. We propose that the β(IV)-spectrin/STAT3 complex is a determinant of fibroblast phenotype and fibrosis, with implications for remodeling response in cardiovascular disease (CVD).

AIMS: Bradycardia contributes to tachy-brady arrhythmias or sinus arrest during heart failure (HF). Sinoatrial node (SAN) adenosine A1 receptors (ADO A1Rs) are upregulated in HF, and adenosine is known to exert negative chronotropic effects on the SAN. Here, we investigated the role of A1R signaling at physiologically relevant ADO concentrations on HF SAN pacemaker cells. MAIN METHODS: Dogs with tachypacing-induced chronic HF and normal controls (CTL) were studied. SAN tissue was collected for A1R and GIRK mRNA quantification. SAN cells were isolated for perforated patch clamp recordings and firing rate (bpm), slope of slow diastolic depolarization (SDD), and maximum diastolic potential (MDP) were measured. Action potentials (APs) and currents were recorded before and after addition of 1 and 10 μM ADO. To assess contributions of A1R and G protein-coupled Inward Rectifier Potassium Current (GIRK) to ADO effects, APs were measured after the addition of DPCPX (selective A1R antagonist) or TPQ (selective GIRK blocker). KEY FINDINGS: A1R and GIRK mRNA expression were significantly increased in HF. In addition, ADO induced greater rate slowing and membrane hyperpolarization in HF vs CTL (p

Background Atrial fibrillation (AF) is the most common type of arrhythmia. Abnormal atrial myocyte Ca2+ handling promotes aberrant membrane excitability and remodeling that are important for atrial arrhythmogenesis. The sequence of molecular events leading to loss of normal atrial myocyte Ca2+ homeostasis is not established. Late Na+ current (INa,L) is increased in atrial myocytes from AF patients together with an increase in activity of Ca2+/calmodulin-dependent kinase II (CaMKII). Objective The purpose of this study was to determine whether CaMKII-dependent phosphorylation at Ser571 on NaV1.5 increases atrial INa,L, leading to aberrant atrial Ca2+ cycling, altered electrophysiology, and increased AF risk. Methods Atrial myocyte electrophysiology, Ca2+ handling, and arrhythmia susceptibility were studied in wild-type and Scn5a knock-in mice expressing phosphomimetic (S571E) or phosphoresistant (S571A) NaV1.5 at Ser571. Results Atrial myocytes from S571E but not S571A mice displayed an increase in INa,L and action potential duration, and with adrenergic stress have increased delayed afterdepolarizations. Frequency of Ca2+ sparks and waves was increased in S571E atrial myocytes compared to wild type. S571E mice showed an increase in atrial events induced by adrenergic stress and AF inducibility in vivo. Isolated S571E atria were more susceptible to spontaneous atrial events, which were abrogated by inhibiting sarcoplasmic reticulum Ca2+ release, CaMKII, or the Na+/Ca2+ exchanger. Expression of phospho-NaV1.5 at Ser571 and autophosphorylated CaMKII were increased in atrial samples from human AF patients. Conclusion This study identified CaMKII-dependent regulation of NaV1.5 as an important upstream event in Ca2+ handling defects and abnormal impulse generation in the setting of AF.

Atrial fibrillation (AF) is the most prevalent form of cardiac arrhythmia. Current treatments for AF remain suboptimal due to a lack of understanding of the underlying atrial structures that directly sustain AF. Existing approaches for analyzing atrial structures in 3-D, especially from late gadolinium-enhanced (LGE) magnetic resonance imaging, rely heavily on manual segmentation methods that are extremely labor-intensive and prone to errors. As a result, a robust and automated method for analyzing atrial structures in 3-D is of high interest. We have, therefore, developed AtriaNet, a 16-layer convolutional neural network (CNN), on 154 3-D LGE-MRIs with a spatial resolution of 0.625 mm $\times0.625$ mm $\times1.25$ mm from patients with AF, to automatically segment the left atrial (LA) epicardium and endocardium. AtriaNet consists of a multi-scaled, dual-pathway architecture that captures both the local atrial tissue geometry and the global positional information of LA using 13 successive convolutions and three further convolutions for merging. By utilizing computationally efficient batch prediction, AtriaNet was able to successfully process each 3-D LGE-MRI within 1 min. Furthermore, benchmarking experiments have shown that AtriaNet has outperformed the state-of-the-art CNNs, with a DICE score of 0.940 and 0.942 for the LA epicardium and endocardium, respectively, and an inter-patient variance of

The sinoatrial node (SAN) is the primary pacemaker in canine and human hearts. The SAN in both species has a unique three-dimensional heterogeneous structure characterized by small pacemaker myocytes enmeshed within fibrotic strands, which partially insulate the cells from aberrant atrial activation. The SAN pacemaker tissue expresses a unique signature of proteins and receptors that mediate SAN automaticity, ion channel currents, and cell-to-cell communication, which are predominantly similar in both species. Recent intramural optical mapping, integrated with structural and molecular studies, has revealed the existence of up to five specialized SAN conduction pathways that preferentially conduct electrical activation to atrial tissues. The intrinsic heart rate, intranodal leading pacemaker shifts, and changes in conduction in response to physiological and pathophysiological stimuli are similar. Structural and/or functional impairments due to cardiac diseases including heart failure cause SAN dysfunctions (SNDs) in both species. These dysfunctions are usually manifested as severe bradycardia, tachy-brady arrhythmias, and conduction abnormalities including exit block and SAN reentry, which could lead to atrial tachycardia and fibrillation, cardiac arrest, and heart failure. Pharmaceutical drugs and implantable pacemakers are only partially successful in managing SNDs, emphasizing a critical need to develop targeted mechanism-based therapies to treat SNDs. Because several structural and functional characteristics are similar between the canine and human SAN, research in these species may be mutually beneficial for developing novel treatment approaches. This review describes structural, functional, and molecular similarities and differences between the canine and human SAN, with special emphasis on arrhythmias and unique causal mechanisms of SND in diseased hearts.

Background: Up to 50% of the adult human sinoatrial node (SAN) is composed of dense connective tissue. Cardiac diseases including heart failure (HF) may increase fibrosis within the SAN pacemaker complex, leading to impaired automaticity and conduction of electric activity to the atria. Unlike the role of cardiac fibroblasts in pathologic fibrotic remodeling and tissue repair, nothing is known about fibroblasts that maintain the inherently fibrotic SAN environment. Methods: Intact SAN pacemaker complex was dissected from cardioplegically arrested explanted nonfailing hearts (non-HF; n=22; 48.7±3.1 years of age) and human failing hearts (n=16; 54.9±2.6 years of age). Connective tissue content was quantified from Masson trichrome–stained head-center and center-tail SAN sections. Expression of extracellular matrix proteins, including collagens 1 and 3A1, CILP1 (cartilage intermediate layer protein 1), and POSTN (periostin), and fibroblast and myofibroblast numbers were quantified by in situ and in vitro immunolabeling. Fibroblasts from the central intramural SAN pacemaker compartment (≈10×5×2 mm 3 ) and right atria were isolated, cultured, passaged once, and treated ± transforming growth factor β1 and subjected to comprehensive high-throughput next-generation sequencing of whole transcriptome, microRNA, and proteomic analyses. Results: Intranodal fibrotic content was significantly higher in SAN pacemaker complex from HF versus non-HF hearts (57.7±2.6% versus 44.0±1.2%; P + /vimentin + /CD31 – (cluster of differentiation 31) fibroblasts were higher in HF SAN. Vimentin + /α-smooth muscle actin + /CD31 – myofibroblasts along with increased interstitial POSTN expression were found only in HF SAN. RNA sequencing and proteomic analyses identified unique differences in mRNA, long noncoding RNA, microRNA, and proteomic profiles between non-HF and HF SAN and right atria fibroblasts and transforming growth factor β1–induced myofibroblasts. Specifically, proteins and signaling pathways associated with extracellular matrix flexibility, stiffness, focal adhesion, and metabolism were altered in HF SAN fibroblasts compared with non-HF SAN. Conclusions: This study revealed increased SAN-specific fibrosis with presence of myofibroblasts, CILP1, and POSTN-positive interstitial fibrosis only in HF versus non-HF human hearts. Comprehensive proteotranscriptomic profiles of SAN fibroblasts identified upregulation of genes and proteins promoting stiffer SAN extracellular matrix in HF hearts. Fibroblast-specific profiles generated by our proteotranscriptomic analyses of the human SAN provide a comprehensive framework for future studies to investigate the role of SAN-specific fibrosis in cardiac rhythm regulation and arrhythmias.

Cross-bridge cycling kinetics play an essential role in the heart’s ability to contract and relax. The rate of tension redevelopment (ktr) slows down as a muscle length is increased in intact human myocardium. We set out to determine the effect of rapid length step changes and protein kinase A (PKA) and protein kinase C-βII (PKC-βII) inhibitors on the ktr in ultra-thin non-failing and failing human right ventricular trabeculae. After stabilizing the muscle either at L90 (90% of optimal length) or at Lopt (optimal length), we rapidly changed the length to either Lopt or L90 and measured ktr. We report that length-dependent changes in ktr occur very rapidly (in the order of seconds or faster) in both non-failing and failing muscles and that the length at which a muscle had been stabilized prior to the length change does not significantly affect ktr. In addition, at L90 and at Lopt, PKA and PKC-βII inhibitors did not significantly change ktr. Our results reveal that length-dependent regulation of cross-bridge cycling kinetics predominantly occurs rapidly and involves the intrinsic properties of the myofilament rather than post-translational modifications that are known to occur in the cardiac muscle as a result of a change in muscle/sarcomere length.

It is well known that moderate exercise training elicits a small increase in ventricular mass (i.e., a physiological hypertrophy) that has many beneficial effects on overall cardiac health. It is also well known that, when a myocardial infarction damages part of the heart, the remaining myocardium remodels to compensate for the loss of viable functioning myocardium. The effects of exercise training, myocardial infarction (MI), and their interaction on the contractile performance of the myocardium itself remain largely to be determined. The present study investigated the contractile properties and kinetics of right ventricular myocardium isolated from sedentary and exercise trained (10-12 wk progressively increasing treadmill running, begun 4 wk after MI induction) dogs with and without a left ventricular myocardial infarction. Exercise training increased force development, whereas MI decreased force development that was not improved by exercise training. Contractile kinetics were significantly slower in the trained dogs, whereas this impact of training was less or no longer present after MI. Length-dependent activation, both evaluated on contractile force and kinetics, was similar in all four groups. The control exercise-trained group exhibited a more positive force-frequency relationship compared with the sedentary control group while both sedentary and trained post-MI dogs had a more negative relationship. Last, the impact of the β-adrenergic receptor agonist isoproterenol resulted in a similar increase in force and acceleration of contractile kinetics in all groups. Thus, exercise training increased developed force but slowed contractile kinetics in control (noninfarcted animals), actions that were attenuated or completely absent in post-MI dogs.

Treatment for heart disease, the leading cause of death in the world, has progressed little for several decades. Here we develop a protein engineering approach to directly tune in vivo cardiac contractility by tailoring the ability of the heart to respond to the Ca2+ signal. Promisingly, our smartly formulated Ca2+-sensitizing TnC (L48Q) enhances heart function without any adverse effects that are commonly observed with positive inotropes. In a myocardial infarction (MI) model of heart failure, expression of TnC L48Q before the MI preserves cardiac function and performance. Moreover, expression of TnC L48Q after the MI therapeutically enhances cardiac function and performance, without compromising survival. We demonstrate engineering TnC can specifically and precisely modulate cardiac contractility that when combined with gene therapy can be employed as a therapeutic strategy for heart disease., Heart contraction, which is decreased in disease, is determined by Ca2+ binding to troponin C. Here, the authors combine a protein engineering approach with gene therapy to modulate heart contractility in mice with the use of rationally designed Troponin C variants, suggesting a new therapy for diseased hearts.

Cross-bridge cycling rate is an important determinant of cardiac output, and its alteration can potentially contribute to reduced output in heart failure patients. Additionally, animal studies suggest that this rate can be regulated by muscle length. The purpose of this study was to investigate cross-bridge cycling rate and its regulation by muscle length under near-physiological conditions in intact right ventricular muscles of nonfailing and failing human hearts. We acquired freshly explanted nonfailing ( n = 9) and failing ( n = 10) human hearts. All experiments were performed on intact right ventricular cardiac trabeculae ( n = 40) at physiological temperature and near the normal heart rate range. The failing myocardium showed the typical heart failure phenotype: a negative force-frequency relationship and β-adrenergic desensitization ( P < 0.05), indicating the expected pathological myocardium in the right ventricles. We found that there exists a length-dependent regulation of cross-bridge cycling kinetics in human myocardium. Decreasing muscle length accelerated the rate of cross-bridge reattachment ( ktr) in both nonfailing and failing myocardium ( P < 0.05) equally; there were no major differences between nonfailing and failing myocardium at each respective length ( P > 0.05), indicating that this regulatory mechanism is preserved in heart failure. Length-dependent assessment of twitch kinetics mirrored these findings; normalized dF/d t slowed down with increasing length of the muscle and was virtually identical in diseased tissue. This study shows for the first time that muscle length regulates cross-bridge kinetics in human myocardium under near-physiological conditions and that those kinetics are preserved in the right ventricular tissues of heart failure patients.

Image inpainting aims to obtain a visually plausible image interpolation in a region of the image in which data is missing due to damage or occlusion. Usually, the only available information is the portion of the image outside the inpainting domain. Besides its numerous applications, the inpainting problem is of theoretical interest since its analysis involves an understanding of the self-similarity present in natural images. In this work, we present a detailed description and implementation of three exemplar-based inpainting methods derived from the variational framework introduced by Arias et al.

Transcription factors TBX5 and PITX2 involve in the regulation of gene expression of ion channels and are closely associated with atrial fibrillation (AF), the most common cardiac arrhythmia in developed countries. The exact cellular and molecular mechanisms underlying the increased susceptibility to AF in patients with TBX5/PITX2 insufficiency remain unclear. In this study, we have developed and validated a novel human left atrial cellular model (TPA) based on the ten Tusscher-Panfilov ventricular cell model to systematically investigate how electrical remodeling induced by TBX5/PITX2 insufficiency leads to AF. Using our TPA model, we have demonstrated that spontaneous diastolic depolarization observed in atrial myocytes with TBX5-deletion can be explained by altered intracellular calcium handling and suppression of inward-rectifier potassium current (IK1). Additionally, our computer simulation results shed new light on the novel cellular mechanism underlying AF by indicating that the imbalance between suppressed outward current IK1 and increased inward sodium-calcium exchanger current (INCX) resulted from SR calcium leak leads to spontaneous depolarizations. Furthermore, our simulation results suggest that these arrhythmogenic triggers can be potentially suppressed by inhibiting sarcoplasmic reticulum (SR) calcium leak and reversing remodeled IK1. More importantly, this study has clinically significant implications on the drugs used for maintaining SR calcium homeostasis, whereby drugs such as dantrolene may confer significant improvement for the treatment of AF patients with TBX5/PITX2 insufficiency.

OBJECTIVE: The electrocardiogram (ECG) provides an effective, non-invasive approach for clinical diagnosis in patients with cardiac diseases, such as atrial fibrillation (AF). AF is the most common cardiac rhythm disturbance and affects ~2% of the general population in industrialized countries. Automatic AF detection in clinics remains a challenging task due to the high inter-patient variability of ECGs, and unsatisfactory existing approaches for AF diagnosis (e.g., atrial or ventricular activity based analyses). APPROACH: We have developed RhythmNet, a 21-layer 1D convolutional recurrent neural network, trained using 8,528 single lead ECG recordings from the 2017 PhysioNet/Computing in Cardiology (CinC) Challenge, to classify ECGs of different rhythms including AF automatically. Our RhythmNet architecture contained 16 convolutions to extract features directly from raw ECG waveforms, followed by three recurrent layers to process ECGs of varying lengths and to detect arrhythmia events in long recordings. Large 15 × 1 convolutional filters were used to effectively learn the detailed variations of the signal within small time-frames such as the P-waves and QRS complexes. We employed residual connections throughout RhythmNet, along with batch-normalization and rectified linear activation units to improve convergence during training. MAIN RESULTS: We evaluated our algorithm on 3,658 testing data and obtained an F(1) accuracy of 82% for classifying sinus rhythm, AF, and other arrhythmias. RhythmNet was also ranked 5(th) in the 2017 CinC challenge. SIGNIFICANCE: Potentially, our approach could aid AF diagnosis in clinics and be used for patient self-monitoring to improve the early detection and effective treatment of AF.

Background In patients with end-stage heart failure, the primary etiology often originates in the left ventricle, and eventually the contractile function of the right ventricle (RV) also becomes compromised. RV tissue-level deficits in contractile force and/or kinetics need quantification to understand involvement in ischemic and non-ischemic failing human myocardium. Methods and results The human population suffering from heart failure is diverse, requiring many subjects to be studied in order to perform an adequately powered statistical analysis. From 2009-present we assessed live tissue-level contractile force and kinetics in isolated myocardial RV trabeculae from 44 non-failing and 41 failing human hearts. At 1 Hz stimulation rate (in vivo resting state) the developed active force was not different in non-failing compared to failing ischemic nor non-ischemic failing trabeculae. In sharp contrast, the kinetics of relaxation were significantly impacted by disease, with 50% relaxation time being significantly shorter in non-failing vs. non-ischemic failing, while the latter was still significantly shorter than ischemic failing. Gender did not significantly impact kinetics. Length-dependent activation was not impacted. Although baseline force was not impacted, contractile reserve was critically blunted. The force-frequency relation was positive in non-failing myocardium, but negative in both ischemic and non-ischemic myocardium, while the β-adrenergic response to isoproterenol was depressed in both pathologies. Conclusions Force development at resting heart rate is not impacted by cardiac pathology, but kinetics are impaired and the magnitude of the impairment depends on the underlying etiology. Focusing on restoration of myocardial kinetics will likely have greater therapeutic potential than targeting force of contraction.

BACKGROUND: Clinical multi-electrode mapping of atrial fibrillation (AF) drivers suffers from variable contact, signal processing, and structural complexity within the 3D human atrial wall, raising questions on the validity of such drivers. OBJECTIVES: To improve AF driver identification by integrating clinical multi-electrode mapping with driver fingerprints defined by high-resolution ex-vivo 3D functional and structural imaging. METHODS: Sustained AF was mapped in coronary-perfused explanted human hearts (n=11) with transmural near-infrared optical mapping (NIOM, ~0.3mm(2) resolution). Simultaneously, custom FIRMap catheters (~9×9mm(2) resolution) mapped endocardial and epicardial surfaces, which were analyzed by Focal Impulse and Rotor Mapping (FIRM) activation and Rotational Activity Profile (RAP). Functional maps were integrated with contrast-enhanced MRI (CE-MRI, ~0.1mm(3) resolution) analysis of 3D fibrosis architecture. RESULTS: During sustained AF, NIOM identified 1-2 intramural, spatially stable reentrant AF drivers per heart. Driver targeted ablation affecting 2.2±1.1% of the atrial surface terminated and prevented AF. Driver regions had significantly higher phase singularity density, and dominant frequency versus neighboring non-driver regions. FIRM had 80% sensitivity to NIOM-defined driver locations (16/20), and matched 14/20 driver visualizations: 10/14 reentries seen with RAP, and 4/6 breakthrough/focal patterns. FIRM detected 1.1±0.9 false-positive RAP per recording, but these regions had lower intramural CE-MRI fibrosis than driver regions (14.9±7.9% vs 23.2±10.5%, p

Background: Genetic variants at the SCN5A / SCN10A locus are strongly associated with electrocardiographic PR and QRS intervals. While SCN5A is the canonical cardiac sodium channel gene, the role of SCN10A in cardiac conduction is less well characterized. Methods: We sequenced the SCN10A locus in 3699 European-ancestry individuals to identify variants associated with cardiac conduction, and replicated our findings in 21,000 individuals of European ancestry. We examined association with expression in human atrial tissue. We explored the biophysical effect of variation on channel function using cellular electrophysiology. Results: We identified 2 intronic single nucleotide polymorphisms in high linkage disequilibrium ( r 2 =0.86) with each other to be the strongest signals for PR (rs10428132, β=−4.74, P =1.52×10 −14 ) and QRS intervals (rs6599251, QRS β=−0.73; P =1.2×10 −4 ), respectively. Although these variants were not associated with SCN5A or SCN10A expression in human atrial tissue (n=490), they were in high linkage disequilibrium ( r 2 ≥0.72) with a common SCN10A missense variant, rs6795970 (V1073A). In total, we identified 7 missense variants, 4 of which (I962V, P1045T, V1073A, and L1092P) were associated with cardiac conduction. These 4 missense variants cluster in the cytoplasmic linker of the second and third domains of the SCN10A protein and together form 6 common haplotypes. Using cellular electrophysiology, we found that haplotypes associated with shorter PR intervals had a significantly larger percentage of late current compared with wild-type (I962V+V1073A+L1092P, 20.2±3.3%, P =0.03, and I962V+V1073A, 22.4±0.8%, P =0.0004 versus wild-type 11.7±1.6%), and the haplotype associated with the longest PR interval had a significantly smaller late current percentage (P1045T, 6.4±1.2%, P =0.03). Conclusions: Our findings suggest an association between genetic variation in SCN10A , the late sodium current, and alterations in cardiac conduction.