Human sinoatrial node structure: 3D microanatomy of sinoatrial conduction pathways

https://doi.org/10.1016/j.pbiomolbio.2015.12.011Get rights and content

Abstract

Introduction

Despite a century of extensive study on the human sinoatrial node (SAN), the structure-to-function features of specialized SAN conduction pathways (SACP) are still unknown and debated. We report a new method for direct analysis of the SAN microstructure in optically-mapped human hearts with and without clinical history of SAN dysfunction.

Methods

Two explanted donor human hearts were coronary-perfused and optically-mapped. Structural analyses of histological sections parallel to epicardium (∼13–21 μm intervals) were integrated with optical maps to create 3D computational reconstructions of the SAN complex. High-resolution fiber fields were obtained using 3D Eigen-analysis of the structure tensor, and used to analyze SACP microstructure with a fiber-tracking approach.

Results

Optical mapping revealed normal SAN activation of the atria through a lateral SACP proximal to the crista terminalis in Heart #1 but persistent SAN exit block in diseased Heart #2. 3D structural analysis displayed a functionally-observed SAN border composed of fibrosis, fat, and/or discontinuous fibers between SAN and atria, which was only crossed by several branching myofiber tracts in SACP regions. Computational 3D fiber-tracking revealed that myofiber tracts of SACPs created continuous connections between SAN #1 and atria, but in SAN #2, SACP region myofiber tracts were discontinuous due to fibrosis and fat.

Conclusions

We developed a new integrative functional, structural and computational approach that allowed for the resolution of the specialized 3D microstructure of human SACPs for the first time. Application of this integrated approach will shed new light on the role of the specialized SAN microanatomy in maintaining sinus rhythm.

Introduction

The sinoatrial node (SAN) is the primary pacemaker of the heart and responsible for initiating and regulating cardiac rhythm (Keith and Flack, 1907, Lewis et al., 1910, James, 1961, Boineau et al., 1988, Opthof, 1988, Boyett et al., 2000, Chandler et al., 2009, Fedorov et al., 2009, Fedorov et al., 2010a). SAN automaticity and conduction depends on the unique heterogeneous distribution of intracellular ion channels, Ca2+ handling proteins and autonomic receptors within the SAN (Monfredi et al., 2010, Dobrzynski et al., 2013, Wu and Anderson, 2014) as well as the unique structure of the SAN complex (Fedorov et al., 2012). The specialized microanatomy allows the small SAN to pace the large atria efficiently by maintaining a balanced source-sink relationship (Joyner et al., 1986). Multiple factors affecting SAN structure could lead to sinus node dysfunction (SND) (Csepe et al., 2015), when the SAN inadequately paces the atria, which may result in a number of cardiac diseases such as heart failure, atrial fibrillation, malignant ventricular arrhythmias, and eventually cardiac arrest (Luu et al., 1989, Sumitomo et al., 2007, Faggioni et al., 2013, Hjortshoj et al., 2013, Alonso et al., 2014, Jensen et al., 2014). SND is the predominant prognosis for electric pacemaker implantation, which is currently the only available treatment (Jensen et al., 2014, Mangrum et al., 2000, Packer et al., 2009, Greenspon et al., 2012). Despite over a century of research on the SAN, limited knowledge of the relationship between the human SAN microarchitecture and SAN function remains a critical barrier to properly understanding SND mechanisms and developing new alternatives to implantable pacemaker therapy.

Since the discovery of the SAN by Keith and Flack in 1907, multiple studies have investigated the SAN structure and its role in the formation and regulation of sinus rhythm in human and different animal hearts (Keith and Flack, 1907, James, 1961, Opthof, 1988, Boyett et al., 2000, Fedorov et al., 2009, Fedorov et al., 2010a, Boineau et al., 1989, Beau et al., 1995, Sanchez-Quintana et al., 2005, Chandler et al., 2011). Located at the junction of the superior vena cava (SVC) and the right atrium, the human SAN structure consists of a compact mass of specialized cardiomyocytes enmeshed in a dense matrix of collagen, fibroblasts and fatty tissue (Csepe et al., 2015). In general, the macrostructural features of the SAN, such as SAN size, the relationship between increased collagen tissue percent with age (Lev and 1954, Alings et al., 1995), the defined SAN artery, and the banana-shaped 3D structure of the SAN, are generally accepted and agreed upon (James, 1961, Sanchez-Quintana et al., 2005, Chandler et al., 2011, Lev and 1954, Truex et al., 1967, Shiraishi et al., 1992) (Fig. 1). However, due to the complexities of this 3D structure, several microstructural features remain disputed and/or undefined.

Among the debates over human SAN microstructure are the contradictory hypotheses of how the SAN is electrically connected to the atria. One hypothesis is that the SAN is electrically insulated from the surrounding atria by a structural border of fibrosis, fat layers and myocyte discontinuity, and that functional and structural connection between the SAN and atria is limited to discrete SAN conduction pathways (SACPs) (James, 1961, Opthof, 1988, Fedorov et al., 2009, Fedorov et al., 2010a, Boineau et al., 1978, Bromberg et al., 1995, Schuessler and 2003). An alternative hypothesis is that SAN and atrial cells are extensively connected by diffuse inter-digitations of the SAN border with the atrial myocardium, and that no discrete pathways exist (Chandler et al., 2009, Sanchez-Quintana et al., 2005, Chandler et al., 2011, Anderson et al., 1998, Sanchez-Quintana et al., 2002). These discrepancies may be explained by methodological limitations of previous SAN studies, such as restricting analyses of SAN structure to 2D instead of utilizing a 3D computational model, insufficient spatial resolution of 3D structural studies, and/or conducting structural studies without functional mapping of SAN conduction.

The importance of the functional-structural SAN to atria connection lies in its fundamental role in the mechanism of atrial activation from SAN pacemaker activity (Fedorov et al., 2012, Joyner et al., 1986, Csepe et al., 2015) and the maintenance of normal sinus rhythm in the human heart. New methodologies need to be developed to resolve the discrepancy of the SAN-atrial connections. In the present study, we developed an integrated approach including high-resolution optical mapping, serial histological sectioning and computational 3D fiber tracking to provide for the first time a detailed description of the functionally-identified SAN structure and a 3D reconstruction of the specialized SACP microstructure in human hearts with and without SND.

Section snippets

Optical mapping of coronary-perfused human atrial preparations

Explanted human hearts were obtained from Lifeline of Ohio in accordance with The Ohio State University Institutional Review Board. Patient-specific data can be found in Table 1. Explanted human hearts were cardioplegically-arrested and cooled to 4 °C in the operating room following cross-clamping of the aorta. Hearts were stored in cold cardioplegic solution (4 °C) during transport, dissection and cannulation. Human atrial preparations were isolated as previously described (Fedorov et al.,

Functional identification of SAN pacemaker complex

Near infrared optical mapping revealed that during baseline conditions, SAN #1 had stable sinus rhythm (61 BPM) with the leading pacemaker in the central region of the SAN tail (Fig. 2A). SAN activation preferentially traveled superiorly from the leading pacemaker at 11.8 ± 3.1 cm/s and slowed to 6.5 ± 1.6 cm/s before exiting SAN through lateral SACP to excite the atria in the superior CT (atrial breakthrough in Fig. 2A). While additional septal and inferior lateral exit and entrance SACPs were

3D histological reconstructions and structural models of the human SAN

Over a century ago, the anatomic structure of the SAN was discovered by Keith and Flack (Keith and Flack, 1907). Since this discovery, animal model and human studies have investigated how cardiac rhythm is initiated and regulated by the SAN (Keith and Flack, 1907, Lewis et al., 1910, James, 1961, Boineau et al., 1988, Opthof, 1988, Boyett et al., 2000, Chandler et al., 2009, Fedorov et al., 2010a). One of the critical components of efficient SAN function as the leading pacemaker of the human

Conclusion

We developed a new integrated approach that allowed us to provide the first 3D structural delineation of human SACPs connecting the human SAN to the atria. We suggest that SACPs are a necessary structural component of proper SAN function and a major contributor for source–sink relationship. Structural remodeling of SACPs due to aging and different cardiac diseases may lead to discontinuous myofiber tracts in SACP regions and ultimately exit block and SND. Future application of this approach

Editors' note

Please see also related communications in this issue by Vigmond and Stuyvers (2016) and George and Poelzing (2016).

Conflicts of interest

There are no conflicts of interest.

Acknowledgments

We sincerely thank the Lifeline of Ohio Organ Procurement Organization for providing the explanted hearts; Dr. Qing Lou and Mr. Benjamin Canan and Mr. Eric Schultz for help with tissue processing.

This work was supported by NIH HL115580 (VVF), HL113084 (PMLJ), HL084583, HL083422, HL114383 (PJM), National Heart Foundation of New Zealand (JZ) #1666 and by funding from the Dorothy M. Davis Heart and Lung Research Institute.

References (74)

  • S. Nakao et al.

    The anatomical basis of bradycardia-tachycardia syndrome in elderly dogs with chronic degenerative valvular disease

    J. Comp. Pathol.

    (2012)
  • Y. Wu et al.

    CaMKII in sinoatrial node physiology and dysfunction

    Front. Pharmacol.

    (2014)
  • N. Akoum et al.

    Atrial fibrosis quantified using late gadolinium enhancement MRI is associated with sinus node dysfunction requiring pacemaker implant

    J. Cardiovasc. Electrophysiol.

    (2012)
  • A.M. Alings et al.

    Age-related changes in structure and relative collagen content of the human and feline sinoatrial node. A comparative study

    Eur. Heart J.

    (1995)
  • A. Alonso et al.

    Association of sick sinus syndrome with incident cardiovascular disease and mortality: the atherosclerosis risk in communities study and cardiovascular health study

    PLoS One

    (2014)
  • R.H. Anderson et al.

    The architecture of the sinus node, the atrioventricular conduction axis, and the internodal atrial myocardium

    J. Cardiovasc. Electrophysiol.

    (1998)
  • O.V. Aslanidi et al.

    Application of micro-computed tomography with iodine staining to cardiac imaging, segmentation, and computational model development

    IEEE Trans. Med. Imaging

    (2013)
  • S.L. Beau et al.

    Relative densities of muscarinic cholinergic and beta-adrenergic receptors in the canine sinoatrial node and their relation to sites of pacemaker activity

    Circ. Res.

    (1995)
  • J.P. Boineau et al.

    Multicentric origin of the atrial depolarization wave: the pacemaker complex. Relation to dynamics of atrial conduction, P-wave changes and heart rate control

    Circulation

    (1978)
  • J.P. Boineau et al.

    Demonstration of a widely distributed atrial pacemaker complex in the human heart

    Circulation

    (1988)
  • J.P. Boineau et al.

    The human atrial pacemaker complex

    J. Electrocardiol.

    (1989)
  • M.R. Boyett et al.

    The sinoatrial node, a heterogeneous pacemaker structure

    Cardiovasc. Res.

    (2000)
  • B.I. Bromberg et al.

    Primary negativity does not predict dominant pacemaker location: implications for sinoatrial conduction

    Am. J. Physiol.

    (1995)
  • T.D. Butters et al.

    A novel computational sheep atria model for the study of atrial fibrillation

    Interface Focus

    (2013)
  • N.J. Chandler et al.

    Molecular architecture of the human sinus node: insights into the function of the cardiac pacemaker

    Circulation

    (2009)
  • N. Chandler et al.

    Computer three-dimensional anatomical reconstruction of the human sinus node and a novel paranodal area

    Anat. Rec. Hob.

    (2011)
  • T.A. Csepe et al.

    Fibrosis: a structural modulator of sinoatrial node physiology and dysfunction

    Front. Physiol.

    (2015)
  • J.C. Demoulin et al.

    Histopathological correlates of sinoatrial disease

    Br. Heart J.

    (1978)
  • M. Disertori et al.

    Electroanatomic mapping and late gadolinium enhancement MRI in a genetic model of arrhythmogenic atrial cardiomyopathy

    J. Cardiovasc. Electrophysiol.

    (2014)
  • M. Faggioni et al.

    Accelerated sinus rhythm prevents catecholaminergic polymorphic ventricular tachycardia in mice and in patients

    Circ. Res.

    (2013)
  • V.V. Fedorov et al.

    Structural and functional evidence for discrete exit pathways that connect the canine sinoatrial node and atria

    Circ. Res.

    (2009)
  • V.V. Fedorov et al.

    Complex interactions between the sinoatrial node and atrium during reentrant arrhythmias in the canine heart

    Circulation

    (2010)
  • V.V. Fedorov et al.

    Conduction barriers and pathways of the sinoatrial pacemaker complex: their role in normal rhythm and atrial arrhythmias

    Am. J. Physiol. Heart Circ. Physiol.

    (2012)
  • A.V. Glukhov et al.

    Calsequestrin 2 deletion causes sinoatrial node dysfunction and atrial arrhythmias associated with altered sarcoplasmic reticulum calcium cycling and degenerative fibrosis within the mouse atrial pacemaker complex

    Eur. Heart J.

    (2015)
  • A.V. Glukhov et al.

    Sinoatrial node reentry in a canine chronic left ventricular infarct model: role of intranodal fibrosis and heterogeneity of refractoriness

    Circ. Arrhythm. Electrophysiol.

    (2013)
  • B.J. Hansen et al.

    Atrial fibrillation driven by micro-anatomic intramural re-entry revealed by simultaneous sub-epicardial and sub-endocardial optical mapping in explanted human hearts

    Eur. Heart J.

    (2015)
  • X. Hao et al.

    TGF-beta1-mediated fibrosis and ion channel remodeling are key mechanisms in producing the sinus node dysfunction associated with SCN5A deficiency and aging

    Circ. Arrhythm. Electrophysiol.

    (2011)
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