Digital twinning of the human ventricular activation sequence to Clinical 12-lead ECGs and magnetic resonance imaging using realistic Purkinje networks for in silico clinical trials

Cardiac in silico clinical trials can virtually assess the safety and efficacy of therapies using human-based modelling and simulation. These technologies can provide mechanistic explanations for clinically observed pathological behaviour. Designing virtual cohorts for in silico trials requires exploiting clinical data to capture the physiological variability in the human population. The clinical characterisation of ventricular activation and the Purkinje network is challenging, especially non-invasively. Our study aims to present a novel digital twinning pipeline that can efficiently generate and integrate Purkinje networks into human multiscale biventricular models based on subject-specific clinical 12-lead electrocardiogram and magnetic resonance recordings. Essential novel features of the pipeline are the human-based Purkinje network generation method, personalisation considering ECG R wave progression as well as QRS morphology, and translation from reduced-order Eikonal models to equivalent biophysically-detailed monodomain ones. We demonstrate ECG simulations in line with clinical data with clinical image-based multiscale models with Purkinje in four control subjects and two hypertrophic cardiomyopathy patients (simulated and clinical QRS complexes with Pearson's correlation coefficients > 0.7). Our methods also considered possible differences in the density of Purkinje myocardial junctions in the Eikonal-based inference as regional conduction velocities. These differences translated into regional coupling effects between Purkinje and myocardial models in the monodomain formulation. In summary, we demonstrate a digital twin pipeline enabling simulations yielding clinically consistent ECGs with clinical CMR image-based biventricular multiscale models, including personalised Purkinje in healthy and cardiac disease conditions.

Protective Role of False Tendon in Subjects with Left Bundle Branch Block: A Virtual Population Study

False tendons (FTs) are fibrous or fibromuscular bands that can be found in both the normal and abnormal human heart in various anatomical forms depending on their attachment points, tissue types, and geometrical properties. While FTs are widely considered to affect the function of the heart, their specific roles remain largely unclear and unexplored. In this paper, we present an in silico study of the ventricular activation time of the human heart in the presence of FTs. This study presents the first computational model of the human heart that includes a FT, Purkinje network, and papillary muscles. Based on this model, we perform simulations to investigate the effect of different types of FTs on hearts with the electrical conduction abnormality of a left bundle branch block (LBBB). We employ a virtual population of 70 human hearts derived from a statistical atlas, and run a total of 560 simulations to assess ventricular activation time with different FT configurations. The obtained results indicate that, in the presence of a LBBB, the FT reduces the total activation time that is abnormally augmented due to a branch block, to such an extent that surgical implant of cardiac resynchronisation devices might not be recommended by international guidelines. Specifically, the simulation results show that FTs reduce the QRS duration at least 10 ms in 80% of hearts, and up to 45 ms for FTs connecting to the ventricular free wall, suggesting a significant reduction of cardiovascular mortality risk. In further simulation studies we show the reduction in the QRS duration is more sensitive to the shape of the heart then the size of the heart or the exact location of the FT. Finally, the model suggests that FTs may contribute to reducing the activation time difference between the left and right ventricles from 12 ms to 4 ms. We conclude that FTs may provide an alternative conduction pathway that compensates for the propagation delay caused by the LBBB. Further investigation is needed to quantify the clinical impact of FTs on cardiovascular mortality risk.

Estimation of Purkinje trees from electro-anatomical mapping of the left ventricle using minimal cost geodesics

The electrical activation of the heart is a complex physiological process that is essential for the understanding of several cardiac dysfunctions, such as ventricular tachycardia (VT). Nowadays, patient-specific activation times on ventricular chambers can be estimated from electro-anatomical maps, providing crucial information to clinicians for guiding cardiac radio-frequency ablation treatment. However, some relevant electrical pathways such as those of the Purkinje system are very difficult to interpret from these maps due to sparsity of data and the limited spatial resolution of the system. We present here a novel method to estimate these fast electrical pathways from the local activations maps (LATs) obtained from electro-anatomical maps. The location of Purkinje-myocardial junctions (PMJs) is estimated considering them as critical points of a distance map defined by the activation maps, and then minimal cost geodesic paths are computed on the ventricular surface between the detected junctions. Experiments to validate the proposed method have been carried out in simplified and realistic simulated data, showing good performance on recovering the main characteristics of simulated Purkinje networks (e.g. PMJs). A feasibility study with real cases of fascicular VT was also performed, showing promising results.

Computational generation of the Purkinje network driven by clinical measurements: the case of pathological propagations

To properly describe the electrical activity of the left ventricle, it is necessary to model the Purkinje fibers, responsible for the fast and coordinate ventricular activation, and their interaction with the muscular propagation. The aim of this work is to propose a methodology for the generation of a patient-specific Purkinje network driven by clinical measurements of the activation times related to pathological propagations. In this case, one needs to consider a strongly coupled problem between the network and the muscle, where the feedback from the latter to the former cannot be neglected as in a normal propagation. We apply the proposed strategy to data acquired on three subjects, one of them suffering from muscular conduction problems owing to a scar and the other two with a muscular pre-excitation syndrome (Wolff-Parkinson-White). To assess the accuracy of the proposed method, we compare the results obtained by using the patient-specific Purkinje network generated by our strategy with the ones obtained by using a non-patient-specific network. The results show that the mean absolute errors in the activation time is reduced for all the cases, highlighting the importance of including a patient-specific Purkinje network in computational models.

Functional, anatomical, and molecular investigation of the cardiac conduction system and arrhythmogenic atrioventricular ring tissue in the rat heart

Background

The cardiac conduction system consists of the sinus node, nodal extensions, atrioventricular (AV) node, penetrating bundle, bundle branches, and Purkinje fibers. Node‐like AV ring tissue also exists at the AV junctions, and the right and left rings unite at the retroaortic node. The study aims were to (1) construct a 3‐dimensional anatomical model of the AV rings and retroaortic node, (2) map electrical activation in the right ring and study its action potential characteristics, and (3) examine gene expression in the right ring and retroaortic node.

Methods and Results

Three‐dimensional reconstruction (based on magnetic resonance imaging, histology, and immunohistochemistry) showed the extent and organization of the specialized tissues (eg, how the AV rings form the right and left nodal extensions into the AV node). Multiextracellular electrode array and microelectrode mapping of isolated right ring preparations revealed robust spontaneous activity with characteristic diastolic depolarization. Using laser microdissection gene expression measured at the mRNA level (using quantitative PCR) and protein level (using immunohistochemistry and Western blotting) showed that the right ring and retroaortic node, like the sinus node and AV node but, unlike ventricular muscle, had statistically significant higher expression of key transcription factors (including Tbx3, Msx2, and Id2) and ion channels (including HCN4, Ca_v3.1, Ca_v3.2, K_v1.5, SK1, K_ir3.1, and K_ir3.4) and lower expression of other key ion channels (Na_v1.5 and K_ir2.1).

Conclusions

The AV rings and retroaortic node possess gene expression profiles similar to that of the AV node. Ion channel expression and electrophysiological recordings show the AV rings could act as ectopic pacemakers and a source of atrial tachycardia.

Characterization and modeling of the peripheral cardiac conduction system

The development of biophysical models of the heart has the potential to get insights in the patho-physiology of the heart, which requires to accurately modeling anatomy and function. The electrical activation sequence of the ventricles depends strongly on the cardiac conduction system (CCS). Its morphology and function cannot be observed in vivo, and therefore data available come from histological studies. We present a review on data available of the peripheral CCS including new experiments. In order to build a realistic model of the CCS we designed a procedure to extract morphological characteristics of the CCS from stained calf tissue samples. A CCS model personalized with our measurements has been built using L-systems. The effect of key unknown parameters of the model in the electrical activation of the left ventricle has been analyzed. The CCS models generated share the main characteristics of observed stained Purkinje networks. The timing of the simulated electrical activation sequences were in the physiological range for CCS models that included enough density of PMJs. These results show that this approach is a potential methodology for collecting knowledge-domain data and build improved CCS models of the heart automatically.

Activation pattern of the avian left ventricle during ventricular pacing

OBJECTIVE

This study was planned to investigate ventricular myocardial excitation in birds in which Purkinje fibres penetrate into the ventricular wall and reach the epicardium to advance our knowledge about the evolution of the ventricular activation process in vertebrates.

METHODS

A depolarization pattern of the left ventricular free wall in seven open-chest laying hens was mapped by 14 seven-electrode plunge needles under ventricular pacing from different sites.

RESULTS

Duration of activation of the left ventricular free wall is significantly increased during ventricular ectopic excitation as compared with sinus rhythm. Its lowest increase occurs during subendocardial pacing of the middle part of the left ventricle, but its greatest increase is observed during subepicardial pacing of the left ventricular base. Multifocality and mosaicity of depolarization of the left ventricular myocardium are expressed in a considerably less degree during ventricular pacing in comparison with sinus rhythm.

CONCLUSION

Ectopic excitation of avian heart ventricles occurs mostly due to successive spreading of the activation wave from a pacing site during both ipsi- and contraventricular pacing. During ipsiventricular pacing at least, ectopic excitation of the heart ventricles with the "rich" Purkinje network behaves like one of the mammalian ventricles with the subendocardial Purkinje network.

[Functional and developmental view on Purkinje fibers system]

The cardiac conducting system is vital for generating and synchronizing the heartbeat. Beginning with Tawara, Einthoven and other pioneering workers, a wealth of information has been collected over the last 100 years on the histologic, morphologic and physiologic characteristics of specialized cardiac tissues. However, in the last ten years considerable effort has been put into understanding the cellular and molecular mechanisms governing its development. During this latter period, controversies have also arisen as to the nature of the signaling mechanisms involved in induction and patterning of the conducting system, particularly with respect to the pathways functioning in mammals. In this review, we will try to summarize the current state of knowledge in this field and point out some of the remaining questions.

[Purkynje fibers of the heart conduction system--history and the present time]

It has been 160 years now since Purkynje published the finding of conduction fibers in the heart in Archiv f. Anatomie u. Physiologie and it has been 166 years since his publication in polish version. Already during Purkynje's life, some anatomists had solved the morphology of these fibers but nobody at that time knew of what great physiological and medical importance this discovery would be for medicine. It was seen as late as in the 20th century and in contemporary times. Purkynje's work indicated the cascade of these discoveries, which were leading in the beginning of the previous century to the formulation of the basic scheme of the conduction system. Purkynje fibers or Purkynje cardiomyocytes are part of the whole complex of the cardiac conduction system which today is classified as specific heart muscle tissue, being responsible for the generation of the heart impulses. From the point of view of ultrastructural composition, the cells of different parts of the cardiac conduction system are partly similar. In contrast to the heart contractile cardiomyocytes, the cells of the cardiac conduction system including Purkynje fibers have a small amount of myofibrils,small mitochondrias, light cytoplasm and a higher glycogen content, but no T-tubular system. They can be detected with some morphological methods. Nevertheless the cells of the conduction system are not completely uniform. They differ in size, number of nexuses-gaps and intercalar discs in individual parts of the conduction system. Nevertheless, these specialized cells work as a whole-unit. Nowadays, the morphology research of all the parts of cardiac conduction system, including Purkynje fibers, is focused on ultrastructural, histochemical and genetical problems. The question is, wheather with future gene/cell therapy disturbances of the conduction system such as arrythmias, can be prevented and cured by replacing the electrical pacemakers with biological ones. If Jan Evangelista Purkynje lived today, he would be surprised but surely delighted with the high degree of research concerning his discovery and its clinical application.

Architectural and functional asymmetry of the His-Purkinje system of the murine heart

OBJECTIVE

The aim of this work was to target a vital reporter gene in the mouse cardiac conduction system (CS) to distinguish this tissue from the surrounding myocardium in the adult heart.

METHODS

A transgenic mouse line has been created in which EGFP is expressed under the control of the Cx40 gene. Correlative investigations associating EGFP imaging and electrophysiological techniques were carried out on the adult heart and isolated cardiomyocytes.

RESULTS

In the heart of the Cx40(EGFP/+) mice, EGFP signal was seen in the coronary arteries, the atria, the atrioventricular (AV) node and the His-Purkinje system. The latter was found to be structurally and functionally asymmetrical. The anatomical asymmetry was apparent in both the number of strands or fasciculi making up the His bundle branches (BBs) (1 strand on the right, 20 or so on the left), and the density (low on the right, high on the left) of the network of Purkinje fibers (PFs) that extends over the ventricular wall surfaces. The profiles of the electrical activation patterns recorded on the right and left flanks of the septum were also asymmetrical, mirroring the architecture of the branches. EGFP made it easy to identify the Purkinje cells in populations of dissociated cardiomyocytes and they were investigated using the patch-clamp technique. The hyperpolarization-activated current (If) was recorded in all spontaneously active Purkinje cells.

CONCLUSIONS

This investigation provides positive evidence of the asymmetry of the His-Purkinje system of the adult mouse, and the first patch-clamp recording data on murine cardiac Purkinje cells. This mouse model opens up new perspectives for investigating the contribution of specific genes to the morphology and function of the His-Purkinje system.

Induction and patterning of the Purkinje fibre network

Impulse-conducting Purkinje cells differentiate from myocytes during embryogenesis. In the embryonic chicken heart, this conversion of contractile myocytes into conduction cells occurs subendocardially and periarterially. The unique sites of Purkinje fibre differentiation suggest that a shear stress-induced paracrine signal from the endocardium and arterial beds may induce adjacent myocytes to differentiate into conduction cells. Consistent with this model, Purkinje fibre marker genes can be induced in cultured embryonic myocytes by endothelin (ET), an endothelial cell-derived signalling peptide. This inductive response is, however, gradually lost from myocytes as embryos develop, and mature myocytes express only genes characteristic of hypertrophy in response to ET. In vivo, active ET is produced, through proteolytic processing, from its precursor by ET-converting enzyme 1 (ECE1) and triggers signalling by binding to its receptors, ETA and ETB. In the embryonic heart, the expression of these ET signalling components changes dynamically, defining the site and timing of Purkinje fibre differentiation within the ventricular myocardium during chick embryogenesis.

A deformable finite element derived finite difference method for cardiac activation problems

We present a finite element (FE) derived finite difference (FD) technique for solving cardiac activation problems over deforming geometries using a bidomain framework. The geometry of the solution domain is defined by a FE mesh and over these FEs a high resolution FD mesh is generated. The difference points are located at regular intervals in the normalized material space within each of the FEs. The bidomain equations are then transformed to the embedded FD mesh which provides a solution space that is both regular and orthogonal. The solution points move in physical space with any deformation of the solution domain, but the equations are set up in such a way that the solution is invariant as it is constructed in material space. The derivation of this new solution technique is presented along with a series of examples that demonstrate the accuracy of this bidomain framework.

Distribution of the Purkinje fibres in the sheep heart

The aim of the present study was to advance our knowledge regarding the anatomy of the purkinje fibres from their origin, at the bundle branches, till their termination within the myocardium. Indian ink injections of the purkinje fibres were carried out in the left ventricle of 25 fresh sheep hearts and in the right ventricle of 20 hearts. Numerous samples were taken from the walls and papillary muscles of the two ventricles for histological analysis and determination of the mode of termination of the fibres. The ventricular conduction system could be injected as far proximally as the bundle branches, thus illustrating the bifascicular nature of the left bundle branch, with numerous interfascicular communications. The purkinje fibres were observed to form an extensive subendocardial network, forming a polygonal arrangement in the left ventricle with a characteristic pattern around the papillary muscles. Deep myocardial branches took origin from this network which penetrated the ventricular wall to reach the epicardium. Histological analysis demonstrated the characteristic features of the purkinje cells, and confirmed the presence ofa perifascicular sheath of connective tissue which surrounded the purkinje fibres until their transition with working cardiomyocytes. The perifascicular connective tissue sheath is important in organising the contraction of the myocardium by preventing lateral spread of conduction and by permitting transmission of the impulse only at the termination of the purkinje fibre. The sheath may also protect the fibres from the stresses and strains originating from contraction of the surrounding myocardium.

The anatomy of the atrioventricular bundle in the heart of camels (Camelus dromedarius)

The anatomy and histology of the atrioventricular bundle (AVB) was studied in the heart of six camels (Camelus dromedarius). The trunk of the atrioventricular bundle (bundle of His) was a direct continuation of the atrioventricular (AV) node with no sharp line of demarcation between the node and the bundle. The atrioventricular bundle ran through the fibrous trigone and entered the lower part of the interventricular membranous septum, beneath the right endocardium, then lay over or slightly to the side of the centre of the muscular interventricular crest. The AVB of camels measured 4.12 +/- 1.00 mm in length, 3.66 +/- 1.13 mm in width and 1.13 +/- 1.85 mm in thickness, its maximum sectional area being 12.68 +/- 6.13 mm2. Histologically, the AVB in the heart of camels comprised multiple strands of Purkinje cells separated by collagen fibres and surrounded by connective tissue. It resembled that in human beings and dogs except that, in camels, intercalated discs were present at the intercellular connections in the AVB.

Anatomy, histology, and pathology of the cardiac conduction system: Part II

Normal anatomic and histologic features of the atrioventricular junction (transitional cell zone, atrioventricular node, penetrating portion of bundle) and the bifurcation of the penetrating portion into bundle branches are reviewed. Terminal ventricular Purkinje fibers are also discussed.

[Tendon of Todaro in the human heart]

The material consists of 50 human hearts without either pathological changes or congenital malformations. Macroscopic and microscopic methods were applied. Histological specimens were stained alternatively with hematoxylin-eosin and according to van Giesson and Masson. In each heart the tendon of Todaro was dissected from its portion situated between the inferior vena cava and coronary sinus up to central part of the membranous septum (Fig. 1). The hearts were divided into 3 groups according to age: fetuses (20 hearts), infants (15) and adults (15) (Table I). In all hearts of fetuses and infants the tendon of Todaro was found to be a well-developed cylindrical structure covered with endocardium. In histologic specimens the tendon was s solid structure well-separated from other tissues (Fig. 3, Fig. 4). In hearts of adults (17-45 years old) the tendon of Todaro was less evident. However, in histological specimens it was present as a compact connective tissue band (Fig. 2). In hearts of subjects older than 50, the endocardium of this area was not elevated and not distinguishable even by palpation. The connective tissue band was formed by rather dissipated fibres, poorly separated from surrounding structures. As a result of our study one may conclude that the tendon of Todaro is present in each human heart. It is well-developed in hearts of fetuses and infants. Later it diminishes gradually becoming almost inconspicuous in old subjects.

Histological organization of the right and left atrioventricular valves of the chicken heart and their relationship to the atrioventricular Purkinje ring and the middle bundle branch

In the avian heart the right and left atrioventricular (AV) valves not only exhibit their own special anatomical characteristics, but they also are in close proximity to the conduction system. The right AV valve is a single, spiral plane of myocardium, in remarkable contrast to the fibrous structure characteristic of the mammalian tricuspid valve. A ring of Purkinje tissue encircles the avian right AV orifice and connects to the muscular valve. The chicken has no crista supraventricularis, its right AV valve serving that function as well as opening and closing the right AV orifice. The left AV valve consists of three leaflets instead of the two typical of mammalian hearts. Its anterior and posterior leaflets are small; its large aortic (medial) leaflet merges with the bases of both the left and noncoronary cusps of the aortic valve by fibrous tissue, resembling that of the mammalian heart. However, unlike in mammals, there is a slim cylinder of continuous myocardium coursing parallel to this fibrous junction. This unusual arc of myocardium in the chicken serves to complete an entire subaortic ring of myocardium and is thus potentially capable of constricting the outflow tract of the chicken's left ventricle. The middle bundle branch connects with both the muscle arch and the AV Purkinje ring. Thus the myocardium in or near both AV valves (and the left ventricular outflow tract) in the chicken heart is so arranged that it may receive direct early activation from the conduction system.

Morphology of electrophysiologically identified junctions between Purkinje fibers and ventricular muscle in rabbit and pig hearts

Purkinje fiber-ventricular muscle (PV) junctions were identified by extracellular recording in isolated, superfused preparations from rabbit and pig hearts. Microelectrode recordings from different cell types at the PV junctions were obtained, and the cells recorded from were retrieved microscopically. To this end 26 tissue blocks were serially sectioned at 4 microns. Microscopic identification of the very cell recorded from was obtained in five of seven Purkinje, five of 16 transitional, and two of two ventricular muscle cell recordings. In addition, some tissue blocks from both junctional and nonjunctional sites identified only by extracellular recording were examined in serial sections. Transitional cells in the rabbit heart are thin, broad bandlike cells (30-35 by 3-5 microns) arranged in one or two sheets in the subendocardium between the Purkinje layer and ventricular mass. Transitional cells are coupled via short, thin strands to both Purkinje and ventricular muscle cells. A second type of PV coupling was observed frequently in the pig, but in only one of 21 cases in the rabbit. Here, a short, linear segment of small transitional cells connected large-diameter Purkinje cells to ventricular muscle cells. Distances found between Purkinje-transitional cell coupling sites and transitional cell-ventricular muscle coupling sites varied from 100 to 1,000 microns in the rabbit heart and from 50 to several hundred micrometers in the pig heart. Action potentials from transitional cells typically showed multiple components in their upstroke. Both our morphological and electrophysiological findings are compatible with the existence of a relatively high-resistance barrier between Purkinje and transitional cells and between transitional and ventricular muscle cells.

The surgical anatomy of the sinoatrial node

The sinoatrial nodes (SAN) were observed, dissected, and measured on 95 adults and 30 child hearts under a dissection microscope. The majority of the SANs in adults are characterized by their pale color, firm consistency, and the location in relation to the penetration of the SAN artery, and they can be located in the superior part of the terminal sulcus. The SANs in children, however, are not easily discerned. The variation of the apex of the right auricular crest and the notch in the superior part of the terminal sulcus have been described, and the present authors suggested that the trigone of the SAN could be used as an important landmark to identify the SAN. The surface features on the SAN, its relationship to the surrounding myocardium and its surgical significance during operation are further discussed.

Skeletonization of the atrioventricular node for AV node reentrant tachycardia: experience with 32 patients

We describe our experience with operative therapy for atrioventricular (AV) node tachycardia using an anatomically guided procedure. The operative rationale was to dissect the AV node from most of its atrial inputs (AV node "skeletonization") with the intent of altering the perinodal substrate and preventing reentry. The anteroseptal and posteroseptal regions were initially approached epicardially to facilitate identification of anatomical structures. Under normothermic cardiopulmonary bypass, the right atrial septum was mobilized and the intermediate AV node was exposed anterior to the tendon of Todaro. Atrioventricular node conduction was monitored electrocardiographically throughout the procedure. Ablation of concomitant accessory pathways was done prior to AV node skeletonization. Thirty-two patients aged 9 to 67 years (mean age, 30 years) underwent operation. Five patients had concomitant accessory pathways in addition to AV node reentry. At electrophysiological study before discharge, no patient had AV block although anterograde and retrograde Wenckebach cycle lengths were significantly prolonged. Six patients had retrograde AV block. Twenty-nine patients are free from arrhythmia and require no antiarrhythmic medication after a follow-up of 1 month to 45 months (mean follow-up, 17 months). Three patients had recurrence of tachycardia ten days, 2 months, and 7 months postoperatively. All patients subsequently had a successful reoperation.

Developmental morphology of vascular and lymphatic capillaries in the working myocardium and Purkinje bundle of the sheep septomarginal band

The normal development of vascular and lymphatic capillaries in the right ventricular septomarginal band of the sheep heart was studied in 9 fetuses aged 60-143 days (term = 147 days), 14 lambs aged 1 day to 16 weeks, and 3 adults. Tissue was fixed by perfusion and examined with light and transmission electron microscopy. The septomarginal band is composed of working myocardium and a well-defined peripheral bundle of Purkinje cells. Vascular capillaries of the working myocardium were closely apposed to myocardial cells. By contrast, vascular capillaries of the Purkinje bundle were situated within the connective tissue sheath and septa, at variable distances from the Purkinje cells. After birth, the capillaries of the Purkinje bundle were also found in grooves and tunnels within the Purkinje strands. The ultrastructure of fetal vascular capillaries associated with myocardial and Purkinje cells was initially similar, and characterized by an abundance of synthetic organelles in endothelial cells and pericytes. However, after 115 days in utero, capillary endothelium with diaphragmed fenestrae, 40-60 nm in width, were observed within the Purkinje bundle. The fenestrae attained an average frequency of 1 per 11 capillary cross sections just before term, and this was maintained in lambs and adults. The ultrastructure of lymphatic capillaries, which were not observed in the septomarginal band until just before term, changed little during development.

Prevalence of the coexistence of left ventricular false tendons and premature ventricular complexes in apparently healthy subjects: a prospective study in the general population

The prevalence of left ventricular false tendons, premature ventricular complexes and their coexistence was evaluated prospectively in 187 healthy company workers aged 21 to 50 (mean 36) years. False tendons were demonstrated echocardiographically in 133 (71%). Eight subjects were withdrawn from the study because of silent mitral valve prolapse. In these 179 healthy subjects, false tendons were detected in 127 (71%) and premature ventricular complexes in 48 (27%). Their coexistence was observed in 40, which showed a significant correlation (p less than 0.05) of false tendons and premature ventricular complexes. In seven of the eight subjects without false tendons, premature ventricular complexes were uniform and infrequent (mean 3 beats/24 h). In the 40 subjects with false tendons, premature ventricular complexes were uniform in 29, multiform in 6 and repetitive in 5, and the mean frequency was 96 beats/24 h. Correlation of premature ventricular complexes with the type of false tendons showed that premature ventricular complexes were significantly associated with thick (greater than or equal to 2 mm) and longitudinal tendons (p less than 0.005). Although it is not certain that left ventricular false tendons are arrhythmogenic, the prevalence of the coexistence of left ventricular false tendons and premature ventricular complexes in the general population, and the special relation between the frequency and the form of premature ventricular complexes and the type of false tendons, suggests that false tendons may play an etiologic role in the genesis of premature ventricular complexes in apparently healthy subjects.

The distribution of terminal sympathetic nerve fibers in bundle branches and false tendons of the bovine heart. An immunohistochemical and catecholamine histofluorescence study

The sympathetic innervation in false tendons as a whole and the distribution of the terminal sympathetic nerve fibers in the conduction tissue in the bundle branches is unclear. Therefore, in the present study, false tendons and bundle branch regions of the bovine heart were examined using tyrosine hydroxylase (TH) immunohistochemistry and the glyoxylic acid induced catecholamine (CA) fluorescence method for demonstration of sympathetic nerve fibers. Acetylcholinesterase (AChE) histochemistry was also applied. Some of the nerve fascicles in the false tendons were found to contain large numbers of sympathetic nerve fibers and such nerve fibers formed plexuses in the walls of arteries and arterioles in these structures. In both false tendons and bundle branches sympathetic nerve fibers 1) were non-homogeneously distributed in the conduction tissue, most regularly occurring in the channels of extracellular space that are present within the bundles of Purkinje fibres, and 2) showed the same pattern of distribution in relation to Purkinje fibre bundle surfaces as the AChE-positive nerve branches. The observations show that there is a substantial sympathetic innervation in false tendons. The final distribution of the nerve fibers in these structures and in the bundle branches are discussed in relation to what is known of tissue morphology and the occurrence of sympathetic nerve influences in these regions. In the present study, previous CA-fluorescence observations of a "marked" sympathetic innervation in bundle branch regions, in terms of the presence of sympathetic nerve fibers in nerve fascicles and vessel walls, were also corroborated by the application of TH-immunohistochemistry.

Cable analysis in quiescent and active sheep Purkinje fibres

Cable properties of sheep cardiac Purkinje fibres were studied under resting and paced conditions. Standard micro-electrode techniques were used to apply intracellular current pulses and record the resultant voltage changes at various distances from the current input. In a parallel set of experiments, fibre dimensions were measured after freezing and serial sectioning. Fibres selected on the basis of a cylindrical appearance had approximately uniform cross-sectional diameters which varied +/- 12% along their length. Electrotonic potentials recorded at rest and in diastole (under conditions that minimized diastolic depolarization) adhered quite closely to the behaviour expected for a unidimensional cable provided voltages were recorded greater than or equal to one fibre diameter from the current source. The unidimensional space constant, input resistance, and membrane time constant were significantly larger during quiescence than in diastole. These differences were accounted for by a 90% increase in membrane resistance at rest. There was no significant change in internal longitudinal resistance nor membrane capacitance associated with activity. The voltage distribution close to the current input (i.e. within one fibre diameter) strongly deviated from the theoretical three-dimensional voltage decay expected for a homogeneous cylinder. This finding suggests that the transverse resistance to current flow is much greater than the longitudinal resistance. The anisotropic behaviour within the cardiac Purkinje fibre may explain several previous observations: (i) the lack of a relationship between conduction velocity and fibre diameter; and (ii) the much shorter liminal length for excitation in Purkinje fibres than for point-stimulated squid axons.

Differentiation of the atrioventricular node, the atrioventricular bundle and the bundle branches in the bovine heart: an immunohistochemical and enzyme histochemical study

The previous observations of differences between different cardiac regions (ventricular myocardium, atrial myocardium, Purkinje fibre system) with respect to the maturation of the M-line region and the establishment of mature metabolic characteristics, have been extended. It was found that M-line maturation proceeds differently also between different regions of the conduction system. The M-line proteins, myomesin and MM-creatine kinase, were detected earlier, by means of immunohistochemistry, in the AV bundle and bundle branch cells than in the AV node cells. Also, a difference was observed in large foetuses. Striations in the AV node were less evident than in the AV bundle and the bundle branches in sections incubated with antibodies against myomesin as well as against MM-creatine kinase. Using enzyme histochemistry it was observed that the differences in metabolic properties between the AV node, the AV bundle and the bundle branches on the one hand, and the ordinary myocardium on the other, of adult hearts, are not established at the early stages. No clear difference in activity of succinate dehydrogenase was seen between the conduction tissues and the ordinary myocardium in the foetal hearts, while the conduction tissues showed a lower activity in the adult hearts. Furthermore, the pattern of activity of mitochondrial glycerol-3-phosphate dehydrogenase between the conduction tissues and the atrial and ventricular myocardium was quite different in early foetal stages compared with the adult stage.

Cardiac conducting system of Gallus domesticus with special reference to the atrial bundles

Gross anatomy and histology of the cardiac conducting system of Gallus domesticus has been studied. Detailed histology of the atrium revealed for the first time, in the avian heart, the presence of three atrial bundles which communicate between the sinuatrial node and the atrioventricular node. Purkinje fibres in the subendocardium of the right atrium are observed. An atrioventricular segment comprising of the posterior end of the interatrial septum and the atrioventricular nodal region has been reported in which the three atrial bundles converge. The role of the atrial bundles in the cardiac contraction has been discussed.