Helical Ventricular Myocardial Band Research Articles

Using Systolic Local Mechanical Load to Predict Fiber Orientation in Ventricles.

A simple rule adopted for myofiber reorientation in the ventricles is pursued by taking the microscopic branching network of myocytes into account. The macroscopic active tension generated on the microscopic branching structure is modeled by a multidirectional active stress tensor, which is defined as a function of the strains in the branching directions. In our reorientation algorithm, the principal direction of the branching network is updated so that it turns in the direction of greater active tension in the isovolumetric systole. Updates are performed step-by-step after the mechanical equilibrium has been attained with the current fiber structure. Starting from a nearly flat distribution of the principal fiber orientation along the circumferential direction, the reoriented fiber helix angles range from 70 to 40° at epicardium and from 60 to 80° at endocardium, in agreement with experimental observations. The helical ventricular myocardial band of Torrent-Guasp’s model and the apical spiral structure of Rushmer’s model are also reconstructed by our algorithm. Applying our algorithm to the infarcted ventricle model, the fiber structure near the infarcted site is remodeled so that the helix angle becomes steeper with respect to the circumferential direction near the epicardial surface. Based on our numerical analysis, we draw the following conclusions. (i) The multidirectional active tension based on the microscopic branching network is potentially used to seek tighter connection with neighboring aggregates. (ii) The thickening and thinning transitions in response to active tension in each myocyte allow the macroscopic principal fiber orientation of the microscopic branching network to move toward the direction of greater active tension. (iii) The force–velocity relationship is the key factor in transferring the fiber shortening strain to the magnitude of active tensions used in the myofiber reorientation. (iv) The algorithm naturally leads to homogeneity in the macroscopic active tension and the fiber shortening strain, and results in near-optimal pumping performance. (v) However, the reorientation mechanism may degrade the pumping performance if there is severely inhomogeneous contractility resulting from infarction. Our goal is to provide a tool to predict the fiber architecture of various heart disease patients for numerical simulations of their treatment plans.

An organosynthetic dynamic heart model with enhanced biomimicry guided by cardiac diffusion tensor imaging.

The complex motion of the beating heart is accomplished by the spatial arrangement of contracting cardiomyocytes with varying orientation across the transmural layers, which is difficult to imitate in organic or synthetic models. High-fidelity testing of intracardiac devices requires anthropomorphic, dynamic cardiac models that represent this complex motion while maintaining the intricate anatomical structures inside the heart. In this work, we introduce a biorobotic hybrid heart that preserves organic intracardiac structures and mimics cardiac motion by replicating the cardiac myofiber architecture of the left ventricle. The heart model is composed of organic endocardial tissue from a preserved explanted heart with intact intracardiac structures and an active synthetic myocardium that drives the motion of the heart. Inspired by the helical ventricular myocardial band theory, we used diffusion tensor magnetic resonance imaging and tractography of an unraveled organic myocardial band to guide the design of individual soft robotic actuators in a synthetic myocardial band. The active soft tissue mimic was adhered to the organic endocardial tissue in a helical fashion using a custom-designed adhesive to form a flexible, conformable, and watertight organosynthetic interface. The resulting biorobotic hybrid heart simulates the contractile motion of the native heart, compared with in vivo and in silico heart models. In summary, we demonstrate a unique approach fabricating a biomimetic heart model with faithful representation of cardiac motion and endocardial tissue anatomy. These innovations represent important advances toward the unmet need for a high-fidelity in vitro cardiac simulator for preclinical testing of intracardiac devices.

What Is the Heart? Anatomy, Function, Pathophysiology, and Misconceptions.

Cardiac dynamics are traditionally linked to a left ventricle, right ventricle, and septum morphology, a topography that differs from the heart’s five-century-old anatomic description of containing a helix and circumferential wrap architectural configuration. Torrent Guasp’s helical ventricular myocardial band (HVMB) defines this anatomy and its structure, and explains why the heart’s six dynamic actions of narrowing, shortening, lengthening, widening, twisting, and uncoiling happen. The described structural findings will raise questions about deductions guiding “accepted cardiac mechanics”, and their functional aspects will challenge and overturn them. These suppositions include the LV, RV, and septum description, timing of mitral valve opening, isovolumic relaxation period, reasons for torsion/twisting, untwisting, reasons for longitudinal and circumferential strain, echocardiographic sub segmentation, resynchronization, RV function dynamics, diastolic dysfunction’s cause, and unrecognized septum impairment. Torrent Guasp’s revolutionary contributions may alter future understanding of the diagnosis and treatment of cardiac disease.

Will the real ventricular architecture please stand up?

Ventricular twisting, essential for cardiac function, is attributed to the contraction of myocardial helical fibers. The exact relationship between ventricular anatomy and function remains to be determined, but one commonly used explanatory model is the helical ventricular myocardial band (HVMB) model of Torrent‐Guasp. This model has been successful in explaining many aspects of ventricular function, (Torrent‐Guasp et al. Eur. J. Cardiothorac. Surg., 25, 376, 2004; Buckberg et al. Eur. J. Cardiothorac. Surg., 47, 587, 2015; Buckberg et al. Eur. J. Cardiothorac. Surg. 47, 778, 2015) but the model ignores important aspects of ventricular anatomy and should probably be replaced. The purpose of this review is to compare the HVMB model with a different model (nested layers). A complication when interpreting experimental observations that relate anatomy to function is that, in the myocardium, shortening does not always imply activation and lengthening does not always imply inactivation.

LEFT VENTRICULAR ROTATION, TWIST AND UNTWIST: PHYSIOLOGICAL ROLE AND CLINICAL RELEVANCE

The helical ventricular myocardial band of Torrent-Guasp is a new concept, which provides strong grounds for reconciliation of some important aspects in cardiovascular medicine. Oblique fiber orientation provides left ventricular rotation, which in addition to radial thickening and longitudinal shortening, is predicted as an essential component of the effective left ventricular pumping. Left ventricular rotation can be measured in clinical practice noninvasively using echocardiography and this provides new opportunities for the assessment of different aspects of left ventricular mechanical function.

Pumping potential of a two-layer left-ventricle-like flexible-matrix-composite structure

The pumping potential, defined as the amount of relative volume reduction engendered by an applied load (relative input stroke), is an important criterion for flexible-body pumps. It is introduced as a measure of the volumetric efficiency of pumping. The PP of many flexible-matrix-composite (FMC) structures has been considered in the literature. Most recently, the PP of a single-layer left-ventricle-like FMC structure, based on the helical ventricular myocardial band hypothesis, has been investigated. A PP of 1.67–1.9 has been achieved. Though reasonably high, it is much smaller than that of the actual left ventricle of the heart (3.33–4.0). In here this work is extended to a two-layer left-ventricle-like FMC structure, which adopts a more accurate fiber orientation. The PP of the two-layer left-ventricle-like FMC structure is determined experimentally and analytically. A FMC flat band is created then twisted and looped to form the flexible-body structure. The analytical investigation using the ANSYS software engenders a PP of 2.5, and the experimental one using a PU/SMA (shape memory alloy) FMC renders a PP of 2.85. These higher values of PP indicate that accurate fiber-orientation in heart-mimicking flexible-structural pumps is very important.

Ventricular structure-function relations in health and disease: Part I. The normal heart.

The heart's structure-function relationships explain normal cardiac dynamics and clarify how they are disrupted by disease. For 500 years, anatomists described circumferential and helical cardiac fibres, yet disagreed about their relationships. One current model is attributed to Torrent Guasp who described functional pathways, the helical ventricular myocardial band (HVMB) with two interconnected loops: an outer basal loop with transverse fibres surrounds an inner apical helical loop that is composed of oblique descending and ascending segments that create a conical apical vortex. This review addresses the potential role of the HVMB in explaining the mechanics of isovolumic contraction, ejection, post-ejection isovolumic interval, rapid filling, torsion and recoiling. During the post-ejection isovolumic interval, a ∼ 90-ms hiatus exists between the end of contraction of the descending and the ascending segments. Compromise of this hiatus by disease disturbs the interdependence between torsion and 'untwisting' and impairs cardiac function. The validity of conventional expressions such as isovolumic relaxation, hyperechogenic septal line, untwisting and mitral valve opening will be revisited.

Ventricular structure-function relations in health and disease: part II. Clinical considerations.

Normal cardiac function of the left and right ventricles, together with the septum, is related to form/function interactions within the helical ventricular myocardial band. This knowledge is a prerequisite to understanding form/function interactions in diseases and for planning new treatments. Topics discussed include congestive heart failure in dilated hearts of ischaemic, valvar or nonischaemic origin as well as diastolic dysfunction. Similar thinking underlies novel treatments for dyssynchrony in pacing, together with focusing upon varying global left or right ventricular anatomy to correct mitral and tricuspid insufficiency caused by tethering of the leaflets. The septum is the lion of the right ventricle and insight is provided into offsetting septal damage during cardiac surgery, rebuilding its anatomical structure in post-tetralogy pulmonary insufficiency, as well as rectifying its dysfunction by decompression in patients with a left ventricular assist device.

Right ventricular architecture responsible for mechanical performance: Unifying role of ventricular septum

The right ventricle (RV) is composed of a free wall containing a wrap-around circumferential muscle at its base and a septum composed of helical fibers that are oblique and cross each other at 60° angles. This structure is defined by the helical ventricular myocardial band and defines RV function because the wrap-around transverse fibers constrict or compress to cause the bellows motion responsible for 20% of RV output, whereas the oblique fibers determine shortening and lengthening that produces 80% of RV systolic function. Clinical shortening is quantified by tricuspid annular plane systolic excursion and measured by echocardiography. Destruction of the free wall by electrocautery or patch replacement does not alter RV function if the septum is intact. Conversely, septal damage causes RV dysfunction if pulmonary vascular resistance is increased. The interaction between structure and function to cause RV failure and how these factors become corrected is defined for RV failure, RV relationship to LV failure, resynchronization, pacing, RV dysplasia, left ventricular assist device, intraoperative septal injury during myocardial protection, the septal role in tricuspid insufficiency, pharmacologic decisions on altering pulmonary vascular resistance in RV failure, congenital heart disease, and adult heart disease is considered in this overview. These structure-function relationships emphasize why clinical decisions must be based on knowledge of normality, recognizing how disease offsets normality, and introducing actions that rebuild normality.

Assessment of the helical ventricular myocardial band using standard echocardiography.

The purpose of this study was to assess the feasibility of morphological and functional evaluation of the helical ventricular myocardial band using standard echocardiographic images. Echocardiographic data were obtained from 132 normal children. We attempted to identify the echogenic bright line serving as the boundary between the ascending and descending segments in the ventricular septum, and between the left and ascending segments in the left ventricular inferior wall in the helical myocardial band model proposed by Torrent-Guasp. Myocardial deformations on both sides of the bright line were compared using speckle tracking echocardiography. The bright line separating the ascending from descending segment was visible in the mid-ventricular septum in the four-chamber view in all subjects. This echogenic boundary was observed obliquely in the parasternal short-axis view in 116 subjects (87.9%). There was no significant difference in peak longitudinal or circumferential strain between the ascending and descending segments. However, the time from the QRS onset to peak circumferential strain was significantly lower in the descending than ascending segment (394.5 ± 37.0 vs. 432.7 ± 33.1 ms, P < 0.0001), whereas there was no significant difference in the time to peak longitudinal strain (394.4 ± 26.4 vs. 393.2 ± 24.1 ms). The bright line between the left and ascending segment was detected in the short-axis view from the subcostal region in 86 subjects (65.2%). The time to peak circumferential strain was significantly lower in the left than ascending segment (380.1 ± 32.0 vs. 435.7 ± 37.9 ms, P < 0.0001). There is a helical ventricular myocardial band that can be observed in standard echocardiographic images.

Estudio tractográfico de la anatomía helicoidal del miocardio ventricular mediante resonancia magnética por tensor de difusión

Introduction and objectivesDeeper understanding of the myocardial structure linking the morphology and function of the heart would unravel crucial knowledge for medical and surgical clinical procedures and studies. Several conceptual models of myocardial fiber organization have been proposed but the lack of an automatic and objective methodology prevented an agreement. We sought to deepen this knowledge through advanced computer graphical representations of the myocardial fiber architecture by diffusion tensor magnetic resonance imaging. MethodsWe performed automatic tractography reconstruction of unsegmented diffusion tensor magnetic resonance imaging datasets of canine heart from the public database of the Johns Hopkins University. Full-scale tractographies have been built with 200 seeds and are composed by streamlines computed on the vector field of primary eigenvectors at the diffusion tensor volumes. We also introduced a novel multiscale visualization technique in order to obtain a simplified tractography. This methodology retains the main geometric features of the fiber tracts, making it easier to decipher the main properties of the architectural organization of the heart. ResultsOutput analysis of our tractographic representations showed exact correlation with low-level details of myocardial architecture, but also with the more abstract conceptualization of a continuous helical ventricular myocardial fiber array. ConclusionsObjective analysis of myocardial architecture by an automated method, including the entire myocardium and using several 3-dimensional levels of complexity, reveals a continuous helical myocardial fiber arrangement of both right and left ventricles, supporting the anatomical model of the helical ventricular myocardial band described by F. Torrent-Guasp.Full English text available from:www.revespcardiol.org/en

Helical Structure of the Cardiac Ventricular Anatomy Assessed by Diffusion Tensor Magnetic Resonance Imaging With Multiresolution Tractography

Introduction and objectivesDeeper understanding of the myocardial structure linking the morphology and function of the heart would unravel crucial knowledge for medical and surgical clinical procedures and studies. Several conceptual models of myocardial fiber organization have been proposed but the lack of an automatic and objective methodology prevented an agreement. We sought to deepen this knowledge through advanced computer graphical representations of the myocardial fiber architecture by diffusion tensor magnetic resonance imaging. MethodsWe performed automatic tractography reconstruction of unsegmented diffusion tensor magnetic resonance imaging datasets of canine heart from the public database of the Johns Hopkins University. Full-scale tractographies have been built with 200 seeds and are composed by streamlines computed on the vector field of primary eigenvectors at the diffusion tensor volumes. We also introduced a novel multiscale visualization technique in order to obtain a simplified tractography. This methodology retains the main geometric features of the fiber tracts, making it easier to decipher the main properties of the architectural organization of the heart. ResultsOutput analysis of our tractographic representations showed exact correlation with low-level details of myocardial architecture, but also with the more abstract conceptualization of a continuous helical ventricular myocardial fiber array. ConclusionsObjective analysis of myocardial architecture by an automated method, including the entire myocardium and using several 3-dimensional levels of complexity, reveals a continuous helical myocardial fiber arrangement of both right and left ventricles, supporting the anatomical model of the helical ventricular myocardial band described by F. Torrent-Guasp.

Ventricular Torsion and Untwisting: Further Insights into Mechanics and Timing Interdependence: A Viewpoint

Ventricular torsion and untwisting are essential for normal ventricular function and their mechanisms are related to the temporal responses of the helical and circular muscle fibers that comprise cardiac architecture. Explanation of the presystolic isovolumic contraction (IVC) period is essential for analysis of these interactions. Structural and imaging studies by magnetic resonance, speckle tracking, velocity vector encoding, and sonomicrometer crystals are described to define why and how different muscular components contract asynchronously. Mechanical and functional relationships are described for pre-systolic IVC, torsion, postejection isovolumic interval, and rapid and slow filling. Circular fibers dominate to cause pre- and posttwisting global counterclockwise and clockwise movement, and helical fibers govern torsion whereby the base rotates clockwise and apex counterclockwise; untwisting cannot begin until torsion is completed. Prolonged torsion extends into the postejection isovolumic interval and delays untwisting, and is caused by prolonged contraction of the right-handed helical arm or descending segment of the helical ventricular myocardial band that narrows the ∼80 ms "timing hiatus" between end of shortening of the descending and the ascending segment or left-handed arm of the helical muscle. Longer torsion duration by this mechanism becomes the common theme for unbalanced torsion and untwisting in diastolic dysfunction, physiological, structural, and electrical disease processes, whose management may be guided by changing the interconnected reasons for these adverse mechanical and timing factors.

Myocyte injury along myofibers in left ventricular remodeling after myocardial infarction

Left ventricular (LV) remodeling following myocardial infarction (MI) is considered to contribute to cardiac dysfunction. Though myofiber organization is a key component of cardiac structure, functional and anatomical features of injured myofiber during LV remodeling have not been fully defined. We investigated myocyte injury after acute MI in a mouse model. Mice were subjected to surgical coronary occlusion/reperfusion by left anterior descending coronary artery (LAD) ligation and examined at 1 week and 4 weeks post-MI. Magnetic resonance imaging (MRI) analysis demonstrated a significant decrease in systolic regional wall thickening (WT) in the border and remote zones at 4 weeks post-MI compared to that at 1 week post-MI (-86% in border zone, P<0.05, and -77% in remote zone, P<0.05). Histological assays demonstrated that a broad fibrotic scar extended from the initial infarct zone to the remote zone along mid-circumferential myofibers. Of particular note was the fact that no fibrosis was found in longitudinal myofibers in the epi- and endo-myocardium. This pattern of the scar formation coincided with the helical ventricular myocardial band (HVMB) model, introduced by Torrent-Guasp. MRI analysis demonstrated that the extension of the fibrotic scar along the band might account for the progression in cardiac dysfunction during LV remodeling.

Reconstruction of the Architecture of Ventricular Myocardial Fibers in Ex Vivo Human Hearts

The 3-dimensional arrangement of the ventricular mass has been controversial. The aim of the present study was to investigate the macroarchitecture of ventricular myocardial fibers and to analyze whether it is consistent with the helical ventricular myocardial band (HVMB) hypothesis. Eight excised human hearts were scanned by diffusion tensor magnetic resonance imaging (DT-MRI). Fiber tracking was then used with the DT-MRI data to reconstruct and visualize the positions of the myocardial fibers to reveal the architecture of ventricular myocardial fibers. The left ventricular myocardial fibers were found to consist of 2 crossed populations that were approximately normal from the epicardium to the endocardium in the tangent plane. The myocardial fibers in the middle of the myocardium had a smooth, linear angular rotation. The ventricular myocardial fibers maintained complete continuity and specific orientations that corresponded to the HVMB structure. The architecture of the ventricular myocardial fibers in the human heart conforms to the HVMB structural hypothesis.

Investigation on the Structure of Ventricular Mass Using Magnetic Resonance Diffusion Tensor Imaging

The 3-dimensional arrangement of the ventricular mass remains controversial. In this study, we used magnetic resonance diffusion tensor imaging (MRDTI) in an attempt to determine whether the ventricular mass is arranged in the form of a helical ventricular myocardial band (HVMB) and what the geometrical features of the HVMB are in postmortem pig hearts. Ten pig hearts were harvested from the slaughterhouse, and their whole-body MR images were obtained. The data were obtained via DTI by single-shot echo planar imaging and sensitivity encoding. The pig hearts were scanned with single-shot echo planar imaging and sensitivity-encoding scans (TE/TRZ78.5/10000 ms) with diffusion-sensitized gradients (b = 800 s/mm2) along 6 directions. Color-coded imaging and fiber-tracking techniques were used to investigate the arrangement of the fibers of ventricular mass on a GE Healthcare Advantage Workstation (Microsoft Windows). Color-coded images showed that the ventricular wall in each section was uniformly divided into 3 layers (subendocardial, middle, and subepicardial) in all samples. Fiber tracking showed that the subendocardial layer ran obliquely from base to apex, turned a circle, and transformed into the middle layer at the apex, and then ran obliquely upward. The ventricular mass was arranged in the form of double-helical coils. The crossing angle between subendocardial layer and middle layer was nearly vertical. Results of our investigation with MRDTI support the theory of Torrent-Guasp et al that the ventricular mass is arranged in the form of an HVMB.

Cardiac Mechanics Revisited

The keynote to understanding cardiac function is recognizing the underlying architecture responsible for the contractile mechanisms that produce the narrowing, shortening, lengthening, widening, and twisting disclosed by echocardiographic and magnetic resonance technology. Despite background knowledge of a spiral clockwise and counterclockwise arrangement of muscle fibers, issues about the exact architecture, interrelationships, and function of the different sets of muscle fibers remain to be resolved. This report (1) details observed patterns of cardiac dynamic directional and twisting motions via multiple imaging sources; (2) summarizes the deficiencies of correlations between ventricular function and known ventricular muscle architecture; (3) correlates known cardiac motions with the functional anatomy within the helical ventricular myocardial band; and (4) defines an innovative muscular systolic mechanism that challenges the previously described concept of "isovolumic relaxation." This new knowledge may open new doors to treating heart failure due to diastolic dysfunction.

Structure and function relationships of the helical ventricular myocardial band

Understanding cardiac function requires knowledge of the architecture responsible for the normal actions of emptying and filling. Newer imaging methods are surveyed to characterize directional (narrowing, shortening, lengthening, and widening) and twisting motions. These movements are defined and then compared with a spectrum of models to introduce a useful "functional anatomy" that explains cardiac spatial and temporal relationships. The sequential nature of normal contraction differs from a synchronous beat. The prior concept of constriction is replaced by understanding that clockwise and counterclockwise helical motions are necessary to cause the predominant twisting motion. The helical ventricular myocardial band model of Torrent-Guasp fulfills the architectural structure to define normal function. Expansion of information from this model allows novel understanding of mechanisms that explains why a component of ventricular suction involves a systolic event, clarifies septum function, determines diastolic dysfunction, introduces new treatments, shows how knowledge of the helical structure influences understanding of atrioventricular and biventricular pacing, and creates novel methods for introducing septal pacing stimuli. Further testing of these spatial anatomic concepts is needed to create a more accurate understanding of the architectural mechanisms that underlie cardiac dynamics to address future problems in unhealthy hearts.

Ventricular Structures Must Be Understood during Surgical Restoration for Heart Failure

Scandinavian Journal of Surgery 96: 164–176, 2007 Ventricular structures must be understood during surgical restoration for heart failure G. D. Buckberg Division of Cardiothoracic Surgery, David Geffen School of Medicine at UCLA, Los Angeles, California, U.S.A. California Institute of Technology, Pasadena, California, U.S.A. Key words: Ventricular restoration; helical ventricular myocardial band; ischemic cardiomyopathy; non-ischemic cardiomyopathy; valvular cardiomyopathy; ��� ���� �� ventricular fiber orientation; SAVE; Pacopexy Introduction The central theme of cardiac surgery is that alteration of structure improves function, and this concept is fundamental during surgical restoration of dilated hearts in cardiac failure. Adherence to this goal re- quires a clear knowledge of the normal conical ven- tricular form, recognition of how disease caused ar- chitectural deterioration towards a spherical shape that impairs performance, and why rebuilding struc- ture returns function towards normal. The keynote knowledge governing surgical decisions relates to understanding how operative modifications affect left ventricle spatial relationships. The conical pattern of normal cardiac size and shape is well known and the underlying spatial ar- rangements are closely linked to the helical ventricu- lar myocardial band (1) comprised of a surrounding wrap of the basal loop with transverse fibers and an apical loop of reciprocal oblique fibers forming a spi- ral vortex at the apex as shown in Fig. 1A. The spher- ical configuration of the enlarged global ventricle widens the apical loop (Fig. 1B) by making the oblique apical loop fibers develop a transverse orien- tation that more closely resembles the horizontal fiber orientation of the basal loop. The bioengineering infrastructure for this mechan- ical change in size and shape reduces ejection frac- tion, which is 60% with oblique fiber direction and lowered to 30% when fiber orientation is transverse (2). This architectural disadvantage limits deforma- tion (or twisting capacity that is visible at operation or by MRI recordings) to negatively impact the natu- ral increment from mid wall to apex (3) and thus impair sequential contraction efficiency which limits performance by producing mechanical dyssynchrony. Furthermore, secondary mitral incompetence follows chamber widening and supplemental modifications during ventricular restoration that offset these c hanges will be described. Symptoms of arrhythmia and cardiac failure, co- equal causes of mortality, worsen as ventricular size progressively increases. Surgical correction of the di- lated heart requires changing the spherical configura- tion architecture into a more normal elliptical form (4), and simultaneously rebuilds more normal inter- nal spatial components. The primary and secondary geometric events will be summarized and serve as the basis of developing procedures used to alter ven- tricular size, shape, the apical tip, papillary muscle inter space basal width, and inferior wall toward nor- mal. The disease and the ventricle and secondary mitral changes Enlarged ventricular size with the spherical shape becomes a unifying theme of dilated cardiomyopathy. (Fig. 2) The underlying pathologic processes respon- sible for spherical configuration range from a) i schemic disease causing an extensive postmyocar- dial infarction scar leading to secondary stretch of compensatory remote fibers within unscarred muscle, or global stretch from multiple small scars, but with- out a large asynergic region, b) non-ischemic disease causing distention stretch from volume loading fol- lowing mitral or aortic valve incompetence, or c) non- ischemic cardiomyopathy destroying segments of re gional myocardial fibers with secondary distention of remaining hypertrophied and thicker viable muscle. The secondary changes that cause associated mi- tral incompetence include ventricular stretch that alters leaflet coaptation, widening of the mitral an- Correspondence: Gerald Buckberg, M.D. David Geffen School of Medicine at UCLA Division of Cardiothoracic Surgery 62-258 Center for the Health Sciences Los Angeles, CA 90095-1701 U.S.A. Email: gbuckberg@mednet.ucla.edu

Analysis of the fiber architecture of the heart by quantitative polarized light microscopy. Accuracy, limitations and contribution to the study of the fiber architecture of the ventricles during fetal and neonatal life☆

To address the advantages and drawbacks of quantitative polarized light microscopy for the study of myocardial cell orientation and to identify its contribution in the field. Quantitative polarized light microscopy allows to measure the orientation of myocardial fibers into the ventricular mass. For each pixel of a horizontal section, this orientation is the mean value of the directions of all myosin filaments contained in the thickness of the section for each pixel of the section and is accounted for by two angles, the azimuth angle, which is the angle of the fiber in the plane of the section, and the elevation angle, which measures the way the fiber escapes from the section. The azimuth is accurately measured, and its range of definition is complete from 0 degrees to 180 degrees . The elevation angle can be defined only in the range 0 degrees to 90 degrees . It is accurately measured between 20 degrees and 70 degrees . From 0 degrees to 20 degrees , there is a systematic bias raising the measured values, and from 70 degrees to 90 degrees , the angle is not accurately measured. With this method, we validated Streeter's conjecture concerning the architecture of the left ventricle. We formulated a pretzel conjecture about the fiber architecture of the whole ventricular mass during fetal period. In our model, elaborated by visual analysis of registered maps of orientation, the fibers run like geodesics on a nested set of 'pretzels'. Next, the validity of the helical ventricular myocardial band model of Torrent-Guasp has been examined. It appears that the band model does not account for the patterns observed in our data during the fetal period. However, after the major events of postnatal cardiovascular adaptation, our data can neither discard nor confirm Torrent-Guasp's model. Present limitations of quantitative polarized light analysis can neither confirm nor discard the existing models of fiber orientation in the whole ventricular mass after the neonatal period. However, the problems of mathematical and experimental validation of these two models have been posed in a rigorous manner. Non-ambiguous fiber tracking and demonstration of these models will require significant improvement of the definition range of the elevation angle that should be extended to 180 degrees .