Reverse re-modelling chronic heart failure by reinstating heart rate variability.

Heart rate variability (HRV) is a crucial indicator of cardiovascular health. Low HRV is correlated with disease severity and mortality in heart failure. Heart rate increases and decreases with each breath in normal physiology termed respiratory sinus arrhythmia (RSA). RSA is highly evolutionarily conserved, most prominent in the young and athletic and is lost in cardiovascular disease. Despite this, current pacemakers either pace the heart in a metronomic fashion or sense activity in the sinus node. If RSA has been lost in cardiovascular disease current pacemakers cannot restore it. We hypothesized that restoration of RSA in heart failure would improve cardiac function. Restoration of RSA in heart failure was assessed in an ovine model of heart failure with reduced ejection fraction. Conscious 24 h recordings were made from three groups, RSA paced (n = 6), monotonically paced (n = 6) and heart failure time control (n = 5). Real-time blood pressure, cardiac output, heart rate and diaphragmatic EMG were recorded in all animals. Respiratory modulated pacing was generated by a proprietary device (Ceryx Medical) to pace the heart with real-time respiratory modulation. RSA pacing substantially increased cardiac output by 1.4 L/min (20%) compared to contemporary (monotonic) pacing. This increase in cardiac output led to a significant decrease in apnoeas associated with heart failure, reversed cardiomyocyte hypertrophy, and restored the T-tubule structure that is essential for force generation. Re-instating RSA in heart failure improves cardiac function through mechanisms of reverse re-modelling; the improvement observed is far greater than that seen with current contemporary therapies. These findings support the concept of re-instating RSA as a regime for patients who require a pacemaker.

Myocardial material parameter estimation: a non-homogeneous finite element study from simple shear tests.

The passive material properties of myocardium play a major role in diastolic performance of the heart. In particular, the shear behaviour is thought to play an important mechanical role due to the laminar architecture of myocardium. We have previously compared a number of myocardial constitutive relations with the aim to extract their suitability for inverse material parameter estimation. The previous study assumed a homogeneous deformation. In the present study we relaxed the homogeneous assumption by implementing these laws into a finite element environment in order to obtain more realistic measures for the suitability of these laws in both their ability to fit a given set of experimental data, as well as their stability in the finite element environment. In particular, we examined five constitutive laws and compare them on the basis of (i) "goodness of fit": how well they fit a set of six shear deformation tests, (ii) "determinability": how well determined the objective function is at the optimal parameter fit, and (iii) "variability": how well determined the material parameters are over the range of experiments. Furthermore, we compared the FE results with those from the previous study.It was found that the same material law as in the previous study, the orthotropic Fung-type "Costa-Law", was the most suitable for inverse material parameter estimation for myocardium in simple shear.

Internet Applications for Computational Biology, the CMISS Web Browser Extension and and Use in Education.

The Internet is becoming increasingly accessible and new technologies are enabling the delivery of more features to end users. It is therefore increasingly compelling to develop technology to facilitate the delivery of educational content and computational tools via the Internet. Here we report on the Internet enabling of the CMISS package as a Web browser extension, and its use in a custom online teaching application for medical students.

Intramural multisite recording of transmembrane potential in the heart.

Heart surface optical mapping of transmembrane potentials has been widely used in studies of normal and pathological heart rhythms and defibrillation. In these studies, three-dimensional spatio-temporal events can only be inferred from two-dimensional surface potential maps. We present a novel optical system that enables high fidelity transmural recording of transmembrane potentials. A probe constructed from optical fibers is used to deliver excitation light and collect fluorescence from seven positions, each 1 mm apart, through the left ventricle wall of the rabbit heart. Excitation is provided by the 488-nm line of a water-cooled argon-ion laser. The fluorescence of the voltage-sensitive dye di-4-ANEPPS from each tissue site is split at 600 nm and imaged onto separate photodiodes for later signal ratioing. The optics and electronics are easily expandable to accommodate multiple optical probes. The system is used to record the first simultaneous measurements of transmembrane potential at a number of sites through the intact heart wall.

A triaxial-measurement shear-test device for soft biological tissues.

A novel shear-test device for soft biological tissue, capable of applying simple shear deformations simultaneously in two orthogonal directions while measuring the resulting forces generated in three axes, is described. We validated the device using a synthetic gel, the properties of which were ascertained from independent tensile and rotational shear tests. Material parameters for the gel were fitted using neo-Hookean analytical solutions to the independent test data, and these matched the results from the device. Preliminary results obtained with rat septal myocardium are also presented to demonstrate the feasibility of the apparatus in determining the shear characteristics of living tissue.

The effect of synthetic patch repair of coarctation on regional deformation of the aortic wall.

BACKGROUND: A long-term complication of synthetic patch repair of coarctation is true aneurysm formation. AIM: An in vitro study was undertaken to determine the effects of patch angioplasty on aortic geometry and strain adjacent to the patch. METHODS: Segments of human descending thoracic aorta were subject to 10 pressure loading cycles (10-120 mm Hg; 1.36-16.32 kPa) before and after simulated coarctation repair with a synthetic patch. Local curvature and strain were estimated by fitting a geometric model to reconstructed three-dimensional surface marker points. RESULTS: In the control aortas, when pressure increased from 11 +/- 1.0 to 124 +/- 4.0 mm Hg (1.5 +/- 0.14 to 16.86 +/- 0.54 kPa), average circumferential curvature decreased from 0.1543 +/- 0.03 to 0.1065 +/- 0.03 mm(-1). The average major extension reached a maximum of 1.43 +/- 0.08. After patch implantation, the average circumferential curvature was reduced relative to control at all pressures. Average major extensions were significantly greater than paired control values and reached a maximum of 1.55 +/- 0.08 at 122 +/- 4.0 mm Hg (16.59 +/- 0. 54 kPa). Substantial strain inhomogeneity was observed and major extensions were greatest immediately adjacent to the patch. INFERENCE: Synthetic patch repair of coarctation of the aorta increases wall strain and produces significant regional gradients in strain. With control aortic material properties there may be a substantial increase in wall stress immediately adjacent to the aorta, which could lead to true aneurysm formation.

3-Dimensional configuration of perimysial collagen fibres in rat cardiac muscle at resting and extended sarcomere lengths.

1. We have used fluorescence confocal laser scanning microscopy to attain the three-dimensional (3-D) microstructure of perimysial collagen fibres over the range of sarcomere lengths (1.9-2.3 micrometers) in which passive force of cardiac muscle increases steeply. 2. A uniaxial muscle preparation (right ventricular trabecula of rat) was used so that the 3-D collagen configuration could be readily related to sarcomere length. Transmission electron microscopy showed that these preparations were structurally homologous to ventricular wall muscle. 3. Trabeculae were mounted on the stage of an inverted microscope and fixed at various sarcomere lengths. After a trabecula was stained with the fluorophore Sirius Red F3BA and embedded in resin, sequential optical sectioning enabled 3-D reconstruction of its perimysial collagen fibres. The area fraction of these fibres, determined from the cross-sections of seven trabeculae, was 10.5 +/- 3.9 % (means +/- s.d.). 4. The reconstructed 3-D images show that perimysial collagen fibres are wavy (as distinct from coiled) cords which straighten considerably as the sarcomere length is increased from 1.85 +/- 0.06 micrometer (near-resting length) to 2.3 +/- 0.04 micrometer (means +/- s.d., n = 4). These observations are consistent with the notion that the straightening of these fibres is responsible for limiting extension of the cardiac sarcomere to a length of approximately 2.3 micrometers.

Extended confocal microscopy of myocardial laminae and collagen network.

Ventricular myocardium has a complex three-dimensional structure which has previously been inferred from two-dimensional images. We describe a technique for imaging the 3D organization of myocytes in conjunction with the collagen network in extended blocks of myocardium. Rat hearts were fixed with Bouin's solution and perfusion-stained with picrosirius red. Transmural blocks from the left ventricular free wall were embedded in Agar 100 resin and mounted securely in an ultramicrotome chuck. Confocal fluorescence laser scanning microscopy was used to obtain 3D images to a depth of 60 microns in a contiguous mosaic across the surface. Approximately 50 microns was then cut off the surface of the block with an ultramicrotome. This sequence was repeated 20 times. Images were assembled and registered in 3D to form an extended volume 3800 x 800 x 800 microns 3 spanning the heart wall from epicardium to endocardium. Examples are given of how digital reslicing and volume rendering methods can be applied to the resulting dataset to provide quantitative structural information about the 3D organization of myocytes, extracellular collagen matrix and blood vessel network of the heart.

Laminar structure of the heart: a mathematical model.

A mathematical description of cardiac anatomy is presented for use with finite element models of the electrical activation and mechanical function of the heart. The geometry of the heart is given in terms of prolate spheroidal coordinates defined at the nodes of a finite element mesh and interpolated within elements by a combination of linear Lagrange and cubic Hermite basis functions. Cardiac microstructure is assumed to have three axes of symmetry: one aligned with the muscle fiber orientation (the fiber axis); a second set orthogonal to the fiber direction and lying in the newly identified myocardial sheet plane (the sheet axis); and a third set orthogonal to the first two, in the sheet-normal direction. The geometry, fiber-axis direction, and sheet-axis direction of a dog heart are fitted with parameters defined at the nodes of the finite element mesh. The fiber and sheet orientation parameters are defined with respect to the ventricular geometry such that 1) they can be applied to any heart of known dimensions, and 2) they can be used for the same heart at various states of deformation, as is needed, for example, in continuum models of ventricular contraction.

Laminar structure of the heart: ventricular myocyte arrangement and connective tissue architecture in the dog.

We have studied the three-dimensional arrangement of ventricular muscle cells and the associated extracellular connective tissue matrix in dog hearts. Four hearts were potassium-arrested, excised, and perfusion-fixed at zero transmural pressure. Full-thickness segments were cut from the right and left ventricular walls at a series of precisely located sites. Morphology was visualized macroscopically and with scanning electron microscopy in 1) transmural planes of section and 2) planes tangential to the epicardial surface. The appearance of all specimens was consistent with an ordered laminar arrangement of myocytes with extensive cleavage planes between muscle layers. These planes ran radially from endocardium toward epicardium in transmural section and coincided with the local muscle fiber orientation in tangential section. Stereological techniques were used to quantify aspects of this organization. There was no consistent variation in the cellular organization of muscle layers (48.4 +/- 20.4 microns thick and 4 +/- 2 myocytes across) transmurally or in different ventricular regions (23 sites in 6 segments), but there was significant transmural variation in the coupling between adjacent layers. The number of branches between layers decreased twofold from subepicardium to midwall, whereas the length distribution of perimysial collagen fibers connecting muscle layers was greatest in the midwall. We conclude that ventricular myocardium is not a uniformly branching continuum but a laminar hierarchy in which it is possible to identify three axes of material symmetry at any point.

Transverse shear along myocardial cleavage planes provides a mechanism for normal systolic wall thickening.

Recent studies in humans and other species show that there is substantial transverse shear strain in the left ventricular myocardium, and others have shown transverse myocardial laminae separated by cleavage planes. We proposed that cellular rearrangement based on shearing along myocardial cleavage planes could account for > 50% of normal systolic wall thickening, since < 50% can be explained by increases in myocyte diameter. To test this hypothesis, we measured strains at two sites with different cleavage-plane anatomy in eight open-chest dogs. Columns of radiopaque markers were implanted in the left ventricular anterior free wall and septum. Markers were tracked with biplane cineradiography, and strains were quantified by using finite deformation techniques. Hearts were perfusion-fixed with glutaraldehyde, and cleavage-plane orientations at the bead sites were measured in three orthogonal planes. At subendocardial sites of the anterior left ventricular wall, where the cleavage planes approach the endocardium obliquely from the apical side of the surface normal in the longitudinal-radial plane (-67 +/- 11 degrees), systolic longitudinal-radial transverse shear (E23) was positive (0.14 +/- 0.08). At the septal sites where the subendocardial cleavage planes approach the endocardium obliquely from above the surface normal (44 +/- 12 degrees), E23 was negative (-0.12 +/- 0.08). The differences in cleavage-plane angle and E23 at the two sites were each highly significant (P < .0005). At both sites, the transverse shear strain accompanied substantial systolic wall thickening at the subendocardium (anterior, E33 = 0.44 +/- 0.16; septum, E33 = 0.22 +/- 0.14). These data are not representative of the behavior in midwall and outer wall sites, where cleavage-plane orientation was not consistently different between anterior left ventricle and septum. Our data indicate that rearrangement of myocytes by slippage along myocardial cleavage planes is in the correct direction and of sufficient magnitude in the subendocardium (inner third) to account for a substantial proportion (> 50%) of systolic wall thickening. Furthermore, three-dimensional reconstruction of the myocardial laminae and local comparison with maximum strain vectors indicate that for the inner third of the ventricular wall the maximum shear deformation is a result of relative sliding between myocardial laminae.

Impaired subendocardial function in tachycardia-induced cardiac failure.

Chronic rapid ventricular pacing (CRVP) in many experimental models induces ventricular dilatation, reduced ejection fraction, and symptomatic congestive heart failure. We have investigated transmural mechanical function in the left ventricular (LV) wall of five Hanford miniature swine before and after CRVP-induced failure. Three columns of radiopaque markers 1 mm in diameter were implanted in the anterior LV wall through a median sternotomy. A pair of LV pacing wires were sutured into the myocardium, a pneumatic cuff was placed around the inferior vena cava (IVC), and two fluid-filled Silastic catheters were implanted into the LV apex. Two weeks after surgery, the pigs were suspended awake in a sling, and markers were tracked with biplane cineradiography. The hearts were paced for 3 wk (225-240 beats/min), and the study was repeated with the pacemaker off. Saline infusion and IVC occlusion were used to vary LV end-diastolic pressure (EDP) so control-to-failure comparisons could be made at matched LV EDPs. End-systolic strains in the circumferential (E11), longitudinal (E22), and transmural (E33) directions were quantified using finite element methods. There was a significant reduction in E11 and E33 for the subendocardium: in E11, from -0.27 to -0.18; in E33, from 0.83 to 0.46. There were no significant changes in subendocardial E22 or in any of the outer wall normal strains. These results indicate that CRVP causes substantial reduction of subendocardial, but not subepicardial, function; taken together with previous data indicating subendocardial hypoperfusion, these results support the contention that an imbalance between blood flow and oxygen demand plays a role in the etiology of heart failure in this model.

An anatomical heart model with applications to myocardial activation and ventricular mechanics.

A three-dimensional finite element model of the mechanical and electrical behavior of the heart is being developed in a collaboration among Auckland University, New Zealand; the University of California at San Diego, U.S.; and McGill University, Canada. The equations of continuum mechanics from the theory of finite deformation elasticity are formulated in a prolate spheroidal coordinate system and solved using a combination of Galerkin and collocation techniques. The finite element basis functions used for the dependent and independent variables range from linear Lagrange to cubic Hermite, depending on the degree of spatial variation and continuity required for each variable. Orthotropic constitutive equations derived from biaxial testing of myocardial sheets are defined with respect to the microstructural axes of the tissue at the Gaussian quadrature points of the model. In particular, we define the muscle fiber orientation and the newly identified myocardial sheet axis orientation throughout the myocardium using finite element fields with nodal parameters fitted by least-squares to comprehensive measurements of these variables. Electrical activation of the model is achieved by solving the FitzHugh-Nagumo equations with collocation at fixed material points of the anatomical finite element model. Electrical propagation relies on an orthotropic conductivity tensor defined with respect to the local material axes. The mechanical constitutive laws for the Galerkin continuum mechanics model are (1) an orthotropic "pole-zero" law for the passive mechanical properties of myocardium and (2) a Wiener cascade model of the active mechanical properties of the muscle fibers. This chapter concentrates on two aspects of the model: first, grid generation, including both the generation of nodal coordinates for the finite element mesh and the generation of orthotropic material axes at each computational point, and, second, the formulation of constitutive laws suitable for numerically intensive finite element computations. Extensions to this model and applications to the mechanical and electrical function of the heart are described in Chapter 16 by McCulloch and co-workers.