Barbara Johnston | Griffith University (original) (raw)

Papers by Barbara Johnston

Accurate determination of cardiac tissue parameters is essential in bidomain models that simulate... more Accurate determination of cardiac tissue parameters is essential in bidomain models that simulate the electrical activity of the heart and thereby contribute to understanding cardiovascular disease. Recent experimental work indicated the need for six parameters, which measure electrical conductivity in two domains (extracellular and intracellular), along and across the cardiac fibres within a sheet and also between sheets. This is in contrast to the available experimentally determined conductivities, which are sets of four values, where it is assumed that conductivities across the fibres within a sheet and between the fibre sheets are equal. This study presents a mathematical model that incorporates six bidomain conductivities. It also discusses the design of a multi-electrode array and inversion method to retrieve these

Accurate cardiac bidomain conductivity values are essential for realistic simulation of various c... more Accurate cardiac bidomain conductivity values are essential for realistic simulation of various cardiac electrophysiological phenomena. A method was previously developed that can determine the conductivities from measurements of potential on a multi-electrode array placed on the surface of the heart. These conductivities, as well as a value for fibre rotation, are determined using a mathematical model and a two-pass process that is based on Tikhonov regularisation. Using simulated potentials, to which noise is added, the inversion method was recently shown to retrieve the intracellular conductivities accurately with up to 15% noise and the extracellular conductivities extremely accurately even with 20% noise. Recent work investigated the sensitivity of the

Mathematical Biosciences, 2013

... 546, 1982. 223 [10] P. Le Guyader, F. Trelles, and P. Savard, Extracel-lular measurement of a... more ... 546, 1982. 223 [10] P. Le Guyader, F. Trelles, and P. Savard, Extracel-lular measurement of anisotropic bidomain myocar-dial conductivities. I. Theoretical analysis, Annals of Biomedical Engineering, 29:862877, 2001. [11] P ...

Four electrode techniques have long been used to determine conductivity parameters in cardiac tis... more Four electrode techniques have long been used to determine conductivity parameters in cardiac tissue. This paper introduces a mathematical model and solution technique to theoretically analyse electrode configurations, specifically allowing for plunge electrodes. In particular, the focus is on using four electrode configurations to determine fibre rotation in cardiac tissue. Two configurations are analysed, the first with the four electrodes collinear and the second consisting of two probes a fixed distance apart, with the current electrodes on one probe and the measuring electrodes on the other. It is found that the second electrode configuration can yield a value for the fibre rotation under the assumptions of the model.

Ischaemic heart disease has been the leading cause of death in Australia for the past 10 years. I... more Ischaemic heart disease has been the leading cause of death in Australia for the past 10 years. Included in this category are angina, blocked arteries and heart attacks. This paper presents a modelling study of the precursors to artery blockage, namely coronary artery disease, or narrowing of the coronary arteries, based on geometrical considerations.

Effects of arterial curvature and lumen diameter on wall shear stress distributions 2 Abstract. T... more Effects of arterial curvature and lumen diameter on wall shear stress distributions 2 Abstract. This study looks at blood flow in four different human right coronary arteries (RCAs), which have been reconstructed from bi-plane angiograms. A Generalised Power Law model of blood viscosity is used to study the blood flow at a particular point in the cardiac cycle. Large differences are found in the wall shear stress magnitude (WSS) distributions in the four arteries, leading to the conclusion that it is not possible to make generalisations based on the study of a single artery. The pattern of WSS is found to be related to the geometry of a particular artery, that is, lumen diameter and arterial curvature as well as a combination of these two factors. There is a strong correlation between WSS and reciprocal radius and a weaker correlation between high curvature and extremes of WSS, with high WSS on the 'inside' of a bend and low WSS on the 'outside' of a bend. This is in contrast to the situation for a simple curved tube with constant radius where the inverse is observed. For each artery, a region proximal to the acute margin is identified where low WSS is found and where WSS is lower on the 'inner' wall of the RCA than on the 'outer' wall. This region is one where several studies have found that the human RCA preferentially exhibits atherogenesis.

Although realistic cardiac electrophysiological simulations require accurate model parameters, no... more Although realistic cardiac electrophysiological simulations require accurate model parameters, no fully experimentally determined sets of six cardiac conductivity values exist. The present authors have recently proposed a method to determine the six bidomain conductivity values, for the extra– and intracellular domains in the longitudinal , transverse and normal directions, from measurements of potential made in cardiac tissue in vivo. The method uses a 3D mathematical model, a microelectrode measuring array and a novel inversion technique, which retrieves the conductivities and the fibre rotation angle from the potential measurements. In this work, a number of different data analysis methods are compared for realistically large sets of these simulated potential measurements and the best method is identified. Using synthetic data it is found that the three ex-tracellular conductivities can be retrieved extremely accurately , with relative errors of less than 5%, even with noise of u...

The distribution of wall shear stress (WSS) in coronary arteries is an initiating factor for coro... more The distribution of wall shear stress (WSS) in coronary arteries is an initiating factor for coronary artery disease. The complicated three dimensional structure of coronary arteries makes it difficult to isolate the factors affecting WSS distributions. Here we present a computational fluid dynamics study of transient flow in simplified S-shaped arteries with in-plane and out-of-plane bends. For in-plane arteries there are two regions of consistently low WSS during the cardiac cycle. The WSS distribution is related to secondary velocities which appear as symmetric counter-rotating eddies and are approximately 8% of the axial velocity. For out-of-plane arteries the WSS distributions are similar, but the eddies are no longer symmetric with secondary velocities only 7% of the axial velocities. The symmetric velocity patterns produced by the in-plane model could only exist in a single bend, which limits the usefulness of this model. The results obtained give useful insights into the nat...

This paper presents a comparison of wall shear stress distri butions and particle traces in a nor... more This paper presents a comparison of wall shear stress distri butions and particle traces in a normal and an ectatic coro nary artery. The comparisons are based on computational fluid dynamics simulations of blood flow through these ar teries. These preliminary results give some insight into an giographic observations of flow in ectatic arteries. Yes Yes

ABSTRACT Accurate determination of cardiac conductivity values is of paramount importance for det... more ABSTRACT Accurate determination of cardiac conductivity values is of paramount importance for detailed simulation of many electrophysiological phenomena. Here we present an approach to determine these values using Tikhonov regularisation applied to potential measurements made on a micro- needle array. Results show that it is possible to accurately retrieve most of the required values with measurement noise as high as 10%.

ABSTRACT An important aspect of numerical integration is to have some knowledge of the truncation... more ABSTRACT An important aspect of numerical integration is to have some knowledge of the truncation error for a given number of integration points. In this paper we determine estimates for these errors in the application of Gauss–Legendre quadrature to evaluate numerically two dimensional integrals which arise in the boundary element method. Expressions for the truncation errors developed here require the approximate evaluation of two integrals in the complex plane. The second integral, which has been termed the “remainder of the remainder”, was assumed small by the authors in a previous attempt in developing error estimates. However, here this integral is included and it is evaluated using a novel approach for the choice of contour. We consider examples where ignoring the “remainder of the remainder” was a reasonable assumption and also consider cases where this remainder dominates the error. Finally, it is shown, for each of the integrals considered, that these new error estimates agree very closely with the actual quadrature error.

Biomedical Engineering, 2013

ABSTRACT Previous work has shown that variations in cardiac conductivity values have significant ... more ABSTRACT Previous work has shown that variations in cardiac conductivity values have significant effects on the resulting epicardial potential distributions arising from subendocardial ischaemia. This work carries on from a previous study in this area by allowing for variation in (the more physiologically realistic) six, rather than four, conductivity values. This is achieved by relaxing the usual assumption that the cardiac conductivity values in both normal directions, one normal to the fibres within the sheet and the other normal to the sheet, are equal. An extended non–dimensional formulation is derived for the passive bidomain equation, leading to a normal direction dimensionless conductivity ratio, in addition to the previously found ratios in the transverse and longitudinal directions. Two six–conductivity datasets are used to set up a parameter space for the above ratios, from which epicardial potential distributions are derived and compared. Results from this study appear to support the conclusions of the previous four–conductivity study; that is, differences in the epicardial potential distributions are best explained by variations in the ratio of the intracellular longitudinal conductivity to the intracellular transverse conductivity.

Tellus B, 2011

... 1997-2007 By Barbara M. Johnston1* and Albert J. Gabric1,, 1 Australian Rivers Institute, Gri... more ... 1997-2007 By Barbara M. Johnston1* and Albert J. Gabric1,, 1 Australian Rivers Institute, Griffith University, Nathan, Queensland, Australia, 4111 10 January 2011 * Corresponding author. email: Barbara.Johnston@griffith.edu.au. ...

Tellus A, 2010

A B S T R A C T Decadal-scale changes in the meridional circulation of the Southern Ocean south o... more A B S T R A C T Decadal-scale changes in the meridional circulation of the Southern Ocean south of Australia are studied, over the period 1958-2005, using Simple Ocean Data Assimilation (SODA) reanalysis data. Upper Circumpolar Deep Water (UCDW) is found to upwell nearer to the surface over time, while the mixed layer (ML) is found to deepen, leading to an increase in the number of times that UCDW intrudes into the ML. This entrainment of nutrients, especially iron, into the ML from UCDW, is crucial for primary production and appears to occur predominantly in summer/autumn, contrary to previous reports.

SIAM Journal on Scientific Computing, 2008