An FPGA-Based Vision Prosthesis Prototype: Implementing an Efficient Multiplexing Method for Addressing Electrodes

A prototype of an epi-retinal vision prosthesis based upon an efficient electrode addressing schema has been developed. This system has the ability to stimulate multiple electrode regions simultaneously, hence greatly improving the maximum rate of stimulation compared to many currently available neural stimulation devices based on serial stimulation protocols. To minimize the problem of cross talk between stimulating electrodes, a hexagon layout of electrodes was implemented. Basic tests were completed using a field programmable gate array logic system driving analogue circuitry to inject current into physiological saline via electrodes in hexagon arrangements and in a simple paired arrangement. The hexagon layout of electrodes was shown to clearly reduce the interaction between multiple current sources and hence cross talk.

A home health monitoring system including intelligent reporting and alerts

We describe the design and implementation of an intelligent reporting and alerts system that has been designed with a specific goal to address the needs of managing chronic and complex disease through the use of home telecare technology. Our approach has been to develop these tools using as far as possible, open standards. Clinical measurement data gathered using home telecare and stored in a relational database in XML format is extracted and converted into a Clinical Document Architecture (CDA) as defined by the Health Level 7 (HL7) organization. Data trends are presented to the clinician as simple graphs and summary statistics (means, standard deviations) over time for an individual patient. Clinicians may receive this data by display through a Web-interface or by email or faxed reports. A Ripple Down Rules (RDR) knowledge base supports more complex decision-making provided in the Alerts module. The RDR output is incorporated into the output reports as a textual statement, and/or a graphical highlighting of key parameters in the trends images and tables. Rule development and validation is part of ongoing research.

Towards photosensor movement--adaptive image analysis in an electronic retinal prosthesis

By way of extracellular, stimulating electrodes, electronic vision prosthesis aims to render discrete light spots--so-called phosphenes--in the visual field, thereby providing a phosphene image serving as a rudimentary remediation of profound blindness. It is proposed that a digital camera, or some other photosensitive array, captures frames, the frames be analyzed, and phosphenes be actuated accordingly. We present a numerical experiment wherein we observed the phosphene image in response to a set of stimuli for various image analysis schemes. We used the mutual-information function to quantify the efficacy of analysis schemes; the function penalizes a scheme for introducing redundancy to the phosphene image, while accounting for the probability of each stimulus. We demonstrate an effective scheme involving Laplacian of Gaussian (nabla(2)G) kernels geometrically transformed in accordance with phosphene layout. Further, we propose adapting the kernels comprising a scheme in accordance with photosensor movement.

Effect on prosthetic vision visual acuity by filtering schemes, filter cut-off frequency and phosphene matrix: a virtual reality simulation

Visual acuity of prosthetic vision was examined under virtual reality simulation. Prosthetic vision was simulated by first filtering an image using circular mean filters or Gaussian smoothing filters of different cut-off frequencies. Pixel values at 100 fixed sites of the filtered image were taken, sampling either with a regular rectangular or hexagonal matrix. Each pixel value was transformed into a Gaussian intensity profile centered at the corresponding position at which the sample was taken to simulate the evoked visual effect of an electric stimulation. Visual acuity scores of three subjects, each completing two sets of results, were recorded across different filtering schemes, cut-off frequencies and sampling matrices. The best mean score recorded was 1.55 logMAR, with the worst being 1.70 logMAR. The difference was mostly attributed to filter cut-off frequency. Differences between filtering schemes were insignificant. Results also showed emerging trends demonstrating differences between rectangular and hexagonal sampling matrices.

Noninvasive Average Flow Estimation for an Implantable Rotary Blood Pump: A New Algorithm Incorporating the Role of Blood Viscosity

The effect of blood hematocrit (HCT) on a noninvasive flow estimation algorithm was examined in a centrifugal implantable rotary blood pump (iRBP) used for ventricular assistance. An average flow estimator, based on three parameters, input electrical power, pump speed, and HCT, was developed. Data were collected in a mock loop under steady flow conditions for a variety of pump operating points and for various HCT levels. Analysis was performed using three-dimensional polynomial surfaces to fit the collected data for each different HCT level. The polynomial coefficients of the surfaces were then analyzed as a function of HCT. Linear correlations between estimated and measured pump flow over a flow range from 1.0 to 7.5 L/min resulted in a slope of 1.024 L/min (R2=0.9805). Early patient data tested against the estimator have shown promising consistency, suggesting that consideration of HCT can improve the accuracy of existing flow estimation algorithms.

Optical imaging of electrically evoked visual signals in cats: I. Responses to corneal and intravitreal electrical stimulation

Microelectronic retinal prostheses have been shown to restore the perception of light to the blind through electrical stimulation. Conventional recording techniques such as recording electrode arrays on the visual cortex can give a basic understanding of the events that occur during such stimulation events, but their finite size and number limits the spatial resolution achievable with them. Optical imaging of intrinsic signals (OIS imaging) allows for greater resolution (approximately 50 microm) of the activity in the cortex. This can be used to facilitate a greater understanding of the complex neurophysiological events that allow prosthetic vision. This paper shows responses to visual and electrical stimulation of the retina, and demonstrates that OIS imaging may be an effective technique in further refining stimulation techniques and implant designs for retinal prostheses.

Identification and Classification of Physiologically Significant Pumping States in an Implantable Rotary Blood Pump

In a clinical setting it is necessary to control the speed of rotary blood pumps used as left ventricular assist devices to prevent possible severe complications associated with over- or underpumping. The hypothesis is that by using only the noninvasive measure of instantaneous pump impeller speed to assess flow dynamics, it is possible to detect physiologically significant pumping states (without the need for additional implantable sensors). By varying pump speed in an animal model, five such states were identified: regurgitant pump flow, ventricular ejection (VE), nonopening of the aortic valve over the cardiac cycle (ANO), and partial collapse (intermittent and continuous) of the ventricle wall (PVC-I and PVC-C). These states are described in detail and a strategy for their noninvasive detection has been developed and validated using (n = 6) ex vivo porcine experiments. Employing a classification and regression tree, the strategy was able to detect pumping states with a high degree of sensitivity and specificity: state VE-99.2/100.0% (sensitivity/specificity); state ANO-100.0/100.0%; state PVC-I- 95.7/91.2%; state PVC-C-69.7/98.7%. With a simplified binary scheme differentiating suction (PVC-I, PVC-C) and nonsuction (VE, ANO) states, both such states were detected with 100% sensitivity.

Classification of a known sequence of motions and postures from accelerometry data using adapted Gaussian mixture models

Accelerometry shows promise in providing an inexpensive but effective means of long-term ambulatory monitoring of elderly patients. The accurate classification of everyday movements should allow such a monitoring system to exhibit greater 'intelligence', improving its ability to detect and predict falls by forming a more specific picture of the activities of a person and thereby allowing more accurate tracking of the health parameters associated with those activities. With this in mind, this study aims to develop more robust and effective methods for the classification of postures and motions from data obtained using a single, waist-mounted, triaxial accelerometer; in particular, aiming to improve the flexibility and generality of the monitoring system, making it better able to detect and identify short-duration movements and more adaptable to a specific person or device. Two movement classification methods were investigated: a rule-based Heuristic system and a Gaussian mixture model (GMM)-based system. A novel time-domain feature extraction method is proposed for the GMM system to allow better detection of short-duration movements. A method for adapting the GMMs to compensate for the problem of limited user-specific training data is also proposed and investigated. Classification performance was considered in relation to data gathered in an unsupervised, directed routine conducted in a three-month field trial involving six elderly subjects. The GMM system was found to achieve a mean accuracy of 91.3%, distinguishing between three postures (sitting, standing and lying) and five movements (sit-to-stand, stand-to-sit, lie-to-stand, stand-to-lie and walking), compared to 71.1% achieved by the Heuristic system. The adaptation method was found to offer a mean accuracy of 92.2%; a relative improvement of 20.2% over tests without subject-specific data and 4.5% over tests using only a limited amount of subject-specific data. While limited to a restricted subset of possible motions and postures, these results provide a significant step in the search for a more robust and accurate ambulatory classification system.

Bank note recognition for the vision impaired

Blind Australians find great difficulty in recognising bank notes. Each note has the same feel, with no Braille markings, irregular edges or other tangible features. In Australia, there is only one device available that can assist blind people recognise their notes. Internationally, there are devices available; however they are expensive, complex and have not been developed to cater for Australian currency. This paper discusses a new device, the MoneyTalker that takes advantage of the largely different colours and patterns on each Australian bank note and recognises the notes electronically, using the reflection and transmission properties of light. Different coloured lights are transmitted through the inserted note and the corresponding sensors detect distinct ranges of values depending on the colour of the note. Various classification algorithms were studied and the final algorithm was chosen based on accuracy and speed of recognition. The MoneyTalker has shown an accuracy of more than 99%. A blind subject has tested the device and believes that it is usable, compact and affordable. Based on the devices that are available currently in Australia, the MoneyTalker is an effective alternative in terms of accuracy and usability.

A comparison of 1-D models of cardiac pacemaker heterogeneity

In this paper, we investigate the role of sinoatrial node (SAN) cellular heterogeneity in two key aspects of normal cardiac pacemaker function: frequency entrainment of the SAN, and propagation of excitation into the atrial tissue. Using detailed ionic models of electrical activity in SAN and atrial myocytes, we have formulated a number of one-dimensional models of SAN heterogeneity based on discrete-region (in which central and peripheral SAN type cell are separated into discrete regions), gradient and mosaic models of SAN organization. Each of the different models were assessed on their ability to achieve frequency entrainment of the SAN and activation of the adjoining atrial tissue in the presence of both uniform and linearly increasing conductivity profiles. Simulation results suggest that the gradient model of SAN heterogeneity, in which cells display a smooth variation in membrane properties from the center to the periphery of the SAN, produces action potential waveshapes and a site of earliest activation consistent with experimental observations in the intact SAN. The gradient model also achieves frequency entrainment of the SAN more easily than other models of SAN heterogeneity. Based on these results, we conclude that the gradient model of SAN heterogeneity, in the presence of a uniform conductivity profile, is the most likely model of SAN organization.

Implementation of a real-time human movement classifier using a triaxial accelerometer for ambulatory monitoring

The real-time monitoring of human movement can provide valuable information regarding an individual's degree of functional ability and general level of activity. This paper presents the implementation of a real-time classification system for the types of human movement associated with the data acquired from a single, waist-mounted triaxial accelerometer unit. The major advance proposed by the system is to perform the vast majority of signal processing onboard the wearable unit using embedded intelligence. In this way, the system distinguishes between periods of activity and rest, recognizes the postural orientation of the wearer, detects events such as walking and falls, and provides an estimation of metabolic energy expenditure. A laboratory-based trial involving six subjects was undertaken, with results indicating an overall accuracy of 90.8% across a series of 12 tasks (283 tests) involving a variety of movements related to normal daily activities. Distinction between activity and rest was performed without error; recognition of postural orientation was carried out with 94.1% accuracy, classification of walking was achieved with less certainty (83.3% accuracy), and detection of possible falls was made with 95.6% accuracy. Results demonstrate the feasibility of implementing an accelerometry-based, real-time movement classifier using embedded intelligence.

Automated non-invasive detection of pumping states in an implantable rotary blood pump

With respect to rotary blood pumps used as left ventricular assist devices (LVADs), it is clinically important to control pump flow to avoid complications associated with over-or under-pumping of the native heart. By employing only the non-invasive observer of instantaneous pump impeller speed to assess flow dynamics, a number of physiologically significant pumping states may be detected. Based on a number of acute animal experiments, five such states were identified: regurgitant pump flow (PR), ventricular ejection (VE), non-opening of the aortic valve (ANO), and partial collapse (intermittent and continuous) of the ventricle wall (PVC-I and PVC-C). Two broader states, normal (corresponding to VE, ANO) and suction (corresponding to PVC-I, PVC-C) were readily discernable in clinical data from human patients implanted with LVADs. Based on data from both the animal experiments (N=6) and the human patients (N=10), a strategy for the automated non-invasive detection of significant pumping states has been developed and validated. Employing a classification and regression tree (CART), this system detects pumping states with a high degree of accuracy: state VE -87.5/100.0% (sensitivity/specificity); state ANO - 98.1/92.5%; state PVC-I - 90.0/90.2%; state PVC-C - 61.2/98.0%. With a simplified binary scheme differentiating suction and normal states, both states were detected without error in data from the animal experiments, and with a sensitivity/specificity, for detecting suction, of 99.2/98.3% in the human patient data.

Microelectronic retinal prosthesis: II. Use of high-voltage CMOS in retinal neurostimulators

This paper presents the design, implementation, and simulated and measured results of a complementary metal-oxide-semiconductor neurostimulator implemented in a 0.35 microm high-voltage process. To allow for a high stimulation voltage, and hence the greatest versatility of the neurostimulator in situ, a high-voltage CMOS process was used. The neurostimulator utilized current sources and sinks to simultaneously deliver and recover charge. It has the ability to deliver stimulus in three output current ranges using a current sink only, current source only, or both a current source and sink combined to provide focused stimulation. The worst case integral non-linearity and differential non-linearity errors were 0.2 LSB and 0.1 LSB respectively, and the current source and sink turn-on times were under 500 ns, providing fast switching time in response to stimuli instructions. The total die area was under 13 mm2, well within the area constraints of our implantable vision prosthesis device.

Psychophysics of prosthetic vision: I. Visual scanning and visual acuity

Recipients of vision prosthesis prototypes have reported electrically elicited visual perceptions as discrete dots of light (phosphenes). Phosphenes construct the scenery in discontinuous small isolated patches, resulting in visual information deficit to a large portion of the visual field. Visual scanning therefore plays an important role in the utility of prosthetic vision. In a psychophysical study, normally sighted subjects undertook a visual acuity task in a simulation of prosthetic vision with scanning facilitated by head movements. Subjects who adopted the circular scanning technique (4/12) correctly identified >60% of the test items, compared to subjects with no particular scanning patterns (3/12) with <50%. Increased head movement velocity was correlated to increased performance; at optimal scanning velocities, we estimated a 50% increase in identification rate or a two-fold improvement in visual acuity threshold compared to otherwise complete lack of scanning movement. Improved performance likely resulted from positive interactions with the temporal processes of the human visual system, which may as much as double the spatial information of that originally afforded by the phosphene lattice.

Psychophysics of prosthetic vision: III. stochastic rendering, the phosphene image, and perception

For pt.I see ibid., p.Z004024-7. For pt.II see ibid., p.Z004558-61. This paper examines the rendering of luminous spots ("phosphenes") in the visual field, and their stochastic positioning as a means of anti-aliasing the resulting spotty image ("phosphene image"). We derive an equation concerning the correlations of pairs of phosphenes comprising the phosphene image, and show the relationship to the statistics governing the stochastic positioning. We present some examples where stochastic rendering assists the veridical perception of textures, and argue for its superiority as cf. ordered rendering. Our preliminary results suggest that it may be perceptually effective to manufacture disordered arrays of stimulating electrodes for intraocular implantation.

Designing Home Telecare: A Case Study in Monitoring Cystic Fibrosis

The objective of this study was to investigate the feasibility of using home telecare for monitoring cystic fibrosis (CF). Five adolescents were asked to use a home telecare system during a routine hospital visit over one week. Frequency of use was measured from computer logs. Unacceptable measurements were identified by visual inspection. User impressions of home telecare and appropriateness of the system for managing CF was determined from observations of user interaction, survey and qualitative analysis. Patients used the system to record lung function measurements without any supervision and indicated that the system was easy to learn and use. The role of home telecare in supporting collaborative self-management appeared to be well understood. Home telecare was seen as a supplement to standard care that would provide a link to the hospital between clinic visits. Participants indicated that feedback provided by the system and ongoing clinical support would determine long-term use and compliance with the monitoring protocol. Clinicians reported the usefulness of home telecare in maintaining a longitudinal record of their patient's health that would supplement verbal description of symptoms and reduce time to treatment by increasing patient self-awareness of health status. Home telecare may be a feasible intervention for monitoring CF. Feedback provided by the system must be presented in a format that is familiar and easily understood by users. Further system refinement and evaluation is required to determine patient compliance with their customized monitoring protocol prior to assessing impact on clinical outcomes.

Architecture for knowledge-based and federated search of online clinical evidence

Background

It is increasingly difficult for clinicians to keep up-to-date with the rapidly growing biomedical literature. Online evidence retrieval methods are now seen as a core tool to support evidence-based health practice. However, standard search engine technology is not designed to manage the many different types of evidence sources that are available or to handle the very different information needs of various clinical groups, who often work in widely different settings.

Objectives

The objectives of this paper are (1) to describe the design considerations and system architecture of a wrapper-mediator approach to federate search system design, including the use of knowledge-based, meta-search filters, and (2) to analyze the implications of system design choices on performance measurements.

Methods

A trial was performed to evaluate the technical performance of a federated evidence retrieval system, which provided access to eight distinct online resources, including e-journals, PubMed, and electronic guidelines. The Quick Clinical system architecture utilized a universal query language to reformulate queries internally and utilized meta-search filters to optimize search strategies across resources. We recruited 227 family physicians from across Australia who used the system to retrieve evidence in a routine clinical setting over a 4-week period. The total search time for a query was recorded, along with the duration of individual queries sent to different online resources.

Results

Clinicians performed 1662 searches over the trial. The average search duration was 4.9 ± 3.2 s (N = 1662 searches). Mean search duration to the individual sources was between 0.05 s and 4.55 s. Average system time (ie, system overhead) was 0.12 s.

Conclusions

The relatively small system overhead compared to the average time it takes to perform a search for an individual source shows that the system achieves a good trade-off between performance and reliability. Furthermore, despite the additional effort required to incorporate the capabilities of each individual source (to improve the quality of search results), system maintenance requires only a small additional overhead.

Learning prosthetic vision: a virtual-reality study

Acceptance of prosthetic vision will be heavily dependent on the ability of recipients to form useful information from such vision. Training strategies to accelerate learning and maximize visual comprehension would need to be designed in the light of the factors affecting human learning under prosthetic vision. Some of these potential factors were examined in a visual acuity study using the Landolt C optotype under virtual-reality simulation of prosthetic vision. Fifteen normally sighted subjects were tested for 10-20 sessions. Potential learning factors were tested at p < 0.05 with regression models. Learning was most evident across-sessions, though 17% of sessions did express significant within-session trends. Learning was highly concentrated toward a critical range of optotype sizes, and subjects were less capable in identifying the closed optotype (a Landolt C with no gap, forming a closed annulus). Training for implant recipients should target these critical sizes and the closed optotype to extend the limit of visual comprehension. Although there was no evidence that image processing affected overall learning, subjects showed varying personal preferences.

A bidomain model of epiretinal stimulation

A computational bidomain model of epiretinal stimulation is presented, consisting of a continuum description of active retinal tissue in contact with bulk vitreous fluid. Results from two-electrode and four-electrode bipolar stimulation suggest that a biphasic cathodic-anodic stimulus sequence is effective in providing targeted focal activation of retinal tissue. Undesired secondary activations beneath each electrode return may be eliminated by using multiple returns for each stimulus electrode.

Reliability of cardiorespiratory measurements with a new ergospirometer during intense treadmill exercise in Thoroughbred horses

This study investigated the reliability of measurements with a new equine ergospirometer (Quadflow). Heart rate and blood lactate responses during exercise in horses wearing the Quadflow and an open flow mask were also compared. The mean percentage error of the oxygen uptake measurements was 8.2% (range 2.1-12.5%). Percent error for peak expiratory flow rates ranged from 6.1% to 9.4 %, and for minute ventilation from 2.5% to 7.4%. The coefficients of variation of the means of four measurements in two horses exercising continuously at 9.0 m/s were <5% for variables related to pulmonary ventilation, and was 7.7% for oxygen uptake. The Quadflow mask resulted in small increases in blood lactate concentration and relative heart rate during submaximal exercise. It was concluded that between- and within-test reliability statistics for important measurements in equine clinical exercise testing were acceptable for routine use in a veterinary practice or research laboratory.

Simulated prosthetic visual fixation, saccade, and smooth pursuit

A visual tracking task was administered to 20 subjects afforded simulated prosthetic vision (a phosphene array); a total of 3h data was taken from each subject over the course of 10 visits. The experiment assessed prosthetic visual fixation, saccade and smooth pursuit and the effect of practice. Further, we demonstrated an image analysis technique that assisted fixation and pursuit (but not saccade) accuracy, and required less vigorous movement of the phosphene array in pursuing the target. As measured by mean deviation from the target, fixation and pursuit accuracies were improved by 8.3 and 3.3 min of visual arc, respectively (35.8% and 6.8%), for inter-phosphene spacing of 1.9 degrees . The analysis technique, involving overlapping Gaussian kernels, was an heuristic design; this is the first step of an iterative, experimental approach to devising effective image analysis to be contained in an electronic vision prosthesis. The approach should ultimately afford implanted patients improved prosthetic visual function.

Visual acuity measurement of prosthetic vision: a virtual-reality simulation study

A virtual-reality simulation tested prosthetic visual acuity for both rectangular and hexagonal phosphene grids. Thirteen normally sighted, untrained subjects were required to identify the Landolt C optotype over ten sessions. Overall performance, performance by filter setting (image analysis), and performance by size and orientation of the Landolt C optotype are reported. The results indicated that the hexagonal grid had a slight (4.1%) but statistically significant (p < 0.0001) performance advantage over the rectangular grid for correct identification of the testing symbol. It was also observed that both the phosphene grids had distinct performance profiles relating to their respective spatial orientation. Over the course of the ten sessions, learning afforded subjects an averaged improved performance of 10%.

Classification of basic daily movements using a triaxial accelerometer

A generic framework for the automated classification of human movements using an accelerometry monitoring system is introduced. The framework was structured around a binary decision tree in which movements were divided into classes and subclasses at different hierarchical levels. General distinctions between movements were applied in the top levels, and successively more detailed subclassifications were made in the lower levels of the tree. The structure was modular and flexible: parts of the tree could be reordered, pruned or extended, without the remainder of the tree being affected. This framework was used to develop a classifier to identify basic movements from the signals obtained from a single, waist-mounted triaxial accelerometer. The movements were first divided into activity and rest. The activities were classified as falls, walking, transition between postural orientations, or other movement. The postural orientations during rest were classified as sitting, standing or lying. In controlled laboratory studies in which 26 normal, healthy subjects carried out a set of basic movements, the sensitivity of every classification exceeded 87%, and the specificity exceeded 94%; the overall accuracy of the system, measured as the number of correct classifications across all levels of the hierarchy, was a sensitivity of 97.7% and a specificity of 98.7% over a data set of 1309 movements.

Accelerometry: providing an integrated, practical method for long-term, ambulatory monitoring of human movement

Accelerometry offers a practical and low cost method of objectively monitoring human movements, and has particular applicability to the monitoring of free-living subjects. Accelerometers have been used to monitor a range of different movements, including gait, sit-to-stand transfers, postural sway and falls. They have also been used to measure physical activity levels and to identify and classify movements performed by subjects. This paper reviews the use of accelerometer-based systems in each of these areas. The scope and applicability of such systems in unsupervised monitoring of human movement are considered. The different systems and monitoring techniques can be integrated to provide a more comprehensive system that is suitable for measuring a range of different parameters in an unsupervised monitoring context with free-living subjects. An integrated approach is described in which a single, waist-mounted accelerometry system is used to monitor a range of different parameters of human movement in an unsupervised setting.

Estimation of pressure pulse amplification between aorta and brachial artery using stepwise multiple regression models

The pressure pulse is amplified between the aorta and peripheral sites. This study compares two methods to estimate pressure pulse amplification (PPA) between the aorta and the brachial artery. Method 1: PPA was determined from a multi-parameter linear regression of subject parameters (gender, age, height, weight, heart rate (HR), brachial systolic pressure (BSP), diastolic pressure (BDP), mean pressure (MP)). Method 2: PPA was calculated from central aortic pressure waveforms (CW) estimated from the same subject parameters. The sample population (1421 male, 992 female) was selected from a database where aortic pressure was estimated by mathematical transformation of a peripheral (radial) pulse calibrated to sphygmomanometric BSP and BDP. The two methods were consistent in showing HR and MP as the most important parameters to estimate PPA. Correlation coefficients (R2) of 0.48 (method 1) and 0.44 (method 2) were obtained using height, weight, HR, BSP, BDP and age. Inclusion of MP increased R2 to 0.77 (method 1) and 0.71 (method 2). This study shows that databases containing peripheral and central aortic pressure waveforms can be used to construct multiple regression models for PPA estimation. These models could be applied to studies of similar subject groups where peripheral waveforms may not be available.

A pilot study of long-term monitoring of human movements in the home using accelerometry

We assessed the feasibility of using a waist-mounted, wireless triaxial accelerometer (TA) to monitor human movements in an unsupervised home setting to detect changes in functional status. A pilot study was carried out with six healthy subjects aged 80-86 years. The subjects wore a TA unit every day for two to three months. Each morning they carried out a short routine of directed movements that included standing, sitting, lying and walking. Important movement variables were measured. During the rest of the day, subjects were monitored for falls, and variables such as metabolic energy expenditure were measured. All subjects remained healthy; there was no overall change in functional status and there were only slight fluctuations in health status. No longitudinal changes were detected in any of the variables measured during the directed routine. There was a moderate correlation between weekly self-reported health status and energy expenditure: subjects reported a lower health status for weeks in which they expended less energy. The TA system was found to be practical for long-term, unsupervised home monitoring. All subjects found the system simple to use and the TA unit unobtrusive and comfortable to wear. High compliance rates were achieved: the TA units were worn on 88% of the days in the study, for an average of 11.2 hours per day.

A gradient model of cardiac pacemaker myocytes

We have formulated a spatial-gradient model of action potential heterogeneity within the rabbit sinoatrial node (SAN), based on cell-specific ionic models of electrical activity from its central and peripheral regions. The ionic models are derived from a generic cell model, incorporating five background and exchange currents, and seven time-dependent currents based on three- or four-state Markov schemes. State transition rates are given by non-linear sigmoid functions of membrane potential. By appropriate selection of parameters, the generic model is able to accurately reproduce a wide range of action potential waveforms observed experimentally. Specifically, the model can fit recordings from central and peripheral regions of the SAN with RMS errors of 0.3987 and 0.7628 m V, respectively. Using a custom least squares parameter optimisation routine, we have constructed a spatially-varying gradient model that exhibits a smooth transition in action potential characteristics from the central to the peripheral region, whilst ensuring individual membrane currents remain physiologically accurate. Smooth transition action potential characteristics include maximum diastolic potential, overshoot potential, upstroke velocity, action potential duration and cycle length. The gradient model is suitable for developing higher dimensional models of the right atrium, in which action potential heterogeneity within nodal tissue may be readily incorporated.

Parameter estimation in cardiac ionic models

We examine the problem of parameter estimation in mathematical models of excitable cell cardiac electrical activity using the well-known Beeler-Reuter (1977) ionic equations for the ventricular action potential. The estimation problem can be regarded as equivalent to the accurate reconstruction of ionic current kinetics and amplitudes in an excitable cell model, given only action potential experimental data. We show that in the Beeler-Reuter case, all ionic currents may be reasonably reconstructed using an experimental design consisting of action potential recordings perturbed by pseudo-random injection currents. The Beeler-Reuter model was parameterised into 63 parameters completely defining all membrane current amplitudes and kinetics. Total membrane current was fitted to model-generated experimental data using a 'data-clamp' protocol. The experimental data consisted of a default action-potential waveform and an optional series of perturbed waveforms generated by current injections. Local parameter identifiability was ascertained from the reciprocal condition value (1/lambda) of the Hessian at the known solution. When fitting to a single action potential waveform, the model was found to be over-determined, having a 1/lambda value of approximately 3.6e-14. This value improved slightly to approximately 1.4e-10 when an additional 2 perturbed waveforms were included in the fitting process, suggesting that the additional data did not overly improve the identifiability problem. The additional data, however, did allow the accurate reconstruction of all ionic currents. This indicates that by appropriate experimental design, it may be possible to infer the properties of underlying membrane currents from observation of transmembrane potential waveforms perturbed by pseudo-random currents.

Contribution to the Theory of Prosthetic Vision

By way of extracellular, electrical stimulation of the visual pathway, the various approaches to vision prosthesis aim to provide crude, patterned vision to individuals with profound blindness. Common to all approaches is the implantable electrode array and the rendering of phosphenes-the actuated percepts occupying the visual field of the implantee. Thus prosthetic vision may be simulated, and underlying theories as to how to render it efficacious developed. We review the field of simulated prosthetic vision. Furthermore, with retinal prosthesis in mind, we suggest a revised approach-an approach with regard to sampling theory, the vertebrate central visual pathway, and eye movements. The parallel development of prosthetic vision theory, explored via simulation and bioengineering issues surrounding neurostimulator design and implantation has bearing on the success of clinical trials by numerous groups in coming years.

Using information technology to improve the management of chronic disease

Information and communications technology (ICT) is increasingly being used in management of chronic illness to facilitate shared services (virtual health networks and electronic health records), knowledge management (care rules and protocols, scheduling, information directories), as well as consumer-based health education and evidence-based clinical protocols. Common applications of ICT include home monitoring of vital signs for patients with chronic disease, as well as replacing home visits by nurses in person with telemedicine videophone consultations. A patient-managed Home Telecare System with integrated clinical signs monitoring, automated scheduling and medication reminders, as well as access to health education and daily logs, is presented as an example of ICT use for chronic disease self-management. A clinical case study demonstrates how early identification of adverse trends in clinical signs recorded in the home can either avoid hospital readmission or reduce the length of hospital stay.

Detection of daily physical activities using a triaxial accelerometer

Triaxial accelerometers have been employed to monitor human movements in a variety of circumstances. The study considered the use of data from a single waist-mounted triaxial accelerometer to distinguish between activity states and rest A method using acceleration magnitude was applied to data collected from 26 normal subjects performing sit-to-stand and stand-to-sit transitions and walking. The effects of three parameters were investigated: the length n of a smoothing median filter, the width w of the averaging window used to process the signal and the value of the acceleration magnitude threshold th. These were found to be inter-related, and sets of parameters that resulted in accurate discrimination were determined by the relationship between th and the product of w and n, and by the relationship between n and w. The subjects were randomly divided into control (N = 13) and test (N = 13) groups. Optimum parameter sets were determined using the control group. Eleven sets of parameters yielded the same optimum results of a sensitivity of 1.0 and a specificity of 0.96 in the control group. Upon application to the test group, using these parameters, the system successfully distinguished between activity and rest, giving sensitivities greater than 0.98 and specificities between 0.88 and 0.94.

Non-invasive flow estimation in an implantable rotary blood pump: a study considering non-pulsatile and pulsatile flows

Non-invasive estimation of flow was investigated in an implantable rotary blood pump (iRBP) with a hydrodynamic bearing. The effects of non-pulsatile and pulsatile flows were studied using in vitro mock loops, and acute (N = 3) and chronic (N = 6) ovine experiments. Using the non-pulsatile and pulsatile mock loops an average flow estimation algorithm was derived from root mean square (RMS) pump impeller speed and RMS input power. These algorithms were programmed into the iRBP controller for subsequent validation in vivo. In the acute experiments, venous return and systemic vascular resistance were adjusted through pharmacological intervention and exsanguination to produce an average range of pump flows from 0.0 to 2.6 l min(-1). Over this range the RMS estimation error was 88 +/- 12 ml, with a linear correlation slope of 0.992 +/- 0.006 (R2 = 0.986 +/- 0.004). In the chronic experiments, animals were monitored daily for up to three months and an average range of flows from 2.8 to 4.8 l min(-1) recorded. A linear correlation between the estimated and measured pump flows yielded a slope of 1.005 +/- 0.006 (R2 = 0.966 +/- 0.004). The RMS estimation error was 120 +/- 11 ml. Using this algorithm it is possible to effectively estimate flow in a rotary blood pump without implanting additional invasive sensors.

Development and surgical implantation of a vision prosthesis model into the ovine eye

PURPOSE

A surgical technique was designed and tested to enable the implantation of an intraocular electrical retinal stimulator.

METHOD

An inoperative perspex and silicone model was constructed to closely resemble the anticipated properties of the proposed visual prosthesis. The animal model chosen for these experiments was the sheep, because the dimensions of its ocular anatomy are approximately 30% larger than the human's, being otherwise grossly similar. The surgical method involved transplanar port-hole lensectomy and vitrectomy, insertion of the model implant through a limbal incision, and fixation of the perspex subunit close to the location of the native crystalline lens, by way of trans-pars plana fixation sutures. Adequate pre-retinal positioning of the implant's silicone extension was obtained by way of its inherent elastic recoil.

RESULTS

The procedure was performed without macroscopic evidence of undue surgical trauma.

CONCLUSION

Although further long-term experiments are required to fully assess the surgical procedure and biocompatibility of the implant, intraoperative assessment and postmortem computed tomographic imaging of the globe has confirmed the successful intraocular positioning and fixation of the implant.

Inhomogeneity of action potential waveshape assists frequency entrainment of cardiac pacemaker cells

In this paper, we have employed ionic models of sinoatrial node cells to investigate the synchronization of a pair of coupled cardiac pacemaker cells from central and peripheral regions of the sinoatrial node. The free-running cycle length of the cell models was perturbed using two independent techniques and the minimum coupling conductance required to achieve frequency entrainment was used to assess the relative ease with which various cell pairs achieve entrainment. The factors effecting entrainment were further investigated using single-cell models paced with an artificial biphasic coupling current. Our simulation results suggest that dissimilar cell types, those with largely different upstroke velocities entrain more easily, that is, they require less coupling conductance to achieve 1:1 frequency entrainment. We, therefore, propose that regional variation in action-potential waveshape within the sinoatrial node assists frequency synchronization in vivo.

Vagal entrainment of heart rate is simulated by an integrator with feedback

Paradoxical stable entrainment of heart rate to inhibitory vagal impulses can be simulated with two distinct mathematical models; a complex ionic current model of sinoatrial node pacemaker activity, as well as a simple integrator with non-linear feedback. We show that both models exhibit similar entrainment characteristics to repetitive vagal stimuli. By applying a sharp disturbance to each model whilst entrained, the subsequent path of cycle length recovery can be described by dynamic phase response curves and phase-phase plots, the properties of which dictate whether stable entrainment is possible.

CMOS neurostimulation ASIC with 100 channels, scaleable output, and bidirectional radio-frequency telemetry

100-channel neurostimulation circuit comprising a complementary metal oxide semiconductor (CMOS), application-specific integrated circuit (ASIC) has been designed, constructed and tested. The ASIC forms a significant milestone and an integral component of a 100-electrode neurostimulation system being developed by the authors. The system comprises an externally worn transmitter and a body implantable stimulator. The purpose of the system is to communicate both data and power across tissue via radio-frequency (RF) telemetry such that externally programmable, constant current, charge balanced, biphasic stimuli may be delivered to neural tissue at 100 unique sites. An intrinsic reverse telemetry feature of the ASIC has been designed such that information pertaining to the device function, reconstruction of the stimulation voltage waveform, and the measurement of impedance may be obtained through noninvasive means. To compensate for the paucity of data pertaining to the stimulation thresholds necessary in evoking a physiological response, the ASIC has been designed with scaleable current output. The ASIC has been designed primarily as a treatment of degenerative disorders of the retina whereby the 100 channels are to be utilized in the delivery of a pattern of stimuli of varying intensity and or duty cycle to the surviving neural tissue of the retina. However, it is conceivable that other fields of neurostimulation such as cochlear prosthetics and functional electronic stimulation may benefit from the employment of the system.

Sensorless flow and head estimation in the VentrAssist rotary blood pump

Flow rate and pressure difference (or head) are key variables needed in the control of implantable rotary blood pumps. However, use of flow and/or pressure probes can decrease reliability and increase system power consumption and expense. For a given fluid viscosity, the flow state is determined by any 2 of the 4 pump variables: Flow, pressure difference, speed, and motor input power can be used. Thus, if viscosity is known or if its influence is sufficiently small, flow rate and pressure difference can be estimated from the motor speed and motor input power. For the VentrAssist centrifugal blood pump, which uses a hydrodynamic bearing, sensorless flow and pressure head estimation accuracy of 2 of our impeller designs were compared for a viscosity range of 1.2 to 4.5 mPas. This showed impeller design optimization can improve estimation accuracy. We also compared estimation accuracy using 2 blood analogues used in vitro, aqueous glycerol and red blood cells suspended in Haemaccel. The nature of the blood analogue and not only the viscosity of the fluid seems to influence estimation accuracy in our pump.

The potential impact of home telecare on clinical practice

Home telecare, in which the health status of patients at home is monitored remotely, has the potential to improve care and reduce costs. Its widespread implementation would require fundamental changes in the healthcare system.

Information technology in primary health care

Appropriate application of information technology in primary health care will extend traditional diagnosis and patient management beyond the doctor's clinic into the everyday living environment. We describe a model of information management in primary health care, and place special emphasis on the emerging areas of clinical decision support, computerised clinical measurements, patient education and network connectivity. Briefly discussed is the design of innovative home monitoring techniques and a telemedicine based communications infrastructure that should improve access to high quality primary health care for all citizens, irrespective of their distance from major urban centres. A preliminary design for a telemedicine-assisted primary health care network is presented, based on this model of information management. The premise is that improvements in health care services and reductions in health care costs can be effected by establishing a continuum of patient care from the patient's home, to the doctor's surgery, to speciality services in hospitals and to other service providers in the health care sector. While, the proposal focuses on new opportunities arising from the imminent introduction of broad band interactive fibre optic networks throughout Australia, the technology and projected data transfers could easily be handled in the short-term using modem access to the standard telephone network. A simple connectivity scheme for system integration is also presented.

A web-based approach for electrocardiogram monitoring in the home

A Web-based electrocardiogram (ECG) monitoring service in which a longitudinal clinical record is used for management of patients, is described. The Web application is used to collect clinical data from the patient's home. A database on the server acts as a central repository where this clinical information is stored. A Web browser provides access to the patient's records and ECG data. We discuss the technologies used to automate the retrieval and storage of clinical data from a patient database, and the recording and reviewing of clinical measurement data. On the client's Web browser, ActiveX controls embedded in the Web pages provide a link between the various components including the Web server, Web page, the specialised client side ECG review and acquisition software, and the local file system. The ActiveX controls also implement FTP functions to retrieve and submit clinical data to and from the server. An intelligent software agent on the server is activated whenever new ECG data is sent from the home. The agent compares historical data with newly acquired data. Using this method, an optimum patient care strategy can be evaluated, a summarised report along with reminders and suggestions for action is sent to the doctor and patient by email.

The bionic eye (electronic visual prosthesis): a review

The concept of a visual prosthesis for the blind or partially sighted is not a new one. Indeed, for more than three decades this technology based treatment for blindness has appeared imminent. Despite the concerted efforts of numerous physicians, scientists and engineers, the successful application of a useful visual prosthesis remains elusive. The present review will endeavour to describe past efforts, investigate the present state of the art and indicate the obstacles that must be overcome in order to bring an electronic visual prosthesis to fruition.

Review of ionic models of vagal-cardiac pacemaker control

Mathematical models of ion currents in pacemaker cells of the heart and their associated modulation by vagal stimulation have provided numerous insights into the ionic mechanisms underlying parasympathetic control of heart rate. In this article, ionic models described in the literature are reviewed and compared, with a view to examining their effectiveness in reproducing known chronotropic responses to vagal stimulation.

Monte Carlo simulation of tuning capacitor selection in a cochlear implant

The principles of Monte Carlo simulation are applied to critical manufacturing process in the construction of cochlear implants. In this application, the tuning of the implant's radio frequency (RF) communication system is dependent upon a "select on test" tuning capacitance. The aim is to accurately predict the usage of capacitors of a given size in future production of cochlear implants using Monte-Carlo simulation. The predicted quantities may then be purchased from a supplier to satisfy the requirements of high volume manufacturing without maintaining unnecessary and expensive stock levels of infrequently or never used capacitors. Simulations indicate that for large production runs, prediction of the necessary quantities of particular tuning capacitor sizes may enable the implant manufacturer to reduce their capacitor stock levels by as much as 82 percent. This significant reduction is in comparison to maintaining a uniform stock level of all sizes of capacitors, assuming random probability of requiring each size of tuning capacitor.

Simulations of postvagal tachycardia at the single cell pacemaker level: a new hypothesis

Simulations performed on a single cell model of rabbit sinoatrial node activity after prolonged vagal stimulation have been able to reproduce the known characteristics of cycle length recovery, including the presence of rapid and slow recovery phases and the transient undershoot phenomenon known as postvagal tachycardia (PVT). In the model, the PVT component has been hypothesized to result from the recovery of background levels of the muscarinic K+ current iK,ACh from desensitization due to prolonged exposure to acetylcholine (ACh) neurotransmitter. Other components of the recovery were found to be due to the inactivation of iK,ACh after the hydrolysis of ACh (rapid phase) and the recovery of the hyperpolarizing-activated current i(f) from its ACh-induced inhibition (slow phase). The magnitudes of both the rapid component and the PVT were found to increase linearly with preceding vagally mediated increase in cycle length, whereas the gain of the slow component was found to saturate, reflecting the limited contribution of i(f) inhibition to cycle prolongation.

Vagal control of sinoatrial rhythm: a mathematical model

The ionic mechanisms underlying vagal control of the cardiac pacemaker were investigated using a new single cell mathematical model of sinoatrial node electrical activity. The model was formulated from a wide range of electrophysiological data available in the literature, with particular reference to whole cell recordings from enzymatically isolated sinoatrial node cells. Development of the model was prompted by the lack of an existing physiologically accurate formulation of sinoatrial node activity that could reproduce the known complex chronotropic response of the pacemaker to brief-burst vagal stimulation, as observed in whole animal and isolated sinus node preparations. Features of the model include the dynamic modulation of the hyperpolarisation-activated current (i(f)) and the L-type calcium current (iCa,L) by acetylcholine, the improved characterisation of the muscarinic potassium current (iK,ACh), assigning the entire background potassium current (ib,K) to spontaneous openings of its channels, and the utilisation of second order kinetics for acetylcholine within the neuroeffector junction. Simulations performed using brief vagal stimuli elicited a strong hyperpolarisation of the membrane which prolonged the cycle in which it was delivered in a phase-dependent manner. This phase-dependency was presented in the form of a standard phase response curve which was characterised by a positive linear slope region, a breakpoint characteristic and a "no effect" zone in which the vagal pulse could no longer prolong the cycle. The breakpoint was manifested as a discontinuity in the curve which was examined by bracketing this point at the limit of the double precision arithmetic employed. At these boundary points on either side of the breakpoint, the vagal stimulus was able to activate outward iK,ACh in such a manner as to finely balance the increasing inward iCa,L trying to generate phase 0 upstroke. On decay of iK,ACh, the membrane either subsequently repolarised or fired to produce an action potential depending on the precise phase of the stimulus. The positive linear slope portion of the PRC was characterised by a strong resetting type behaviour in which the membrane hyperpolarised to approximately the same value, irrespective of the phase of stimulus delivery. For vagal stimulus bursts applied throughout the "no effect" zone, outward iK,ACh was not sufficiently activated in order to overcome the strong inward drive of iCa,L and could not prevent upstroke occurring. For these vagal stimuli, the subsequent cycle was hyperpolarised and prolonged. The size of the "no effect" zone was directly related to the inherent latency incorporated in the activation characteristic of iK,ACh. In contrast to previous models of vagal pacemaker control, our new model was able to reproduce the classical triphasic chronotropic response to brief vagal stimulation characterised by a primary inhibition response, a postinhibitory rebound and a secondary inhibition response. In particular, the postinhibitory rebound was due to activation of the inward hyperpolarisation-activated current by the vagally-induced membrane hyperpolarisation, whilst the secondary inhibition phase resulted from the inhibition of the hyperpolarisation-activated current by acetylcholine. The model suggests that the complex chronotropic responses of the cardiac pacemaker to brief vagal stimulation arises from inherent ionic mechanisms operating within the sinoatrial node.

Remote monitoring of health status of the elderly at home. A multidisciplinary project on aging at the University of New South Wales

This paper discusses the design and implementation of a multidisciplinary research project and associated field trials to test the hypothesis that functional health status amongst the elderly can be accurately determined remotely by continuously monitoring relatively simple parameters that measure the interaction between participants and their environment. In this study we propose that changes in such simple measures as mobility, sleep patterns, and utilisation of cooking, washing and toilet facilities, can identify changes in functional health status. One of the primary end goals of the project will be to automatically prompt appropriate, timely and cost-effective intervention of medical and community based services to help reduce morbidity and maintain an independent high quality of life for the elderly. Targeted intervention will diminish the demand for high cost medical services. This will have large potential economic implications in helping to contain and reduce the increasing cost of providing health care services to the aged.

Exogenous arginine vasopressin does not enhance carotid baroreflex control in the conscious dog

Arginine vasopressin (AVP) has profound effects on the cardiovascular system, yet has minimal pressor activity at physiological levels in intact subjects. We designed an investigation to delineate the effects of AVP on open-loop carotid baroreflex control of mean arterial pressure (MAP), total peripheral resistance (TPR), and cardiac output (CO) in conscious, chronically instrumented dogs. During graded infusions of AVP (0.5-2.0 ng.kg-1.min-1), the open-loop hemodynamic responses to controlled changes in isolated carotid sinus pressure (CSP) were determined. Increasing levels of AVP infusion led to significant increases in plasma AVP levels (P < 0.01). Increasing doses of AVP led to significant increases in TPR at all levels of CSP (P < 0.01). The overall range and gain of the response were not significantly different at any level of AVP infusion. Despite this increase in systemic resistance, there was no significant change in the MAP-CSP relationship. Infusion of AVP led to a dose-dependent depression in CO (P < 0.01) and heart rate (HR; P < 0.05) at all levels of CSP with no significant effect on open-loop baroreflex control. We conclude that although exogenous AVP induces profound changes in cardiovascular function, it does not alter carotid baroreflex control of MAP, TPR, CO, and HR.