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Biomedical subjects

N H Lovell

Publications and source records attributed to N H Lovell.

At least 19 recordsLinked to original sources
PubMed2005

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%.

Artificial Intelligence↗
PubMed2005

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.

Animals↗
PubMed2004

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.

Acceleration↗
PubMed2004

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.

Adult↗
PubMed2003

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.

Acceleration↗
PubMed2003

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.

Algorithms↗
PubMed2001

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.

Feedback↗
PubMed2001

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.

Electric Impedance↗
PubMed2001

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.

Action Potentials↗
PubMed2000

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.

Blood Viscosity↗
PubMed1999

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.

Activities of Daily Living↗
PubMed1999

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.

Data Collection↗
PubMed1999

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.

Computer Systems↗
PubMed1998

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.

Computer Simulation↗
PubMed1998

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.

Bionics↗
PubMed1998

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.

Cochlear Implants↗
PubMed1996

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.

Acetylcholine↗