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Biomedical subjects

L Tarassenko

Publications and source records attributed to L Tarassenko.

At least 19 recordsLinked to original sources
PubMed2001

Tracking poles with an autoregressive model: a confidence index for the analysis of the intrapartum cardiotocogram.

Clinicians often rely upon the cardiotocogram, a display of the fetal heart rate and maternal uterine activity (UA) over time, as a means of monitoring fetal health during labour. Fetal health can be monitored adequately only when the signal quality of the cardiotocogram is good. We propose an automated assessment of UA signal quality in order to create a confidence index for subsequent analysis of the intrapartum cardiotocogram. We use an autoregressive (AR) model of the UA to estimate the power at the contraction frequency, with high power indicative of "good" UA signal quality. 5th, 10th, and 15th-order AR models are used to assess the signal quality of 12 intrapartum UA traces as "good/medium" or "poor". We compare our results to two experts' visual assessments of signal quality. The 10th-order model exhibits the highest percent agreement rate of 62%. It also exhibits the most balanced false positive and false negative rates, where "good" or "medium" signal quality is considered a positive and "poor" signal quality a negative. The 10th-order model can therefore be used as a confidence index to reduce the errors made in the identification of uterine contractions in the UA trace and in the subsequent analysis of the cardiotocogram as a whole.

Cardiotocography↗
PubMed1999

Second by second patterns in cortical electroencephalograph and systolic blood pressure during Cheyne-Stokes.

Little is known about how arousal develops during the ventilatory phase of Cheyne-Stokes breathing. This study employs neural network analysis of electroencephalograms (EEGs) to describe these changes and relate them to changes in systolic blood pressure, which is probably a subcortical marker of arousal. Six patients with Cheyne-Stokes respiration (apnoea/hypopnoea index 32-69 h(-1)) caused by stable chronic heart failure underwent polysomnography including arterial beat-to-beat systolic blood pressure determination. Periods of 15 sequential apnoeas during nonrapid eye movement sleep were identified for each subject. For each apnoea, the EEG was examined second-by-second using neural net analysis from 28 s before to 28 s after apnoea termination (first return of oronasal airflow), and this was compared with the systolic blood pressure pattern. During the apnoeic phase, sleep deepened progressively. Arousal started to develop at or just before apnoea termination and progresses through the breathing phase. The rise and fall in the systolic blood pressure closely followed the rise and fall in electroencephalographic sleep depth. In conclusion, during Cheyne-Stokes breathing, cortical electroencephalographic arousal begins at or just before the resumption of breathing. Cortical electroencephalographic sleep depth changes are closely mirrored by changes in arterial systolic blood pressure, suggesting that the state changes in the cortical and basal brain structures may be synchronous.

Arousal↗
PubMed1998

Sleep studies of adults with severe or profound mental retardation and epilepsy.

Sleep patterns of people with mental retardation have received little research attention. This is an important gap in knowledge because understanding the relation between sleep and wakefulness may be critical to care provision. Descriptive sleep information on 28 people with severe or profound mental retardation and epilepsy was presented here. Sleep EEG data, studied both conventionally and by means of a neural network-based sleep analysis system suggest atypical sleep stages with significant depletion of REM sleep and a predominance of "indiscriminate" non-REM sleep. Sleep diaries completed by caregivers reveal lengthy sleep period times, especially among those with profound mental retardation. Possible explanations for these results and their implications were discussed.

Adolescent↗
PubMed1997

Analysis of dynamic MR breast images using a model of contrast enhancement.

We describe a model of dynamic contrast enhancement in breast MRI designed to aid the radiologist in cases for which X-ray mammography is ineffective. The breasts are segmented from the image slices by a dynamic programming algorithm after morphological opening. A pharmacokinetic model has been derived to fit the rise in intensities after injection of a contrast agent, in a way that facilitates investigation of the effects of different models of bolus injection. The pharmacokinetic model is used in a modified Horn-Schunck algorithm to correct for motion effects during the seven minute acquisition period. The results show significant localization of tumours and enable discrimination of cancerous tissue. In particular, we illustrate the approach with an image that shows a carcinoma, whose appearance and localization are greatly improved by the registration algorithm.

Algorithms↗
PubMed1996

A review of parametric modelling techniques for EEG analysis.

This review provides an introduction to the use of parametric modelling techniques for time series analysis, and in particular the application of autoregressive modelling to the analysis of physiological signals such as the human electroencephalogram. The concept of signal stationarity is considered and, in the light of this, both adaptive models, and non-adaptive models employing fixed or adaptive segmentation, are discussed. For non-adaptive autoregressive models, the Yule-Walker equations are derived and the popular Levinson-Durbin and Burg algorithms are introduced. The interpretation of an autoregressive model as a recursive digital filter and its use in spectral estimation are considered, and the important issues of model stability and model complexity are discussed.

Biomedical Engineering↗
PubMed1996

A new approach to the analysis of the human sleep/wakefulness continuum.

The conventional approach to the analysis of human sleep uses a set of pre-defined rules to allocate each 20 or 30-s epoch to one of six main sleep stages. The application of these rules is performed either manually, by visual inspection of the electroencephalogram and related signals, or, more recently, by a software implementation of these rules on a computer. This article evaluates the limitations of rule-based sleep staging and then presents a new method of sleep analysis that makes no such use of pre-defined rules and stages, tracking instead the dynamic development of sleep on a continuous scale. The extraction of meaningful features from the electroencephalogram is first considered, and for this purpose a technique called autoregressive modelling was preferred to the more commonly-used methods of band-pass filtering or the fast Fourier transform. This is followed by a qualitative investigation into the dynamics of the electroencephalogram during sleep using a technique for data visualization known as a self-organizing feature map. The insights gained using this map led to the subsequent development of a new, quantitative method of sleep analysis that utilizes the pattern recognition capabilities of an artificial neural network. The outputs from this network provide a second-by-second quantification of the sleep/wakefulness continuum with a resolution that far exceeds that of rule-based sleep staging. This is demonstrated by the neural network's ability to pinpoint micro-arousals and highlight periods of severely disturbed sleep caused by certain sleep disorders. Both these phenomena are of considerable clinical value, but neither are scored satisfactorily using rule-based sleep staging.

Adult↗
PubMed1996

Effect of short term graded withdrawal of nasal continuous positive airway pressure on systemic blood pressure in patients with obstructive sleep apnoea.

It is debated whether obstructive sleep apnoea (OSA) is a significant independent risk factor for sustained hypertension or cardiovascular morbidity and mortality. In an attempt to avoid the problem of confounding variables we have investigated whether withdrawing nasal continuous positive airway pressure (NCPAP) from patients with OSA for different proportions of the night leads to a subsequent rise in their morning blood pressures. Six patients with treated OSA had their NCPAP automatically varied between 3 cms H2O and a therapeutic pressure over 5 successive nights. The proportion of therapeutic NCPAP given was kept constant over the 5 nights and blood pressure measured the morning after the 5th night. Each patient had 5 different levels of sleep disruption, from no therapeutic NCPAP at all, through to 100% NCPAP. The nocturnal consequences of these different proportions of NCPAP were quantified both by oximetry and by a new EEG analysis that provides an objective estimate of the periodicity (fluctuations in the EEG depth) of the time course seen in patients with OSA. Increasing degrees of nocturnal hypoxic dipping and EEG periodicity were positively correlated with the subsequent morning systolic and diastolic blood pressures (p < 0.02). About 20% of the variance in systolic and diastolic blood pressure could be accounted for by the amount of either hypoxic dipping or EEG periodicity. The results of this study suggest that acute changes in awake blood pressure can be caused by sleep apnoea. It agrees with other data suggesting that OSA can have an independent influence on morning BP, but that this effect may have worn off by the afternoon and evening. Some of the discrepancies between the numerous studies in this area may be due to the timing of blood pressure measurements.

Aged↗
PubMed1993

Pulse oximetry: theoretical and experimental models.

In the paper a pulse oximetry model is developed using an approach which combines both theoretical and empirical modelling. The optical properties of whole blood are measured as a function of cuvette depth by transmission spectrophotometry using red (660 nm) and infra-red (950 nm) light-emitting diodes as light sources. Twersky's theoretical model gives the best fit to the experimental data. A simple theoretical model which takes into account the nonlinear relationship between optical density and cuvette depth is then used to obtain an expression for the R:IR ratio, which relates the measurement of transmission at the two wavelengths. The R:IR ratio is found to be more or less independent of cuvette depth (SD = 0.14 at 100 per cent SaO2). To validate the predictions of the theoretical model, the results of a previous experiment in which the relationship between SaO2 and the R:IR ratio was recorded using a flexible cuvette are used. The experimental values are found to lie within one standard deviation from the theoretical curve relating SaO2 and the R:IR ratio. It is argued that a reasonably accurate model for pulse oximetry which is based on whole blood and not haemoglobin solutions has been developed.

Humans↗
PubMed1993

Reflectance pulse oximetry measurements from the retinal fundus.

Conventional transmission pulse oximetry is a noninvasive technique for the continuous monitoring of arterial oxygen saturation (SaO2) from peripheral vascular beds such as the finger tip or earlobe. In this paper we propose to exploit the unique transparency of the ocular media to make reflectance pulse oximetry measurements on the retinal fundus. This technique potentially offers significant advantages over conventional pulse oximetry, primarily the ability to monitor cerebral, as opposed to peripheral, oxygen saturation. We have developed an in vitro system to stimulate the retinal circulation and ocular optics. This system consists of a flexible cuvette located in a model eye and an extracorporeal blood circuit to stimulate arterial blood flow. The system was used to investigate the relationship between SaO2 and the R/IR ratio in reflectance pulse oximetry. To enable in vivo measurements to be made, we also modified a standard haptic contact lens to hold the pulse oximeter probe in front of the pupil. In a preliminary study, the lens was fitted to an awake volunteer and cardiac-synchronous signals were detected by the retinal pulse oximeter.

Contact Lenses↗
PubMed1993

On-chip learning with analogue VLSI neural networks.

Results from simulations of weight perturbation as an on-chip learning scheme for analogue VLSI neural networks are presented. The limitations of analogue hardware are modelled as realistically as possible. Thus synaptic weight precision is defined according to the smallest change in the weight setting voltage which gives a measurable change at the output of the corresponding neuron. Tests are carried out on a hard classification problem constructed from mobile robot navigation data. The simulations show that the degradation in classification performance on a 500-pattern test set caused by the introduction of realistic hardware constraints is acceptable: with 8-bit weights, updated probabilistically and with a simplified output error criterion, the error rate increases by no more than 7% when compared with weight perturbation implemented with full 32-bit precision.

Algorithms↗
PubMed1992

New method of automated sleep quantification.

Since its discovery some 50 years ago, the electro-encephalogram (EEG) has formed the basis for classification of sleep into several stages, either laboriously performed by visual examination of the EEG and related signals or, more recently, by automated techniques. Both visual scoring and most automated analyses are highly subjective and rely on application of a predefined set of rules. A method of analysing the EEG which requires no such application of rules and aims to give some indication of the dynamics of sleep in humans is proposed in the paper.

Adult↗
PubMed1992

Quantitative optical determination of the viability of platelet concentrates.

An optical method to assess platelet viability in platelet transfusion concentrates has been developed which is based on the observation that healthy, discoid platelets transmit more light in shear flow than aged, spherical cells. Using a specially designed glass channel, the rise in transmission of light through platelet samples with agitation is referenced against a measurement of transmission without flow. Referencing the measurements may minimize the effects of red blood cell contamination, as well as other variations among platelet packs such as plasma characteristics and plastic container variability. At an optimal frequency of agitation, the ratio of the transmission of light with agitation to that of the sample at rest is found to correlate highly with the concentration of discoid platelets, r = 0.92 to 0.95 (P less than 0.001), for light sources including a HeNe laser and two LEDs. Using this index, the discoid platelet counts of twelve platelet packs are estimated and compared with manual counts. The mean difference between the two methods was 0.009 x 10(9) discoid platelets ml-1 with a standard deviation of 0.11 x 10(9) cells ml-1. We conclude that the method may be applied to the development of a non-invasive device to monitor the concentration of viable, discoid platelets in storage.

Blood Platelets↗
PubMed1992

An optical method for the determination of platelet count in platelet samples contaminated with red blood cells.

We present a simple photometric method to determine the total concentration of platelets present in a sample independently of red blood cell concentration. Standard optical density curves for platelet samples ranging in concentration from 0 to 1.5 x 10(9) cells/ml and contaminated with red blood cells ranging in concentration from 0 to 0.03 x 10(9) cells/ml are determined. A study of the absorbance spectra of red blood cells and platelets suggests that by calculating the absorbance difference between two wavelengths, an estimate of red blood cell concentration can be made. Then, in the second step of this two-step method, the individual absorbance measurements at the two wavelengths are matched to the standard values determined previously to derive an estimate of platelet concentration. In a trial of 62 unknown platelet samples contaminated with red blood cells, the standard deviation for the error in platelet count was 0.16 x 10(9) cells/ml with a mean difference of 0.011 x 10(9) platelets/ml. We conclude that our method may be useful in laboratories not equipped with electronic cell counters as well as in applications such as the development of noninvasive measurements of platelet concentration in platelet transfusion packs.

Erythrocytes↗
PubMed1991

In vitro investigation of the factors affecting pulse oximetry.

The effect of a number of physiological parameters on pulse oximetry accuracy has been investigated in an in vitro model. We have found that above 50% saturation, pulse oximeters will not be affected by variations in haematocrit, blood flow rate, tissue blood content and pulse amplitude. At low saturations, however, it is known that the accuracy of pulse oximeters decreases and our in vitro results suggest how this may be corrected.

Blood Flow Velocity↗
PubMed1991

Temperature dependence of led and its theoretical effect on pulse oximetry.

Ambient temperature is known to affect the emission spectrum of a light-emitting diode (LED). This study has investigated the effect of changes in ambient temperature on the emission spectra of two LED with peak emission wavelengths similar to those used in pulse oximetry. There was a 5.5-nm increase in the peak wavelength for a 660-nm LED, and a 7.8-nm increase in the peak wavelength for a 950-nm LED as temperature increased from 0 to 50 degrees C. Using a simple theoretical model based on the Beer-Lambert law, the effect of these shifts in wavelength on pulse oximeter accuracy was examined and found to be negligible over the temperature range studied.

Electrodes↗
PubMed1991

Absorption and multiple scattering by suspensions of aligned red blood cells.

The analytical theory of multiple scattering [J. Opt. Soc. Am. 60, 1084 (1970)] permits predictions of scattering patterns by homogeneous suspensions of aligned and randomly oriented particles. Predictions for randomly oriented particles have been tested previously. Using an optical system involving a He-Ne laser and suspensions of red blood cells, we tested the theory's predictions for scattering by suspensions in two distinct alignments. The qualitative effects of cell alignment on light scattering are consistent with those predicted, although measured differences in scattering between the two alignments exceed those predicted. We conclude that the theory may provide an optical means of distinguishing particle orientation in multiple scattering suspensions.

Absorption↗
PubMed1988

Quantitative analysis of immunological reactions on silicon surfaces by multiple-angle Brewster angle reflectometry.

An optical biosensing instrument has been developed for the quantitative analysis of immunological reactions on biochemically sensitized surfaces. It is based on the measurement of reflectance changes of polarized laser light incident on high refractive index substrates at angles close to the pseudo-Brewster angle. Multiple-angle Brewster angle reflectometry (MABAR) has been used to investigate the binding of anti-human serum albumin (a-HSA) to human serum albumin (HSA) coated silicon surfaces. The concentration dependence and reaction kinetics of antibody-antigen complex formation have been studied using red and green He-Ne laser light. A significant increase in sensitivity has been observed with green light.

Antibodies↗