Showing papers in "Physiological Measurement in 1998"

The impedivity of freshly excised human breast tissue

Abstract: A set of 120 impedivity spectra was collected in breast tissue immediately after excision from 64 patients undergoing breast surgery. The measurements were made at 12 discrete frequencies halving from 1 MHz to 488 Hz. The spectra were sorted into three groups of normal tissue, mammary gland, connective tissue and adipose tissue, and three groups of pathological tissue, mastopathy, fibroadenoma and carcinoma. Intergroup multiple comparisons of the components of impedivity and admittivity were systematically carried out at every measurement frequency. The low-frequency-limit resistivity, the fractional power and characteristic frequencies were calculated from the experimental data. No significant correlation between impedivity and admittivity and patient's age was observed, except for mastopathy. No significant difference between groups of normal tissue and benign pathology (mammary gland, mastopathy and fibroadenoma) was found. The group of carcinoma differed from all the other groups principally by the low-frequency-limit resistivity, the fractional power and the phase angle at frequency above 125 kHz. These results indicate that impedance spectroscopy is appropriate for the detection of breast cancer.

The variability of the photoplethysmographic signal - a potential method for the evaluation of the autonomic nervous system

Abstract: The heart rate variability is composed of low- and high-frequency fluctuations, which are mediated by the sympathetic and the parasympathetic nervous systems. The baseline and the amplitude of the photoplethysmographic (PPG) signal also show fluctuations in the same frequencies. In the current study, PPG examinations were performed on the fingers of normal subjects and diabetic patients, and three parameters were derived from each PPG pulse: the baseline of the pulse, its amplitude and its period (which is equal to the heart period). The level of the variability of each PPG pulse parameter was measured by the ratio of the standard deviation of the parameter to its mean value. The level of the low-frequency fluctuations for the PPG amplitude and for the heart cycle period did not differ between males and females, but was lower for diabetic patients, indicating lower activity of the autonomic nervous system. The curves of the baseline and the amplitude of the PPG signal for the non-diabetic subjects showed high correlation between the left and the right hands. For most of the diabetic patients the right-left correlation coefficients were significantly lower than those for the non-diabetic subjects. Our initial results have shown that the variability of the PPG parameters shows promise for the assessment of the function of the autonomic nervous system.

Ventilation and perfusion imaging by electrical impedance tomography : a comparison with radionuclide scanning

Abstract: Electrical impedance tomography (EIT) is a technique that makes it possible to measure ventilation and pulmonary perfusion in a volume that approximates to a 2D plane. The possibility of using EIT for measuring the left-right division of ventilation and perfusion was compared with that of radionuclide imaging. Following routine ventilation (81mKr) and perfusion scanning (99mTc-MAA), EIT measurements were performed at the third and the sixth intercostal level in 14 patients with lung cancer. A correlation (r = 0.98, p < 0.005) between the left-right division for the ventilation measured with EIT and that with 81mKr was found. For the left-right division of pulmonary perfusion a correlation of 0.95 (p < 0.005) was found between the two methods. The reliability coefficient (RC) was calculated for estimating the left-right division with EIT. The RC for the ventilation measurements was 94% and 96% for the perfusion measurements. The correlation analysis for reproducibility of the EIT measurements was 0.95 (p < 0.001) for the ventilation and 0.93 (p < 0.001) for the perfusion measurements. In conclusion, EIT can be regarded as a promising technique to estimate the left-right division of pulmonary perfusion and ventilation.

A model of artefacts produced by stray capacitance during whole body or segmental bioimpedance spectroscopy

Abstract: We have developed a novel model for the simulation of artefacts which are produced by stray capacitance during bioimpedance spectroscopy. We focused on whole body and segmental measurements in the frequency range 5-1000 kHz. The current source was assumed to by asymmetric with respect to ground as is the case for many commercial devices. We considered the following stray pathways: 1, cable capacitance; 2, capacitance between neighbouring electrode leads; 3. capacitance between different body segments and earth; 4, capacitance between signal ground of the device and earth. According to our results the pathways 3 and 4 cause a significant spurious dispersion in the measured impedance spectra at frequencies > 500 kHz. During segmental measurements the spectra have been found to be sensitive to an interchange of the electrode cable pairs. The sensitivity was also observed in vivo and is due to asymmetry of the potential distribution along the segment with respect to earth. In contrast to previously published approaches, our model renders possible the simulation of this effect. However, it is unable to fully explain the deviations of in vivo measured impedance spectra from a single Cole circle. We postulate that the remaining deviations are due to a physiologically caused superposition of two dispersions from two different tissues.

Non-invasive microwave radiometry thermometry

Abstract: Near-field microwave radiometry and radiometric imaging are non-invasive techniques that are able to provide temperature information at a depth of up to several centimetres in subcutaneous tissues They are based on the measurement of microwave electromagnetic thermal noise This paper describes the basic principles, measurement methods and limitations of the techniques and the results of clinical studies, and it reviews recent progress

Human perspiration measurement

Abstract: We review various methods developed for human perspiration measurement and their physiological applications, with special reference to the performance and application of a new home-made ratemeter and instrumentation with a microscope. Many kinds of humidity sensor based on humidity-sensitive electrical properties have been investigated and placed on the market. Recently a capacitive thin-film humidity sensor was constructed and confirmed to be one of the best humidity sensors for accurately and quickly detecting changes in the relative humidity of gas-flow perfused through a ventilated chamber for human perspiration measurement. In this paper we also introduce a new home-made ratemeter with a capacitive humidity sensor, the electrical output of which is not disturbed by changes in ambient temperature, and new instrumentation for directly observing drops of sweat secreted from eccrine glands in human skin and simultaneously measuring the change in amount of perspiration at the same area of skin. Finally, we review physiological applications of the methods for measuring human palmar perspiration including emotional sweating. Keywords: human perspiration, palmar sweating, sweat drops, skin potential

Sources of error in bioimpedance spectroscopy.

Abstract: Two different makes of bioimpedance spectrometer (UniQuest-SEAC SFB-3 and Xitron 4000B) were used for a series of measurements on volunteers and patients in intensive care. Although each machine was accurate over the frequency range 5 to 500 kHz when bench tested on model resistor-capacitor circuits, significant differences in their recorded impedance parameters appeared when used in vivo, especially on intensive care patients. A series of laboratory tests was performed on each machine simulating the situation in vivo to identify possible reasons for these differences. Whilst stray capacitance in the environment was identified as the major contributor to variability in high-frequency performance, interaction between electrode impedance and lead positioning was also a factor. The observed phase shift with frequency or time delay (Td) used in the Xitron modeling software appears to be the result of a time constant caused by stray capacitance and so is unlikely to have any biological meaning. Significant differences in the in vivo numerical values produced by bioimpedance spectrometers may be attributed to instrument design, data processing and, in particular, the clinical environment.

Pulmonary perfusion measured by means of electrical impedance tomography

Abstract: Electrical impedance tomography (EIT) is a recent imaging technique based on electrical impedance, offering the possibility of measuring pulmonary perfusion In the present study the influence of several pulmonary haemodynamical parameters on the EIT signal were investigated First, the influence on the systolic wave of the EIT signal (delta Zsys) of stroke volume, large pulmonary artery distensibility (both assessed by means of MRI) and the extent of the pulmonary peripheral vascular bed in 11 emphysematous patients (reduced peripheral vascular bed) and 9 controls (normal peripheral vascular bed) was investigated Second, the influence of hypoxic pulmonary vasoconstriction on delta Zsys was examined in 14 healthy subjects Finally, the origin of the diastolic wave was examined in three patients with atrioventricular dissociation Multiple regression analysis showed that delta Zsys was only dependent on the variable emphysema (p 09) The mean value of delta Zsys for emphysematous patients (131 +/- 32 arbitrary units (AU)) was significantly lower (p < 0001) than in the control group (200 +/- 39) In the group of healthy subjects delta Zsys decreased significantly (p < 0001) during hypoxia (193 +/- 38 AU) compared with rest measurements (260 +/- 62 AU) The absence of the diastolic wave in the cardiological patients suggests the influence of reverse venous blood flow on the EIT signal It is concluded that volume changes in the small pulmonary vessels contribute significantly to the EIT signal Moreover, the hypoxia induced decrease in delta Zsys indicates the potential of EIT for measuring pulmonary vascular responses to external stimuli

Potential errors in the application of mixture theory to multifrequency bioelectrical impedance analysis

Abstract: Potential errors in the application of mixture theory to the analysis of multiple-frequency bioelectrical impedance data for the determination of body fluid volumes are assessed. Potential sources of error include: conductive length; tissue fluid resistivity; body density; weight and technical errors of measurement. Inclusion of inaccurate estimates of body density and weight introduce errors of typically but incorrect assumptions regarding conductive length or fluid resistivities may each incur errors of up to 20%. Keywords: mixture theory, errors, bioelectrical impedance analysis, multiple-frequency bioelectrical impedance analysis

Design of Pulse Oximeters (Medical Science Series)

Abstract: Design of Pulse Oximeters is the latest in IOPPs Medical Science Series, the official book series of IFMBE and IOMP. The book is edited by Professor John Webster from the University of Wisconsin who has a substantial international reputation in bioengineering for producing lucid text books. With this sort of pedigree I looked forward to reviewing it. I was not disappointed. This is an excellent book, comprehensive, up-to-date and to be recommended to anyone involved in detailed use of pulse oximetry data or developing pulse oximetry techniques themselves. The first three chapters set the scene. The book starts with the physiology of oxygen transport itself and moves on to consider why the measurement is made and to set it in the context of blood oxygen measurement in general. This start gives the book firm roots which reduces the chance of the reader losing track of the physiological or diagnostic reasons for pursuing the detailed technical methods reported in the following chapters. After a chapter on the details of light absorption in blood and tissue that describes all the relevant physical limitations to the method, the book launches into detailed discussion of electronic design, signal processing, instrument control and probe design. The last few chapters are concerned with calibration, accuracy, user interface design and applications. The chapter on user interface design is particularly welcome. It is very easy for designers of equipment to lose sight of this aspect of design in instruments that will be used in environments where bad interface design can have a deleterious effect on human factors performance. Throughout the book instructional objectives are given for each chapter and the bibliographies of the majority of chapters are long enough to give students using the book starting points for further reading. From a teaching point of view the book could be used as an extended, in-depth case study of the implementation of a physiological measurement technology. It is a pity that the price of £60.00 in the UK will make it difficult to make the book compulsory in post-graduate physiological measurement courses.

Patient-instrument connection errors in bioelectrical impedance measurement

Abstract: Bioelectrical impedance in vivo measurement errors due to the connection between instrument and patient are analysed theoretically and experimentally. Special attention is paid to the patient cable capacitances and to the reduction of unwanted common-mode signals at the voltage amplifier input. Experiments with single-ended and symmetrical current sources, with different simulated body impedance values, with five types of cable connection and with three types of cable screen driving are carried out. The use of a balanced voltage-to-current transverter with a floating common point is recommended. Four separate patient cables should be used with separately driven screens by high-quality unity-gain buffers. Thus the impedance module measurement error can be reduced to below 1% in the range of 20 to and the phase measurement error to less than , for frequencies from 1 kHz to 300 kHz. Keywords: bioelectrical impedance, impedance spectroscopy, instrumentation, patient-instrument interface, measurement errors

A model for ultrasonic scattering in cancellous bone based on velocity fluctuations in a binary mixture

Abstract: A scattering model based on velocity fluctuations in a binary mixture (marrow fat and cortical matrix) was used to estimate the ultrasonic attenuation in cancellous bone as a function of volume fraction. The calculation of velocity fluctuations alone seems to be suitable for the qualitative estimation of attenuation. The predicted values of the attenuation were of the same order of magnitude as experimentally determined values from the literature. This agreement was achieved with only a small number of variables (the velocities of the two components and the scatterer size) in the model, representing a major advantage compared with other theories. Hence the suggested approach appears to be a good starting point for further theoretical investigations using scattering theories. However, this has to be accompanied by accurate ultrasonic and microstructural measurements. Keywords: ultrasound, bone, scattering, attenuation, velocity fluctuations

Comparison of mean skin temperature using `covered' versus `uncovered' contact thermistors

Abstract: The purpose of this study was to compare the mean skin temperature obtained using `covered' versus `uncovered' contact thermistors in a variety of environmental conditions. Ten male subjects walked on a treadmill at 6.4 kph for 30 min in three different environments: thermoneutral , hot-humid , and hot-dry . was measured using three `uncovered' contact thermistors, and three `covered' thermistors. The `uncovered' probes were attached using acrylic rings that allowed the outer surface of the thermistors to have free exchange with the environment. The `covered' probes were affixed to the skin using foam patches and tape. The latter procedure is commonly used in many research and clinical laboratories. The `covered' probes resulted in a significantly (p < 0.05) higher under all three environmental conditions. The largest difference occurred in the thermoneutral condition with the `covered' being higher than the `uncovered' . These results suggest that covering a contact skin thermistor hinders heat loss and results in an artificially high . It is hoped that the results of this study will encourage the adoption of `uncovered' contact skin thermistors in the future. Keywords: mean skin temperature, forearm skin temperature, contact themistors

Data analysis in multiple-frequency bioelectrical impedance analysis

Abstract: The performance of three analytical methods for multiple-frequency bioelectrical impedance analysis (MFBIA) data was assessed. The methods were the established method of Cole and Cole, the newly proposed method of Siconolfi and co-workers and a modification of this procedure. Method performance was assessed from the adequacy of the curve fitting techniques, as judged by the correlation coefficient and standard error of the estimate, and the accuracy of the different methods in determining the theoretical values of impedance parameters describing a set of model electrical circuits. The experimental data were well fitted by all curve-fitting procedures (r = 0.9 with SEE 0.3 to 3.5% or better for most circuit-procedure combinations). Cole-Cole modelling provided the most accurate estimates of circuit impedance values, generally within 1-2% of the theoretical values, followed by the Siconolfi procedure using a sixth-order polynomial regression (1-6% variation). None of the methods, however, accurately estimated circuit parameters when the measured impedances were low (< 20 omega) reflecting the electronic limits of the impedance meter used. These data suggest that Cole-Cole modelling remains the preferred method for the analysis of MFBIA data.

An assessment of the radiation dose to patients and staff from a Lunar Expert-XL fan beam densitometer

Abstract: Dual-energy x-ray absorptiometry (DXA) is a widely used technique for measuring bone mineral density for the identification and management of osteoporotic subjects. The original DXA pencil beam systems expose patients to an effective dose of ionizing radiation of around and require no additional protective shielding for staff. The new fan beam densitometers incorporate solid state detectors and have a higher photon flux, enabling faster acquisition times and giving improved resolution. However, this may be at the expense of higher radiation dose. This study was conducted to assess the radiation dose to patients and staff from the standard scan modes using a Lunar Expert-XL fan beam densitometer. This is, we believe, the first dose assessment of the Expert-XL. The results indicate that the scatter dose at 1 m from the scan table, assuming four AP spine and femoral neck examinations per hour, is about . This is well below the limit of set by the UK's Ionising Radiation Regulations for defining a Controlled Area but above the lesser limit of for a Supervised Area. Typical effective doses to patients are for an AP lumbar spine scan, up to for AP femoral neck, for lateral spine morphometry and for whole body. Although exceeding those of pencil beam DXA machines, these doses are less than for standard radiographic procedures, particularly of the lumbar spine. Where reduced scan time, improved image resolution or morphometric analysis of the spine are required, the patient doses from the Lunar Expert-XL are not prohibitive. Keywords: DXA, dosimetry, fan beam, BMD

Quantification of blood volume by electrical impedance tomography using a tissue-equivalent phantom.

Abstract: An in vivo electrical impedance tomography (EIT) system was designed to accurately estimate quantities of intra-peritoneal blood in the abdominal cavity. For this it is essential that the response is relatively independent of the position of the high conductivity anomaly (blood) in the body. The sensitivity of the system to the introduction of blood-equivalent resistivity anomalies was assessed by using a cylindrical tissue-equivalent phantom. It was found that a satisfactorily uniform response of the system in both radial (transverse) and axial (longitudinal) directions in the phantom could be achieved by filtering resistivity profile images obtained by EIT measurement, and by using extended electrodes to collect data. Post-processing of single impedance images gave rise to a quantity denoted the resistivity index. A filter was then used to remove the remaining radial variation of the resistivity index. It was calculated by evaluating the resistivity index of a number of theoretically calculated images, and constructing a correction filter similar to those used to remove lens imperfections, such as coma, in optical components. The 30% increase in the resistivity index observed when an anomaly was moved to the maximum extent allowed by the filter calculation (0.75 of the phantom radius) was reduced by the filter to 6%. A study of the axial dependence observed in the resistivity index using electrodes extended in the axial direction by found that the variation in resistivity index with axial position was about half of that observed using small circular electrodes similar to those used in the Sheffield mark I system. Keywords: electrical impedance tomography, volume estimation, linearized backprojection, intra-peritoneal bleeding

Quantifying changes in retinal circulation: the generation of parametric images from fluorescein angiograms

Abstract: Fluorescein angiography is an established technique for examining the functional integrity of the retinal circulation. The ability to quantify this function offers the possibility of early detection of changes due to retinopathy. We have developed a technique to generate functional, parametric images of the retinal circulation. A given angiogram is first registered to align consecutive frames. At each point in the retina, a graph of fluorescein intensity versus time is then constructed and fitted with a gamma variate curve. Parameters are extracted from these curves and formed into parametric images showing the variation in fluorescein passage across the entire area of the angiogram. Parameters examined to date include time to maximum intensity, time of arrival and rise time. The technique has been demonstrated using photographic and scanning laser ophthalmoscopic angiograms of both normal subjects and patients with a variety of retinopathies. The time to maximum images of the normal subjects reveals a similar filling pattern in each case, whilst the pathologies present in the abnormal angiograms are clearly identified. The generation of functional time to maximum images enables the health of the retinal circulation to be quantified with respect to the rate at which the vasculature fills with fluorescein. This offers a potential tool for detecting the onset of retinopathy and monitoring its progression. Keywords: fluorescein angiography, scanning laser ophthalmoscope, image processing, retinal circulation

Oesophageal transit time evaluated by a biomagnetic method

Abstract: The aim of this work was to determine the oesophageal transit time (OTT) of a bolus using the biomagnetic technique and compare the results to those obtained by means of scintigraphy. For the biomagnetic evaluation, a test meal (yoghurt) uniformly labelled with 5 g of powder ferrite was swallowed in a single gulp by 19 normal volunteers in the upright position. One sensor (first order gradiometer) was placed at the furcula and a second one at the xiphoid process to detect the passage of the test meal and the magnetic signal output was recorded in a computer. The OTT was determined by plotting the voltage signal against time. The scintigraphic technique was used in the same volunteers: the test meal was labelled with less than 350 MBq of 99mTc-phytate and swallowed in the same way. The bolus transit was recorded at 4 frames s(-1) (100-120 frames acquisition) and the OTT was determined by drawing two regions of interest in the same areas as the sensors. The results were determined by plotting counts against time. The averages for OTTs were 3.8 +/- 0.8 s for the scintigraphic technique and 4.6 +/- 0.9 s for the biomagnetic technique. Although scintigraphic OTT was significantly shorter than magnetic OTT, there was a significant correlation between them. We conclude that the biomagnetic study may be used to evaluate OTT.

Influences of lung parenchyma density and thoracic fluid on ventilatory EIT measurements

Abstract: Ventilatory impedance changes can be measured by electrical impedance tomography (EIT). Several studies have pointed out that the ventilatory-induced impedance change measured over the lungs shows a linear relationship with tidal volume. However, EIT measures the ventilatory impedance changes relative to a reference. Therefore, changes in the reference due to lung parenchyma destruction (increase of thoracic impedance) or lung water (decrease of thoracic impedance) might influence ventilatory EIT measurements. A study was designed to evaluate the influence of the density of lung parenchyma and the thoracic fluid content on ventilatory EIT measurements. Eleven emphysema patients with a variable degree of lung parenchyma destruction, nine haemodialysis patients with general fluid overload and ten healthy subjects were measured. The impedance changes were measured with the subject in the supine position breathing a constant tidal volume of 1 litre starting at the maximum end-expiratory level. In the emphysema group a significantly lower impedance change between ins- and expiration was found in comparison with the healthy subjects ( versus , p < 0.05), whereas the haemodialysis group showed a significantly larger impedance change between ins- and expiration before haemodialysis (, p < 0.05). A significant decrease in ventilation-induced impedance change during dialysis was found ( versus , p < 0.01). Furthermore, a significant correlation between lung function parameters, which indicate the severity of lung parenchyma destruction, and the measured impedance change was found in emphysema patients. From these results it can be concluded that the density of lung parenchyma and the thoracic fluid content have a serious impact on the ventilation-induced impedance change. Keywords: electrical impedance, emphysema, haemodialysis, tomography

Repeatability of body sway measurements; day-to-day variation measured by sway magnetometry

Abstract: The postural stability of 30 normal subjects in the age range 15 to 64 years was measured on five separate occasions, using the sway magnetometry technique, to determine the day-to-day repeatability of this measurement technique. On each occasion the pathlength and area enclosed by movement of the hips in a horizontal plane were measured over two 30 s periods with eyes open and eyes closed, with subjects standing on a firm base. Day-to-day repeatability had a standard deviation of 14% for pathlength and 42% for area. Neither the age nor the sex of the subject had any demonstrable effect on the results.

Reliable Design of Medical Devices

Abstract: The development of a medical device is a complex process. It requires the integration of a range of diverse disciplines, activities, and regulatory requirements, all of which must be achieved to ensure that the final result is a safe, effective and competitive product. While much of the information required in the process of device development is already in the pubic domain, it is scattered throughout many documents and not easily assimilated by the busy design engineer. The author of this recently published volume, an employee of a major medical equipment manufacturing company, is clearly aware of the problem. He has used his extensive experience of the process to bring together the necessary information in a convenient and easily accessible format. The subject material is presented in some twenty-eight chapters, dealing in turn with the basic concepts of reliability, medical device regulation and standards, product specification and definition, testing and data analysis, and a discussion of issues around manufacturing and maintenance. The style is highly structured and concise, with extensive use of bullet points and tables. There is considerable emphasis on the problems associated with the use of software in medical devices, a subject currently receiving much attention as we approach the millennium. Each chapter includes a useful list of source material, although the specific contribution of each is not always made clear. This book will appeal to designers of medical equipment as it contains a wealth of both factual information and guidance on how to apply it. At one point the author discusses the role that trade secrets play in protecting competitive advantage; perhaps in writing a book such this he has disregarded his own advice.

Measurement of the volume of a leg ulcer using a laser scanner

Abstract: Effective management of leg ulcer healing depends on accurate and reliable measurements of wound size. Current techniques usually rely on estimates of surface area or circumference which do not fully describe the healing process. A novel instrument has been developed that is capable of measuring the variations in the surface contours (topography) of a solid object. Its primary application is to measure the size of a leg ulcer by scanning a laser displacement sensor over the affected area. There is no contact with the wound and scanning takes approximately 2 minutes to perform. Volume is calculated by subtracting the measured topography from one calculated using an algorithm to reconstruct a healthy leg surface. A study was carried out where patients had their ulcers scanned during their visits to the leg ulcer clinic. Data are presented from two venous leg ulcers showing the calculated volume reducing over time.

Biosensors in the Body. Continuous In Vivo Monitoring

Abstract: Biosensors involve the integration of a biological recognition molecule with a physical or chemical transducer such that the biomolecule can be exploited for transduction purposes. This basic idea has seen myriad developments and a fast-expanding literature. Of all the niche areas, in vivo monitoring is one where few other technologies can compete. However, both for sensors and patients alike, there are dangers, and the issues merit detailed analysis. Yet there is no comprehensive text marking out the relevant scientific territory, so this multi-author, multidisciplinary volume is a valuable first which gives due cognizance to implant biology. The editor has put together a creditable list of practitioners with the requisite level of real experience to contribute new insights. That said, there is a certain abstraction to the choice of topics; rather predictably, glucose monitoring takes the lion's share, but an opportunity has been missed in not covering a large and important literature on sensors for ions and gases. These latter, not strictly biosensors, throw up all the issues relating to blood and tissue compatibility and safety, and yet have very limited coverage in the final chapter. Chapter one is a self-contained, well referenced review of many of the measurement opportunities, written by the editor, clearly an enthusiast for his subject. This chapter can serve as a stand-alone introductory text to those new to the field, and succeeds in giving the right balance of sensor chemistry and material science needed. For once, due emphasis is given to the latter, by contrast to quite weak descriptions in the literature hitherto. In chapter two, a review of glucose monitoring is provided, with a succinct comparative assessment of different electrochemical strategies. There is sufficient technical detail to allow the reader to reach an independent view. Chapter three gives the non-invasive counterweight to glucose monitoring using near infrared. The method is, of course, attractive, but the problems are many, and the goal elusive. If one accepts that such a technology has any place at all in a text on biosensors, then the theory, instrumentation and biological outcomes given here is a reasonable summary. Chapter four returns to glucose sensors, but now deals with the needs of long-term fully implanted telemetry-based devices. Practical design needs and quite respectable performance data are detailed in an animal model; certainly this lends some support to the proposition for long-term biochemical monitoring. The devil, however, is in the detail, extensive review of the host tissue response is an acute reminder of how little we really understand of either the markers or the effectors of the tissue rejection process. A concerted effort using mini- and micro-sensors for following stimulated brain metabolite and neurotransmitter changes has been made in recent years. Because of the relative low grade adverse response of brain tissue, meaningful data and some new insights have emerged. A full chapter (six) given over to this area does cover the literature well, though someone not familiar with the underlying electrochemistry may find it difficult to follow. Chapter seven gives a concise summary of microfabrication methods and structural outcomes for different transducer types. Though of practical interest, most of the sensors covered are unlikely to be used in vivo for reasons of innate chemistry. Chapter 8 is an overview of optical sensors, mainly for blood gas analysis, with yet more on near infrared, this time with suggested opportunities for imaging. Anyone in the business of developing medical sensors will find this a useful source book for well referenced, recent advances. The glucose sensor preoccupation certainly makes for some repetition, but enthusiastic coverage of topics by most of the contributors makes this a worthy statement about the field. If nothing else, the book will at least raise the `biomaterials' aspects of in vivo sensors.

Three methods compared for detecting the onset of alpha wave synchronization following eye closure

Abstract: Recent work indicates that the variation in the occipital alpha wave component of the EEG spectrum, controlled through eye closure, can be used by an untrained person to effect reliable activation of electrical devices. Here we describe and compare three real-time strategies, based on analogue and digital signal processing methods, of detecting the onset of alpha wave synchronization during eye closure. The goal of this work is to establish a method which satisfies the condition of rapid detection of alpha wave enhancement, thereby allowing for the efficient activation of devices, while simultaneously registering few or no false positives due to the natural variation in the alpha signal with eyes open. This work, based on measurements on 15 subjects in the age range 12 to 40 years, indicates that renal-time analysis of the EEG power spectrum provides for rapid detection of the onset of alpha wave synchronization while maintaining low counts of false positives.

Continuous monitoring of sweating by electrical conductivity measurement

Abstract: A method of continuous monitoring of sweating was developed in which sweat was detected by changes in the conductivity of perfusing water. An ion-free solution was perfused at a constant flow rate through a chamber attached to the skin surface. The chamber was designed so that the electrodes were installed inside at the inlet and outlet, and a 14 channel was constructed at the bottom to wash out sweat. The 90% response time was 0.12 s. Attaching the chamber to the palm allowed measurements to be made with the subject seated in a comfortable environment. The sweat rate and heart rate were measured simultaneously with an air-ventilation chamber and a heart rate counter, respectively, with the subjects at rest, and under stresses such as grasping hands and doing mental arithmetic. This method yields sweat responses similar to those obtained with an air-ventilation chamber and simultaneous heart rate measurements. The main advantages of this method are faster response time and smaller observation area. Keywords: electrolyte content of sweat, sweat rate, heart rate, mental stress

Diffraction and interface losses in broadband ultrasound attenuation measurements of the calcaneum.

Abstract: Ultrasonic investigation of bone disease most frequently involves measurements on the heel, and the parameter most often used for this is the broadband ultrasound attenuation (BUA) which is the slope of the attenuation as a function of frequency between 0.2 and 0.6 MHz. In this study, the possible losses in the BUA measurement due to diffraction and interfaces have been discussed and evaluated using existing data and a standard diffraction model. The loss due to diffraction was found to depend critically upon whether a contact or immersion technique is used. For a contact method, we estimate that the diffraction loss can be greater than , whereas insertion devices typically will have losses of up to , and it not always clear whether or how manufacturers have attempted to correct for this. Dispersion which is found in the os calcis has only a small effect on the diffraction loss, but it can cause a frequency dependent interface loss of about . It may be impossible to correct for these discrepancies in vivo as the necessary data for the individual components of the heel are not known. However the losses should be borne in mind as factors limiting the accuracy of the measurements and, in the case of in vitro investigations, may merit further study. Keywords: ultrasound, bone, broadband ultrasound attenuation, diffraction, interface loss

The influence of extravascular lung water on cardiac output measurements using thoracic impedance cardiography

Abstract: The purpose of this study was to investigate the influence of pulmonary oedema as measured with the double indicator dilution technique on the accuracy of cardiac output (CO) measurement using thoracic impedance cardiography (TIC) compared with thermodilution in thirteen sepsis patients. Differences in the Kubicek and Sramek-Bernstein equation with respect to pulmonary oedema were explored theoretically and experimentally. From a parallel two cylinder model a hypothesis can be derived that CO determined with the Kubicek equation is oedema independent, whereas CO determined using the Sramek-Bernstein equation is oedema dependent. Experimentally, CO determined using Kubicek's equation correlated better with thermodilution CO (r = 0.75) than CO determined with the Sramek-Bernstein equation (r = 0.25). The effect of oedema on the accuracy of TIC was investigated by comparing the differences in the CO of impedance and thermodilution to the extravascular lung water index. For the Kubicek equation the difference was not influenced by oedema (r = 0.04, p = 0.84), whereas for the Sramek-Bernstein equation the difference was affected by oedema (r = 0.39, p = 0.05). Thus, the effects of pulmonary oedema on the accuracy of TIC measurements can better be understood with the parallel cylinder model. Moreover, the Kubicek equation still holds when pulmonary oedema is present, in contrast to the Sramek-Bernstein equation.

Postural stability of normal subjects measured by sway magnetometry: pathlength and area for the age range 15 to 64 years

Abstract: Fifty-eight subjects with no known vestibular pathology, postural problems or cerebrovascular disease were studied to determine the range of sway values in a normal working-age population. Sway was measured with a magnetometry system, previously shown to be highly successful in distinguishing sway with and without the eyes open in individual subjects. 30 subjects were aged 15-29 years and 28 aged 30-64 years. With the subject standing on a firm base the pathlength and area enclosed by movement of the hips in the horizontal plane were measured over 30 second periods with eyes open and eyes closed. There were no significant differences between the two age ranges. We obtained the following upper limits of normal values for the age range 15 to 64 years: pathlength with eyes open 140 mm, pathlength with eyes closed 200 mm, area with eyes open 150 mm2, and area with eyes closed 300 mm2. The mean and standard deviation of the Romberg coefficient (sway with eyes open/sway with eyes closed) was 0.73 +/- 0.09 for pathlength measurement and 0.70 +/- 0.26 for area measurement.

Beat-to-beat wavelet variance of the QRS complex as a marker of arrhythmogenic substrate in ventricular tachycardia patients

Abstract: This study proposes a wavelet transform based technique to assess the beat-to-beat variation of the QRS signal in post-myocardial infarction patients with sustained monomorphic ventricular tachycardia Recent electrophysiological investigations suggested that the diminished synchrony between the normal myocardium and the scarred arrhythmogenic tissue bordering a myocardial infarction area gives rise to beat-variable ECG signal components Using a mathematical model of small variations in a largely repetitive waveform, we show that the inherent alignment errors (trigger jitter) of the high-resolution ECG (HRECG) can artificially increase the value of the time-domain beat-to-beat variance, making it less valuable as a marker of beat-variable signal components To overcome this drawback, we propose the wavelet based approach which discriminates between the different factors responsible for the beat variability (the alignment error and the beat-variable signal components) The Morlet wavelet transform is performed on HRECG signals from normal individuals (control group) and post-myocardial infarction patients with documented ventricular tachycardia Electrical variability is quantitatively assessed via the beat-to-beat wavelet variance measurements A marker of arrhythmogenic induced variance which achieves a good performance in discrimination of ventricular tachycardia patients from normal subjects was found between 200 Hz and 300 Hz This finding is in agreement with the proposed mathematical model which states that the useful part of the time-frequency map is shifted upward in a precise mathematical way, as the variance induced by the beat-variable arrhythmogenic signals depend on the frequency characteristics of the first derivative of these signals We conclude that the dynamics of the arrhythmogenic substrate as revealed by the beat-to-beat wavelet variance can be a new estimator of ventricular tachycardia risk Keywords: beat-to-beat variance, wavelet transform, ventricular tachycardia

Extra-cellular volume estimation by electrical impedance--phase measurement or curve fitting: a comparative study.

Abstract: In order to determine body fluid shifts between the intra- and extra-cellular spaces, multifrequency impedance measurement is performed. According to the Cole-Cole extrapolation, lumped values of intra- and extra-cellular conduction can be estimated which are commonly expressed in resistances and respectively. For this purpose the magnitude and phase of the impedance under study are determined at a number of frequencies in the range between 5 kHz and 1 MHz. An approach to determine intra- and extra-cellular conduction on the basis of Bode analysis is presented in this article. On this basis, estimation of the ratio between intra- and extra-cellular conduction could be performed by phase measurement only, midrange in the bandwidth of interest. An important feature is that the relation between intra- and extra-cellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required. Based on a two frequency measurement determining at 4 kHz and at 64 kHz it proved possible to estimate extra-cellular volume (ECV) more accurately compared with the estimation based on extrapolation according to the Cole-Cole model in 26 patients. Reference values of ECV were determined by sodium bromide. The results show a correlation of 0.90 with the reference method. The average error of ECV estimation was -3.6% (SD 8.4), whereas the Cole-Cole extrapolation showed an error of 13.2% (SD 9.5). An important feature of the proposed approach is that the relation between intra- and extra-cellular conduction can be continuously monitored by phase measurement and no curve fitting whatsoever is required. Keywords: electrical impedance plethysmography, electrical bio-impedance analysis, Cole-Cole modelling, extra-cellular volume