Showing papers in "Physiological Measurement in 2004"

Nonlinear model predictive control of glucose concentration in subjects with type 1 diabetes

Abstract: A nonlinear model predictive controller has been developed to maintain normoglycemia in subjects with type 1 diabetes during fasting conditions such as during overnight fast. The controller employs a compartment model, which represents the glucoregulatory system and includes submodels representing absorption of subcutaneously administered short-acting insulin Lispro and gut absorption. The controller uses Bayesian parameter estimation to determine time-varying model parameters. Moving target trajectory facilitates slow, controlled normalization of elevated glucose levels and faster normalization of low glucose values. The predictive capabilities of the model have been evaluated using data from 15 clinical experiments in subjects with type 1 diabetes. The experiments employed intravenous glucose sampling (every 15 min) and subcutaneous infusion of insulin Lispro by insulin pump (modified also every 15 min). The model gave glucose predictions with a mean square error proportionally related to the prediction horizon with the value of 0.2 mmol L(-1) per 15 min. The assessment of clinical utility of model-based glucose predictions using Clarke error grid analysis gave 95% of values in zone A and the remaining 5% of values in zone B for glucose predictions up to 60 min (n = 1674). In conclusion, adaptive nonlinear model predictive control is promising for the control of glucose concentration during fasting conditions in subjects with type 1 diabetes.

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

Abstract: 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.

EIT reconstruction algorithms: pitfalls, challenges and recent developments

Abstract: We review developments, issues and challenges in electrical impedance tomography (EIT) for the 4th Conference on Biomedical Applications of Electrical Impedance Tomography, held at Manchester in 2003. We focus on the necessity for three-dimensional data collection and reconstruction, efficient solution of the forward problem, and both present and future reconstruction algorithms. We also suggest common pitfalls or 'inverse crimes' to avoid.

On the suitability of near-infrared (NIR) systems for next-generation brain?computer interfaces

Abstract: A brain–computer interface (BCI) gives those suffering from neuromuscular impairments a means to interact and communicate with their surrounding environment. A BCI translates physiological signals, typically electrical, detected from the brain to control an output device. A significant problem with current BCIs is the lengthy training periods involved for proficient usage, which can often lead to frustration and anxiety on the part of the user. Ultimately this can lead to abandonment of the device. The primary reason for this is that relatively indirect measures of cognitive function, as can be gleaned from the electroencephalogram (EEG), are harnessed. A more suitable and usable interface would need to measure cognitive function more directly. In order to do this, new measurement modalities, signal acquisition and processing, and translation algorithms need to be addressed. In this paper, we propose a novel approach, using non-invasive near-infrared imaging technology to develop a user-friendly optical BCI. As an alternative to the traditional EEGbased devices, we have used practical non-invasive optical techniques to detect characteristic haemodynamic responses due to motor imagery and consequently created an accessible BCI that is simple to attach and requires little user training.

Nanosecond pulsed electric fields modulate cell function through intracellular signal transduction mechanisms.

Abstract: These studies describe the effects of nanosecond (10-300 ns) pulsed electric fields (nsPEF) on mammalian cell structure and function. As the pulse durations decrease, effects on the plasma membrane (PM) decrease and effects on intracellular signal transduction mechanisms increase. When nsPEF-induced PM electroporation effects occur, they are distinct from classical PM electroporation effects, suggesting unique, nsPEF-induced PM modulations. In HL-60 cells, nsPEF that are well below the threshold for PM electroporation and apoptosis induction induce effects that are similar to purinergic agonistmediated calcium release from intracellular stores, which secondarily initiate capacitive calcium influx through store-operated calcium channels in the PM. NsPEF with durations and electric field intensities that do or do not cause PM electroporation, induce apoptosis in mammalian cells with a well-characterized phenotype typified by externalization of phosphatidylserine on the outer PM and activation of caspase proteases. Treatment of mouse fibrosarcoma tumors with nsPEF also results in apoptosis induction. When Jurkat cells were transfected by electroporation and then treated with nsPEF, green fluorescent protein expression was enhanced compared to electroporation alone. The results indicate that nsPEF activate intracellular mechanisms that can determine cell function and fate, providing an important new tool for probing signal transduction mechanisms that modulate cell structure and function and for potential therapeutic applications for cancer and gene therapy.

Cerebrospinal fluid dynamics

Abstract: Hydrocephalus is far more complicated than a simple disorder of CSF circulation. Historically, it has been diagnosed using clinical and psychomotor assessment plus brain imaging. The role of physiological measurement to aid diagnosis becomes more appreciated in current clinical practice. This has been reflected by recently formulated guidelines for the management of normal pressure hydrocephalus. Clinical measurement in hydrocephalus is mainly related to intracranial pressure (ICP) and cerebral blood flow. This review lists and discusses most common forms of the methods: CSF infusion study, overnight ICP monitoring, assessment of slow ICP waves, testing pressure reactivity, cerebral autoregulation, CO2 reactivity and PET-CBF studies combined with MRI co-registration. The basics of CSF dynamics modelling are presented and the principles of the assessment of functioning of the implanted hydrocephalus shunts are also discussed. The descriptions of multiple forms of measurement along with clinical illustrations are mainly based on in-house experience of a multidisciplinary group of scientists and clinicians from Cambridge, UK.

Validation of a new dielectric device to assess changes of tissue water in skin and subcutaneous fat.

Abstract: Easily applicable and inexpensive water-specific techniques to evaluate local oedema, swollen tissue problems and fluid retention in humans are not available. In the present investigation a recently constructed non-invasive device for a local measurement of changes in tissue water in human skin and subcutaneous fat (SSF) was validated. The instrument transmits an ultra high-frequency electromagnetic (EM) wave of 300 MHz into a coaxial line and further into an open-ended coaxial probe which is in contact with the skin. Due to the dimensions of the applied probe the penetration of the EM field extends to subcutaneous fat. A major part of the EM energy is absorbed by tissue water while the rest is reflected back into a coaxial line. From the information of the reflected wave an electrical parameter, directly proportional to tissue water content, called a dielectric constant of SSF, was calculated. For system validation, the decrease of water content in SSF measured with the dielectric technique in the volar forearm of seven patients during haemodialysis treatment was compared with the decrease of the circumference of the forearm and the amount of fluid removed. Statistically highly significant correlations were obtained between the decreasing dielectric constant (i.e. water content) of the SSF and the fluid removed during haemodialysis treatment (r = -0.99, p < 0.01) and between the decreasing dielectric constant and the circumference of the arm (r = 0.97, p < 0.05). The sensitivity of the dielectric method was four-fold compared with the circumferential measurement. The repeatability 3.0% was not dependent on the phase of haemodialysis. The new device allows an easy and non-invasive measurement technique to assess changes of tissue water in SSF.

The effect of contacting force on photoplethysmographic signals.

Abstract: Photoplethysmography (PPG) has been used in oxygen saturation measurement, heart rate monitoring, and the assessment of peripheral circulation and large artery compliance However, the waveform of the photoplethysmographic signal may be affected by the contacting force between the sensor and the measurement site The aim of this study is to investigate the change in pulse amplitude (AC), DC amplitude, ratio of AC/DC and normalized pulse area of the reflective photoplethysmographic signals with increasing contacting force, from 02 N to 18 N Signals were recorded from the fingers of fifteen healthy subjects With increasing contacting force, the DC amplitude increased and the normalized pulse area decreased, whereas the pulse amplitude and the ratio of AC/DC increased first and then decreased For different subjects, the pulse amplitude and the ratio of AC/DC peaked at different contacting forces, from 02 N to 10 N, and most of the subjects achieved their maximum pulse amplitude within 02–04 N Over the range of contacting force between 02 N and 08 N, the DC amplitude and the normalized pulse area had significant changes (p < 0001) The results suggest that the effects of contacting force should be carefully examined in the design of photoplethysmography-based health care devices

Comprehensive analysis of cardiac health using heart rate signals.

Abstract: The electrocardiogram is a representative signal containing information about the condition of the heart. The shape and size of the P-QRS-T wave, the time intervals between its various peaks, etc may contain useful information about the nature of disease affecting the heart. However, the human observer cannot directly monitor these subtle details. Besides, since bio-signals are highly subjective, the symptoms may appear at random in the time scale. Therefore, the heart rate variability signal parameters, extracted and analyzed using computers, are highly useful in diagnostics. Analysis of heart rate variability (HRV) has become a popular noninvasive tool for assessing the activities of the autonomic nervous system. The HRV analysis is based on the concept that fast fluctuations may specifically reflect changes of sympathetic and vagal activity. It shows that the structure generating the signal is not simply linear, but also involves nonlinear contributions. These signals are essentially nonstationary; may contain indicators of current disease, or even warnings about impending diseases. The indicators may be present at all times or may occur at random in the time scale. However, to study and pinpoint abnormalities in voluminous data collected over several hours is strenuous and time consuming. This paper deals with the analysis of eight types of cardiac abnormalities and presents the ranges of linear and nonlinear parameters calculated for them with a confidence level of more than 90%.

Reverse iontophoresis for non-invasive transdermal monitoring

Abstract: Iontophoresis is the application of a small electric current to enhance the transport of both charged and polar, neutral compounds across the skin. Manipulation of either the total charge delivered and/or certain electrode formulation parameters allows control of electromigration and electroosmosis, the two principal mechanisms of transdermal iontophoresis. While the approach has been mainly used for transdermal drug delivery, 'reverse iontophoresis', by which substances are extracted to the skin surface, has recently been the subject of considerable effort. Glucose monitoring has been extensively studied and other applications, including therapeutic drug monitoring, are contributing to the development of the technique. An internal standard calibration procedure may ultimately render this novel monitoring technique completely non-invasive.

Automated detection of obstructive sleep apnoea at different time scales using the electrocardiogram

Abstract: An automated classification algorithm is presented which processes short-duration epochs of surface electrocardiogram data derived from polysomnography studies, and determines whether an epoch is from a period of sleep disordered respiration (SDR) or normal respiration (NR). The epoch lengths considered were 15, 30, 45, 60, 75, and 90 s. Epochs were labeled as 'NR' or 'SDR' by a human expert, based on standard polysomnography interpretation rules. The automated classification algorithm was trained and tested on a database of 70 overnight ECG recordings from subjects with and without obstructive sleep apnoea syndrome (35 used for training, 35 for independent validation). Depending on the epoch length, the classifier correctly labeled between 87% (15 s epochs) and 91% (60 s epochs) of the epochs in the test set. Accuracy was lowest for the shortest (15 s) and longest (90 s) epoch lengths, but the analysis was relatively insensitive to choice of epoch length. The classifications from these epochs were combined to form an overall summary measure of minutes-of-SDR, allowing per-subject classification.

Investigation of cerebral haemodynamics by near-infrared spectroscopy in young healthy volunteers reveals posture-dependent spontaneous oscillations

Abstract: Autonomic reflexes enable the cardiovascular system to respond to gravitational displacement of blood during changes in posture. Spontaneous oscillations present in the cerebral and systemic circulation of healthy subjects have demonstrated a regulatory role. This study assessed the dynamic responses of the cerebral and systemic circulation upon standing up and the posture dependence of spontaneous oscillations. In ten young healthy volunteers, blood pressure and cerebral haemodynamics were continuously monitored non-invasively using the Portapres and near-infrared spectroscopy (NIRS), respectively. Oscillatory changes in the cerebral NIRS signals and the diastolic blood pressure (DBP) signal have been identified by the fast Fourier analysis. Blood pressure increased during standing and returned to basal level when volunteers sat on a chair. The mean value of cerebral tissue oxygen index (TOI) as measured by NIRS did not demonstrate any significant changes. Oscillatory changes in DBP, oxyhaemoglobin concentration [O2Hb] and TOI showed a significant increase when subjects were standing. Investigation of the low frequency component (approximately 0.1 Hz) of these fluctuations revealed posture dependence associated with activation of autonomic reflexes. Systemic and cerebral changes appeared to preserve adequate blood flow and cerebral perfusion during standing in healthy volunteers. Oscillatory changes in [O2Hb] and TOI, which may be related to the degree of cerebral sympathetic stimulation, are posture dependent in healthy subjects.

Detection of brain oedema using magnetic induction tomography: a feasibility study of the likely sensitivity and detectability.

Abstract: The detection and continuous monitoring of brain oedema is of particular interest in clinical applications because existing methods (invasive measurement of the intracranial pressure) may cause considerable distress for the patients A new non-invasive method for continuous monitoring of an oedema promises the use of multi-frequency magnetic induction tomography (MIT) MIT is an imaging method for reconstructing the changes of the conductivity Δκ in a target object The sensitivity of a single MIT-channel to a spherical oedematous region was analysed with a realistic model of the human brain The model considers the cerebrospinal fluid around the brain, the grey matter, the white matter, the ventricle system and an oedema (spherical perturbation) Sensitivity maps were generated for different sizes and positions of the oedema when using a coaxial coil system The maps show minimum sensitivity along the coil axis, and increasing values when moving the perturbation towards the brain surface Parallel to the coil axis, however, the sensitivity does not vary significantly When assuming a standard deviation of 10−7 for the relative voltage change due to the system's noise, a centrally placed oedema with a conductivity contrast of 2 with respect to the background and a radius of 20 mm can be detected at 100 kHz At higher frequencies the sensitivity increases considerably, thus suggesting the capability of multi-frequency MIT to detect cerebral oedema

Carotid flow rates and flow division at the bifurcation in healthy volunteers

Abstract: In nine healthy subjects, magnetic resonance imaging was used to measure blood flow waveforms in the common (CCA), internal (ICA) and external (ECA) carotid arteries. Useful data were acquired from 14 carotid arteries in total. Flow rates were determined from regions of interest placed over the arteries in CINE-phase contrast velocity encoded images. Use of a normalized cardiac cycle allowed the combination of flow waveforms from individuals. Time-averaged group mean flow rates were 6.16, 4.14 and 1.59 ml s(-1) for the CCA, ICA and ECA, respectively. Time-averaged values for the flow division ratios ICA/CCA, ECA/ICA and ECA/CCA were 0.70, 0.39 and 0.26, respectively. The data will be of use in future physiological studies and in computational modelling of carotid artery haemodynamics.

Cortical entropy changes with general anaesthesia: theory and experiment.

Abstract: Commonly used general anaesthetics cause a decrease in the spectral entropy of the electroencephalogram as the patient transits from the conscious to the unconscious state. Although the spectral entropy is a configurational entropy, it is plausible that the spectral entropy may be acting as a reliable indicator of real changes in cortical neuronal interactions. Using a mean field theory, the activity of the cerebral cortex may be modelled as fluctuations in mean soma potential around equilibrium states. In the adiabatic limit, the stochastic differential equations take the form of an Ornstein-Uhlenbeck process. It can be shown that spectral entropy is a logarithmic measure of the rate of synaptic interaction. This model predicts that the spectral entropy should decrease abruptly from values approximately 1.0 to values of approximately 0.7 as the patient becomes unconscious during induction of general anaesthesia, and then not decrease significantly on further deepening of anaesthesia. These predictions were compared with experimental results in which electrocorticograms and brain concentrations of propofol were recorded in seven sheep during induction of anaesthesia with intravenous propofol. The observed changes in spectral entropy agreed with the theoretical predictions. We conclude that spectral entropy may be a sensitive monitor of the consciousness-unconsciousness transition, rather than a progressive indicator of anaesthetic drug effect.

What is the adequate sampling interval of the ECG signal for heart rate variability analysis in the time domain

Abstract: Heart rate variability (HRV) analysis is considered a simple method for investigating neurocardiac regulation. However, measures of HRV may be corrupted by technical artifacts. In order to investigate the consequences of digitization errors on the time domain parameters, HRV measures from model tachograms resampled at different rates were compared. Two 375-element tachograms from human ECG tracings were shifted to a mean of 800 ms and stretched to standard deviations (SD) of 5-120 ms in 5 ms steps. All were resampled at 1-10 ms in 1 ms steps, 10 times repetitively at each interval. The mean, SD, relative accuracy error (RAE) and relative precision error (RPE) were calculated from the mean RR-interval, SD (SDNN), root mean square of successive RR-differences (RMSSD) and the percentage of consecutive RR-interval differences greater than 50 ms (pNN50). The RAE and RPE of the mean heart rate were below 0.1%. In the series with 5 ms SD, the SDNN-RAE exceeded 30% at 10 ms SI, its RPE was lower than 2% all through. Resampling the 15 ms SD tachogram at 1, 2, 4, or 10 ms resulted in RMSSD-RAE of 0.7%, 2.5%, 7.8% and 45.1%, respectively, while its RPE remained below 5%. The pNN50 shows poor accuracy and precision. An ECG sampling interval of 1 ms is recommended for HRV analysis without interpolation in order to get accurate time domain measures even in seriously reduced-variability samples. However, a lower sampling rate may be satisfactory in cases where higher variability is expected.

Ranking of pattern recognition parameters for premature ventricular contractions classification by neural networks.

Abstract: Detection and classification of ventricular complexes from a limited number of ECG leads is of considerable importance in critical care or operating room patient monitoring. Beat-to-beat detection allows the heart rhythm evolution to be followed and various types of arrhythmia to be recognized. A quantitative analysis is proposed of pattern recognition parameters for classification of normal QRS complexes and premature ventricular contractions (PVC). Twenty-six parameters have been defined: the width of the QRS complex, three vectorcardiogram parameters and 11 from two ECG leads. These parameters include: amplitudes of positive and negative peaks, area of positive and negative waves, various time-interval durations, amplitude and angle of the QRS vector, etc. They are measured for all QRS complexes annotated as 'normals' and 'PVCs' from the 48 ECG recordings of the MIT-BIH arrhythmia database. Neural networks (NN) are shown to be a useful instrument for the analysis of large quantities of parameters. Separate ranking of any parameter and homogeneous group ranking (amplitude, area, interval, slope and vector) were performed. From the two ECG leads, the first three ranked parameter groups for clustering of PVCs are amplitude, slope and interval, while for N clustering they are vector, amplitude and area. Considering the entire parameter set, we obtained N = 99.7% correct detection of normal QRS complexes and PVC = 98.5% of premature ventricular complexes. The study also shows that simultaneous analysis of two ECG channels yields better accuracy compared to using a single channel: the improvement is 0.1% in the classification of N beats and 4.5% for PVC beats.

Algebraic reconstruction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density

Abstract: Magnetic resonance–electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured magnetic flux density is developed. In this method, the imaging problem is formulated as the solution of a non-linear matrix equation which is solved iteratively to reconstruct resistivity. Numerical simulations are performed to test the algorithm both for noise-free and noisy cases. The uniqueness of the solution is monitored by looking at the singular value behavior of the matrix and it is shown that at least two current injection profiles are necessary. The method is also modified to handle region-of-interest reconstructions. In particular it is shown that, if the image of a certain xy-slice is sought for, then it suffices to measure the z-component of magnetic flux density up to a distance above and below that slice. The method is robust and has good convergence behavior for the simulation phantoms used.

Real time detection of ventricular fibrillation and tachycardia.

Abstract: The automatic external defibrillator (AED) is a lifesaving device, which processes and analyses the electrocardiogram (ECG) and delivers a defibrillation shock to terminate ventricular fibrillation or tachycardia above 180 bpm. The built-in algorithm for ECG analysis has to discriminate between shockable and non-shockable rhythms and its accuracy, represented by sensitivity and specificity, is aimed at approaching the maximum values of 100%. An algorithm for VF/VT detection is proposed using a band-pass digital filter with integer coefficients, which is very simple to implement in real-time operation. A branch for wave detection is activated for heart rate measurement and an auxiliary parameter calculation. The method was tested with ECG records from the widely recognized databases of the American Heart Association (AHA) and the Massachusetts Institute of Technology (MIT). A sensitivity of 95.93% and a specificity of 94.38% were obtained.

Bioelectrical impedance vector analysis in haemodialysis patients: relation between oedema and mortality

Abstract: In this work, bioelectrical impedance vector analysis (BIVA) method is used in a sample of haemodialysis patients in stable (without oedema) and critical (hyperhydrated and malnutrition) states, in order to establish the relation between hyperhydration (oedema) and mortality. The measurements obtained were single frequency (50 kHz), tetrapolar (hand–foot) complex impedance measurements (vector components are: resistance R and reactance Xc). The impedance components were standardized by the height H of the subjects, (R/H and Xc/H) to obtain de impedance vector Z/H, that is represented in the RXc plot (abscise R/H, ordinate Xc/H ). Measurements were performed on a sample of 74 patients (30 men and 44 women, 18–70 year, body mass index (BMI), 19–30 kg m −2 ) at the Saturnino Lora University Hospital in Santiago de Cuba. The 46 stable patients comprised 28 men and 18 women; the 28 critical patients 16 men and 12 women. The reference population consisted of 1196 healthy adult subjects living in Santiago de Cuba (689 men and 507 women, 18–70 year, BMI 19–30 kg m −2 ). We used the RXc plot with the BIVA method to characterize the reference population using the 50%, 75% and 95% tolerance ellipses. Student’s t-test and Hotelling’s T 2 -test were used to analyse the separation of groups obtained by means of clinical diagnosis and those obtained by BIVA. We obtained a significant difference (P < 0.05) in R/H ,X c/H and phase angle (PA) in men as in women between the location of Z/H vectors in the RXc graph and the separation made by the doctors between stable and critical patients. Critical (hyperhydrated) patients were located below the inferior pole of the 75% tolerance ellipse, whereas stable patients were within the tolerance ellipses. Some cases classified as stable by

A physiologically representative in vitro model of the coronary circulation

Abstract: With the development of clinical diagnostic techniques to investigate the coronary circulation in conscious humans, the in vitro validation of such newly developed techniques is of major importance. The aim of this study was to develop an in vitro model that is able to mimic the coronary circulation in such a way that coronary pressure and flow signals under baseline as well as hyperaemic conditions are approximated as realistically as possible and are in accordance with recently gained insights into such signals in conscious man. In the present in vitro model the heart, the systemic and coronary circulation are modelled on the basis of the elements of a lumped parameter mathematical model only consisting of elements that can be represented by segments in an experimental set-up. A collapsible tube, collapsed by the ventricular pressure, represents the variable resistance and volume behaviour of the endocardial part of the myocardium. The pressure and flow signals obtained are similar to physiological human coronary pressure and flow, both for baseline and hyperaemic conditions. The model allows for in vitro evaluation of clinical diagnostic techniques.

Static conductivity imaging using variational gradient Bz algorithm in magnetic resonance electrical impedance tomography

Abstract: A new image reconstruction algorithm is proposed to visualize static conductivity images of a subject in magnetic resonance electrical impedance tomography (MREIT). Injecting electrical current into the subject through surface electrodes, we can measure the induced internal magnetic flux density B = (Bx, By, Bz) using an MRI scanner. In this paper, we assume that only the z-component Bz is measurable due to a practical limitation of the measurement technique in MREIT. Under this circumstance, a constructive MREIT imaging technique called the harmonic Bz algorithm was recently developed to produce high-resolution conductivity images. The algorithm is based on the relation between inverted delta2Bz and the conductivity requiring the computation of inverted delta2Bz. Since twice differentiations of noisy Bz data tend to amplify the noise, the performance of the harmonic Bz algorithm is deteriorated when the signal-to-noise ratio in measured Bz data is not high enough. Therefore, it is highly desirable to develop a new algorithm reducing the number of differentiations. In this work, we propose the variational gradient Bz algorithm where Bz is differentiated only once. Numerical simulations with added random noise confirmed its ability to reconstruct static conductivity images in MREIT. We also found that it outperforms the harmonic Bz algorithm in terms of noise tolerance. From a careful analysis of the performance of the variational gradient Bz algorithm, we suggest several methods to further improve the image quality including a better choice of basis functions, regularization technique and multilevel approach. The proposed variational framework utilizing only Bz will lead to different versions of improved algorithms.

Activity-based sleep–wake identification in infants

Abstract: Actigraphy offers one of the best-known alternatives to polysomnography for sleep-wake identification. The advantages of actigraphy include high accuracy, simplicity of use and low intrusiveness. These features allow the use of actigraphy for determining sleep-wake states in such highly sensitive groups as infants. This study utilizes a motion sensor (accelerometer) for a dual purpose: to determine an infant's position in the crib and to identify sleep-wake states. The accelerometer was positioned over the sacral region on the infant's diaper, unlike commonly used attachment to an ankle. Opposed to broadly used discriminant analysis, this study utilized logistic regression and neural networks as predictors. The accuracy of predicted sleep-wake states was established in comparison to the sleep-wake states recorded by technicians in a polysomnograph study. Both statistical and neural predictors of this study provide an accuracy of approximately 77-92% which is comparable to similar studies achieving prediction rates of 85-95%, thus validating the suggested methodology. The results support the use of body motion as a simple and reliable method for determining sleep-wake states in infants. Nonlinear mapping capabilities of the neural network benefit the accuracy of sleep-wake state identification. Utilization of the accelerometer for the dual purpose allows us to minimize intrusiveness of home infant monitors.

Beat-to-beat detection of fetal heart rate: Doppler ultrasound cardiotocography compared to direct ECG cardiotocography in time and frequency domain.

Abstract: In order to obtain power spectral information on the fetal heart rate in stages of pregnancy earlier than labor an algorithm has been developed to calculate the fetal heart rate on a beat-to-beat basis from Doppler ultrasound cardiotocographic signals. The algorithm was evaluated by comparing the calculated fetal heart rate with the heart rate determined from direct ECG signals measured with a scalp electrode. Heart rates were compared both in time and frequency domain. In the time domain the results achieved by both methods correlate well (correlation coefficient = 0.977 (p < 0.001)), in the frequency domain the results correlate even better (correlation coefficient = 0.991 (p < 0.001)). Based on these findings, it can be concluded that the developed algorithm provides a valuable tool for obtaining power spectral information on the fetal heart rate in stages of pregnancy earlier than labor.

Design and implementation of a high frequency electrical impedance tomography system.

Abstract: Electrical impedance tomography is an imaging modality being investigated for use in detection of breast cancer. Use of higher frequencies than have typically been employed may benefit the detection processes. In this current work we discuss the design and initial implementation of a system having a bandwidth of 10 MHz. Previous investigations into high frequency designs have proven more difficult than anticipated and shown that careful selection of systems architecture is critical to achieving broadband performance above 1 MHz. The design for this new system is based on a digital signal processor (DSP) which is used for control, signal generation and signal processing. Signal generation and detection, software design and preliminary system specifications are discussed.

Peripheral arterial tonometry, oximetry and actigraphy for ambulatory recording of sleep apnea.

Abstract: Autonomous nervous functions change with sleep stages and show characteristic changes associated with sleep disorders Therefore, continuous monitoring of autonomous nervous functions during sleep can be used for diagnostic purposes Recently, the peripheral arterial tonometry (PAT) has been introduced to determine peripheral arterial vascular tone on the finger being determined by sympathetic activity We investigate a new ambulatory recording system which uses PAT, oximetry and actigraphy (Watch-PAT) in order to detect sleep apnea and arousal The Watch-PAT is battery operated and attached to the wrist and has two finger sensors Twenty-one patients with suspected sleep apnea were recorded with cardiorespiratory polysomnography and the new system in parallel Seventeen recordings could be evaluated The correlation for the apnea/hypopnea index derived from the sleep laboratory and the respiratory disturbance index derived from the Watch-PAT was r = 089 (p < 001) and between arousals and the respiratory disturbance index was r = 077 (p < 001) The correlation for the total sleep time compared between the two systems was r = 015 (ns) The Watch-PAT detects apneas and hypopneas with a reasonable reliability and it is very sensitive to arousals The number of Watch-PAT events lies between the sum of apneas plus hypopneas and arousals Arousals are not unique to apnea events and therefore the specifity of the Watch-PAT is limited In conclusion, the Watch-PAT is well suited to perform therapy control studies in patients suffering from sleep apnea and being treated

A primary field compensation scheme for planar array magnetic induction tomography.

Abstract: In biomedical magnetic induction tomography (MIT), measurement precision may be improved by incorporating some form of primary field compensation/cancellation scheme. Schemes which have been described previously include gradiometric approaches and the use of 'back-off' coils. In each of these methods, however, the primary field cancellation was achieved only for a single transmitter/receiver combination. For the purpose of imaging, it would be desirable for a fully electronically scanned MIT system to provide a complete set of measurements, all with the primary field cancelled. A single channel suitable for incorporation into an MIT system with planar-array geometry is described. The transmitter is a 6-turn coil of wire 5 cm in diameter. The receiver is a surface mount inductor, of inductance 10 µH, mounted such that, in principle, no net primary field flux threads it. The results of measurements carried out with the single channel system suggest that the signal due to the primary excitation field can be reduced on average by a factor of 298 by the sensor geometry over the operating frequency range 1–10 MHz. The standard deviation and drift of the signal with the system adjusted for maximum primary field cancellation, expressed as a percentage of the signal when the receiver coil was rotated until its axis of sensitivity lay along the primary field, were 0.0009% and 0.009%, respectively. The filter time constant used was 30 ms.

A new method for measurement of gas exchange during anaesthesia using an extractable marker gas.

Abstract: A new method for the measurement of pulmonary gas exchange during inhalational anaesthesia is described which measures fresh gas and exhaust gas flows using carbon dioxide as an extractable marker gas. The theoretical precision of the method was compared by Monte Carlo modelling with other approaches which use marker gas dilution. A system was constructed for automated measurement of uptake of oxygen, nitrous oxide, volatile anaesthetic agent and elimination of carbon dioxide by an anaesthetized patient. The accuracy and precision of the method was tested in vitro on a lung gas exchange simulator, by comparison with simultaneous measurements made using nitrogen as marker gas and the Haldane transformation. Good agreement was obtained for measurement of simulated uptake or elimination of all gases studied over a physiologically realistic range of values. Mean bias for oxygen and nitrous oxide uptake was 0.003 l min−1, for isoflurane 0.0001 l min−1 and for carbon dioxide 0.001 l min−1. Limits of agreement lay within 10% of the mean uptake rate for nitrous oxide, within 5% for oxygen and isoflurane and within 1% for carbon dioxide. The extractable marker gas method allows accurate and continuous measurement of gas exchange in an anaesthetic breathing system with any inspired gas mixture.

A computational study of the interaction between coronary blood flow and myocardial mechanics.

Abstract: An anatomically based computational model of coronary blood flow, coupled to cardiac mechanics, is verified with experimental data and used to investigate the mechanisms by which myocardial contraction inhibits coronary blood flow. From finite deformation mechanics solutions the regional variation in intramyocardial pressure (IMP) exerted on coronary vessels embedded in the ventricular wall is calculated. This pressure is then coupled to a haemodymanic model of vascular blood flow to predict the spatial–temporal characteristics of perfusion throughout the myocardium. The calculated IMP is shown to vary approximately linearly between ventricular pressure at the endocardium and atmospheric pressure at the epicardium through the diastolic loading and isovolumic contraction phases. During the ejection and isovolumic relaxation phases IMP values rise slightly above ventricular pressure. The average radius of small arterial vessels embedded in the myocardium decreases during isovolumic contraction (18% at left ventricular endocardium) before increasing during ejection (10% at left ventricular endocardium) due to a rise in inflow pressure. Embedded venous vessels show a reduction in radius through both phases of contraction (35% at left ventricular endocardium). Calculated blood flows in both the large epicardial and small myocardial vessels show a 180° phase difference between arterial and venous velocity patterns with arterial flow occurring predominantly during diastole and venous flow occurring predominantly during systole. These results indicate that the transmission of ventricular cavity pressure through the myocardium is the dominant mechanism by which coronary blood flow is reduced during the isovolumic phase of contraction. In the ejection phase of contraction myocardial stiffening plays a more significant role in inhibiting blood flow.

Image-based carotid flow reconstruction: a comparison between MRI and ultrasound

Abstract: Atherosclerosis is a major cause of morbidity and mortality. Its apparent link with wall shear stress (WSS) has led to considerable interest in the in vivo estimation of WSS. Determining WSS by combining medical images with computational fluid dynamics (CFD) simulations can be performed both with magnetic resonance imaging (MRI) and three-dimensional ultrasound (3DUS). This study compares predicted 3D flow patterns based on black blood MRI and 3DUS. Velocity fields in the carotid arteries of nine subjects have been reconstructed, and the haemodynamic wall parameters WSS, oscillatory shear index (OSI), WSS gradients (WSSG) and angle gradients (WSSAG) were compared between the two imaging techniques. There was a good qualitative agreement between results derived from MRI and 3DUS (average correlation strength above 0.60). The root mean square error between haemodynamic wall parameters was comparable to the range of the expected variability of each imaging technique (WSS: 0.411 N m(-2); OSI: 0.048; temporal WSSG: 150 N s(-1) m(-2); spatial WSSG: 2.29 N m(-3); WSSAG: 87.6 rad m(-1)). In conclusion, MRI and 3DUS are capable of providing haemodynamic parameters when combined with CFD, and the predictions are in most cases qualitatively and quantitatively similar. The relatively high cost of MRI and continuing improvement in ultrasound favour US to MRI for future haemodynamic studies of superficial arteries.