Showing papers in "Physiological Measurement in 2007"

Photoplethysmography and its application in clinical physiological measurement.

Abstract: Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system. There has been a resurgence of interest in the technique in recent years, driven by the demand for low cost, simple and portable technology for the primary care and community based clinical settings, the wide availability of low cost and small semiconductor components, and the advancement of computer-based pulse wave analysis techniques. The PPG technology has been used in a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease. The introductory sections of the topical review describe the basic principle of operation and interaction of light with tissue, early and recent history of PPG, instrumentation, measurement protocol, and pulse wave analysis. The review then focuses on the applications of PPG in clinical physiological measurements, including clinical physiological monitoring, vascular assessment and autonomic function.

Geometry of the Poincaré plot of RR intervals and its asymmetry in healthy adults.

Abstract: The geometry of the Poincare plot of RR intervals is considered and its basic descriptors are defined in terms of the second moment of a distribution of points in a plane. One of the standard descriptors, SD1, is redefined and used to define two new descriptors, SD1(UP) and SD1(DOWN), whose squares partition SD1(2) (the variance corresponding to short-term heart rate variability) into contributions from decelerations and accelerations of heart rate. It is shown that there is a visible and statistically highly significant asymmetry in the Poincare plot, with the upper part, corresponding to decelerations of heart rate, larger than the lower part, which corresponds to accelerations. The effect is shown in one hundred 30 min long time series of RR intervals derived from the ECG recordings of 100 young (19-32 years old) and healthy adults. After shuffling the data to random order the asymmetry disappears, which shows that this is a genuine physiological phenomenon rather than an artefact of the method.

Novel dry electrodes for ECG monitoring.

Abstract: The development, fabrication and characterization of two novel dry bioelectrodes—conductive and capacitive ones—for biopotential monitoring are presented. The new electrodes have the potential to improve the applicability of dry electrodes in ambulant recording of ECG by reducing motion artifacts as well as the contact impedance to the skin. Furthermore, a passive filter network is integrated into the new electrodes to suppress slow offset fluctuation of the ECG signal caused e.g. by motions like breathing or changes in the electrode–skin interface properties. Compared to standard gel electrodes these new electrodes exhibit equivalent and superior contact impedances and biosignals. The integrated filter network effectively suppresses fluctuating offset potentials.

A robust fetal ECG detection method for abdominal recordings

Abstract: In this paper, we propose a new method for FECG detection in abdominal recordings. The method consists of a sequential analysis approach, in which the a priori information about the interference signals is used for the detection of the FECG. Our method is evaluated on a set of 20 abdominal recordings from pregnant women with different gestational ages. Its performance in terms of fetal heart rate (FHR) detection success is compared with that of independent component analysis (ICA). The results show that our sequential estimation method outperforms ICA with a FHR detection rate of 85% versus 60% of ICA. The superior performance of our method is especially evident in recordings with a low signal-to-noise ratio (SNR). This indicates that our method is more robust than ICA for FECG detection.

An efficient method for snore/nonsnore classification of sleep sounds.

Abstract: A new method to detect snoring episodes in sleep sound recordings is proposed. Sleep sound segments (i.e., 'sound episodes' or simply 'episodes') are classified as snores and nonsnores according to their subband energy distributions. The similarity of inter- and intra-individual spectral energy distributions motivated the representation of the feature vectors in a lower dimensional space. Episodes have been efficiently represented in two dimensions using principal component analysis, and classified as snores or nonsnores. The sound recordings were obtained from individuals who are suspected of OSAS pathology while they were connected to the polysomnography in Gulhane Military Medical Academy Sleep Studies Laboratory (GMMA-SSL), Ankara, Turkey. The data from 30 subjects (18 simple snorers and 12 OSA patients) with different apnoea/hypopnea indices were classified using the proposed algorithm. The system was tested by using the manual annotations of an ENT specialist as a reference. The accuracy for simple snorers was found to be 97.3% when the system was trained using only simple snorers' data. It drops to 90.2% when the training data contain both simple snorers' and OSA patients' data. (Both of these results were obtained by using training and testing sets of different individuals.) In the case of snore episode detection with OSA patients the accuracy is 86.8%. All these results can be considered as acceptable values to use the system for clinical purposes including the diagnosis and treatment of OSAS. The method proposed here has been used to develop a tool for the ENT clinic of GMMA-SSL that provides information for objective evaluation of sleep sounds.

Measurement of EMG activity with textile electrodes embedded into clothing

Abstract: Novel textile electrodes that can be embedded into sports clothing to measure averaged rectified electromyography (EMG) have been developed for easy use in field tests and in clinical settings. The purpose of this study was to evaluate the validity, reliability and feasibility of this new product to measure averaged rectified EMG. The validity was tested by comparing the signals from bipolar textile electrodes (42 cm(2)) and traditional bipolar surface electrodes (1.32 cm(2)) during bilateral isometric knee extension exercise with two electrode locations (A: both electrodes located in the same place, B: traditional electrodes placed on the individual muscles according to SENIAM, n=10 persons for each). Within-session repeatability (the coefficient of variation CV%, n=10) was calculated from five repetitions of 60% maximum voluntary contraction (MVC). The day-to-day repeatability (n=8) was assessed by measuring three different isometric force levels on five consecutive days. The feasibility of the textile electrodes in field conditions was assessed during a maximal treadmill test (n=28). Bland-Altman plots showed a good agreement within 2SD between the textile and traditional electrodes, demonstrating that the textile electrodes provide similar information on the EMG signal amplitude to the traditional electrodes. The within-session CV ranged from 13% to 21% in both the textile and traditional electrodes. The day-to-day CV was smaller, ranging from 4% to 11% for the textile electrodes. A similar relationship (r(2)=0.5) was found between muscle strength and the EMG of traditional and textile electrodes. The feasibility study showed that the textile electrode technique can potentially make EMG measurements very easy in field conditions. This study indicates that textile electrodes embedded into shorts is a valid and feasible method for assessing the average rectified value of EMG.

Synchronization between arterial blood pressure and cerebral oxyhaemoglobin concentration investigated by wavelet cross-correlation

Abstract: Wavelet cross-correlation (WCC) is used to analyse the relationship between low-frequency oscillations in near-infrared spectroscopy (NIRS) measured cerebral oxyhaemoglobin (O2Hb) and mean arterial blood pressure (MAP) in patients suffering from autonomic failure and age-matched controls. Statistically significant differences are found in the wavelet scale of maximum cross-correlation upon posture change in patients, but not in controls. We propose that WCC analysis of the relationship between O2Hb and MAP provides a useful method of investigating the dynamics of cerebral autoregulation using the spontaneous low-frequency oscillations that are typically observed in both variables without having to make the assumption of stationarity of the time series. It is suggested that for a short-duration clinical test previous transfer-function-based approaches to analyse this relationship may suffer due to the inherent nonstationarity of low-frequency oscillations that are observed in the resting brain.

A review of errors in multi-frequency EIT instrumentation

Abstract: Multi-frequency electrical impedance tomography (MFEIT) was proposed over 10 years ago as a potential spectroscopic impedance imaging method. At least seven systems have been developed for imaging the lung, heart, breast and brain, yet none has yet achieved clinical acceptance. While the absolute impedance varies considerably between different tissues, the changes in the spectrum due to physiological changes are expected to be quite small, especially when measured through a volume. This places substantial requirements on the MFEIT instrumentation to maintain a flat system frequency response over a broad frequency range (dc-MHz). In this work, the EIT measurement problem is described from a multi-frequency perspective. Solutions to the common problems are considered from recent MFEIT systems, and the debate over four-terminal or two-terminal (multiple source) architecture is revisited. An analysis of the sources of MFEIT errors identifies the major sources of error as stray capacitance and common-mode voltages which lead to a load dependence in the frequency response of MFEIT systems. A system that employs active electrodes appears to be the most able to cope with these errors (Li et al 1996). A distributed system with digitization at the electrode is suggested as a next step in MFEIT system development.

Assessment of pressure wave reflection: getting the timing right!

Abstract: Assessment of timing and magnitude of wave reflection is ideally based on wave separation analysis (WSA). In clinical practice, however, waveform analysis (WFA) is often used to study wave reflection, with different coexisting approaches to assess 'landmarks' on the waveform which are indicative for return of the reflected wave. The aim of this work was to compare WSA and WFA. Data were obtained from 2132 subjects (1093 women) aged between 35 and 56 and free from overt cardiovascular disease. Carotid pressure and aortic flow waveforms, and carotid-femoral pulse wave velocity were measured non-invasively. WSA yielded the timing of return of reflected wave (Tf-b), the ratio of forward and backward pressure wave (Pb/Pf), and the effective length of the arterial tree (Leff). WFA resulted in identification of the shoulder (Tsho) or inflection point (Tinf) as landmark points, with subsequently derived augmentation index and Leff (AIxsho and Leff,sho, AIxinf and Leff,inf, respectively). (i) Neither Tinf nor Tsho corresponded with the timing obtained from WSA. (ii) Measurements of Leff were found to decrease with age (conforming with current physiological insights) whilst Leff,inf was found to increase with age in women, and mixed results were obtained for Leff,sho. (iii) Both AIxinf and AIxsho showed a persistent gender difference which was not present in Pb/Pf. Using the pressure at Tf-b to calculate AIx, the systematic gender difference in AIxf-b was greatly reduced. Analysis of pressure wave reflection is optimally based on measurement of pressure and flow, rather than on waveform analysis alone.

The suitability of Tedlar bags for breath sampling in medical diagnostic research

Abstract: Tedlar bags are tested for their suitability for breath sampling for medical diagnostic purposes. Proton-transfer reaction-mass spectrometry was used to monitor the changes in composition of various mixtures contained in custom-made black-layered Tedlar bags. Characteristic ions at m/z 88 and 95 amu reflect considerable pollution from the bag material. The pollutant found on m/z 88 amu is most probably N,N-dimethylacetamide, a latent solvent used in the production of Tedlar film. Gas composition losses during filling were found to range from 5 to 47%, depending on the compound. Once stored, the half-lives of methanol, acetaldehyde, acetone, isoprene, benzene, toluene and styrene were estimated between 5 and 13 days. Losses from breath samples (52 h after filling) were found to be less than 10%. No observable decrease was found for ethylene over 3 days, using laser-based photoacoustic detection. For the use of Tedlar bags, a standardized protocol is advised, where the time point of analysis is fixed for all samples and should be kept as close as possible to the time of sampling.

Evaluation of viscoelastic parameters of the skeletal muscles in junior triathletes.

Abstract: Five male triathletes of the Estonian national junior team were observed during a seven-week competition period. The Myoton-2 equipment was used to describe the viscoelastic parameters of the skeletal muscles. The frequency of damped mechanical oscillation of the muscle tissue (Hz - indicating the tension in the muscle), logarithmic decrement of the oscillations (Theta - indicating the elasticity of the muscle) and stiffness (N m(-1)) of the muscle tissue were registered bilaterally in eight muscles in both the relaxed and the contracted states: BB - biceps brachii (caput longum); TB - triceps brachii (caput longum); BF - biceps femoris (caput longum); RF - rectus femoris; TA - tibialis anterior; GC - gastrocnemius (caput mediale); LD - latissimus dorsi; PM - pectoralis major (pars sternocostalis). A portable massage table was used for the subject to rest on during the measuring. For the measurement of the anterior muscles, the subject lay supine; for the posterior muscles the prone position was used. The (isometric) contraction was standardized simply by the same measuring position of the limb-the subject raised his arm or leg to an angle of 45 degrees from the horizontal level, using a 2.3 kg dumb-bell as an additional weight for the upper limb. The tarsal dorsiflexion and plantarflexion was performed against a fixed table to contract the crural muscles. The elasticity of the skeletal muscle is higher for the contracted state with respect to the relaxed one (p < 0.0001) and is described by decline of the value of logarithmic decrement, the stiffness and the tension in the muscle increases (p < 0.0001 for both parameters). The measured skeletal muscles differ significantly (p < 0.0018) by the viscoelastic properties in the relaxed state. In the relaxed state, TA was the most elastic (mean +/- SD; Theta-0.74 +/- 0.13), stiff (mean +/- SD; 346.68 +/- 60.34 N m(-1)) and tense muscle (mean +/- SD; 18.72 +/- 1.55 Hz). In the contracted state, the elasticity of TA did not change (0.76 +/- 0.14) while the stiffness and the tension in this muscle rose significantly (93% and 38%, accordingly). Personal differences (p < 0.005) exist if pooled data from the muscles are compared between the subjects.

Multi-frequency EIT system with radially symmetric architecture: KHU Mark1.

Abstract: We describe the development of a multi-frequency electrical impedance tomography (EIT) system (KHU Mark1) with a single balanced current source and multiple voltmeters. It was primarily designed for imaging brain function with a flexible strategy for addressing electrodes and a frequency range from 10 Hz–500 kHz. The maximal number of voltmeters is 64, and all of them can simultaneously acquire and demodulate voltage signals. Each voltmeter measures a differential voltage between a pair of electrodes. All voltmeters are configured in a radially symmetric architecture in order to optimize the routing of wires and minimize cross-talk. We adopted several techniques from existing EIT systems including digital waveform generation, a Howland current generator with a generalized impedance converter (GIC), digital phase-sensitive demodulation and tri-axial cables. New features of the KHU Mark1 system include multiple GIC circuits to maximize the output impedance of the current source at multiple frequencies. The voltmeter employs contact impedance measurements, data overflow detection, spike noise rejection, automatic gain control and programmable data averaging. The KHU Mark1 system measures both in-phase and quadrature components of trans-impedances. By using a script file describing an operating mode, the system setup can be easily changed. The performance of the developed multi-frequency EIT system was evaluated in terms of a common-mode rejection ratio, signal-to-noise ratio, linearity error and reciprocity error. Time-difference and frequency-difference images of a saline phantom with a banana object are presented showing a frequency-dependent complex conductivity of the banana. Future design of a more innovative system is suggested including miniaturization and wireless techniques.

Protective ventilation using electrical impedance tomography.

Abstract: Dynamic thoracic EIT is capable of detecting changes of the ventilation distribution in the lung. Nevertheless, it has yet to become an established clinical tool. Therefore, it is necessary to consider application scenarios wherein fast and distinct changes of the tissue conductivities are to be found and also have a clear diagnostic significance. One such a scenario is the artificial ventilation of patients suffering from the acute respiratory distress syndrome (ARDS). New protective ventilation strategies involving recruitment manoeuvres are associated with noticeable shifts of body fluids and regional ventilation, which can quite easily be detected by EIT. The bedside assessment of these recruitment manoeuvres will help the attending physician to optimize treatment. Hence, we performed an animal study of lavage-induced lung failure and investigated if EIT is capable of qualitatively as well as quantitatively monitoring lung recruitment during a stepwise PEEP trial. Additionally, we integrated EIT into a fuzzy controller-based ventilation system which allows one to perform automated recruitment manoeuvres (open lung concept) based on online PaO2 measurements. We found that EIT is a useful tool to titrate the proper PEEP level after fully recruiting the lung. Furthermore, EIT seems to be able to determine the status of recruitment when combining it with other physiological parameters. These results suggest that EIT may play an important role in the individualization of protective ventilation strategies.

Full-field stimulus testing (FST) to quantify visual perception in severely blind candidates for treatment trials

Abstract: Incurable genetic retinal blindness is in readiness for clinical trials to attempt restoration of vision. Early trials will involve those patients with the most severe visual losses, but there are no standard methods to quantify extremely abnormal visual perception. Full-field stimulus testing (FST) has been devised to fill this need. The current work advances FST to assay vision in the most severely blind patients using achromatic and chromatic stimuli in an LED-based visual stimulator. Our studies of this new FST in a cohort of retinal degeneration patients show the feasibility of the technique in a clinical setting. Results and variability of the new FST were comparable to our earlier FST device when vision was measurable with both. The extension of the dynamic range of chromatic and achromatic stimuli in the new FST by 2 log units allowed vision that was previously not measurable to be quantified. With this advance, inclusion and exclusion criteria can be developed and safety as well as efficacy outcomes measured in upcoming treatment trials of severe retinal blindness.

Detecting REM sleep from the finger: an automatic REM sleep algorithm based on peripheral arterial tone (PAT) and actigraphy.

Abstract: Scoring of REM sleep based on polysomnographic recordings is a laborious and time-consuming process. The growing number of ambulatory devices designed for cost-effective home-based diagnostic sleep recordings necessitates the development of a reliable automatic REM sleep detection algorithm that is not based on the traditional electroencephalographic, electrooccolographic and electromyographic recordings trio. This paper presents an automatic REM detection algorithm based on the peripheral arterial tone (PAT) signal and actigraphy which are recorded with an ambulatory wrist-worn device (Watch-PAT100). The PAT signal is a measure of the pulsatile volume changes at the finger tip reflecting sympathetic tone variations. The algorithm was developed using a training set of 30 patients recorded simultaneously with polysomnography and Watch-PAT100. Sleep records were divided into 5 min intervals and two time series were constructed from the PAT amplitudes and PAT-derived inter-pulse periods in each interval. A prediction function based on 16 features extracted from the above time series that determines the likelihood of detecting a REM epoch was developed. The coefficients of the prediction function were determined using a genetic algorithm (GA) optimizing process tuned to maximize a price function depending on the sensitivity, specificity and agreement of the algorithm in comparison with the gold standard of polysomnographic manual scoring. Based on a separate validation set of 30 patients overall sensitivity, specificity and agreement of the automatic algorithm to identify standard 30 s epochs of REM sleep were 78%, 92%, 89%, respectively. Deploying this REM detection algorithm in a wrist worn device could be very useful for unattended ambulatory sleep monitoring. The innovative method of optimization using a genetic algorithm has been proven to yield robust results in the validation set.

Stroke volume response during exercise measured by acetylene uptake and MRI.

Abstract: The intra-breath technique to measure acetylene absorption offers the possibility to determine augmentation of the pulmonary blood flow per heart beat (Q(C)) as an estimate of the stroke volume response during exercise. However, this method has not been compared with a validated test until now. Therefore, the aim of this study was to compare Q(C) with stroke volume (SV(MRI)) determined by magnetic resonance imaging (MRI) at rest and during exercise in healthy subjects and patients. For this purpose, ten healthy subjects and ten patients with idiopathic pulmonary arterial hypertension (iPAH) with expected impaired stoke volume response during exercise were measured by both methods. Exercise-induced changes in Q(C) and SV(MRI) were correlated in healthy controls (r = 0.75, p < 0.05). Compared to healthy controls, Q(C) increased less during exercise in iPAH patients (11 +/- 17 ml versus 33 +/- 12 ml, p < 0.05). A similar difference in stroke volume response to exercise between the two groups was measured by MRI (-0.6 +/- 8 ml versus 23 +/- 12 ml, p < 0.05, respectively). Hence, intra-breath and MRI measurements showed similar differences in exercise-induced changes in stroke volume between controls and patients. From these results it can be concluded that the intra-breath measurement of acetylene absorption might be of value as a non-invasive tool to estimate stroke volume augmentation during exercise and can detect differences in stroke volume responses between iPAH patients and healthy subjects.

Temporal image reconstruction in electrical impedance tomography.

Abstract: Electrical impedance tomography (EIT) calculates images of the body from body impedance measurements. While the spatial resolution of these images is relatively low, the temporal resolution of EIT data can be high. Most EIT reconstruction algorithms solve each data frame independently, although Kalman filter algorithms track the image changes across frames. This paper proposes a new approach which directly accounts for correlations between images in successive data frames. Image reconstruction is posed in terms of an augmented image x and measurement vector y, which concatenate the values from the d previous and future frames. Image reconstruction is then based on an augmented regularization matrix R, which accounts for a model of both the spatial and temporal correlations between image elements. Results are compared for reconstruction algorithms based on independent frames, Kalman filters and the proposed approach. For low values of the regularization hyperparameter, the proposed approach performs similarly to independent frames, but for higher hyperparameter values, it uses adjacent frame data to reduce reconstructed image noise.

Assessing nonlinear properties of heart rate variability from short-term recordings: are these measurements reliable?

Abstract: Several parameters assessing nonlinear properties of heart rate variability (HRV) from short-term ( 140%. Relative reliability was substantial (0.6 < ICC < 0.8) in half of the indices, moderate in one and poor in the remaining. Compared to classical linear indices, nonlinear HRV parameters seem more suitable for individual test–retest evaluations but, due to a reduced ICC, they need increased sample size in comparative studies involving two groups of subjects.

Reproducibility of regional lung ventilation distribution determined by electrical impedance tomography during mechanical ventilation

Abstract: Electrical impedance tomography (EIT) has the potential to become a new tool for bedside monitoring of regional lung ventilation. The aim of our study was to assess the reproducibility of regional lung ventilation distribution determined by EIT during mechanical ventilation under identical ventilator settings. The experiments were performed on 10 anaesthetized supine pigs ventilated in a volume-controlled mode. EIT measurements were performed with the Goe-MF II device (Viasys Healthcare, Hochberg, Germany) during repeated changes in positive end-expiratory pressure (PEEP) from 0 to 10 cm H2O. Regional lung ventilation was determined in the right and left hemithorax as well as in 64 regions of interest evenly distributed over each chest side in the ventrodorsal direction. Ventilation distributions in both lungs were visualized as ventrodorsal ventilation profiles and shifts in ventilation distribution quantified in terms of centres of ventilation in relation to the chest diameter. The proportion of the right lung on total ventilation in the chest cross-section was 0.54+/-0.04 and remained unaffected by repetitive PEEP changes. Initial PEEP increase resulted in a redistribution of ventilation towards dorsal lung regions with a shift of the centre of ventilation from 45+/-3% to 49+/-3% of the chest diameter in the right and from 47+/-2% to 50+/-2% in the left hemithorax. Excellent reproducibility of the results in the individual regions of interest with almost identical patterns of ventilation distribution was found during repeated PEEP changes.

Sample entropy of electrocardiographic RR and QT time-series data during rest and exercise.

Abstract: Sample entropy (SampEn) is a measure of the complexity of data. Few studies have compared the SampEn of electrocardiographic cardiac interval (RR) data (SampEn-RR) during differing physiological states, and none have examined SampEn for the corresponding QT interval (SampEn-QT). The aim of this study was to quantify SampEn-RR and SampEn-QT during rest and for a range of exercise workloads. Specific objectives were to assess both the utility of SampEn for discriminating between physiological states and the relationship of SampEn-RR with traditional measures of heart rate variability (HRV). Twelve males of similar age, mass and aerobic fitness participated. A three-lead ECG was recorded continuously during pre-exercise, progressive bicycle exercise and recovery, and beat-to-beat RR and QT intervals were quantified for sinus beats. SampEn and HRV were calculated within consecutive 1 min periods throughout. Consistent estimation of SampEn-RR and SampEn-QT was possible with an appropriate choice of SampEn parameters. SampEn-RR was sensitive to differing physiological conditions, but its discriminating ability was poorer than that of linear HRV indices. SampEn-RR was also negatively correlated with normalized LF and LF/HF parameters. We interpret changes in SampEn for RR and QT data in terms of the altered autonomic nervous system (ANS) control of either the atrial or the ventricular myocardium (or both) during discrete physiological states. We speculate that greater complexity in QT data might be explained by a direct ANS influence on the ventricular myocardium.

Research on the relationship between brain anoxia at different regional oxygen saturations and brain damage using near-infrared spectroscopy

Abstract: The objective of this paper is to investigate the difference in physiological parameters, EEG and morphology of brain tissues in newborn pigs with different regional oxygen saturations of brain (rSO(2)) and provide a basis for the determination of brain injury and degree of injury with the rSO(2) in clinical practice. A noninvasive near-infrared spectroscopy (NIRS) technique was used to monitor the rSO(2) of 27 newborn pigs. After mechanical ventilation and inhalation of 3-11% oxygen for 30 min by the newborn pigs, the pigs were grouped according to the rSO(2) in the brain caused by inhalation of different concentrations of oxygen. There were six animals each in rSO(2) 60% group (normal control). The physiological parameters and the EEG were monitored during the experiment. The animals were sacrificed by decollation at 72 hours after brain injury, and light microscope examination and pathological analysis of the ultrastructure were conducted on the brain tissues in the CA1 zone of hippocampi. In rSO(2) > 40% groups, the mean arterial pressure (MAP) was stable and there were no significant changes in blood lactic acid, amplitudes of the EEG, light microscopic findings and ultrastructure after hypoxia. When the rSO(2) was between 30% and 40%, the MAP was stable, the level of blood lactic acid increased, metabolic acidosis occurred, there was no significant change in the amplitudes of the EEG, there were ischemic changes in brain tissues under a light microscope and there was an injury of mitochondria in the neurons in the CA1 zone of hippocampi. When the rSO(2) was less than 30%, circulatory failure occurred, the level of blood lactic acid increased, there was serious metabolic acidosis, the amplitudes of the EEG significantly decreased, there were vacuolization and broken fragments of cells under the light microscope and the mitochondria in the neurons in the CA1 zone of hippocampi were seriously injured. Under varying degrees of hypoxia, when the rSO(2) is between 30% and 40%, brain injury occurs and the functional zones of mitochondria are injured in newborn pigs. When the rSO(2) is less than 30%, the brain functions are seriously abnormal, and the serious morphological impairment in the functional zones of mitochondria is the basis for the disturbance of energy metabolism in brain neurocytes after hypoxia and the sequelae of the nervous system.

Real time ECG artifact removal for myoelectric prosthesis control.

Abstract: The electrocardiogram (ECG) artifact is a major noise source contaminating the electromyogram (EMG) of torso muscles. This study investigates removal of ECG artifacts in real time for myoelectric prosthesis control, a clinical application that demands speed and efficiency. Three methods with simple and fast implementation were investigated. Removal of ECG artifacts by digital high-pass filtering was implemented. The effects of the cutoff frequency and filter order of high-pass filtering on the resulting EMG signal were quantified. An alternative adaptive spike-clipping approach was also developed to dynamically detect and suppress the ECG artifacts in the signal. Finally, the two methods were combined. Experimental surface EMG recordings with different ECG/EMG ratios were used as testing signals to evaluate the proposed methods. As a key parameter for clinical myoelectric prosthesis control, the average rectified amplitude of the signal was used as the performance indicator to quantitatively analyze the EMG content distortion and the ECG artifact suppression imposed by the two methods. Aiming at clinical application, the optimal parameter assignment for each method was determined on the basis of the performance using the suite of testing signals with various ECG/EMG ratios.

Electrode placement configurations for 3D EIT

Abstract: This paper investigates several configurations for placing electrodes on a 3D cylindrical medium to reconstruct 3D images using 16 electrode EIT equipment intended for use with a 2D adjacent drive protocol. Seven different electrode placement configurations are compared in terms of the following figures of merit: resolution, radial and vertical position error, image magnitude, immunity to noise, immunity to electrode placement errors, and qualitative evaluation of image artefacts. Results show that for ideal conditions, none of the configurations considered performed significantly better than the others. However, when noise and electrode placement errors were considered the planar electrode placement configuration (two rings of vertically aligned electrodes with electrodes placed sequentially in each ring) had the overall best performance. Based on these results, we recommend planar electrode placement configuration for 3D EIT lung imaging of the thorax.

Measurement of induced magnetic flux density using injection current nonlinear encoding (ICNE) in MREIT.

Abstract: Magnetic resonance electrical impedance tomography (MREIT) measures induced magnetic flux densities subject to externally injected currents in order to visualize conductivity distributions inside an electrically conducting object. Injection currents induce magnetic flux densities that appear in phase parts of acquired MR image data. In the conventional current injection method, we inject currents during the time segment between the end of the first RF pulse and the beginning of the reading gradient in order to ensure the gradient linearity. Noting that longer current injections can accumulate more phase changes, we propose a new pulse sequence called injection current nonlinear encoding (ICNE) where the duration of the injection current pulse is extended until the end of the reading gradient. Since the current injection during the reading gradient disturbs the gradient linearity, we first analyze the MR signal produced by the ICNE pulse sequence and suggest a novel algorithm to extract the induced magnetic flux density from the acquired MR signal. Numerical simulations and phantom experiments show that the new method is clearly advantageous in terms of the reduced noise level in measured magnetic flux density data. The amount of noise reduction depends on the choice of the data acquisition time and it was about 24% when we used a prolonged data acquisition time of 10.8 ms. The ICNE method will enhance the clinical applicability of the MREIT technique when it is combined with an appropriate phase artefact minimization method.

Micro-PIV measurements of blood flow in extraembryonic blood vessels of chicken embryos.

Abstract: The hemodynamic characteristics of blood flow are important in the diagnosis of circulatory diseases, since such diseases are related to wall shear stress of cardiovascular vessels. In chicken embryos at early stages of development, it is possible to directly visualize blood flow inside blood vessels. We therefore employed a micro-PIV technique to assess blood flow in extraembryonic venous and arterial blood vessels of chicken embryos, using red blood cells (RBCs) as tracers and obtaining flow images of RBCs using a high-speed CMOS camera. The mean velocity field showed non-Newtonian flow characteristics. The blood flow in two venous vessels merged smoothly into the Y-shaped downstream vein without any flow separation or secondary flow. Vorticity was high in the inner regions, where the radius of curvature varied greatly. A periodic variation of temporally resolved velocity signals, due to beating of the heart, was observed in arterial blood vessels. The pulsating frequency was obtained by fast Fourier transform analysis using the measured velocity data. The measurement technique used here was useful in analyzing the hemodynamic characteristics of in vivo blood flow in chicken embryos.

Microcirculatory function monitoring at the bedside—a view from the intensive care

Abstract: Microcirculatory dysfunction plays a key role in the pathophysiology of various disease states and may consequently impact patient outcome. Until recently, the evaluation of the microcirculation using different measurement techniques has been mostly limited to animal and human research. With technical advances, microcirculatory monitoring nowadays becomes more and more available for application in clinical praxis. Unfortunately, measurements within the microcirculation are mostly limited to easily accessible surfaces, such as skin, muscle and tongue. Due to major differences in the physiologic regulation of microcirculatory blood flow and in metabolism between organs and even within different tissues in one organ, the clinical importance of regional microcirculatory measurements remains to be determined. In addition, technical methods available demonstrate large differences in the measured parameters and sampling volume, making interpretation of data even more difficult. Nonetheless, the monitoring of the microcirculation may, ahead of time, alert physicians that tissue oxygen supply becomes compromised and it may lead to a better understanding of basic pathophysiological aspects of disease. In the present review, we describe available non-invasive microcirculatory measurement techniques which can be applied clinically at the bedside. After a short discussion of physiologic and pathophysiologic basics related to microcirculatory monitoring, the measuring principles, applications, strengths and limitations of different monitoring systems are discussed.

Analysis of surface EMG signal morphology in Parkinson's disease.

Abstract: A novel approach is presented for the analysis of surface electromyogram (EMG) morphology in Parkinson's disease (PD) The method is based on histogram and crossing rate (CR) analysis of the EMG signal In the method, histograms and CR values are used as high-dimensional feature vectors The dimensionality of them is then reduced using the Karhunen-Loeve transform (KLT) Finally, the discriminant analysis of feature vectors is performed in low-dimensional eigenspace Histograms and CR values were chosen for analysis, because Parkinsonian EMG signals typically involve patterns of EMG bursts Traditional methods of EMG amplitude and spectral analysis are not effective in analyzing impulse-like signals The method, which was tested with EMG signals measured from 25 patients with PD and 22 healthy controls, was promising for discriminating between these two groups of subjects The ratio of correct discrimination by augmented KLT was 86% for the control group and 72% for the patient group On the basis of these results, further studies are suggested in order to evaluate the usability of this method in early stage diagnostics of PD

Reply to "Comment on 'Analysis of electroencephalograms in Alzheimer's disease patients with multiscale entropy'"

Abstract: We appreciate the interest of Dr Tang in our article. Certainly, our previous results should be taken with caution due to the small database size. Nevertheless, it must be noted that this limitation was clearly recognized in our article. Furthermore, our hypothesis is completely justified from the current state of the art in the analysis of electroencephalogram (EEG) signals from Alzheimer's disease (AD) patients. We evaluated whether the multiscale entropy (MSE) analysis of the EEG background activity was useful to distinguish AD patients and controls. We do believe that further discussions about risk factors or related clinicophysiological protein aspects are clearly beyond the scope of our article. For the sake of completeness, we now detail some results that complement our previous analysis. They suggest that the MSE analysis can provide relevant information about the dynamics of AD patients' EEG data. Thus, we must reaffirm our conclusions, although we again acknowledge that further studies are needed.

Estimating stroke volume from oxygen pulse during exercise.

Abstract: This investigation aimed at verifying whether it was possible to reliably assess stroke volume (SV) during exercise from oxygen pulse (OP) and from a model of arterio-venous oxygen difference (a-vO(2)D) estimation. The model was tested in 15 amateur male cyclists performing an exercise test on a cycle-ergometer consisting of a linear increase of workload up to exhaustion. Starting from the analysis of previous published data, we constructed a model of a-vO(2)D estimation (a-vO(2)D(est)) which predicted that the a-vO(2)D at rest was 30% of the total arterial O(2) content (CaO(2)) and that it increased linearly during exercise reaching a value of 80% of CaO(2) at the peak workload (W(max)) of cycle exercise. Then, the SV was calculated by applying the following equation, SV = OP/a-vO(2)D(est), where the OP was assessed as the oxygen uptake/heart rate. Data calculated by our model were compared with those obtained by impedance cardiography. The main result was that the limits of agreement between the SV assessed by impedance cardiography and the SV estimated were between 22.4 and -27.9 ml (+18.8 and -24% in terms of per cent difference between the two SV measures). It was concluded that our model for estimating SV during effort may be reasonably applicable, at least in a healthy population.

The slow wave component of retinal activity in rd/rd mice recorded with a multi-electrode array.

Abstract: We investigated the differences in the retinal activity between normal and degenerate retina. Multi-electrode recordings were performed in in vitro mice retinas. Only short duration (<2 ms) retinal spikes were recorded in normal mice by postnatal day 28. However, in rd/rd mice, a slow wave component with approximately 100 ms duration was also recorded along with the spikes. We attempted to understand the mechanism of this slow wave component in degenerate retina by applying various synaptic blockers. With CNQX/AP-7, the glutamate antagonist (n = 7), the slow wave component disappeared while the normally less-dominant retinal spikes became more apparent. With strychnine, the glycine antagonist (n = 3) or picrotoxin, GABA antagonist (n = 3), the amplitude of the slow wave component increased. These suggest that a stronger excitatory glutamate input from bipolar cells to ganglion cells is the main contributor to this slow wave component in rd/rd mice.