A new movement artifact detector for photoplethysmographic signals

Oximeters are commonly used in abbreviated cardiorespiratory studies (ACS) to monitor blood oxygen saturation and heart rate using the photoplethysmography (PPG) signal. These data are prone to movement artifacts, especially in infants who move or need to be handled often. Therefore segments of PPG data contaminated by movement artifact must be detected as a first stage of analysis. In ACS this identification is generally done manually, by having an expert visually assess the quality of the signal. This is subjective and very time consuming, especially for long data records. For this reason we present a novel detector of PPG movement artifacts that uses moving average filters to remove trends, reduce the effect of white noise, and notch filter pulse-related information. The normalized root mean square of the filtered signal is then used as a detection statistic. We demonstrate its detection properties using a data set from infants recovering from anesthesia, and show that it performs better than other automated methods based on entropy or higher-order statistics. Furthermore, the new method is more robust than the other methods in the presence of large noise.

Large-scale expansion of no-take closures within the Great Barrier Reef has not enhanced fishery production

A rare opportunity to test hypotheses about potential fishery benefits of large-scale closures was initiated in July 2004 when an additional 28.4% of the 348 000 km2 Great Barrier Reef (GBR) region of Queensland, Australia was closed to all fishing. Advice to the Australian and Queensland governments that supported this initiative predicted these additional closures would generate minimal (10%) initial reductions in both catch and landed value within the GBR area, with recovery of catches becoming apparent after three years. To test these predictions, commercial fisheries data from the GBR area and from the two adjacent (non-GBR) areas of Queensland were compared for the periods immediately before and after the closures were implemented. The observed means for total annual catch and value within the GBR declined from preclosure (2000-2003) levels of 12780 Mg and Australian $160 million, to initial post-closure (2005-2008) levels of 8143 Mg and $102 million; decreases of 35% and 36% respectively. Because the reference areas in the non-GBR had minimal changes in catch and value, the beyond-BACI (before, after, control, impact) analyses estimated initial net reductions within the GBR of 35% for both total catch and value. There was no evidence of recovery in total catch levels or any comparative improvement in catch rates within the GBR nine years after implementation. These results are not consistent with the advice to governments that the closures would have minimal initial impacts and rapidly generate benefits to fisheries in the GBR through increased juvenile recruitment and adult spillovers. Instead, the absence of evidence of recovery in catches to date currently supports an alternative hypothesis that where there is already effective fisheries management, the closing of areas to all fishing will generate reductions in overall catches similar to the percentage of the fished area that is closed.

Automated analysis of respiratory behavior in extremely preterm infants and extubation readiness

BACKGROUND

Rates of extubation failure of extremely preterm infants remain high. Analysis of breathing patterns variability during spontaneous breathing under endotracheal tube continuous positive airway pressure (ETT-CPAP) is a potential tool to predict extubation readiness.

OBJECTIVE

To investigate if automated analysis of respiratory signals would reveal differences in respiratory behavior between infants that were successfully extubated or not.

METHODS

Respiratory Inductive Plethysmography (RIP) signals were recorded during ETT-CPAP just prior to extubation. Signals were digitized, and analyzed using an Automated Unsupervised Respiratory Event Analysis (AUREA). Extubation failure was defined as reintubation within 72 hr. Statistical differences between infants who were successfully extubated or failed were calculated.

RESULTS

A total of 56 infants were enrolled and one was excluded due to instability during the ETT-CPAP; 11 out of 55 infants studied failed extubation (20%). No differences in demographics were observed between the success and failure groups. Significant differences on the variability of some respiratory parameters or 'metrics' estimated by AUREA were observed between the 2 groups. Indeed, a simple classification using the variability of two metrics of respiratory behavior predicted extubation failure with high accuracy.

CONCLUSION

Automated analysis of respiratory behavior during a short ETT-CPAP period may help in the prediction of extubation readiness in extremely preterm infants.

Detection of breathing segments in respiratory signals

The typical approach for analysis of respiratory records consists of detection of respiratory pauses and elimination of segments corrupted by movement artifacts. This is motivated by established rules used for manual scoring of respiratory events, which focus on pause segmentation and do not define criteria to identify breathing segments. With this strategy, breathing segments can only be inferred indirectly from the absence of abnormalities, yielding an unclear and ambiguous definition. In this work we present novel detectors for synchronous and asynchronous breathing, and compare them with AUREA, a novel system for Automated Unsupervised Respiratory Event Analysis, which performs indirect classification of breathing. Results from analysis of real infant respiratory data show an improvement in the identification of synchronous and asynchronous breathing of 9% and 27% respectively, demonstrating that direct detection of breathing enhances the classification performance.

Subspace identification of Hammerstein systems using B-splines

This paper presents an algorithm for the identification of Hammerstein cascades with hard nonlinearities. The nonlinearity of the cascade is described using a B-spline basis with fixed knot locations; the linear dynamics are described using a state-space model. The algorithm automatically estimates both the order of the linear system and the number and locations of the knots used to characterize the nonlinearity. Therefore, it significantly reduces the a priori knowledge about the underlying system required for identification. A simulation study on a model of reflex stiffness shows that the new method estimates the nonlinearity accurately in the presence of output noise.

System identification of biomedical systems from short transients using space methods

System identification technology has been a popular tool to analysis the dynamic behavior of biomedical systems. A segment of data with enough information must be collected in order to obtain unbiased estimate of the systems. However, some biomedical systems have short transients, such as Vestibulo-Ocular Reflex (VOR). Systems often cannot be identified from the short transients collected in one experiment. Thus, the experiments must be repeated, and an ensemble of input and output recorded. This paper presents a subspace method to identify Multiple Input and Multiple Output (MIMO) state space models for biomedical systems from short transients using ensemble data. A simulating ankle joint stiffness experiment demonstrates that the algorithm provides accurate results.

Closed-loop system identification of ankle dynamics with compliant loads

Joint stiffness, defined as the relation between the angular position of a joint and the torque acting about it, can be used to describe the dynamic behavior of the human ankle during posture and movement. Joint stiffness can be separated into intrinsic stiffness and reflex stiffness, which are modeled as a linear system and a LNL system, respectively. With a compliant load, joint stiffness can be viewed as being operated in closed-loop because the torque is fed back through the load to change the position. In this paper, we present a new method to estimate the intrinsic and reflex stiffness from the total torque measurement. An EIV (Errors-In-Variables) subspace system identification method is used to estimate the dynamics of each pathway directly from measured data. Simulation and experiment studies demonstrate that the method produces accurate results.

Power-based segmentation of respiratory signals using forward-backward bank filtering

We present an automated method for the segmentation of ribcage and abdominal signals measured by noninvasive respiratory inductance plethysmography (RIP) into quiet breathing and artifact-corrupted segments. This procedure, which involves forward-backward filtering, is applicable to the automated off-line analysis of long records of respiratory signals. Examples of applications include home and sleep laboratory studies of cardiorespiratory data. The new procedure was successfully applied to the segmentation of cardiorespiratory signals acquired post-operatively from infants in the recovery room of the Montreal Children's Hospital (MCH).

Nondestructive online in vitro monitoring of pre-osteoblast cell proliferation within microporous polymer scaffolds

We present a system for the online, in vitro, nondestructive monitoring of tissue growth within microporous polymer scaffolds. The system is based on measuring the admittance of the sample over a frequency range of 10-200 MHz using an open-ended coaxial probe and impedance analyzer. The sample admittance is related to the sample complex permittivity (CP) by a quasi-static model of the probe's aperture admittance. A modified effective medium approximation is then used to relate the CP to the cell volume fraction. The change of cell volume fraction is used as a measure of tissue growth inside the scaffold. The system detected relative cell concentration differences between microporous polymer scaffolds seeded with 0.4, 0.45, 0.5, and 0.6 x 10(6) pre-osteoblast cells. In addition, the pre-osteoblast proliferation within 56 scaffolds over 14 days was recorded by the system and a concurrent DNA assay. Both techniques produced cell proliferation curves that corresponded to those found in literature. Thus, our data confirmed that the new system can assess relative cell concentration differences in microporous scaffolds enabling online nondestructive tissue growth monitoring.

Identification of time-varying intrinsic and reflex joint stiffness

We have developed a time-varying, parallel- cascade system identification algorithm to separate joint stiffness into intrinsic and reflex components at each point in time throughout rapid movements. The components are identified using an iterative algorithm in which intrinsic and reflex dynamics are identified using separate time-varying (TV) techniques based on ensemble methods. An ensemble of input-output records having the same TV behavior is acquired and used to identify the system dynamics as impulse response functions at time increments corresponding to the sampling interval. Simulation studies showed that the time-varying, parallel-cascade algorithm performed well under realistic conditions with 99.9% VAF between simulated and predicted torque. To evaluate the performance of the algorithm under realistic conditions we applied it to an ensemble of experimental data acquired under stationary conditions. Results demonstrated that the TV estimates converged to those of the established time-invariant algorithm and allowed us to determine how variance of the TV estimates varied with the number of realizations in the ensemble.

Decomposition of a parallel cascade model of ankle stiffness using subspace methods

Joint stiffness, defined as the relation between the angular position of a joint and the torque acting about it, can be used to describe the dynamical behavior of the human ankle during posture and movement. Joint stiffness can be separated into intrinsic stiffness and reflex stiffness, which are modeled as a linear system and a Hammerstein system, respectively. A two-pathway parallel cascade model, with the intrinsic stiffness on one pathway and the reflex stiffness on the other, can be used to describe the joint stiffness. In this paper, we present a new method to separate the torque from each pathway from the total torque measurement. A subspace based system identification method is used to estimate the dynamics of each pathway directly from measured data without iteration. Simulation studies demonstrate that the method produces accurate results without the need of iteration.

A Highly Responsive System for On-line in vitro Assessment of Tissue Growth within MicroPorous Polymer Scaffolds

We have proposed a highly responsive system for the on-line in vitro assessment of tissue growth within microporous polymer scaffolds that obviates any compromise of sample integrity. The system's function is based on the sample's loss factor: the imaginary part of the complex permittivity. Reflection measurements were performed using an open-ended coaxial probe and impedance analyzer; they were then related to the sample's complex permittivity by a quasi-static model of the probe's aperture admittance. Measurements of saline solutions showed that the real part of permittivity was corrupted by apparent polarization effects. Consequently, we developed a simplified formulation of the imaginary part of the Hanai-Wagner effective medium approximation to eliminate its dependence on the real part of complex permittivity measurement. This formulation allows the sample's cell concentration to be determined. The variation of a sample's cell concentration over time was used as a measure of tissue growth. Measurements in the frequency range of 10-200 MHz were performed on micro-porous polymer scaffolds seeded with progressively greater number of cells. Results demonstrated that the system detected concentration differences between cell-seeded scaffolds.

Time-varying parallel-cascade system identification of ankle stiffness from ensemble data

Measurement of joint dynamic stiffness during time-varying conditions is crucial to understand the role of joint mechanics during movement. Stiffness can be separated into intrinsic and reflex components, and are modeled as linear dynamic and Hammerstein systems, respectively. Time-varying identification methods using ensemble data have been developed previously for both pathways and were tested separately on simulated data. In this study, these algorithms were integrated into the time-varying, parallel-cascade identification method. Ankle dynamics were modeled during a ramp input and simulated impulse response functions (IRFs) were generated. Gaussian white noise was low-pass filtered and was convolved with the simulated systems over 500 realizations. The ensemble data was used to evaluate the new identification technique. The mean variances accounted for (VAFs) between the true and identified IRFs for the intrinsic and reflex pathways were 99.9% and 97.7%, respectively, demonstrating the technique's strong ability to predict the system's dynamics.

Determination of the systematic and random measurement error in an LC-FTICR mass spectrometry analysis of a partially characterized complex peptide mixture

In high-throughput proteomics, a promising approach presently being explored is the use of liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry (LC-FTICR-MS) to provide measurements of the masses of tryptic peptides in complex mixtures, which can then be used to identify the proteins which gave rise to those peptides. In order to apply this method, it is necessary to account for any systematic measurement error, and it is useful to have an estimate of the random error in measured masses. In this investigation, a complex mixture of peptides derived from a partially characterized sample was analyzed by LC-FTICR-MS. Through the application of a Bayesian probability model of the data, partial knowledge of the composition of the sample is sufficient both to determine any systematic error and to estimate the random error in measured masses.

Detection of movement artifacts in respiratory inductance plethysmography: performance analysis of a Neyman-Pearson energy-based detector

In [3] we developed a method for the automated estimation of the phase relation between thoracic and abdominal signals measured by noninvasive respiratory inductance plethysmography (RIP). In the present paper, we improve on the phase estimator by including an automated procedure for the detection of periods of gross body movements. We assume that the number of sleep obstructive events during periods of gross body movements is zero in probability. We hope that combining the phase estimator with the gross body movement detector should yield improved diagnostic tools for the automated classification of obstructive hypopnea events.

Modulation of stretch reflex excitability during quiet human standing

Stretch reflex excitability was measured during quiet standing by using a bilateral electro-hydraulic actuator to apply perturbations of angular position to the ankle. Subjects were instructed to stand quietly while pulse displacements were applied at random time intervals. Position, torque, gastrocnemius soleus EMG, tibialis anterior EMG, heel position, tibia angle, femur angle, and sacrum angle were measured. Activation level and reflex excitability varied substantially from trial to trial - reflex torque decreased as the background torque level increased; while reflex EMG increased when background torque increased. This behavior is consistent with previous findings in prone subjects. Reflex torque for a given activation level was found to vary with the initial torque derivative. Negative torque derivatives produced greater reflex excitation then their positive counterparts. These findings suggest that reflex excitability in quiet human standing is modulated to optimize balance.

Surface analysis of peptide mass spectra to improve time and mass localization

Current peak detections algorithms for processing mass spectrometry (MS) spectra generally rely on two dimensional techniques for identifying the location and intensity of peaks from a single spectrum. However, when high performance liquid chromatography (HPLC) is coupled with mass spectrometry, a third dimension, retention time, is introduced. The ensemble of MS spectra may then be regarded as a 3D surface where spectral intensity is a function of m/z (mass-to-charge) and time. This suggests that peak localization can be improved by incorporating the time domain data and average data across both dimensions. This work describes a surface intensity analysis algorithm and the results of its use.

Fetal heart rate deceleration detection using a discrete cosine transform implementation of singular spectrum analysis

OBJECTIVES

To develop a singular-spectrum analysis (SSA) based change-point detection algorithm applicable to fetal heart rate (FHR) monitoring to improve the detection of deceleration events.

METHODS

We present a method for decomposing a signal into near-orthogonal components via the discrete cosine transform (DCT) and apply this in a novel online manner to change-point detection based on SSA. The SSA technique forms models of the underlying signal that can be compared over time; models that are sufficiently different indicate signal change points. To adapt the algorithm to deceleration detection where many successive similar change events can occur, we modify the standard SSA algorithm to hold the reference model constant under such conditions, an approach that we term "base-hold SSA". The algorithm is applied to a database of 15 FHR tracings that have been preprocessed to locate candidate decelerations and is compared to the markings of an expert obstetrician.

RESULTS

Of the 528 true and 1285 false decelerations presented to the algorithm, the base-hold approach improved on standard SSA, reducing the number of missed decelerations from 64 to 49 (21.9%) while maintaining the same reduction in false-positives (278).

CONCLUSIONS

The standard SSA assumption that changes are infrequent does not apply to FHR analysis where decelerations can occur successively and in close proximity; our base-hold SSA modification improves detection of these types of event series.

Experimental evaluation of computerised tomography point spread function variability within the field of view: parametric models

The objective of the paper was to validate non-linear parametric models of computerised tomography point spread function (PSF), to investigate the role of model parameters and to verify the effect of different imaging conditions on estimated parameters. These models were then to be used experimentally to estimate the variation of PSF shape within the field of view of a scanner. Two parametric models of the PSF are presented. The Gaussian model is appropriate when PSF values are positive, and the damped cosine model can account for negative values. These models are non-linear and fully two-dimensional and do not assume radial symmetry. The models were fitted to images of a point source. The models accounted for over 99% of the variance in the PSF signal. Errors in modulation transfer function were limited to 5% when the appropriate model was selected. The difference in the blurring characteristics of three image reconstruction filters was well quantified by shape parameters, and position parameters located the PSF with subpixel accuracy. With a point source located 50mm directly above the centre of the field of view, the PSF was found to be anisotropic.

Predicting surgical outcome in temporal lobe epilepsy patients using MRI and MRSI

OBJECTIVE

To develop a classifier that uses MR data to predict surgical outcome in patients with temporal lobe epilepsy (TLE).

METHODS

Eighty-one patients with medically refractory TLE who underwent surgical treatment were studied. Patients underwent comprehensive presurgical investigation, including ictal video-EEG recording, 1H MRS imaging, and volumetric MRI. Outcome was measured using Engel's classification system, condensed into two outcome groups. Two approaches were taken. First, outcome was defined as experiencing worthwhile improvement with >90% reduction of seizure frequency (Classes I, II, and III) or not (Class IV). A second approach was to define outcome as experiencing freedom from seizures following surgery (Class I) or not (Classes II, III, and IV). For each approach, a Bayesian classifier was constructed to predict outcome by calculating the probability of a patient's pattern of results from spectroscopic analysis of the temporal lobes and volumetric analysis of the amygdala and hippocampus being associated with the various outcome groups.

RESULTS

The worthwhile improvement classifier correctly predicted the surgical outcomes of 60 of 65 (92%) of patients who experienced worthwhile improvement and 10 of 16 (63%) of patients who did not. The seizure-free classifier correctly predicted the surgical outcomes of 39 of 52 (75%) of patients who became seizure free and 21 of 29 (72%) of patients who did not.

CONCLUSION

MR features are important markers of surgical outcome in patients with TLE and can provide assistance in identifying surgical candidates.

Identification of time-varying Hammerstein systems from ensemble data

In this paper, we describe a new technique to identify rapidly time-varying Hammerstein systems from ensembles of input-output realizations. The technique involves two steps. A correlation approach is first used to obtain initial estimates of the linear subsystem parameters for every sampling time. An iterative optimization algorithm is then employed to produce final estimates of the system parameters. The input does not need to be white. The technique was tested on simulated data and was found to produce excellent results under realistic conditions.

Intrinsic and reflex stiffness in normal and spastic, spinal cord injured subjects

Mechanical changes underlying spastic hypertonia were explored using a parallel cascade system identification technique to evaluate the relative contributions of intrinsic and reflex mechanisms to dynamic ankle stiffness in healthy subjects (controls) and spastic, spinal cord injured (SCI) patients. We examined the modulation of the gain and dynamics of these components with ankle angle for both passive and active conditions. Four main findings emerged. First, intrinsic and reflex stiffness dynamics were qualitatively similar in SCI patients and controls. Intrinsic stiffness dynamics were well modeled by a linear second-order model relating intrinsic torque to joint position, while reflex stiffness dynamics were accurately described by a linear, third-order system relating half-wave rectified velocity to reflex torque. Differences between the two groups were evident in the values of four parameters, the elastic and viscous parameters for intrinsic stiffness and the gain and first-order cut-off frequency for reflex stiffness. Second, reflex stiffness was substantially increased in SCI patients, where it generated as much as 40% of the total torque variance, compared with controls, where reflex contributions never exceeded 7%. Third, differences between SCI patients and controls depended strongly on joint position, becoming larger as the ankle was dorsiflexed. At full plantarflexion, there was no difference between SCI and control subjects; in the mid-range, reflex stiffness was abnormally high in SCI patients; at full dorsiflexion, both reflex and intrinsic stiffness were larger than normal. Fourth, differences between SCI and control subjects were smaller during the active than the passive condition, because intrinsic stiffness increased more in controls than SCI subjects; nevertheless, reflex gain remained abnormally high in SCI patients. These results elucidate the nature and origins of the mechanical abnormalities associated with hypertonia and provide a better understanding of its functional and clinical implications.

Separable least squares identification of nonlinear Hammerstein models: application to stretch reflex dynamics

The Hammerstein cascade, consisting of a zero-memory nonlinearity followed by a linear filter, is often used to model nonlinear biological systems. This structure can represent some high-order nonlinear systems accurately with relatively few parameters. However, it is not possible, in general, to estimate the parameters of a Hammerstein cascade in closed form. The most effective method available to date uses an iterative approach, which alternates between estimating the linear element from a crosscorrelation, and then fitting a polynomial to the nonlinearity via linear regression. This paper proposes the use of separable least squares optimization methods to estimate the linear and nonlinear elements simultaneously in a least squares framework. A separable least squares algorithm for the identification of Hammerstein cascades is developed and used to analyze stretch reflex electromyogram data from two experimental subjects. The results show that in each case the proposed algorithm produced a better model, in that it predicted the system's response to novel inputs more accurately, than did models estimated using the traditional iterative algorithm. Monte-Carlo simulations demonstrated that when the input is a non-Gaussian, nonwhite signal, as is often the case experimentally, the traditional iterative identification approach produces biased models, whereas the separable least squares approach proposed in this paper does not.

Identification of physiological systems: estimation of linear time-varying dynamics with non-white inputs and noisy outputs

A new technique to identify linear time-varying systems from ensembles of input-output realisations is presented. First, a correlation-based least-squares method is derived. This method consists of solving, for each sampling time, a matrix equation involving estimates of the input autocorrelation and input-output cross-correlation functions computed from data across the ensemble. Then, the matrix inverse needed to solve this matrix equation is replaced with a pseudo-inverse. The model is thus constrained to describe only those components of the dynamics that can be reliably identified. Ignoring 'unidentifiable' components has virtually no adverse effect on the predicted outputs. Simulation results demonstrate that the pseudoinverse technique yields more reliable estimates of the dynamics than a previously proposed least-squares technique when the inputs are coloured and the output signal-to-noise ratio (SNR) is low. With the input spectrum flat up to approximately 10% of the sampling rate and an output SNR of 5dB, the mean variance accounted for (VAF) between the true instantaneous impulse response functions (IRFs) and the instantaneous IRFs estimated with the least-squares technique was 0.2%. In contrast, the mean VAF between the true instantaneous IRFs and the instantaneous IRFs estimated with the pseudoinverse technique was 89.0%.

Intrinsic and reflex contributions to human ankle stiffness: variation with activation level and position

A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness over a wide range of tonic voluntary contraction levels and ankle positions in healthy human subjects. Intrinsic stiffness dynamics were described well by a linear pathway having elastic, viscous, and inertial properties. A velocity-sensitive pathway comprising a delay, a static non-linearity, resembling a half-wave rectifier, followed by a low-pass filter, described reflex stiffness dynamics. The absolute magnitude of intrinsic and reflex stiffness parameters varied from subject to subject but the relative changes with contraction level and position were consistent. Intrinsic stiffness increased monotonically with contraction level while reflex stiffness was maximal at low contraction levels and then decreased. Intrinsic and reflex stiffness both increased as the ankle was dorsiflexed. As a result, reflex mechanics made their largest relative contributions near the neutral position at low levels of activity. The size of the maximum reflex contribution varied widely among subjects, in some it was so small (ca 1%) that it would be unlikely to have any functional importance; however, in other subjects, reflex contributions were large enough (as high as 55% in one case) to play a significant role in the control of posture and movement. This variability may have arisen because stretch reflexes were not useful for the torque-matching task in these experiments. It will be of interest to examine other tasks where stretch reflexes would have a direct impact on performance.

Modulation of stretch reflexes during imposed walking movements of the human ankle

Our overall objectives were to examine the role of peripheral afferents from the ankle in modulating stretch reflexes during imposed walking movements and to assess the mechanical consequences of this reflex activity. Specifically we sought to define the changes in the electromyographic (EMG) and mechanical responses to a stretch as a function of the phase of the step cycle. We recorded the ankle position of a normal subject walking on a treadmill at 3 km/h and used a hydraulic actuator to impose the same movements on supine subjects generating a constant level of ankle torque. Small pulse displacements, superimposed on the simulated walking movement, evoked stretch reflexes at different phases of the cycle. Three major findings resulted: 1) soleus reflex EMG responses were influenced strongly by imposed walking movements. The response amplitude was substantially smaller than that observed during steady-state conditions and was modulated throughout the step cycle. This modulation was qualitatively similar to that observed during active walking. Because central factors were held constant during the imposed walking experiments, we conclude that peripheral mechanisms were capable of both reducing the amplitude of the reflex EMG and producing its modulation throughout the movement. 2) Pulse disturbances applied from early to midstance of the imposed walking cycle generated large reflex torques, suggesting that the stretch reflex could help to resist unexpected perturbations during this phase of walking. In contrast, pulses applied during late stance and swing phase generated little reflex torque. 3) Reflex EMG and reflex torque were modulated differently throughout the imposed walking cycle. In fact, at the time when the reflex EMG response was largest, the corresponding reflex torque was negligible. Thus movement not only changes the reflex EMG but greatly modifies the mechanical output that results.

Fitting cytosolic ADP recovery after exercise with a step response function

Phosphorus magnetic resonance (MR) spectroscopy was used to measure the recovery kinetics of calculated cytoplasmic metabolically active adenosine diphosphate (ADP) after exercise in normal subjects and patients with mitochondrial myopathies. These kinetics have previously been fitted with a single exponential function, despite a complex time-dependent undershoot in many subjects. By considering the transition from ischemic-exercise to perfused-recovery as a step function input, a second-order linear system was developed yielding a step response function to fit the ADP recovery. Using this method, an average improvement in fit of 23% resulted in a significant improvement in the characterization of ADP recovery for all normal subjects with substantial undershoot. The patient group had a comparable improvement in fit of 11%. Fitting the ADP recovery with a second-order step response function can provide significantly better characterization of muscle oxidative metabolism in vivo.

Benign symmetric lipomatosis (Madelung's disease)

Benign symmetric lipomatosis, also known as Madelung's disease, is a rare condition characterized by massive fatty deposits arranged symmetrically around the neck, shoulders, and arms. These patients might present for liposuction and body contouring. Although infrequently encountered in the average plastic surgeon's practice, this condition should be considered when evaluating candidates for these procedures. The deformity is associated with chronic alcohol use and also with malignant tumors of the upper airways. The deformity is prone to recurrence and its surgical treatment often results in less than optimal aesthetic outcomes. Despite this fact, surgical removal via either direct excision or suction-assisted lipectomy provides the only real hope of palliation. This report describes a patient with this deformity and a review of the literature.

Nonparametric block-structured modeling of lung tissue strip mechanics

Very large amplitude pseudorandom uniaxial perturbations containing frequencies between 0.125 and 12.5 Hz were applied to five dog lung tissue strips. Three different nonlinear block-structured models in nonparametric form were fit to the data. These models consisted of (1) a static nonlinear block followed by a dynamic linear block (Hammerstein model); (2) the same blocks in reverse order (Wiener model); and (3) the blocks in parallel (parallel model). Both the Hammerstein and Wiener models performed well for a given input perturbation, each accounting for greater than 99% of the measured stress signal variance. However, the Wiener and parallel model parameters showed some dependence on the strain amplitude and the mean stress. In contrast, a single Hammerstein model accounted for the data at all strain amplitudes and operating stresses. A Hammerstein model featuring a fifth-order polynomial static nonlinearity and a linear impulse response function of 1 s duration accounted for the most output variance (99.84%+/-0.13%, mean+/-standard deviations for perturbations of 50% strain at 1.5 kPa stress). The static nonlinear behavior of the Hammerstein model also matched the quasistatic stress-strain behavior obtained at the same strain amplitude and operating stress. These results show that the static nonlinear behavior of the dog lung tissue strip is separable from its linear dynamic behavior.

An adaptive filter to reduce cardiogenic oscillations on esophageal pressure signals

Measurements of pressure swings in the esophagus (Pes) can be used to estimate variables of clinical importance, e.g., intrinsic positive end-expiratory pressure (PEEPi). Unfortunately, cardiogenic oscillations frequently corrupt Pes and complicate further analysis. Due to significant band overlap with the respiratory component of Pes, cardiogenic oscillations cannot be suppressed adequately using standard filtering techniques. In this article, we present an adaptive filter that employs the electrocardiogram to identify and suppress the cardiogenic oscillations. This filter was tested using simulated data, where the variance accounted for relative to the simulated respiratory pressure swings increased from as low as 55% for the unfiltered Pes signal to over 95% when the adaptive filter was used. In patient data, the adaptive filter reduced the apparent cardiogenic oscillations without noticeably distorting the sharp deflections in Pes due to respiration. Furthermore, the filter suppressed peaks in the Fourier transform of Pes at integer multiples of the heart rate, while the remaining frequencies remained largely unchanged. During stable breathing, the standard deviation of PEEPi was reduced by between 44% and 71% in these four patients when the filter was used. We conclude that our filter removes a significant fraction of the cardiogenic oscillations that corrupt records of Pes.

Generalized eigenvector algorithm for nonlinear system identification with non-white inputs

Traditional methods for nonlinear system identification require a white, Gaussian, test input, a restriction that has limited their usability in many fields. In this study, we address the problem of identifying the dynamics of a nonlinear system when the input is highly colored-a restriction commonly encountered in the study of physiological systems. An extension of the parallel cascade method is developed that is optimal in a constrained minimum mean squared error sense and exactly corrects for the distortion induced by the non-white input spectrum. However, this correction is a deconvolution, which may become extremely ill-conditioned if the input spectrum departs significantly from whiteness; to confront this, we develop a low-rank projection operation that stabilizes the deconvolution. The overall algorithm is robust and places few requirements on the nature of the test input. Practical application of this new method is demonstrated by using it to identify a known analog nonlinear system from experimental data.

Identification of intrinsic and reflex contributions to human ankle stiffness dynamics

We have examined dynamic stiffness at the human ankle using position perturbations which were designed to provide a wide-bandwidth input with low average velocity. A parallel-cascade, nonlinear system identification technique was used to separate overall stiffness into intrinsic and reflex components. Intrinsic stiffness was described by a linear, second-order system similar to that demonstrated previously. Reflex stiffness dynamics were more complex, comprising a delay, a unidirectional rate-sensitive element and then lowpass dynamics. Reflex mechanisms were found to be most important at frequencies of 5-10 Hz. The gain and dynamics of reflex stiffness varied strongly with the parameters of the perturbation, the gain decreasing as the mean velocity of the perturbation increased. Under some conditions, torques generated by reflex mechanisms were of the same magnitude as those from intrinsic mechanisms. It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.

Identification of time-varying stiffness dynamics of the human ankle joint during an imposed movement

The time-varying stiffness dynamics of the human ankle joint were identified during a large stretch imposed upon the active triceps surae muscles. Small stochastic position perturbations were superimposed upon many repetitions of the larger movement and an ensemble time-varying identification technique was then used to characterize the relationship between the small perturbation and the torque it evoked at each sample in time throughout the movement. This technique was found to provide an excellent description of the ankle stiffness dynamics throughout this movement, with the identified stiffness impulse response functions accounting for more than 80% of the torque variance at all times. The average low-frequency stiffness values (K(low)) derived from the stiffness impulse responses at each sample in time are believed to reflect primarily the instantaneous elastic properties of active crossbridges. These properties, which reflect the contractile state of the muscles more directly than force or torque measurements, have not been obtained previously from an intact muscle-joint system. We found that stiffness actually increased during the later portion of the large imposed stretch, indicating the triceps surae muscles did not yield significantly, and that the post-stretch steady-state stiffness level was approximately 60% higher than prior to the stretch. Reflex activity evoked by the large stretch did not produce a detectable change in K(low), even though this activity did produce a clear twitch-like response in joint torque beginning approximately 60 ms following stretch onset. A second-order mechanical model was found to provide an adequate characterization of stiffness dynamics for steady-state periods before and well after the imposed movement, but it could not adequately describe the observed changes in stiffness dynamics during the movement itself. However, the variation of model parameters indicated that the torque evoked by the stochastic displacement was predominantly elastic in nature. The stiffness behavior during stretch observed here for the intact human ankle joint is largely consistent with previous studies performed in isolated muscle preparations.

Identification of physiological systems: a robust method for non-parametric impulse response estimation

The identification of non-parametric impulse response functions (IRFs) from noisy finite-length data records is analysed using the techniques of matrix perturbation theory. Based on these findings, a method for IRF estimation is developed that is more robust than existing techniques, particularly when the input is non-white. Furthermore, methods are developed for computing confidence bounds on the resulting IRF estimates. Monte Carlo simulations are used to assess the capabilities of this new method and to demonstrate its superiority over classical techniques. An application to the identification of dynamic ankle stiffness in humans is presented.

Experimental correlation-based identification of X-ray CT point spread function. Part 1: Method and experimental results

Knowledge of the point spread function (PSF) of an imaging system is important when studying the characteristics of the blur present in the images. Published experimental PSF identification techniques adapt classical one-dimensional linear system identification strategies using impulse, step function or periodic input signals. This study proposes and successfully applies a correlation method based on the Wiener-Hopf equation to identify the PSF of a CT scanner. The input consists of a series of pseudorandomly located holes. Results are found to be statistically equivalent to those obtained with the impulse method at a 90% two-sided confidence interval. Like the impulse method, it readily yields two-dimensional estimate, but the larger input circumvents the major objection to the use of a wire input. Furthermore, it is relatively immune to output noise, offering an advantage over edge methods. This resistance to noise may prove helpful for nuclear medicine imaging techniques, for which the signal-to-noise ratio is much lower than that X-ray for CT.

Experimental correlation-based identification of X-ray CT point spread function. Part 2: Simulation and design of input signal

The preferred signals for non-parametric correlation-based point spread function identification are white noise or pseudo-random binary sequences (PRBSs). Given the difficulty of building a phantom based on either of these signals, a new input is devised that corresponds to pseudo-randomly located holes. The positions of the holes correspond to zeros in a 2-D PRBS. To optimise the design of the phantom and to ensure proper imaging procedure, a number of simulations are conducted. The effects of the following parameters on identification quality are investigated: the size of the holes and their minimum separation, the period of the PRBS, input-output translational and rotational mis-registration, pixel size and the presence of cupping. The factors affecting identification quality the most are rotational alignment, hole size and separation, as well as sequence length. During simulations, a point spread function offering characteristics similar to the Philips Tomoscan CX is identified. Optimal results are obtained when the signal consists of 0.6 mm holes, separated by 0.9 mm, whose position is based on a 32 x 32 PRBS generated with a ten-stage shift-register. When adequate rotational alignment is provided, it is shown that the pseudo-randomly located holes signal is a good substitute for a purely white signal when identifying the PSF of a CT scanner.

Bioavailability of fluconazole in surgical intensive care unit patients: a study comparing routes of administration

OBJECTIVE

Anticandidal therapy is commonly used in the surgical intensive care unit (SICU). Unfortunately, it is expensive because it is generally given intravenously, as acute trauma and abdominal surgery are often accompanied by impaired gastrointestinal function. We compared the systemic availability of fluconazole given enterally or intravenously in trauma and surgery SICU patients to determine the reliability of enteral administration.

METHODS

Nine adult trauma (Injury Severity Score (> or = 18) and nine adult abdominal surgery SICU patients were randomized to receive fluconazole 100 mg via the intravenous (IV) or enteral route. Patients with a bilirubin > 4.0 mg/dL or creatinine clearance < 60 mL/min were excluded. Enteral fluconazole was crushed, dissolved, and flushed through a nasogastric or feeding tube. Eleven serial blood samples were drawn over 72 hours. Area under the curve (AUC), elimination rate constant (Ke), and terminal half-life (T1/2) were determined and compared (t test). Relative bioavailability was estimated (AUC(enteral)/AUCIV).

RESULTS

Peak concentrations occurred within 2 hours after enteral dosing and 15 minutes after IV dosing. The relative bioavailability was 77%. Weight, AUC, Ke, and T1/2 did not differ between enteral and IV dosing.

CONCLUSIONS

Fluconazole is significantly absorbed when crushed, dissolved, and given via a nasogastric or feeding tube in SICU patients. Nonsignificant trends toward lower systemic availability with enteral administration can be overcome with slightly higher doses. Since enteral administration of fluconazole costs 10% of fluconazole given intravenously, more liberal use of enteral administration offers tremendous savings. Such savings moderate the cost concerns of antifungal therapy.

Psychological and rheumatic symptoms of women requesting silicone breast implant removal

In this case series, we describe the psychological and rheumatic symptoms of 52 women requesting breast implant removal. Main outcome measures include psychological evaluation with the Brief Symptom Inventory and the Beck Depression Inventory. Clinical evaluation was performed using a rheumatic symptom survey. Results noted elevation of all Brief Symptom Inventory subscales (highest in Somatization and Obsessive-Compulsive), and the Beck Depression Inventory showed mild depression. Rheumatic survey noted a mean of 12 of 23 symptoms. In conclusion, psychological distress and mild depression are noted in this group, who also may experience rheumatic symptoms of connective tissue disease. There was a moderate positive correlation between rheumatic symptoms and psychological distress.

Effect of maintained stretch on the range of motion of the human ankle joint

The effectiveness of maintained stretch in expanding the range of motion of the human ankle joint was assessed in a population of normal adults. Controlled movements were imposed upon the ankle, and triceps surae and tibialis anterior electromyograms were monitored to ensure that only passive joint properties generated ankle torque. We found that a majority of subjects (7 of 12) showed evidence of muscle activity sufficient to distort a subjective assessment of changes in range of motion. For the remaining five subjects, a 60-s maintained stretch produced a small decrease in the torque subsequently generated by an imposed dorsiflexing movement, but this effect was transient and largely disappeared following 300 s of rest at a neutral position. This short-term effect is consistent with the viscoelastic properties of collagenous material stretched during such treatment and is unlikely to lead to long-term increases in range of motion. RELEVANCE: The results of these experiments indicate that subjective assessments of changes in joint range of motion may be distorted by voluntary and reflexive muscle activation. Moreover the presumed increases in range of motion produced by maintained stretch in our normal subjects were small and transient. These results suggest that future assessments of the long-term efficacy of this treatment must monitor muscle activity and take into account known viscoelastic properties of collagenous materials.

Nonlinear behavior of muscle reflexes at the human ankle joint

1. Pulse inputs (similar to tendon jerks) were applied to the human ankle joint with the use of a hydraulic actuator. Inputs of only 1-2 degrees could elicit large responses (> 20% of maximum voluntary contraction). The magnitude of the response depended nonlinearly on a number of factors: the amplitude, direction, and duration of the pulse; the angle of the ankle; and the level of voluntary activation of the ankle muscles. 2. Pulses that flexed or extended the ankle could both produce reflex torques in the same direction (extensor torque). Although an extension of the ankle did not itself produce a response, it could affect the response to a subsequent flexion for up to 1 s. 3. The influence of random perturbations on the stretch reflex at the ankle was assessed. Responses to pulse displacements alone and to pulses superimposed on random perturbations were compared at the same level of voluntary activity. Reflex responses decreased in a graded manner with increasing amplitude or bandwidth of the random perturbations. 4. These results demonstrate that stretch reflexes can generate substantial torques, but in a highly nonlinear manner. In particular, passive joint movements markedly alter stretch reflex gain, and these changes must be considered in interpreting the functional significance of reflex actions.

Identification of time-varying dynamics of the human triceps surae stretch reflex. II. Rapid imposed movement

We examined the time-varying dynamics of the human triceps surae stretch reflex before, during, and after a large stretch was imposed upon the ankle joint, during a constant voluntary contraction of 15% of maximum voluntary contraction. Stretch reflex dynamics were estimated by superimposing a small stochastic displacement on many such stretches and using an "ensemble-based" time-varying identification procedure to compute impulse response functions relating the perturbation to the evoked electromyogram (EMG) at each point throughout the task. We found that stretch reflex magnitude (relating joint velocity to EMG) varied directly with baseline EMG activity during steady-state conditions before and after the large imposed stretch. Following the large stretch and the reflex activity it evoked, both background EMG and stretch reflex magnitude declined for up to 100 ms; changes in the stretch reflex were substantially greater in magnitude and followed a different time course from the corresponding changes in background EMG, however, indicating that stretch reflex properties were modulated independently of motoneuron pool activation level. Based on timing and the invariance of stretch reflex dynamics across time, it is argued that this behavior is largely mediated via peripheral neural mechanisms. This peripheral modulation of the stretch reflex presumably supplements various descending influences to adjust reflex properties.

Identification of time-varying dynamics of the human triceps surae stretch reflex. I. Rapid isometric contraction

We have examined the time variations of stretch reflex dynamics throughout rapid voluntary changes in the isometric contraction level of the human triceps surae muscles. This was achieved by superimposing a small stochastic displacement upon many such changing contractions and then identifying the time-varying relationship between the perturbation and the evoked electromyograms (EMGs). An "ensemble" time-varying system identification technique was used to estimate these input-output dynamics as a set of impulse response functions, one for each time before, during, and after the change in contraction level, with a temporal resolution equal to the data acquisition rate. Three main findings resulted. First, stretch reflex gain (relating joint velocity to EMG) was significantly modulated during changes in voluntary contraction level, increasing as the subject contracted the muscles and decreasing as the subject relaxed. Second, stretch reflex dynamics did not change with contraction level, even when its gain varied substantially. Third, the time course of the gain changes closely followed the level of the EMG, even though the subjects used rather different activation and deactivation patterns. These results suggest that, for the behavior studied (i.e., rapid changes in isometric contraction level), stretch reflex gain and motoneuron pool activation level were controlled by a common descending command rather than being independently specified.

Quantitative analysis of four EMG amplifiers

Four typical EMG amplifiers were tested quantitatively to observe the diversity and specificity of available equipment. Gain, phase, common mode rejection ratio (CMRR) and noise characteristics were measured for each device. Various gain and phase responses were observed, each best suited to specific application areas. For all amplifiers, the CMRR was shown to decrease dramatically in the presence of input impedance mismatches of more than 10 k omega between the two electrodes. Because such impedance mismatches are common on the skin surface, these results indicate that proper skin preparation is required to maximize the noise rejection capabilities of the tested amplifiers.

Nonparametric two-dimensional point spread function estimation for biomedical imaging

The problem of identifying optical system point spread functions (PSFs) arises frequently in the area of image processing and restoration. The paper presents a method for determining two-dimensional PSFs from input/output image signals. The PSF of the system is determined from a set of linear equations involving elements of the input autocorrelation function and the input/output cross-correlation function. The resulting PSF is the one that minimises the sum of squares difference between the actual output image and the predicted one.

Identification of time-varying biological systems from ensemble data

The theory underlying a new method for the identification of time-varying systems is described. The method uses singular value decomposition to obtain least-squares estimates of time-varying impulse response functions from an ensemble of input-output realizations. No a priori assumptions regarding the system structure or form of the time-variation are required and there are few restrictions on the input signal. Simulation studies, using a model of time-varying joint dynamics, show that the method can track rapid changes in system dynamics accurately and is robust in the presence of output noise. An application of the method is demonstrated by using it to track dynamic ankle stiffness during a rapid, voluntary, isometric contraction. During the transient phase of the contraction, low-frequency ankle stiffness gain decreased in a manner which could not be described with the second-order model of joint dynamics often used under stationary conditions.

Wound Complications Following Vertical Banded Gastroplasty: report of experience and review of the literature

This prospective study was undertaken to evaluate wound management and outcome in 76 morbidly obese patients undergoing vertical banded gastroplasty through a detailed protocol to determine the nature and incidence of wound complications and to evaluate these results relative to the surgical literature. Early wound complications can be infrequent. Late wound failures often occur in patients not experiencing early wound complications. Mechanical factors, such as size-related tension across the incision, undoubtedly play a major role in late wound failure. A long suture length to wound length ratio mitigates mechanical factors involved in late wound failure. With attention to technical factors and detail, wound complications can be minimized, though not eliminated.

Diagnostic peritoneal lavage. Limited indications due to evolving concepts in trauma care

This study was undertaken to determine the appropriateness of celiotomy in 100 consecutive patients who underwent celiotomy solely because of positive diagnostic peritoneal lavage (DPL) following blunt (B) or stab (S) abdominal trauma. A total of 32 (32%) patients had positive DPL by laboratory criteria: blunt trauma: greater than 100K RBC/mm3, greater than 500 WBC/mm3; stab trauma: greater than 50K RBC/mm3, greater than 250 WBC/mm3. DPL in 68 patients was positive by gross inspection; 18 of these 68 patients' DPL laboratory results returned after surgery and did not satisfy the laboratory definition of positive DPL. In all 61 per cent underwent therapeutic celiotomy (TC) and 39 per cent underwent nontherapeutic celiotomy (NTC). Grade I and II spleen and/or liver injuries led to 79 per cent of NTCs. Positive DPL, determined by gross inspection or by laboratory testing, has a very poor accuracy rate when evaluated in light of evolving beliefs that promote nonoperative therapy for grade I and II liver and spleen injuries. When positive DPL is the sole indication for celiotomy in patients with blunt or stab abdominal trauma, an unacceptably large number of NTCs will be performed. DPL should have a limited role in the evaluation of patients with abdominal trauma.

Prospective study of a prosthetic H-graft portacaval shunt

This study was undertaken to prospectively evaluate the 8-mm Gore-Tex interposition H-graft portacaval shunt. Thirty-six high-risk patients at the University of South Florida-affiliated hospitals received small-diameter shunts because of bleeding esophagogastric varices over a recent 2-year period. Portal vein and portal vein-inferior vena cava gradients were significantly reduced after shunting. These pressure changes were manifested clinically by the absence of variceal rebleeding and improvement of ascites; in addition, the incidence of encephalopathy was low. The 8-mm graft maintained hepatopedal flow in 67% of the patients, but reversal of flow did not result in complications commonly associated with poor portal perfusion. Graft thrombosis occurred in four (11%) patients. All grafts were successfully revised, three by operative revision and one by an interventional radiologist. Operative mortality was low (11%), and morbidity was unusual. The small-diameter H-graft portacaval shunt is a safe and effective method of treatment for bleeding esophagogastric varices.