Biomedical subjects

Lotfi Senhadji

Publications and source records attributed to Lotfi Senhadji.

14 recordsLinked to original sources

PubMed2006

Quantitative evaluation of linear and nonlinear methods characterizing interdependencies between brain signals.

Brain functional connectivity can be characterized by the temporal evolution of correlation between signals recorded from spatially-distributed regions. It is aimed at explaining how different brain areas interact within networks involved during normal (as in cognitive tasks) or pathological (as in epilepsy) situations. Numerous techniques were introduced for assessing this connectivity. Recently, some efforts were made to compare methods performances but mainly qualitatively and for a special application. In this paper, we go further and propose a comprehensive comparison of different classes of methods (linear and nonlinear regressions, phase synchronization, and generalized synchronization) based on various simulation models. For this purpose, quantitative criteria are used: in addition to mean square error under null hypothesis (independence between two signals) and mean variance computed over all values of coupling degree in each model, we provide a criterion for comparing performances. Results show that the performances of the compared methods are highly dependent on the hypothesis regarding the underlying model for the generation of the signals. Moreover, none of them outperforms the others in all cases and the performance hierarchy is model dependent.

Brain↗

PubMed2006

Blind source separation for ambulatory sleep recording.

This paper deals with the conception of a new system for sleep staging in ambulatory conditions. Sleep recording is performed by means of five electrodes: two temporal, two frontal and a reference. This configuration enables to avoid the chin area to enhance the quality of the muscular signal and the hair region for patient convenience. The electroencephalopgram (EEG), eletromyogram (EMG), and electrooculogram (EOG) signals are separated using the Independent Component Analysis approach. The system is compared to a standard sleep analysis system using polysomnographic recordings of 14 patients. The overall concordance of 67.2% is achieved between the two systems. Based on the validation results and the computational efficiency we recommend the clinical use of the proposed system in a commercial sleep analysis platform.

Algorithms↗

PubMed2005

Acquisition time reduction in magnetic resonance spectroscopic imaging using discrete wavelet encoding.

This paper describes a new magnetic resonance spectroscopic imaging (MRSI) technique based upon the discrete wavelet transform to reduce acquisition time and cross voxel contamination. Prototype functions called wavelets are used in wavelet encoding to localize defined regions in localized space by dilations and translations. Wavelet encoding in MRSI is achieved by matching the slice selective RF pulse profiles to a set of dilated and translated wavelets. Single and dual band slice selective excitation and refocusing pulses, with profiles resembling Haar wavelets, are used in a spin-echo sequence to acquire 2D-MRSI wavelet encoding data. The 2D space region is spanned up to the desired resolution by a proportional number of dilations (increases in the localization gradients) and translations (frequency shift) of the Haar wavelets (RF pulses). Acquisition time is reduced by acquiring successive MR signals from regions of space with variable size and different locations with no requirement for a TR waiting time between acquisitions. An inverse wavelet transform is performed on the data to produce the correct spatial MR signal distribution.

Algorithms↗

PubMed2005

Time-frequency characterization of interdependencies in nonstationary signals: application to epileptic EEG.

For the past decades, numerous works have been dedicated to the development of signal processing methods aimed at measuring the degree of association between electroencephalographic (EEG) signals. This interdependency parameter, which may be defined in various ways, is often used to characterize a functional coupling between different brain structures or regions during either normal or pathological processes. In this paper, we focus on the time-frequency characterization of the interdependency between signals. Particularly, we propose a novel estimator of the linear relationship between nonstationary signals based on the cross correlation of narrow band filtered signals. This estimator is compared to a more classical estimator based on the coherence function. In a simulation framework, results show that it may exhibit better statistical performances (bias and variance or mean square error) when a priori knowledge about time delay between signals is available. On real data (intracerebral EEG signals), results show that this estimator may also enhance the readability of the time-frequency representation of relationship and, thus, can improve the interpretation of nonstationary interdependencies in EEG signals. Finally, we illustrate the importance of characterizing the relationship in both time and frequency domains by comparing with frequency-independent methods (linear and nonlinear).

Algorithms↗

PubMed2003

Beat-to-beat blood pressure variability and patent ductus arteriosus in ventilated, premature infants.

The aim of the present study was to test the hypothesis that a relationship exists between respiratory-induced blood pressure variability (BPV) and transductal shunting in premature infants with respiratory distress. Ten premature infants (27-32 weeks gestation) with respiratory distress ventilated in the synchronised, positive-pressure mode were examined. The interrelations between blood pressure and transthoracic impedance were described using time and frequency domain analysis. Haemodynamic effects of left-to-right transductal shunting were assessed using Doppler echocardiography (ratio of diastolic flow to systolic flow in the subdiaphragmatic aorta). The dependence of blood pressure fluctuations on the respiratory cycle was seen consistently in both time-domain and cross-spectral analysis. The amplitude of these fluctuations varied between infants. In the time domain, the beat-to-beat pressure difference was 0.69-4.1 mmHg for diastolic and 0.99-5.24 mmHg for systolic blood pressure. There was a positive linear relationship between the respiratory-related BPV indicators and the extent of left-to-right transductal shunting ( r=0.86 for diastolic and 0.80 for systolic pressure, P<0.01). Respiratory-related BPV was not correlated to the indicators of left ventricle preload. It is concluded that respiratory related BPV involves both diastolic and systolic blood pressure and is correlated to the magnitude of left-to-right transductal shunting in the population studied.

Ductus Arteriosus, Patent↗

PubMed2003

Lactate doublet quantification and lipid signal suppression using a new biexponential decay filter: application to simulated and 1H MRS brain tumor time-domain data.

A new postprocessing filter based on the continuous wavelet transform (CWT) method modeled as a biexponential decay function to isolate the lactate doublet from overlapping lipid resonance(s) and estimate its magnetic resonance spectroscopy (MRS) parameters (signal amplitude, resonance frequencies, and apparent relaxation time (T(*) (2))) is proposed. The new filter employs the same iterative process used in the previously single exponential decay filter. A comparison of the results obtained from application of both filters to simulated data and real (1)H MRS data collected from human blood plasma and brain tumors demonstrates that the new filter provides a better estimate of MRS parameters of lactate, with less computation time. Furthermore, the results show that the new filter is less sensitive to noise and provides a direct estimate of J-coupling value of the lactate doublet.

Adipose Tissue↗

PubMed2003

The relationship between expired concentration of sevoflurane and sympathovagal tone in children.

UNLABELLED: In children, sevoflurane depresses parasympathetic tone during induction more than halothane. The effects of sevoflurane on parasympathetic activity could explain the difference in heart rate (HR) changes described between infants and children. In this study, we sought to determine the relationship between the end-tidal concentration of sevoflurane and sympathetic and parasympathetic tone in children by spectral analysis of RR intervals. Thirty-three children, ASA physical status I, who required elective surgery were studied. In 10 children (Group A), recordings were performed while gradually decreasing the inspired sevoflurane concentration from 8% to the beginning of clinical awakening. In 23 other children (Group B), recordings were performed while children were awake and at a steady-state of 1 and 2 minimum alveolar anesthetic concentration of sevoflurane. A time-varying autoregressive modeling of the interpolated RR sequences was performed, and spectral density in low-frequency (LF; 0.04-0.15 Hz) and high-frequency (HF; 0.15-0.55 Hz) bands was calculated. In Group A, HR slowing paralleled the decrease in expired sevoflurane concentration. Conversely, the decrease in expired concentration of sevoflurane led to an increase in systolic blood pressure (SBP), HF, LF, and LF/HF. The increase in LF/HF preceded the increase in HF. In Group B, the baseline HF power spectrum and normalized values HFnu (HFnu = HF/LF + HF) were significantly increased in children older than 3 yr. Changes in HR induced by sevoflurane were negatively correlated with baseline HF and HFnu (R(2) = 0.6; P < 0.001). These results demonstrate that withdrawal of parasympathetic tone is the main determinant for the change in HR induced by sevoflurane. IMPLICATIONS: The effects of sevoflurane on parasympathetic activity could explain the difference in heart-rate changes described between infants and children during induction. This study describes the changes in heart rate and its variability induced by sevoflurane in children and shows that these changes are related to parasympathetic tone before the induction of anesthesia.

Anesthesia Recovery Period↗

PubMed2002

Localized proton spectroscopy without water suppression: removal of gradient induced frequency modulations by modulus signal selection.

Most Magnetic Resonance Spectroscopy (MRS) localization methods can generate gradient vibrations at acoustic frequencies and/or magnetic field oscillation, which can cause a time-varying magnetic field superimposed onto the static one. This effect can produce frequency modulations of the spectral resonances. When localized MRS data are acquired without water suppression, the associated frequency modulations are manifested as a manifold of spurious peaks, called sidebands, which occur symmetrically around the water resonance. These sidebands can be larger than the small metabolite resonances and can present a problem for the quantitation of the spectra, especially at short echo times. Furthermore, the resonance lineshapes may be distorted if any low frequency modulations are present. A simple solution is presented which consists of selecting the modulus of the acquired Free Induction Decay (FID) signal. Since the frequency modulations affect only the phase of the FID signal, the obtained real spectrum of the modulus is free from the spurious peaks where quantitative results may be directly obtained. Using this method, the distortions caused by the sidebands are removed. This is demonstrated by processing proton MRS spectra acquired without water suppression collected from a phantom containing metabolites at concentrations comparable to those in human brain and from a human subject using two different localization methods (PRESS and Chemical Shift Imaging PRESS-(CSI)). The results obtained illustrate the ability of this approach to remove the spurious peaks. The corrected spectra can then be fit accurately. This is confirmed by the results obtained from both the relative and the absolute metabolites concentrations in phantoms and in vivo.

Brain Chemistry↗

PubMed2002

Epileptic transient detection: wavelets and time-frequency approaches.

This paper is aimed at presenting the two main classes of nonstationary signal transforms that are currently used to analyze and to characterize EEG observations. Time-scale methods, or wavelet transforms, allow a time versus duration analysis to be performed whereas time-frequency methods allow spectral contents to be analyzed as a function of time. These two types of transform are well suited to the study of changes either localized or progressive that may be observed in EEG signal dynamics and that sign the evolution of underlying physiological mechanisms. The potential interest of these methods in nonstationary signal representation is illustrated through several academic examples. Then, methods are applied on real EEG signals to solve problems such that the detection of interictal transient signals (like spikes or spike-waves) and the recognition of signatures during ictal periods.

Algorithms↗

PubMed2002

Extrapolation of cardiac index from analysis of the left ventricular outflow velocities in children: implication of the relationship between aortic size and body surface area.

BACKGROUND: It has recently been reported in critically ill patients that a linear relationship exists between cardiac index (CI) measured with thermodilution and mean aortic blood flow velocity (MAFV). This hypothesis can be validated mathematically only if the aortic area (AA index) indexed to body surface area (BSA) remains constant and if the relationship between aortic diameter (PhiAo) and BSA is nonlinear. However, several other equations have described the relationship between BSA and, respectively, PhiAo and aortic area (AA) in children. The aim of this study was to determine if the relationships calculated between BSA and aortic size in children (without left ventricular outflow tract abnormality) could validate the hypothesis that MAFV and CI are well linked linearly, leading to its use to determine CI. METHODS: Two hundred and thirty-two measurements performed in 126 children and infants were retrospectively analysed. PhiAo was measured in the long axis view at the annulus using two-dimensional mode echocardiography with a 5-MHz transducer. Various linear and nonlinear relationships between BSA and, respectively, PhiAo, PhiAoindex, AA and AAindex were determined based on a nonlinear regression method with a model as follows: y=a(xc) + b. The comparisons between regressions were conducted based on the estimation error. RESULTS: The relationships between PhiAo and BSA appeared nonlinear and was well described by: PhiAo=2.96(BSA1/4) - 1.31 with a non-zero y-intercept and PhiAo=1.64(BSA1/2) with a zero y-intercept. In contrast, the relationships between AA and BSA were linear. The AAindex was not linked to BSA and can be considered as constant. The coefficient a of the equation appeared similar to those obtained mathematically with the relationship previously described between MAFV and CI. CONCLUSIONS: The hypothesis that CI can be extrapolated to the measurement of MAFV appears valid as regards the relationships calculated between aortic size and BSA in children without left ventricular outflow tract abnormality.

Adolescent↗

PubMed2002

Monitoring approaches in general anesthesia: a survey.

The aim of this article is to give an overview of recent developments in the field of general anesthesia monitoring. We measure different physiological signals related to the functioning of several nervous systems. Using statistical or signal processing methods, monitors are derived and correlated to the dosage of anesthetic agents and to the status of the patient. Comparisons among existing monitors show that there is no one universal monitor applicable to all general anesthesia--each has its own characteristics and might be useful in particular clinical situations. Modeling of the underlying physiological mechanisms of the anesthesia may help for better understanding the interactions between anesthetic agents and nervous systems. Physiological-model-based general anesthesia monitoring and control can then be considered and optimized for each patient.

Adolescent↗

PubMed2001

Impedance cardiographic waveforms in children: Involvement in left ventricular ejection time determination during anesthesia.

OBJECTIVE: The physiologic basis of the impedance cardiographic (ICG) signal is still a matter of debate. An accurate determination of left ventricular ejection time (LVET) is needed to calculate stroke volume. In children, several shapes of the ICG signal are observed during anesthesia or in critical care. The aim of this study was to determine the relationship between the shape of the ICG signal and various morphologic and hemodynamic determinants obtained with Doppler echocardiography to highlight the effect of the various shapes in the accuracy of LVET determination. DESIGN: Prospective study. SETTING: Pediatric surgery in a university hospital. PATIENTS: 103 children, ASA physical status I or II. INTERVENTIONS: General anesthesia for elective surgery. Measurements: Electrocardiography, ICG, and Doppler echocardiography were recorded simultaneously. Classic hemodynamic variables, such as heart rate, LVET, stroke volume, or systemic vascular resistance, were measured or calculated. A mathematical model of the ICG signal was used to determine the shape of the ICG signal and its potential relationship to various morphologic and hemodynamic determinants. The ICG signal shape was then modeled with the normalized amplitudes (NHA) and phase (NHP) of the three harmonics of the fundamental signal. Analysis was performed with a multiple correspondence analysis. RESULTS: ICG method LVET did not significantly correlate with LVET determined with Doppler or with heart rate. NHA and NHP of the ICG signal were not significantly related to LVET determined with Doppler or to stroke volume. NHA were linked to heart rate and the relative length of the cardiac diastolic period. CONCLUSION: The absence of relationship between LVET determined with Doppler and the main variables defining the shape of the ICG signal highlight the relative limit of stroke volume determination with ICG method. The involvement of the arterial compliance in the ICG signal shape can be discussed in regard to the links between NHA and both heart rate and the relative length of the cardiac diastolic period, respectively.

Journal Article↗

PubMedDate not supplied

Comparison of heart rate response to an epinephrine test dose and painful stimulus in children during sevoflurane anesthesia: heart rate variability and beat-to-beat analysis.

BACKGROUND AND OBJECTIVES: During regional anesthesia, various stimuli leading to an adrenergic response can occur. However, simulation of an epidural test dose by using intravenous administration of epinephrine (EPI) has always been compared with an intravenous saline infusion as the control. The aim of this study was to evaluate the possibility of distinguishing in children the effect on HR by an intravascular epinephrine infusion and a painful stimulus, using heart rate variability (HRV) and beat-to-beat analysis of HR. METHODS: Thirty American Society of Anesthesiologists physical status P I children who required elective surgery were studied. At 1 minimum alveolar concentration (MAC) of sevoflurane, electrocardiogram was recorded continuously. Systolic blood pressure (SBP) was measured every minute. Measurements were performed after an intravenous administration of 0.5 microg/kg of epinephrine and during a small skin surgical incision (SI). Time-varying auto-regressive modeling of the interpolated RR sequences was performed for estimating power spectrum (msec(2)). The HF bands were defined by (0.15-0.4 Hz). RESULTS: Median (range) age and weight of all children were 3.5 (1-10) years and 16 (9-30) kg. EPI produced a lower increase in HR than did SI. SBP increased significantly more than did after SI. T-wave amplitude increased significantly after EPI but not after SI. Sixty seconds after the first change in HR, a secondary decrease (in comparison to control value) can be detected with EPI in contrast to SI. HF spectral power increased significantly after EPI administration but decreased after SI. The sensitivity, specificity, and positive and negative predictive value were respectively for DeltaHR >10 beats per minuteof 56%, 26%, 43%, and 38%; for DeltaSBP >15 mm Hg of 60%, 86%, 81%, and 67%; and for DeltaT-wave amplitude >25% of 86%, 73%, 76%, and 84%. Using detection of the secondary decrease of HR, 60 seconds after the first change in HR, sensitivity, specificity, and positive and a negative predictive value were respectively 96%, 100%, 100%, and 96%. CONCLUSIONS: Detection of the secondary HR decrease, 60 seconds after the first change in HR, allows us to distinguish the effects of a painful stimulus from those related to the epinephrine test dose at 1 MAC of sevoflurane. This secondary HR decrease induced by epinephrine appears primarily because of a compensatory increase in parasympathetic tone.

Adrenergic Agonists↗

PubMedDate not supplied

Beat-to-beat analysis of the relation between RT and RR intervals in newborns.

OBJECTIVE: To evaluate the dynamic RT (QRS apex-end of T wave) rate dependence in newborns. STUDY DESIGN: A Digital Holter ECG was acquired on day 15 in nine full-term and eight preterm infants. Ten-minute periods were recorded during wakefulness and sleep. The accuracy of fit with RT-RR pairs was individually assessed by 14 regression formulas (r coefficient, Akaike score, residual analysis). The medians of RT and Bazett's RT correction were calculated for each 10 milliseconds of RR. RESULTS: The mean RR and RT were 429+/-51 and 263+/-18 milliseconds. None of the prediction formulas were sufficiently accurate to describe RT over the whole range of RR (r<0.56). The Bazett correction produced differences of more than 50 milliseconds at different RR. Prematurity, sleep state and heart rate variability did not influence RT-RR relation. CONCLUSION: None of the parametric formulas were found to be accurate in describing RT rate dependence in newborns.

Electrocardiography, Ambulatory↗