Showing papers by "Polytechnic University of Milan published in 1987"

Heart Rate Variability as an Index of Sympathovagal Interaction After Acute Myocardial Infarction

Abstract: By analysis of spectral components of heart rate variability, sympathovagal interaction was assessed in patients after acute myocardial infarction (AMI). At 2 weeks after AMI (n = 70), the low-frequency component was significantly greater (69 +/- 2 vs 53 +/- 3 normalized units [NU], p less than 0.05) and the high-frequency component was significantly smaller (17 +/- 1 vs 35 +/- 3 NU) than in 26 age-matched control subjects. This difference was likely to reflect an alteration of sympathovagal regulatory outflows with a predominance of sympathetic activity. At 6 (n = 33) and 12 (n = 29) months after AMI, a progressive decrease in the low- (62 +/- 2 and 54 +/- 3 NU) and an increase in the high-frequency (23 +/- 2 and 30 +/- 2 NU) spectral components was observed, which suggested a normalization of sympathovagal interaction. An increase in sympathetic efferent activity induced by tilt did not further modify the low-frequency spectral component (78 +/- 3 vs 74 +/- 3 NU) in a subgroup of 24 patients at 2 weeks after AMI. Instead, 1 year after AMI, this maneuver was accompanied by an increase in the low-frequency component (77 +/- 3 vs 53 +/- 3 NU, p less than 0.05) of a magnitude similar to the one observed in control subjects (78 +/- 3 vs 53 +/- 3 NU). These data indicate that the sympathetic predominance that is detectable 2 weeks after AMI is followed by recovery of vagal tone and a normalization of sympathovagal interaction, not only during resting conditions, but also in response to a sympathetic stimulus.

Excitability of the soleus H-reflex arc during walking and stepping in man

Abstract: In eight normal subjects, the excitability of the soleus (Sol) H-reflex was tested in parallel with Sol length changes, EMGs of leg and thigh muscles and ground contact phases, during three different pacing movements: bipedal treadmill walking, single limb treadmill walking, and single-limb stepping on one spot. A computerized procedure was used which compensated for changes in stimulus effectiveness that occurred during free motion. In the three paradigms examined, significant excitability modulations were observed with respect to a control level determined in standing weight-bearing position. During bipedal treadmill walking, excitability was decreased in the early stance, maximally enhanced in the second half of the stance, and again decreased during the end-stance and the whole swing phase, with a minimum value around the toe off period. The main modulation pattern was retained during single-limb treadmill walking. During single-limb stepping on one spot, the stance-phase increase in excitability and the swing phase depression were still present. However, in the second half of the swing phase, reflex responsiveness returned to reference level, which was maintained during the subsequent contact period. Moreover, a decrease in reflex excitability was detected around the mid-stance. The time course of the described modulations was only partly correlated with the EMG and length changes of the Sol muscle. Furthermore, in the three movements tested, during the early stance phase, the excitability of the H-reflex arc did not correspond to the one expected on the basis of the available H-reflex studies performed under static conditions. It is suggested that, at least in certain stride phases (e.g. around the early contact period), an active regulation affects the transmission in the Sol myotatic arc during the pacing movements investigated.

Heart rate variability signal processing: a quantitative approach as an aid to diagnosis in cardiovascular pathologies

Abstract: The heart rate variability (HRV) signal carries important information about the systems controlling heat rate and blood pressure, mainly elicited by autonomic nervous system (sympathetic and parasympathetic) controls. The present paper illustrates methods of HRV signal processing by using autoregressive (AR) modeling and power spectral density estimate. The information enhanced in this way seems to be particularly sensitive in discriminating various cardiovascular pathologies (hypertension, myocardial infarction, diabetic neuropathy, etc.). This method provides a simple non-invasive analysis, based on the processing of spontaneous oscillations in heart rate. Particular emphasis is directed to the algorithms used and to their direct application by using proper computerized techniques: only a few paradigmatical examples will be illustrated as preliminary results.

Back analysis procedures for the interpretation of field measurements in geomechanics

Abstract: SUMMARY A survey is presented of some recent developments of the numerical techniques for back analysis in the field of geomechanics, with particular reference to tunnelling problems. In the spirit of Terzaghi's observational design method, these techniques are seen as practical tools for interpreting the available field measurements, in order to reduce the uncertainties that in many instances affect the parameters governing the solution of complex geomechanics problems. Both deterministic and probabilistic viewpoints are considered and some significant applications to practical problems are illustrated.

Dynamics of electrons in drift detectors

Abstract: The dynamics of electrons generated in silicon drift chambers by ionizing events is studied. Diffusion effects, mutual electrostatic repulsion between the electrons as well as the focusing properties of the depletion field during the electron transport to the anode are considered. The theoretical computations show that the effect of the mutual electrostatic repulsion in the spreading of the charge packet are not negligible even at charge densities produced by a minimum ionizing particle traversing 300 μm of the detector. Experimental results support theoretical expectations.

Single sweep analysis of visual evoked potentials through a model of parametric identification

Abstract: An original method is presented for the single sweep analysis of visual evoked potentials (VEP's). The introduced algorithm bases upon an AutoRegressive with eXogenous input (ARX) modelling. A Least Squares procedure estimates the coefficients of the model and allows to obtain a complete black-box description of the signal generation mechanism, besides providing a filtered version of the single sweep potential. The performance of the algorithm is verified on proper simulation tests and the experimental results put into evidence the noticeable improvement of signal-to-noise ratio with a consequent better recognition of the classical parameters of the peaks (latencies and amplitudes). The possibility of measuring these parameters on a single sweep basis enables to evaluate the dynamics of the Central Nervous System response during the entire course of the examination. A classification of the estimated evoked potentials in a small number of subsets, on the basis of their morphology, is also possible.

A Flexible Modelling Approach for Software Reliability Growth

Abstract: In this paper, we present a model for software reliability which is flexible enough to describe a variety of reliability trends. Flexibility is achieved by allowing a variable Fault Exposure Coefficient (with linear dependence on the number of remaining faults).

Distributed database design methodologies

Abstract: This paper surveys methodological approaches for distributed database design. The design of distribution can be performed top-down or bottom-up; the first approach is typical of a distributed database developed from scratch, while the second approach is typical of the development of a multi-database as the aggregation of existing databases. We review the design problems and methodologies along both directions, and we describe DATAID-D, a top-down methodology for distribution design. We indicate how the methodology is part of a global approach to database design; how to collect the requirements about the distribution of data and applications; and how to progressively build the distribution of a schema. Our approach is exemplified through one case study.

Carbon fibre thermoplastic matrix adhesion

Abstract: Adhesion between carbon fibres and amorphous thermoplastic matrices with high distortion temperature (polyetherimide and polyethersuIphone) is studied. Shear strength at the fibre-matrix interface has been accurately evaluated according to a statistical method based on the measurement of the mean critical fibre length. In the evaluation of critical length, the tensile strength due to the fibre defects, may be accounted for using Weibull's model. On the basis of these and other tests, such as scanning electron microscopy, X-ray photoelectron spectroscopy and punch tool, adhesion is discussed: its prevailing mechanism is attributed to the differential thermal shrinkage of fibre and matrices.

Avalanche semiconductor detector for single optical photons with a time resolution of 60 ps

Abstract: Single optical photons can be detected by semiconductor diodes, that can operate in the triggered avalance mode. Physical properties and structural requirements of such single photon avalance diodes (SPADs) are analyzed. A simple silicon device which has interesting performances (60 ps resolution in single-photon timing, and low dark count rate (less than 10 3 pps at room temperature)) is described. Possible applications are discussed and experimental results are reported (measurements of fast fluorescent decays, and optical time-domain reflectometry in optical fibers with 1 cm resolution). Relations between the device performance and physical phenomena are considered. Criteria are derived for designing and implementing SPAD devices with improved performances. Possible new structures are presented and evaluated.

A probabilistic framework for resource-constrained multi-agent planning

Abstract: In this paper, we consider the problem of temporally coordinating the resource demands of a set of independent agents. We assume that resources are unreliable, making it necessary to retain imprecision in the execution times assigned to specific agent operations. To this end, a probabilistic model of resource allocation is developed for use in estimating the consequences of execution intervals (representing sets of possible resource allocation decisions). This leads to a probabilistic representation of requests for resource usage for which resource congestion constraints can be defined. We consider two applications of the framework: prediction of bottleneck resources and time bound scheduling.

Naturally occurring perylenequinones.

Abstract: Perylenequinones, together with a few compounds containing a partially reduced perylene skeleton, form a relatively small but rapidly increasing group of chemically interesting, biologically active pigments obtainable from natural sources.

Radially variable reflectivity output coupler of novel design for unstable resonators.

Abstract: An output coupler with radially variable reflectivity, based on a Fabry-Perot interferometer made of two suitable spherical mirrors, is proposed. Application to a pulsed Nd:YAG laser with an unstable resonator has produced diffraction-limited output beams with energy up to 300 mJ.

Design Criteria and Efficiency Prediction for Radial Inflow Turbines

Abstract: A theoretical investigation was performed to predict the maximum achievable efficiency of radial inflow turbines for different design conditions. The analytical tool used in the investigation is a computer code able to perform the contemporary optimization of the main design variables, in order to obtain maximum efficiency. Since the results are strictly dependent on the loss correlations, reliability of the efficiency predictions was tested at first by comparison with several test-cases available in literature: good agreement with experimental data was found, pointing to the validity of the results presented here.A large number of cases were analyzed: the efficiency and the main design parameters, obtained after the optimization process, are presented for optimum specific speed. Turbine efficiency was found to be dependent both on compressibility effects, related to the volume expansion ratio, and on actual turbine size, accounting for geometric non-similarity effects. Influence of non-optimum specific speed is also discussed.By means of similarity rules, the results enable turbine design to be performed in a simple way, for a variety of working fluids and design conditions.Copyright © 1987 by ASME

Quantum thermodynamics of nonequilibrium. Onsager reciprocity and dispersion-dissipation relations

Abstract: A generalized Onsager reciprocity theorem emerges as an exact consequence of the structure of the nonlinear equation of motion of quantum thermodynamics and is valid for all the dissipative nonequilibrium states, close and far from stable thermodynamic equilibrium, of an isolated system composed of a single constituent of matter with a finite-dimensional Hilbert space. In addition, a dispersion-dissipation theorem results in a precise relation between the generalized dissipative conductivity that describes the mutual interrelation between dissipative rates of a pair of observables and the codispersions of the same observables and the generators of the motion. These results are presented together with a review of quantum thermodynamic postulates and general results.

Finite covariance functions

Abstract: Because of the full covariance matrices and the computer storage limitations the number of measurements which can be handled by the collocation method simultaneously, is limited. This paper presents a method to compute covariance functions with a finite support yielding sparse covariance matrices. The theoretical background is pointed out and, for the one- and two-dimensional case, special functions are developed which can be combined with the usually used covariance functions to get a “finite covariance function”. Simulated examples to demonstrate the behaviour of different solution methods to solve these special, sparse covariance matrices supplement our investigations.

Contact density and hyperfine splitting of positronium in naphthalene

Abstract: The contact density |Ψ(0)|2 and the hyperfine splitting of positronium in naphthalene have been determined by means of a magnetic quenching experiment. Results show that the perturbing effect of the medium on the hyperfine splitting is not interpretable as a change of the contact density alone, since on passing from vacuum to the medium, the contact density and the splitting do not scale together. The first is reduced by a factor of 0.25, the second by a factor of 0.067. A simple phenomenological model is presented.

Some models of catastrophic behavior in exploited forests

Abstract: This paper presents some models which show that small variations of the human exploitation of a natural forest can give rise to dramatic changes in the forest biomass. Two simple mechanisms for catastrophes, already pointed out for other ecosystems, are briefly discussed at the beginning of the paper. A more interesting mechanism for catastrophes is then presented. It is based on a model of the dynamics of forest biomass and soil nutrients and on the fact that mortality may become very high when soil acidity exceeds a threshold. In particular, it is shown that an increase of the exploitation may give rise to a catastrophic collapse of the forest if the exogenous nutrient inflow, which brings about also acidic deposition, is sufficiently high. Moreover, such a catastrophe is irreversible, i.e., regeneration is not possible, if the nutrient inflow is too high.

Moment-preserving spline approximation on finite intervals

Abstract: Continuing previous wotk, we discuss the problem of approximating a functionf on the interval [0, 1] by a spline function of degreem, withn (variable) knots, matching, as many of the initial moments off as possible. Additional constraints on the derivatives of the approximation at one endpoint of [0, 1] may also be imposed. We show that, if the approximations exist, they can be represented in terms of generalized Gauss-Lobatto and Gauss-Radau quadrature rules relative to appropriate moment functionals or measures (depending off). Pointwise convergence asn→∞, for fixedm>0, is shown for functionsf that are completely monotonic on [0, 1], among others. Numerical examples conclude the paper.

Existence of global solutions for the space inhomogeneous Enskog equation

Abstract: The Enskog equation for a gas of hard smooth spheres is written under the assumption that correlations are negligible but space separation of the spheres at the moment of collision is not. Existence of global solutions is proved when the initial data are in a weighted L1 and depend on the space variable.

Quantitative analysis of intracellular behaviour of porphyrins.

Abstract: Fluorometric analysis performed on L 1210 cells after treatment with Photofrin indicated that the interactions with cellular structures induce a significant modification of the equilibria among the different porphyrin species. This modification turned out to be dependent on the uptake and release processes. Thus, a comparative analysis of the dynamic aspects of the drug accumulation process was performed on cells treated with hematoporphyrin, Photofrin and Photofrin II. The results obtained were interpreted taking into account the different chemical composition of the drugs employed. The porphyrin species mainly released seem to be the monomeric ones and ‘unfolded oligomers’. The release process results in further modifications of the aggregation and/or configu-rational state of intracellular porphyrins due to altered internal equilibrium.

Full Digital Adaptive Equalization in 64-QAM Radio Systems

Abstract: In view of a digital implementation of most of the signal processing required in a QAM demodulation-equalization Structure, a performance analysis as a function of equalizer complexity, A/D conversion resolution, and precision of internal arithmetic is presented. The analysis is focused on a 140 Mbit/s 64-QAM modulation scheme (corresponding to CEPT-4 or 3 \times DS3 systems), but extension to other QAM constellations is straightforward. Considerations of currently available technology, overall computational complexity, and related optimal quantization formats are presented. For high-speed 64 QAM, it is shown that satisfactory performance may be expected from a full digital implementation of reasonable complexity, using current technology.

Photophysical properties of porphyrin-chlorin systems in the presence of surfactants.

Abstract: The therapeutic efficacy of PDT is related to the capability of the photosensitizer to absorb light at a wavelength that can penetrate into tissues. We have synthesized two systems, a haematoporphyrin-chlorin (HPC) and a dihaematoporphyrin ether or ester (DHE) with the terminal ring converted to a chlorin (DHEC). The presence of the chlorin moiety provides an extra band at ˜ 660 nm with a relative amplitude from 5 to 10 times larger than that of the porphyrin at 630 nm. Since both HPC and DHEC strongly aggregate in buffer, we have studied their photophysical properties in the presence of cationic surfactants at different concentrations below and above the critical micelle concentrations. Absorption spectra were measured together with emission spectra and fluorescence decays at different observation wavelengths under excitation at 364 nm. The results were compared with those obtained for DHE in the same environmental conditions. As for DHE, the presence of micelles disaggregate both compounds, resulting in a large increase in the relative emission intensity at ˜ 670 nm due to the presence of the chlorin moiety. The fluorescence decays could be fitted by two or three exponential components indicating the presence of more than one molecular species and/ or conformations. On the basis of our measurements the chlorin molecule does not seem to modify appreciably the photophysical properties of the porphyrin molecules but does superimpose its absorption and emission spectrum onto that of the porphyrin. This result may be of relevance in the possible use of these compounds in PDT.