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Biomedical subjects

R Livi

Publications and source records attributed to R Livi.

At least 19 recordsLinked to original sources
PubMed2003

Relationship between directed percolation and the synchronization transition in spatially extended systems.

We study the nature of the synchronization transition in spatially extended systems by discussing a simple stochastic model. An analytic argument is put forward showing that, in the limit of discontinuous processes, the transition belongs to the directed percolation (DP) universality class. The analysis is complemented by a detailed investigation of the dependence of the first passage time for the amplitude of the difference field on the adopted threshold. We find the existence of a critical threshold separating the regime controlled by linear mechanisms from that controlled by collective phenomena. As a result of this analysis, we conclude that the synchronization transition belongs to the DP class also in continuous models. The conclusions are supported by numerical checks on coupled map lattices too.

Journal Article↗
PubMed2003

Mechanical model of normal and anomalous diffusion.

The overdamped dynamics of a charged particle driven by an uniform electric field through a random sequence of scatterers in one dimension is investigated. Analytic expressions of the mean velocity and of the velocity power spectrum are presented. These show that above a threshold value of the field normal diffusion is superimposed to ballistic motion. The diffusion constant can be given explicitly. At the threshold field, the transition between conduction and localization is accompanied by an anomalous diffusion. Our results exemplify that, even in the absence of time-dependent stochastic forces, a purely mechanical model equipped with a quenched disorder can exhibit normal as well as anomalous diffusion, the latter emerging as a critical property. Via another interpretation, as the motion of a particle on an inclined rough surface, our results are relevant for the problem of segregation by flow.

Journal Article↗
PubMed2002

Renal functional reserve is impaired in patients with systemic sclerosis without clinical signs of kidney involvement.

OBJECTIVE: To evaluate the functional response of the kidney to an amino acid challenge (the so called renal functional reserve (RFR)) in patients with systemic sclerosis (SSc) with no clinical sign of renal involvement. METHODS: Before and after an intravenous amino acid load (Freamine III Baxter, 8.5% solution, 4.16 ml/min for two hours), glomerular filtration rate (GFR, as creatinine clearance), effective renal plasma flow (ERPF, as para-aminohyppurate clearance), and calculated total renal vascular resistance (TRVR) were measured in 21 patients with SSc with apparently normal renal function and 10 normal controls. RESULTS: In basal conditions, patients had lower ERPF (403.5 (SD 43.8) v 496.4 (SD 71.3) ml/min, p<0.0002) and higher TRVR (10 822 (SD 2044) v 8874 (SD 1639) dyne/sxcm(-5), p<0.014) than controls. The RFR, evaluated as the percentage increase of GFR after the amino acid load, was significantly reduced in patients with SSc (SSc +1.9 (SD18.6)%, controls +34.8 (SD 13.9)%; p<0.0002). However, the response of patients was not uniform. Multiple regression analysis showed that the RFR was inversely dependent on the patients' mean arterial pressure at admission and basal GFR (R(2)=65%, p<0.0001). CONCLUSIONS: Most patients with SSc cannot increase renal filtration under the challenge of a protein overload. This defective renal response to the amino acid load test sustains the concept of the prevalence of vasoconstrictor over vasodilating factors in the kidney of these patients.

Amino Acids↗
PubMed2001

Transition to stochastic synchronization in spatially extended systems.

Spatially extended dynamical systems, namely coupled map lattices, driven by additive spatio-temporal noise are shown to exhibit stochastic synchronization. In analogy with low-dimensional systems, synchronization can be achieved only if the maximum Lyapunov exponent becomes negative for sufficiently large noise amplitude. Moreover, noise can suppress also the nonlinear mechanism of information propagation, which may be present in the spatially extended system. An example of phase transition is observed when both the linear and the nonlinear mechanisms of information production disappear at the same critical value of the noise amplitude. The corresponding critical properties cannot be estimated numerically with great accuracy, but some general argument suggests that they could be ascribed to the Kardar-Parisi-Zhang universality class. Conversely, when the nonlinear mechanism prevails on the linear one, another type of phase transition to stochastic synchronization occurs. This one is shown to belong to the universality class of directed percolation.

Journal Article↗
PubMed2000

Finite thermal conductivity in 1D lattices

We discuss the thermal conductivity of a chain of coupled rotators, showing that it is the first example of a 1D nonlinear lattice exhibiting normal transport properties in the absence of an on-site potential. Numerical estimates obtained by simulating a chain in contact with two thermal baths at different temperatures are found to be consistent with those based on linear response theory. The dynamics of the Fourier modes provides direct evidence of energy diffusion. The finiteness of the conductivity is traced back to the occurrence of phase jumps. Our conclusions are confirmed by the analysis of two variants of this model.

Journal Article↗
PubMed2000

First-order phase transition in a (1+1)-dimensional nonequilibrium wetting process

A model for nonequilibrium wetting in 1+1 dimensions is introduced. It comprises adsorption and desorption processes with a dynamics that generically does not obey detailed balance. Depending on the rates of the dynamical processes the wetting transition is either of first or second order. It is found that the wet (unbound) and the nonwet (pinned) states coexist and are both thermodynamically stable in a domain of the dynamical parameters that define the model. This is in contrast with equilibrium transitions where coexistence of thermodynamically stable states takes place only on the transition line.

Journal Article↗
PubMed1998

Reproducibility of ambulatory blood pressure monitoring results in pregnancy.

Blood pressure (BP) differences between two consecutive 24-h monitoring periods (P1, P2) were analyzed in 159 hospitalized pregnant women. Reproducibility index, or twice the standard deviation of differences between individual systolic and diastolic means, was better for 24-h (7.8 and 5.8) and daytime (8.7 and 6.3) than for nighttime (12.2 and 9.7) BP means. It did not depend on BP level or gestation week. Alert reaction to monitoring procedure increased BP only in the first 2 h of P1. The high overall reproducibility of ambulatory BP in pregnancy supports the expanding use of the technique also in this condition.

Adult↗
PubMed1996

Simultaneous and sequential same-arm measurements in the validation studies of automated blood pressure measuring devices.

The oscillometric ambulatory blood pressure recorder Daypress 500 was validated according to the British Hypertension Society protocol. Both sequential and simultaneous measurements were used. Multiple regression analysis demonstrated a significant influence of subject pulse pressure and arm circumference on device-observer systolic pressure differences. Differences between observer consecutive readings were inversely related to heart rate. Device and observer blood pressure readings were closer at simultaneous than at sequential measurements. However, both kinds of measurement led to the same final evaluation (A for diastolic and B for systolic blood pressure), provided that the appropriate grading criteria were applied for each method.

Adult↗
PubMed1992

Gold salts and somatostatin: a new combined analgesic treatment for psoriatic arthritis.

In a group of patients affected with psoriatic arthritis the effects of the association between gold salts (GS) and somatostatin (SOM), in comparison with two groups treated with SOM and GS respectively, were investigated. Sixty patients with psoriatic arthritis were selected and randomly allocated in three groups of twenty patients each. Group 1 received SOM infusion (250 micrograms/h for 96 h) and was assessed at baseline and 1, 15, 30, 60, 90 and 120 days after; Group 2 received intramuscular GS and was assessed at baseline, four months later, and then every month for four months; Group 3 received GS for 8 months; at the fourth month SOM was infused (as in Group 1) and the patients assessed at baseline four months later and then as Group 1. Assessment was made with the Ritchie index, pain scale and morning stiffness evaluation. Growth hormone was assayed in Group 1 every 4 h for 24 h the day before and the day after SOM infusion. The association between GS and SOM demonstrated a particular analgesic activity, effective on joint pain and tenderness, that lasted for all four months of follow-up. SOM showed a good response only after 15 and 30 days, and GS proved to be effective at about the sixth month of treatment. Side effects were reported in Group 1 (abdominal cramps, mild erythrodermia and supraventricular arrhythmia). A growth hormone circadian rhythm was found in psoriatic patients both before and after SOM treatment. The beneficial effect of the SOM/GS combination is demonstrated in psoriatic arthritis.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Knowledge-based system for the diagnosis and treatment of hypertension.

A knowledge-based system to assist the physician in the diagnosis and treatment of hypertension has been developed as the result of cooperation between the Department of Electronic Engineering of the University of Florence and the Interuniversity Centre of Clinical Chronobiology. The system input consists of the data recorded over a 24 h (or longer) period by monitoring (automatically or through self-measurements) the blood pressure of the subject undergoing the system analysis, and the associated anamnestic data. The process results in a report that states from which kind of hypertensive syndrome, if any, the subject is suffering and which anti-hypertensive therapy appears to be most suitable. The system consists of three modules: the first diagnoses hypertension by applying cluster analysis to a set of parameters derived from the principal components of the time series resulting from the subject's blood pressure monitoring; the other two classify hypertension and offer advice about the most advisable treatment, respectively, by using high-level data representation and processing. The knowledge embedded in the system is internally represented by means of frames and rules. This paper describes the structure of the system, illustrates the techniques that have been used for its development and discusses the results of its application.

Artificial Intelligence↗
PubMed1988

Echocardiographic evaluation of children with and without family history of essential hypertension.

A family history of hypertension is considered a risk factor for developing hypertension. We studied two groups of normotensive children (aged 14 years): one comprising 14 subjects with family history of hypertension, the other comprising 15 subjects without family history of hypertension. Children were comparable with respect to age, weight, height, body surface area, heart rate, and arterial blood pressure. M-mode echocardiography demonstrated higher interventricular septum/posterior wall ratio in progeny of hypertensive subjects. Interestingly, all the parameters evaluated were within the normal limits. Our data suggest that a certain degree of cardiac changes is present in children with positive family history of hypertension, though further studies are needed before considering these findings predictive of future essential hypertension.

Adolescent↗
PubMed1987

Autometrically and automatically monitored, iatrogenous, inhalant-decongestant-associated high blood pressure quantified by rhythmometry.

Blood pressure and heart rate were oscillometrically monitored with an automatic Nippon Colin instrument (Komaki, Japan) and were also self-measured by a 20-year-old man treated with an inhalant-decongestant containing an adrenergic and a corticosteroid analog. The subject also collected 24-hr urines for aldosterone determination. An elevation of urinary aldosterone excretion was observed compared to the usual value range of a peer group. After removal of the drug, urinary aldosterone dropped as did urinary potassium, whereas plasma potassium rose. Chronobiologic serial sections, carried out to follow the time course of blood pressure and heart rate, showed decreasing trends in MESOR after discontinuance of treatment. A decrease in the hyperbaric index, a measure of blood pressure excess over 24 hr, was also observed. Another subject treated with the same and a third treated with a similar inhalant-decongestant, who also monitored their blood pressures automatically and/or with self-measurements, showed similar effects: a decrease in blood pressure after removal of treatment and an increase in blood pressure when (for testing only) treatment was briefly resumed.

Adult↗