Biomedical subjects

S Fishman

Publications and source records attributed to S Fishman.

At least 19 recordsLinked to original sources

PubMed2003

Semiclassical quantization of separatrix maps.

Quantization of energy balance equations, which describe a separatrixlike motion is presented. The method is based on an exact canonical transformation of the energy-time pair to the action-angle canonical pair, (E,t)-->(I,theta). Quantum mechanical dynamics can be studied in the framework of the new Hamiltonian. This transformation also establishes a relation between a wide class of the energy balance equations and dynamical localization of classical diffusion by quantum interference, that was studied in the field of quantum chaos. An exact solution for a simple system is presented as well.

Journal Article↗

PubMed2003

Exercise stress testing, myocardial perfusion imaging and stress echocardiography for detecting restenosis after successful percutaneous transluminal coronary angioplasty: a review of performance.

When chest symptoms recur in a patient who underwent percutaneous transluminal coronary angioplasty (PTCA), it is necessary to rule out restenosis (R). Three main noninvasive tests suggest the presence of R: exercise stress test (XT), myocardial perfusion imaging (MPI) and stress echocardiography (s-echo). The objectives of this review were: (1) to estimate the pretest probability of R as a function of time after PTCA in symptomatic patients and (2) to obtain an approximation of the diagnostic parameters of the XT, MPI and s-echo for detecting R. A MEDLINE search (English-language, years: 1980-2001) was conducted to identify studies examining post-PTCA functional testing for diagnosing R. Data from the studies were pooled. Comparing studies was often difficult due to varying methodology in the studies. Pretest probability of R in symptomatic patients increases in a nonlinear fashion from 20% or less at 1 month, to nearly 90% at 1-year postangioplasty. The approximated accuracy of the XT, MPI, and s-echo for detecting R was 62, 82 and 84%, respectively. During the first month after PTCA, none of the noninvasive modalities is able to accurately detect R. Late (7-9 months) after PTCA, the pretest probability of R is high and therefore the noninvasive measure may be spared. Our analysis suggests that MPI and s-echo should be preferred over the XT for diagnosing R.

Angioplasty, Balloon, Coronary↗

PubMed2002

Quantum localization for a kicked rotor with accelerator mode islands.

Dynamical localization of classical superdiffusion for the quantum kicked rotor is studied in the semiclassical limit. Both classical and quantum dynamics of the system become more complicated under the conditions of mixed phase space with accelerator mode islands. Recently, long time quantum flights due to the accelerator mode islands have been found. By exploration of their dynamics, it is shown here that the classical-quantum duality of the flights leads to their localization. The classical mechanism of superdiffusion is due to accelerator mode dynamics, while quantum tunneling suppresses the superdiffusion and leads to localization of the wave function. Coupling of the regular type dynamics inside the accelerator mode island structures to dynamics in the chaotic sea proves increasing the localization length. A numerical procedure and an analytical method are developed to obtain an estimate of the localization length which, as it is shown, has exponentially large scaling with the dimensionless Planck's constant (tilde)h<<1 in the semiclassical limit. Conditions for the validity of the developed method are specified.

Journal Article↗

PubMed2001

Theoretical analysis of systematic errors introduced by a pedicel-girdling technique used to estimate separately the xylem and phloem flows.

The water budget of fruits was analysed by means of a biophysical model of fruit growth, built and calibrated recently for peaches [Prunus persica (L.) Batsch]. This analysis was applied to the evaluation of systematic errors introduced by a pedicel-girdling method (with the observations being treated by means of a subtractive technique) used to separate the contributions of xylem and phloem flow to the total water inflow to the fruit. The flows were considered as solution transport through composite membranes and were calculated by means of equations drawn from non-equilibrium thermodynamics. The total inflow of water was simulated as dependent on the water status in the tree. The hourly time step was used for the simulation. The flows obtained by simulation of the pedicel-girdled fruit were compared with those found by simulation of the intact-pedicel fruit. The error introduced by the pedicel-girdling technique was evaluated theoretically and was shown to vary during the day, ranging from very small (relative error of 3-7%) at the period when the rate of fruit growth is maximal to 100% when the fruit volume does not change. The vascular flows obtained from the "girdling experiments" are discussed in relation to the possible theoretically estimated errors.

Biological Transport↗

PubMed2001

The effect of Losartan on insulin resistance and beta cell function in chronic hemodialysis patients.

Insulin resistance (IR) is prevalent in hemodialysis patients. IR and hyperinsulinemia have an important role in the development of atherosclerosis, which is the most common cause of morbidity and mortality in hemodialysis patients. Thus, antihypertensive drugs that lower IR, may have an additional beneficial effect in the treatment of cardiovascular diseases in these patients. In this preliminary study we examined the effect of Losartan (an angiotensin II receptor antagonist) treatment on IR and beta cell function in five hypertensive non-diabetic chronic hemodialysis patients. All other known causes of IR in end stage renal failure were excluded. After a washout period of two weeks, Losartan 50 mg, was administered for 6 weeks. Fasting blood glucose (FBG) and insulin levels were measured before and after the treatment IR and beta cell function were calculated using the "homeostasis model assessment"-HOMA. Systolic and diastolic blood pressure (BP) have not changed significantly throughout the study. FBG increased significantly from 76 mg/dL +/- 1 to 89 mg/dL +/- 4 (p < 0.01), however, insulin levels have not changed significantly. Calculated IR values did not show a difference, but calculated beta cell function decreased significantly after Losartan treatment from 291% +/- 50 to 146% +/- 10, (p < 0.016). These preliminary results suggest that in chronic hemodialysis hypertensive non-diabetic patients short treatment with Losartan has deleterious effect on glucose homeostasis mediated via a decrease in beta cell function.

Aged↗

PubMed2001

Variation in surface conductance to water vapor diffusion in peach fruit and its effects on fruit growth assessed by a simulation model.

Surface conductance to water vapor diffusion was measured in individual peach fruits (Prunus persica (L.) Batsch) and plotted as a function of fresh fruit mass for four cultivars. Surface conductance increased with fresh fruit mass, but the pattern differed with cultivar, and fruit-to-fruit variation occurred. Relationships between fruit mass and surface conductance were modeled by fitting mathematical equations to the data. The simulation model of Fishman and Génard (1998) was used to study dry mass and water components of fruit growth (1) when surface conductance varied with fruit size or was constant, and (2) when surface conductance values were high, moderate or low with respect to fruit mass. Increased surface conductance with fresh fruit mass resulted in fruit growth cessation. Fruits differing in surface conductance had similar dry mass. However, under well-watered conditions (stem water potential between -1 and -0.2 MPa), the water balance components of growth (osmotic and hydrostatic pressure, water potential and water balance) differed greatly and, as a result, the lower the surface conductance the greater the fresh fruit mass. These differences were buffered under drought conditions (stem water potential between -2.4 and -0.6 MPa).

Fruit↗

PubMed2001

Laparoscopic and thoracoscopic esophagomyotomy for children with achalasia.

BACKGROUND: Minimally invasive esophagomyotomy, consisting of a laparoscopic or thoracoscopic approach, has become a preferred surgical treatment for adults with achalasia. This multicenter study reports on the clinical status of children who have undergone minimally invasive esophagomyotomy for achalasia. METHODS: Symptomatology for achalasia was assessed in 22 pediatric patients who underwent minimally invasive esophagomyotomy for achalasia between 1995 and 2000. All patients were evaluated for duration of hospitalization, postoperative resumption of feeds, postoperative complications, and symptomatic relief. Participants were assigned pre-and postoperative symptom severity scores ranging from 0 (no symptoms) to 3 (severe). RESULTS: The median age of the 10 females and 12 males at time of surgery was 11.3 years +/- 3.4 (standard deviation). Transabdominal laparoscopic esophagomyotomy with fundoplication was performed in 18 patients, and thoracoscopic esophagomyotomy without fundoplication was performed in 4. Two patients required conversion from transabdominal laparoscopic esophagomyotomy to open esophagomyotomy because of intraoperative esophageal perforation. The mean duration of postsurgical follow-up was 17 +/- 16 (standard deviation) months (range, 1-54 months). Mean duration of hospitalization (days +/- standard error or mean) was less for transabdominal laparoscopic esophagomyotomy than for converted open esophagomyotomy (2.7 +/- 0.3 vs. 9.0 +/- 3.0 days; P < 0.05) or for thoracoscopic esophagomyotomy (4.8 +/- 1.7 days; P = not significant). Mean time to resumption of soft feedings (days +/- standard error or mean) occurred sooner after transabdominal laparoscopic esophagomyotomy than after converted open esophagomyotomy (2.0 +/- 0.2 vs. 5.5 +/- 0.5 days; P < 0.001) or after thoracoscopic esophagomyotomy (4.0 +/- 1.3 days; P = not significant). Patients experienced significant pre-to postoperative improvement in mean severity score with regard to dysphagia (2.6 vs. 0.4; P < 0.001) and regurgitation (1.7 vs. 0.2; P < 0.001). CONCLUSIONS: Minimally invasive esophagomyotomy can provide excellent symptomatic relief from dysphagia and regurgitation for children with achalasia.

Adolescent↗

PubMed2001

A biophysical analysis of stem and root diameter variations in woody plants.

A comprehensive model of stem and root diameter variation was developed. The stem (or root) was represented using two coaxial cylinders corresponding with the mature xylem and the extensible tissues. The extensible tissues were assumed to behave as a single cell separated from the mature xylem by a virtual membrane. The mature xylem and the extensible tissues are able to dilate with temperature and grow. Moreover, the extensible tissues are able to shrink and swell according to water flow intensity. The model is mainly based on the calculation of water volume flows in the "single cell" that are described using the principles of irreversible thermodynamics. The elastic response to storage volume and plastic extension accompanying growth are described. The model simulates diameter variation due to temperature, solute accumulation, and xylem, water potential. The model was applied to the peach (Prunus persica) stem and to the plum (Prunus domestica x Prunus spinosa) root. The simulation outputs corresponded well with the diameter variation observed. The model predicts that variations of turgor pressure and osmotic potential are smaller than the variations of xylem water potential. It also demonstrates correlations between the xylem water potential, the turgor pressure, the elastic modulus, and the osmotic potential. The relationship between the diameter and the xylem water potential exhibits a substantial hysteresis, as observed in field data. A sensitivity analysis using the model parameters showed that growth and shrinkage were highly sensitive to the initial values of the turgor pressure and to the reflection coefficient of solutes. Shrinkage and growth were sensitive to elastic modulus and wall-yielding threshold pressure, respectively. The model was not sensitive to changes in temperature.

Biophysical Phenomena↗

PubMed2001

[Diagnosis of idiopathic autonomic neuropathy].

This is a case report of a 51 years old woman admitted to the hospital due to abdominal pain and intractable diarrhea of two months duration. Her history and her physical examination revealed signs and symptoms leading to the diagnosis of the rare syndrome of idiopathic autonomic neuropathy. In this case report we briefly describe the characteristics of the syndrome and its natural history and treatment. We also raise the important issue of common error patterns in diagnostic reasoning such as "omission" and "anchoring" and how these patterns are reflected in the present case.

Abdominal Pain↗

PubMed2001

[The approach to a patient with hypokalemia].

Potassium is the most important intracellular cation in a humans body. Potassium gradient between the intracellular fluid to the extra cellular fluid is crucial for normal nerve-muscle activity. Hypokalemia is a frequent abnormality among patients, exposing them to fatigue and arrhythmia. Understanding the pathophysiology of hypokalemia may help the clinician to treat it properly. The approach to the patient with hypokalemia is presented through the description of a 25-year-old woman who was admitted to the department due to palpitation and hypokalemia. A simple primary investigation is suggested in these cases.

Adult↗

PubMed2000

Relaxation and diffusion for the kicked rotor

The dynamics of the kicked rotor, which is a paradigm for a mixed system, where the motion in some parts of phase space is chaotic and in other parts is regular, is studied statistically. The evolution operator of phase space densities in the chaotic component is calculated in the presence of noise, and the limit of vanishing noise is taken in the end. The relaxation rates to the equilibrium density are calculated analytically within an approximation that improves with increasing stochasticity. The results are tested numerically. A global picture is presented of relaxation to the equilibrium density in the chaotic component when the system is bounded and to diffusive behavior when it is unbounded.

Journal Article↗

PubMed2000

Correlations in the adiabatic response of chaotic systems

Adiabatic variation of the parameters of a chaotic system results in a fluctuating reaction force. The quantum analog of a classical dissipative force, proportional to the time integral of the force-force correlation function, vanishes. We study this quantum-classical crossover for random matrix models. For the Gaussian unitary ensemble the crossover is found to take place on the Heisenberg time scale and the finite time integral practically vanishes for longer times. For the Gaussian orthogonal case, there is no such time scale and the integral falls off inversely proportional to time.

Journal Article↗

PubMed2000

Experimental observation of localization in the spatial frequency domain of a kicked optical system

An experimental realization of an optical "kicked" system is presented. It exhibits localization analogous to that of the quantum "kicked-rotor." In the experiment, free space propagating light is periodically kicked by thin sinusoidal phase gratings, which produce high order diffractions and tend to increase the spatial frequency band. The wave property suppresses this diffusive spread. The localization is realized in a regime near anti-resonance of the system, which is also studied theoretically. The behavior in this regime is similar to that of electronic motion in incommensurate potentials. A crucial part of the experimental system is the grating in-phase positioning, which is done by using the Talbot effect.

Journal Article↗

PubMed2000

Relaxation to the invariant density for the kicked rotor

The relaxation rates to the invariant density in the chaotic phase space component of the kicked rotor (standard map) are calculated analytically for a large stochasticity parameter K. These rates are the logarithms of the poles of the matrix elements of the resolvent, Rinsertion mark(z)=(z-Uinsertion mark)(-1), of the classical evolution operator Uinsertion mark. The resolvent poles are located inside the unit circle. For hyperbolic systems this is a rigorous result, but little is known about mixed systems such as the kicked rotor. In this work, the leading relaxation rates of the kicked rotor are calculated in the presence of noise, to the leading order in 1/sqrt[K]. Then the limit of vanishing noise is taken and the relaxation rates are found to be finite, corresponding to poles lying inside the unit circle. It is found that the slow relaxation rates, in essence, correspond to diffusion modes in the momentum direction. Faster relaxation modes intermix the motion in the momentum and the angle space. The slowest relaxation rate of distributions in the angle space is calculated analytically by studying the dynamics of inhomogeneities projected down to this space. The analytical results are verified by numerical simulations.

Journal Article↗

PubMed2000

Semiclassical cross section correlations

We calculate within a semiclassical approximation the autocorrelation function of cross sections. The starting point is the semiclassical expression for the diagonal matrix elements of an operator. For general operators with a smooth classical limit the autocorrelation function of such matrix elements has two contributions with relative weights determined by classical dynamics. We show how the random matrix result can be obtained if the operator approaches a projector onto a single initial state. The expressions are verified in calculations for the kicked rotor.

Journal Article↗

PubMed1999

Human brain activation under controlled thermal stimulation and habituation to noxious heat: an fMRI study.

Brain activity was studied with functional magnetic resonance imaging (fMRI) following thermal stimulation. Two groups (n = 6/group) of human male volunteers were given up to four noxious (46 degrees C) and four non-noxious (41 degrees C) stimuli. In the 46 degrees C experiment, positive signal changes were found in the frontal gyri, anterior and posterior cingulate gyrus, thalamus, motor cortex, somatosensory cortex (SI and SII), supplementary motor area, insula, and cerebellum. Low-level negative signal changes appeared in the amygdala and hypothalamus. All regions activated by 46 degrees C were also activated by 41 degrees C. However, except for SI and thalamus, significantly more activation was observed for the 46 degrees C stimulus. A significant attenuation of the signal change was observed by the third stimulus for the 46 degrees C, but not for 41 degrees C experiment. Similar findings were replicated in the second group. These fMRI findings specify differences between somatosensory and pain sensation and suggest a number of rich avenues for future research.

Adult↗

PubMed1999

Elevated serum vascular endothelial growth factor in children and young adults with Crohn's disease.

Vascular endothelial growth factor (VEGF) is a cytokine released by fibroblasts, epithelial cells, and leukocytes that potentiates vascular permeability and growth of new capillaries. Because of these multiple effects, VEGF has been postulated to play a role in the pathogenesis of autoimmune disease, as well as in wound healing. We hypothesized that VEGF was potentially important in mediating the vascular permeability and angiogenesis seen in Crohn's disease, and therefore that VEGF would be increased in the serum of children with Crohn's disease. Serum was obtained from 73 children and young adults with Crohn's disease, 47 with ulcerative colitis, and 29 controls. VEGF levels were measured by enzyme-linked immunosorbent assay. Mean VEGF levels were significantly higher in patients with Crohn's disease (436.4 +/- 37.2 pg/ml) than in ulcerative colitis (306 +/- 41.1 pg/ml) or control (167.8 +/- 29.6 pg/ml) patients. Serum VEGF also correlated significantly with disease activity, being elevated in patients with moderate/severe Crohn's disease and ulcerative colitis. We conclude that serum VEGF is released by inflamed tissues in children with Crohn's disease. This multifunctional cytokine could promote inflammation by increasing vascular permeability or promote wound healing by mediating capillary growth.

Adolescent↗

PubMed1999

Nissen fundoplication and Boix-Ochoa antireflux procedure: comparison between two surgical techniques in the treatment of gastroesophageal reflux in children.

Over a period of 19 years an antireflux procedure was performed for gastroesophageal reflux in 59 children. Thirty-two patients underwent Nissen fundoplication and 27 children underwent the Boix-Ochoa antireflux procedure. Six patients died between two and 15 months post surgery of unrelated causes. Follow-up period from six months to 18 years was available in 45 (85%) of the surviving patients. This report summarizes the complications and long-term results with the two surgical procedures and their comparisons. The follow-up evaluation included parental interview and physical examination. Upper GI series and pH monitoring were performed only in children with signs and symptoms of recurrent GER or other post-operative complications. At follow-up with a mean period of 8.7 years following Nissen fundoplication, 87.5% showed good results without any residual symptoms. However, the overall complication rate was as high as 50%. Following the Boix-Ochoa antireflux procedure, 17 (81%) children showed excellent results while four children had recurrent GER. This occurred in two neurologically impaired children and two patients following esophageal atresia repair. No other post-operative complications were encountered with the Boix-Ochoa antireflux procedure. In our experience, the Boix-Ochoa antireflux procedure should be the procedure of choice in the surgical treatment of GER in otherwise normal children while the Nissen fundoplication is preferable in neurologically impaired children and in patients with GER following esophageal atresia repair.

Female↗