PubMed Health⌕ Search

Biomedical subjects

M Porto

Publications and source records attributed to M Porto.

At least 19 recordsLinked to original sources
PubMed2001

Rectification by hopping motion through nonsymmetric potentials: local versus global bias.

The hopping motion of noninteracting classical particles through nonsymmetric potentials is investigated in the case of alternating local bias. Different to the case of alternating global bias, a resonancelike behavior is observed. The system exhibits rectification effects in the regime of small to medium bias, whereas rectification effects are absent in the strong bias regime. Possible generalizations and applications are outlined.

Journal Article↗
PubMed2001

Molecular motor based entirely on the Coulomb interaction.

A molecular motor is introduced which is composed of charges only and which does not contain any potential or interaction besides the Coulomb one. The motor is shown to transform efficiently a driven random rotation into a directed translational motion. The direction of translational motion can be chosen dynamically, so that a "forward gear," a "reverse gear," and even a "neutral gear" exist. The high efficiency stems from the fact that this motor only steps in the direction determined by the chosen gear.

Journal Article↗
PubMed2001

Self-generated power-law tails in probability distributions.

We consider random processes characterized by the presence of correlations in their variance, or more generally in some of their moments. Typical examples are constituted by autoregressive conditional heteroskedasticity (ARCH) processes which are known to display power-law tails in the associated probability distributions. Here, we determine the corresponding exponents exactly and extend these results to relaxation phenomena which can be expected to play a role in natural sciences.

Journal Article↗
PubMed2001

"Generalized des Cloizeaux" exponent for self-avoiding walks on the incipient percolation cluster.

We study the asymptotic shape of self-avoiding random walks (SAW) on the backbone of the incipient percolation cluster in d-dimensional lattices analytically. It is generally accepted that the configurational averaged probability distribution function for the end-to-end distance r of an N step SAW behaves as a power law for r-->0. In this work, we determine the corresponding exponent using scaling arguments, and show that our suggested "generalized des Cloizeaux" expression for the exponent is in excellent agreement with exact enumeration results in two and three dimensions.

Journal Article↗
PubMed2001

Pregnancy outcome in Hispanic patients with unexplained positive triple marker screening for Down syndrome.

OBJECTIVE: The objective of this study was to compare pregnancy outcomes in Hispanic patients with a positive serum triple marker screen for Down syndrome and normal fetal karyotype with Hispanic women who had a negative triple marker screen. METHODS: This prospective investigation involved Hispanic gravidas who underwent maternal serum screening. A power analysis was performed to determine the sample size. Fifty women with false-positive screens for Down syndrome were matched with a control group of 100 women with a negative screen. Adverse pregnancy outcomes were compared between the two groups. RESULTS: An adverse pregnancy outcome occurred in 14% of the study group and in 13% of controls. There were no statistically significant differences between the two groups in the incidence of preterm labor (p > 0.5), pre-eclampsia (p > 0.1), intrauterine growth restriction (p > 0.5), or fetal demise (p > 0.5). CONCLUSION: Hispanic patients with unexplained positive triple marker screen for Down syndrome do not appear to be at increased risk for adverse pregnancy outcomes.

Adult↗
PubMed2000

Molecular motor that never steps backwards.

We investigate the dynamics of a classical particle in a one-dimensional two-wave potential composed of two periodic potentials that are time independent and of the same amplitude and periodicity. One of the periodic potentials is externally driven and performs a translational motion with respect to the other. It is shown that, if one of the potentials is of the ratchet type, translation of the potential in a given direction leads to motion of the particle in the same direction, whereas translation in the opposite direction leaves the particle localized at its original location. Moreover, even if the translation is random, but still has a finite velocity, an efficient directed transport of the particle occurs.

Journal Article↗
PubMed2000

Atomic scale engines: cars and wheels.

We introduce a new approach to build microscopic engines on the atomic scale that move translationally or rotationally and can perform useful functions such as the pulling of a cargo. Characteristic of these engines is the possibility to determine dynamically the directionality of the motion. The approach is based on the transformation of the fed energy to directed motion through a dynamical competition between the intrinsic lengths of the moving object and the supporting carrier.

Journal Article↗
PubMed2000

Influence of dipolar interaction on magnetic properties of ultrafine ferromagnetic particles

We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature T(B) clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at T(B), the relaxational behavior of the magnetic moments below T(B) is not in accordance with the picture of cooperative freezing.

Journal Article↗
PubMed2000

Multifractal behavior of linear polymers in disordered media.

The scaling behavior of linear polymers in disordered media modeled by self-avoiding random walks (SAWs) on the backbone of two- and three-dimensional percolation clusters at their critical concentrations p(c) is studied. All possible SAW configurations of N steps on a single backbone configuration are enumerated exactly. We find that the moments of order q of the total number of SAWs obtained by averaging over many backbone configurations display multifractal behavior; i.e., different moments are dominated by different subsets of the backbone. This leads to generalized coordination numbers mu(q) and enhancement exponents gamma(q), which depend on q. Our numerical results suggest that the relation mu(1)=p(c)mu between the first moment mu(1) and its regular lattice counterpart mu is valid.

Journal Article↗
PubMed2000

Critical packing fraction of rectangular particles on the square lattice

The random packing of identical and nonoverlapping rectangular particles of size nxm (1</=n,m</=10) is studied numerically on the square lattice, and the corresponding packing fractions p(f) and percolation probabilities P(infinity) are determined. We find that for randomly oriented particles there is a critical packing fraction p(c)(f)=0.67+/-0.01, such that for all particles sizes nxm for which p(f) 0 for L-->infinity, while when p(f)>p(c)(f),P(infinity)-->1 when L-->infinity and an infinite cluster exists. The value for p(c)(f) is found to be consistent with the continuum percolation threshold p(c) congruent with0.67 for overlapping particles in two dimensions.

Journal Article↗
PubMed1999

Maternal corticotropin-releasing hormone and habituation in the human fetus.

Elevated concentrations of maternal corticotrophin-releasing hormone (CRH) during the 2nd and early 3rd trimester of human pregnancy are associated with spontaneous preterm birth, but the effects of maternal CRH on the fetus are unknown. Maternal plasma was collected for analysis of CRH concentration, m = 156.24 +/- 130.91 pg/ml, from 33 pregnant women during Weeks 31-33 of gestation. Immediately after collection of plasma, fetal heart rate (FHR) measures were obtained in response to a challenge with a series of vibroacoustic stimuli. Fetuses of mothers with highly elevated CRH did not respond significantly to the presence of a novel stimulus in a repeated series, p = 0.016. These effects on the FHR response were not related to parity, fetal gender, medical (antepartum) risk, or eventual birth outcomes. Impaired dishabituation in these fetuses of mothers with high concentrations of CRH suggests that neurological systems rich with CRH receptors that support learning and memory, such as parahippocampal regions, may be targets for maternal/placental CRH, with implications for fetal neurological development.

Acoustic Stimulation↗
PubMed1999

The incidence and severity of shoulder dystocia correlates with a sonographic measurement of asymmetry in patients with diabetes.

The objective of this paper is to examine the relationship between fetal asymmetry measured sonographically and the incidence and severity of shoulder dystocia in diabetic patients. Ultrasound data were collected retrospectively from examinations of women with gestational and pregastational diabetes who delivered at University of California, Irvine Medical Center from 1993-1995. Sonographic fetal asymmetry was quantified by calculating the difference between the abdominal diameter and the biparietal diameter in centimeters (AD-BPD). The residual AD-BPD was a patient's actual AD-BPD at the time of the ultrasound minus the mean AD-BPD obtained in our population at the patient's gestational age. The correlations between fetal asymmetry and the incidence and severity of shoulder dystocia were assessed using an analysis of variance as well as a logistic regression analysis. Mild shoulder dystocia was defined as a delivery requiring McRobert's maneuver and/or suprapubic pressure, while severe shoulder dystocia was assessed when delivery of the posterior arm with Wood's corkscrew maneuver was required. One hundred twenty-three women met the inclusion criteria for the study. Dividing the cohort into three groups based on AD-BPD residual values resulted in the following AD-BPD residual ranges and incidences of shoulder dystocia: Group I, -1.80 to -0.32 cm (9.8%), Group II, -0.31 to 0.32 cm (19.5%), and Group III .33 to 2.0 cm (34.1%), (p <0.03). The residual AD-BPD difference correlated with the incidence of shoulder dystocia after controlling for maternal age, weight, parity, birth weight, and the gestational age at ultrasound (P <0.03). Similar results were found with regards to dystocia severity as the mean residual AD-BPD difference between those with no dystocia, mild dystocia, and severe shoulder dystocia was -0.09, 0.23, and 0.46 cm, respectively, (p <0.006). The residual AD-BPD correlated with the severity of shoulder dystocia after controlling for the above-mentioned confounding variables (p <0.05) in a regression analysis. There is a direct correlation in diabetic patients between the level of fetal truncal asymmetry measured sonographically and the incidence and severity of shoulder dystocia.

Abdomen↗
PubMed1999

Optimal paths in disordered media: scaling of the crossover from self-similar to self-affine behavior.

We study optimal paths in disordered energy landscapes using energy distributions of the type P(log(10) E)=const that lead to the strong disorder limit. If we truncate the distribution, so that P(log(10) E)=const only for E(min) < or =E < or =E(max), and P(log(10) E)=0 otherwise, we obtain a crossover from self-similar (strong disorder) to self-affine (moderate disorder) behavior at a path length l(x). We find that l(x) proportional, variant[log(10)(E(max)/E(min))](kappa), where the exponent kappa has the value kappa=1.60 +/- 0.03 both in d=2 and d=3. We show how the crossover can be understood from the distribution of local energies on the optimal paths.

Journal Article↗
PubMed1999

Corticotrophin-releasing hormone and fetal responses in human pregnancy.

During human pregnancy, maternal and fetal compartments of the human placenta produce and release corticotrophic-releasing hormone (CRH). Elevations of placental CRH are associated with decreased gestational length (including preterm delivery). The effects of elevated placental CRH on human fetal neurological development are not known. Pregnant women in the 31st and 32nd week of gestation consented to procedures for collection of blood and measurement of fetal heart rate (FHR) in response to a series of 40 vibro-acoustic stimuli (VAS). Measures of habituation and dishabituation were calculated from the FHR. All subjects were followed to delivery. Fetuses (N = 33) of women with highly elevated CRH were least responsive (p < .03) to stimulation after presentation of a novel (dishabituating) stimulus with control for parity, fetal gender, medical (antepartum) risk, and gestational length at term. In a larger sample (N = 156) a polynomial model predicted the pattern of FHR reactivity for the first 15 trials. Placental CRH concentration significantly predicted FHR reactivity after controlling for the effects of trial number, baseline FHR, inter-trial interval, and presence of uterine contractions. Increased maternal CRH levels were significantly related to the length of gestation after controlling for the effects of fetal gender, parity, and medical risk (p = .05). The relationship between length of gestation and FHR was not significant suggesting separate actions of CRH on these events. Elevated placental CRH appears to accelerate certain developmental events (gestational length) and may influence the fetal nervous system. The impaired fetal responses to novelty and increased arousal observed in this study suggest that neurological systems may be targets for placental CRH during sensitive developmental periods.

Body Constitution↗
PubMed1998

Pregnancy in a Jehovah's witness with cervical cancer and anemia.

A 34-year-old Jehovah's Witness presented with vaginal bleeding and anemia at 23 weeks gestation. She was diagnosed with a FIGO Stage IB2 squamous cell carcinoma of the cervix. The patient refused transfusion of blood products and strongly desired to continue the pregnancy. She was hospitalized and at 33 weeks gestation underwent a Cesarean-radical hysterectomy with measures that minimized blood loss.

Adult↗
PubMed1998

Maternal corticotropin-releasing hormone levels in the early third trimester predict length of gestation in human pregnancy.

OBJECTIVE: Corticotropin releasing hormone, a hypothalamic neuropeptide, plays a major role in regulating pituitary-adrenal function and the physiologic response to stress. During pregnancy corticotropin-releasing hormone is synthesized in large amounts by the placenta and released into the maternal and fetal circulations. Various endocrine, autocrine, and paracrine roles have been suggested for placental corticotropin-releasing hormone. The aim of this study was to prospectively assess the relationship between maternal plasma concentrations of corticotropin-releasing hormone in the early third trimester of pregnancy and the length of gestation in two groups of deliveries, with and without spontaneous labor. STUDY DESIGN: In a sample of 63 women with singleton intrauterine pregnancies, maternal plasma samples were collected between 28 and 30 weeks' gestation and corticotropin-releasing hormone concentrations were determined by radioimmunoassay. Each pregnancy was dated on the basis of last menstrual period and early ultrasonography. Parity, antepartum risk conditions, presence or absence of spontaneous labor, and birth outcomes were abstracted from the medical record. RESULTS: Maternal corticotropin-releasing hormone levels between 28 and 30 weeks' gestation significantly and negatively predicted gestational length (P < .01) after adjustment for antepartum risk. Moreover, subjects who were delivered preterm had significantly higher corticotropin-releasing hormone levels in the early third trimester (P < .01) than did those who were delivered at term. In deliveries preceded by spontaneous onset of labor, maternal third-trimester corticotropin-releasing hormone levels significantly and independently predicted earlier onset of labor (P < .01) and preterm labor (P < .05), whereas in deliveries effected by induction of labor or cesarean delivery, maternal corticotropin-releasing hormone levels were a marker of antepartum risk but not a statistically independent predictor of gestational length. CONCLUSION: These findings support the premise that placental corticotropin-releasing hormone is potentially implicated in the timing of human delivery in at least two ways. First, placental corticotropin-releasing hormone may play a role in the physiology of parturition. Premature or accelerated activation of the placental corticotropin-releasing hormone system, as reflected by precocious elevation of maternal corticotropin-releasing hormone levels, may therefore be associated with earlier onset of spontaneous labor and resultant delivery. Second, placental corticotropin-releasing hormone may be a marker of antepartum risk for preterm delivery and therefore an indirect predictor of earlier delivery. The implications of these findings are discussed in the context of the neuroendocrinology of placental corticotropin-releasing hormone and human parturition. Furthermore, the role of corticotropin-releasing hormone as a possible effector of prenatal stress in producing alterations in the timing of normal delivery is detailed.

Adult↗