Barotropic thin shells with linear EOS as models of stars and circumstellar shells in general relativity

The spherically symmetric thin shells of the barotropic fluids with the linear equation of state are considered within the frameworks of general relativity. We study several aspects of the shells as completely relativistic models of stars, first of all the neutron stars and white dwarfs, and circumstellar shells. The exact equations of motion of the shells are obtained. Also we calculate the parameters of the equilibrium configurations, including the radii of static shells. Finally, we study the stability of the equilibrium shells against radial perturbations.Comment: final version; ps-version of figure is available by email request to [email protected]

Tipificación de dos nombres en el género Asperula (Rubiaceae)

The typification of two names in the genus Asperula, A. cynanchica subsp. pyrenaica and A. paui subsp. dianensis (Rubiaceae), applicable to species endemic to the Iberian Peninsula and France, is discussed. Specimens from UPS (Herbarium Burser, Uppsala University) and BC (Institut Botànic of Barcelona) are indicated and designated as lectotypes of, respectively, A. pyrenaica and A. paui var. dianensisSe discute la tipificación de dos nombres en el género Asperula, A. cynanchica subsp. pyrenaica y A. paui subsp. dianensis (Rubiaceae) aplicables a especies endémicas de la Península Ibérica y Francia. Especímenes de UPS (Herbario Burser, Universidad de Uppsala) y BC (Institut Botànic de Barcelona) son indicados y designados como lectotipos de A. pyrenaica and A. paui var. dianensis, respectivamente

Static Pairwise Annihilation in Complex Networks

We study static annihilation on complex networks, in which pairs of connected particles annihilate at a constant rate during time. Through a mean-field formalism, we compute the temporal evolution of the distribution of surviving sites with an arbitrary number of connections. This general formalism, which is exact for disordered networks, is applied to Kronecker, Erd\"os-R\'enyi (i.e. Poisson) and scale-free networks. We compare our theoretical results with extensive numerical simulations obtaining excellent agreement. Although the mean-field approach applies in an exact way neither to ordered lattices nor to small-world networks, it qualitatively describes the annihilation dynamics in such structures. Our results indicate that the higher the connectivity of a given network element, the faster it annihilates. This fact has dramatic consequences in scale-free networks, for which, once the ``hubs'' have been annihilated, the network disintegrates and only isolated sites are left.Comment: 7 Figures, 10 page

A Time-Varying Non-Parametric Methodology for Assessing Changes in QT Variability Unrelated to Heart Rate Variability

OBJECTIVE: To propose and test a novel methodology to measure changes in QT interval variability (QTV) unrelated to RR interval variability (RRV) in non-stationary conditions. METHODS: Time-frequency coherent and residual spectra representing QTV related (QTVrRRV) and unrelated (QTVuRRV) to RRV, respectively, are estimated using time-frequency Cohen's class distributions. The proposed approach decomposes the non-stationary output spectrum of any two-input one-output model with uncorrelated inputs into two spectra representing the information related and unrelated to one of the two inputs, respectively. An algorithm to correct for the bias of the time-frequency coherence function between QTV and RRV is proposed to provide accurate estimates of both QTVuRRV and QTVrRRV. Two simulation studies were conducted to assess the methodology in challenging non-stationary conditions and data recorded during head-up tilt in 16 healthy volunteers were analyzed. RESULTS: In the simulation studies, QTVuRRV changes were tracked with only a minor delay due to the filtering necessary to estimate the non-stationary spectra. The correlation coefficient between theoretical and estimated patterns was >0.92 even for extremely noisy recordings (SNR in QTV =-10dB). During head-up tilt, QTVrRRV explained the largest proportion of QTV, whereas QTVuRRV showed higher relative increase than QTV or QTVrRRV in all spectral bands (P<0.05 for most pairwise comparisons). CONCLUSION: The proposed approach accurately tracks changes in QTVuRRV. Head-up tilt induced a slightly greater increase in QTVuRRV than in QTVrRRV. SIGNIFICANCE: The proposed index QTVuRRV may represent an indirect measure of intrinsic ventricular repolarization variability, a marker of cardiac instability associated with sympathetic ventricular modulation and sudden cardiac death

Comparison of ECG T-wave Duration and morphology restitution markers for sudden cardiac death prediction in chronic heart Failure

An index ofT-wave morphology restitution, TMR, has previously shown to be a sudden cardiac death (SCD) predictor in a population of chronic heart failure (CHF) patients. The aim of this study is to compare the predictive value of TMR, T-wave width restitution (TWR), T-peak-to-end (Tpe) morphology restitution (TpeMR) and Tpe duration restitution (TpeR) indices in the same CHF population. Holter ECG recordings from 651 CHF patients of the MUSIC study, including SCD victims and survivors, were analyzed. TMR was significantly correlated with TWR (ρ=0.66), TpeMR (ρ=0.70) and TpeR (ρ=0.42). SCD victims showed significantly higher values of TMR, TWR and TpeMR than the rest of patients, with TMR being the index most strongly associated with SCD (p=0.002, p=0.006 and p=0.011, respectively). TpeR values were only borderline significantly higher in SCD victims (p=0.061). Univariate Cox analysis showed that TMR was the restitution index with the strongest predictive value (hazard ratio (HR) of 1.466, p < 0.001), followed by TWR (HR of 1.295, p=0.005), TpeR (HR of 1.297, p=0.004) and TpeMR (HR of 1.164, p=0.020). In conclusion, considering the predictive value of the four T-wave restitution indices, TMR is the preferred index for SCD risk stratification, followed by TpeMR. However, the marker TWR could also be used for SCD prediction when computational efficiency is an issue

Quantification of T-wave Morphological Variability Using Time-warping Methods

The aim of this study is to quantify the variation of the T-wave morphology during a 24-hour electrocardiogram (ECG) recording. Two ECG-derived markers are presented to quantify T-wave morphological variability in the temporal, dw, and amplitude, da, domains. Two additional markers, dNLw and dNLa, that only capture the non-linear component of dw and da are also proposed. The proposed markers are used to quantify T-wave time and amplitude variations in 500 24-hour ECG recordings from chronic heart failure patients. Additionally, two mean warped T-waves, used in the calculation of those markers, are proposed to compensate for the rate dependence of the T-wave morphology. Statistical analysis is used to evaluate the correlation between dw, dNLw, da and dNLa and the maximum intra-subject RR range, ΔRR. Results show that the mean warped T-wave is able to compensate for the morphological differences due to RR dynamics. Moreover, the metrics dw and dNLw are correlated with ΔRR, but da and dNLa are not. The proposed dw and dNLw quantify variations in the temporal domain of the T-wave that are correlated with the RR range and, thus, could possibly reflect the variations of dispersion of repolarization due to changes in heart rate

Calcium-Activated Potassium Channels Inhibition in Autonomically Stimulated Human Atrial Myocytes

The autonomic nervous system has been reported to play a major role in the generation and maintenance of atrial fibrillation. Various investigations have suggested small-conductance calcium-activated potassium (SK) channels as potential targets for more effective pharmacological therapies. In this study, we used in silico modeling and simulation to investigate the effects of SK channel inhibition on the action potential (AP) of autonomically stimulated human atrial cardiomyocytes. The Grandi AP model, with a new formulation for the ISK current, was used to represent human atrial electrophysiology. Choliner-gic stimulation by different concentrations of acetylcholine (ACh) hyperpolarized the AP and shortened the AP duration (APD) in a dose-dependent manner, with up to 7 mV resting membrane potential elevation and &gt;200 ms APD shortening for 1 µM ACh at 1 Hz pacing frequency. Additional ß-adrenergic stimulation by 1 µM Isoproterenol (Iso) partially attenuated the effects of cholinergic stimulation by prolonging the APD by 41.6%. ISK inhibition was able to reverse the effects of cholinergic activation, but only for moderate ACh doses and when combined with 1 µM Iso, leading to 58.3% prolongation of the AP stimulated with 0.01 µM ACh. In conclusion, ISK inhibition combined with ß-adrenergic stimulation can be effective in antagonizing cholinergic effects on human atrial myocytes

Cardiovascular Changes Induced by Acute Emotional Stress Estimated from the Pulse Transit Time Difference

In the present work, the difference between arrival times of photoplethysmographic (PPG) pulses from two different sites of the arterial tree (PTTD) is calculated. As the PTTD is considered to be related with arterial pulse wave velocity, an evaluation of it''s usefulness to identify states of acute mental stress is performed. A subset of fourteen healthy volunteers undergoing a stress protocol was analysed and a statistical test was carried out to evaluate the validity of the proposed method. The results showed that the standard deviation of the PTTD had sufficient statistical capacity to discern between states of stress and relaxation. Furthermore, a generalised descending trend of the mean PTTD can be seen from relaxation to stress

Variability of Ventricular Repolarization Dispersion Quantified by Time-Warping the Morphology of the T-waves

We propose two electrocardiogram (ECG)-derived markers of T-wave morphological variability in the temporal, dw , and amplitude, da , domains. Two additional markers, d(NL)w and d(NL)a , restricted to measure the non-linear information present within dw and da are also proposed. METHODS: We evaluated the accuracy of the proposed markers in capturing T-wave time and amplitude variations in 3 situations: (1) In a simulated set up with presence of additive Laplacian noise, (2) when modifying the spatio-temporal distribution of electrical repolarization with an electro-physiological cardiac model and (3) in ECG records from healthy subjects undergoing a tilt table test. RESULTS: The metrics dw , da , d(NL)w and d(NL)a followed T-wave time and amplitude induced variations under different levels of noise, were strongly associated with changes in the spatiotemporal dispersion of repolarization, and showed to provide additional information to differences in the heart rate, QT and Tpe intervals, and in the T-wave width and amplitude. CONCLUSION: The proposed ECG-derived markers robustly quantify T-wave morphological variability, being strongly associated with changes in the dispersion of repolarization. SIGNIFICANCE: The proposed ECG-derived markers can help to quantify the variability in the dispersion of ventricular repolarization, showing a great potential to be used as arrhythmic risk predictors in clinical situations

Respiratory Rate Derived from Pulse Photoplethysmographic Signal by Pulse Decomposition Analysis

A novel technique to derive respiratory rate from pulse photoplethysmographic (PPG) signals is presented. It exploits some morphological features of the PPG pulse that are known to be modulated by respiration: amplitude, slope transit time, and width of the main wave, and time to the first reflected wave. A pulse decomposition analysis technique is proposed to measure these features. This technique allows to decompose the PPG pulse into its main wave and its subsequent reflected waves, improving the robustness against noise and morphological changes that usually occur in long-term recordings. Proposed methods were evaluated with a data base containing PPG and plethysmography-based respiratory signals simultaneously recorded during a paced-breathing experiment. Results suggest that normal ranges of spontaneous respiratory rate (0.1-0.5 Hz) can be accurately estimated (median and interquartile range of relative error less than 5%) from PPG signals by using the studied features