Comprehensive Kinetics of Hydrolysis of Organotriethoxysilanes by 29Si NMR.

The kinetics of several representative hybrid precursors were studied via 29Si NMR: three alkyl precursors, methyltriethoxysilane, ethyltriethoxysilane, and propyltriethoxysilane; as well as two unsaturated radicals, vinyltriethoxysilane and phenyltriethoxysilane. The reaction rate is related to the chemical shift of 29Si in the NMR spectra, which gives information about the electronic density of the Si atoms and the inductive effects of substituents. The concentration of the precursors decreased exponentially with time, and the intermediate products of hydrolysis and the beginning of the condensation reactions showed curves characteristic of sequential reactions, with a similar distribution of the species as a function of the fractional conversion. For all of the precursors, condensation started when the most hydrolyzed species reached a maximum concentration of 0.30 M, when the precursor had run out. A prediction following the developed mathematical model fits the experimental results in line with a common pathway described by eight parameters.

ECG scaling properties of cardiac arrhythmias using detrended fluctuation analysis.

We applied detrended fluctuation analysis to characterize at very short time scales during episodes of cardiac arrhythmias the raw electrocardiogram (ECG) waveform, aiming to get a global insight into its dynamical behaviour in patients who experienced sudden death. We found that in 15 recordings involving different types of arrhythmias (taken from PhysioNet's Sudden Cardiac Death Holter Database), the ECG waveform, besides showing a less-random dynamics, becomes more regular during bigeminy, ventricular tachycardia or even atrial fibrillation and ventricular fibrillation. The ECG waveform scaling properties thus suggest that reduced complexity dominates the underlying mechanisms of arrhythmias. Among other explanations, this may result from shorted or restricted (i.e. less diverse) pathways of conduction of the electrical activity within ventricles.

Scaling patterns of heart rate variability data.

Detrended fluctuation analysis (DFA) is becoming a widely used technique for exploring the structure of correlations in heart rate variability (HRV) data. This method provides a scaling or fractal exponent alpha(x) derived from the behaviour of the root-mean-square fluctuations along different time scales n. Rather than just finding a single exponent, covering either short (alpha(1)) or long range (alpha(2)), we recently suggested tracking the local evolution of alpha(x), as in this way scaling patterns (SP), which seem to provide more detailed characterizations of HRV data, are revealed. Here, we evaluate such potential advantage by classifying long-term data from 51 subjects in normal sinus rhythm and 29 congestive heart failure patients. Using the SP we achieved a significantly better classification of these data than using alpha(x), or the statistic pNN20, thereby confirming that the SP provide a useful assessment of the correlation structure in HRV data.

Comparison of RR-interval scaling exponents derived from long and short segments at different wake periods.

Heartbeat fluctuations show fractal-like correlations that are associated with highly adaptive cardiovascular regulatory systems. Moreover, the short-range fractal or scaling exponent alpha(1) extracted from these correlations has been found to be a predictor of mortality for subjects with an impaired left ventricular function. In general, the RR-interval data required for this analysis are derived from long-term ECG recordings during free-running conditions. Yet short-term recordings are more likely to be obtained in some practical circumstances, so becoming relevant to assess the possibility of obtaining representative alpha(1) exponents from these recordings. Here, we compare the alpha(1) exponents extracted from the RR-interval series (9:00 AM-6:00 PM) of 51 healthy adults in normal sinus rhythm and the alpha(1) calculated from three shorted segments of only 700 beats obtained from the same series at 9:00 AM, 1:30 PM and 5:00 PM. We found no significant differences between the scaling exponents derived from the whole 9 h series and the short segments at 9:00 AM and 5:00 PM, but did find significant differences when comparing the whole series with the short segment at 1:30 PM. Thus, only if the time of day is taken into consideration can short segments of heartbeat fluctuation data be used to obtain representative alpha(1) exponents.

Respiratory sinus arrhythmia in growth restricted fetuses with normal Doppler hemodynamic indices.

BACKGROUND: The autonomic behavior of growth-restricted fetuses at different evolving hemodynamic stages has not been fully elicited. AIM: To analyze the respiratory sinus arrhythmia (RSA) of growth-restricted fetuses that despite this severe condition show normal Doppler hemodynamics. SUBJECTS: 10 growth-restricted fetuses (FGR group) with normal arterial pulsatility indices (umbilical, uterine, middle cerebral, ductus venosus and aortic isthmus), and 10 healthy fetuses (Control group), 32-37weeks of gestation. METHOD: B-mode ultrasound images for visualizing fetal breathing movements (FBM) or breathing akinesis (FBA), and the simultaneous RR-interval time series from maternal abdominal ECG recordings were obtained. The root-mean-square of successive differences of RR-intervals (RMSSD) was considered as a RSA-related parameter among the instantaneous amplitude of the high-frequency component (AMPHF) and its corresponding instantaneous frequency (IFHF), both computed by using empirical mode decomposition. Mean fetal heart-periods and RSA-related parameters were assessed during episodes of FBM and FBA in 30s length windows. RESULTS: FGR and Control groups presented RSA-related fluctuations during FBM and FBA. Also, both groups showed significant higher (p<0.001) values for the mean heart-period, RMSSD and AMPHF during FBM. No-significant differences (p>0.05) were found for the IFHF regardless of breathing activity (FBM vs. FBA). CONCLUSION: Growth-restricted fetuses without evident hemodynamic compromise exhibit a preserved autonomic cardiovascular regulation, characterized by higher values of RSA and mean heart-period in the presence of FBM. This physiological response reflects a compensatory strategy that may contribute to preserve blood flow redistribution to vital organs.

Isotherms and sequential extraction procedures for evaluating sorption and distribution of heavy metals in soils.

Heavy metals are potentially toxic to human life and the environment. Their contaminating effect in soils depends on chemical associations. Hence, determining the chemical form of a metal in soils is important to evaluate its mobility and bioavailability. We utilized a sequential extraction procedure and sorption isotherms (monometal and competitive) to evaluate the mobility and distribution of Cd, Cu, Ni, Pb, and Zn in four soils differing in their physicochemical properties: Calcixerollic Xerochrepts (Cx1 and Cx2), Paralithic Xerorthent (Px) and Lithic Haplumbrept (Lh). Most of the metals retained under point B conditions of sorption isotherms were extracted from the more mobile fractions: exchangeable and carbonates, in contrast with the profiles of the original soils where metals were preferently associated with the residual fraction. In soils having carbonate concentration under 6% (Cx1 and Lh), the exchangeable fraction was predominant, whereas in calcareous soils (Cx2 and Px) metals extracted from carbonates predominated. Partitioning profiles were in accordance with the affinity sequences deduced from the initial slope of isotherms and showed that the soils had a greater number of surface sites and higher affinity for Pb and Cu than for Cd, Ni, or Zn. In general, the simultaneous presence of the cations under study increased the percentages of metals released in the exchangeable fraction. The tendency towards less specific forms was more noticeable in Cx2 and Px soils and for Ni, Zn, and Cd. The affinity of inorganic surfaces was larger for Zn than for Cd or Ni, but the affinity of organic surfaces was larger for Cd or Ni than for Zn.

Competitive sorption of heavy metal by soils. Isotherms and fractional factorial experiments.

Competing ions strongly affect heavy metal sorption onto the solid surfaces of soil. This study evaluated competitive sorption of Cd, Cu, Ni, Pb and Zn on three soils: Calcixerollic Xerochrept, Paralithic Xerorthent and Lithic Haplumbrept. Monometal and competitive sorption isotherms were obtained at 25 degrees C. The individual effect of ions on retention of the others was ascertained by a fractional factorial analysis design. Most of the sorption isotherms belonged to type L subtype 2 in the classification of Giles. In competitive sorption the initial linear part was shorter and the knee sharper when compared with monometal sorption isotherms. Parameters related to sorptive capacity, such as Point B, Langmuir monolayer and Freundlich distribution coefficient, were higher in monometal than in competitive sorption, and in basic soils than in acidic soil. Calcium desorbed at different points of the sorption isotherms indicated that cationic exchange with Ca was the main retention mechanism in calcareous soils. For Pb, the ratio Ca desorbed/Pb sorbed was close to one; for Cu, Ni and Zn the ratio ranged from 1.20 to 1.37, probably due to partial dissolution of calcium carbonates by hydrolytic processes during retention. On the other hand, Cd had a ratio around 0.6 reflecting another additional retention mechanism, probably surface complexation. Fractional factorial design confirmed that the presence of the cations investigated reduced the amount of the five metals retained, but the presence of Cu and Pb in the system depressed Ni, Cd and Zn sorption more than the inverse. Cation mobility was enhanced when equilibrium concentration increased and the effect was higher in Ca-saturated soils.

Detrended fluctuation analysis: a suitable method for studying fetal heart rate variability?

We evaluate the suitability of an enhanced detrended fluctuation analysis for studying fetal heart rate series involving imperfect quality of information. Our results indicate that to explore persistent long-range correlations, or fractality, the collection requirements of the data can be relaxed by allowing the possibility of using averaged fetal heart rate series. In addition, it also appears feasible to employ, without producing major alterations in the long-range scaling behaviour, fragmented fetal heart rate series involving up to 50% of random missing values, or up to 50 min of consecutive missing samples in recordings of approximately equal to 8 h length. These are crucial advantages to overcome the often variable quality of fetal data. Consequently, these findings may open the possibility of obtaining information concerning the development of neural processes from fetal heart rate series, despite their non-stationary and fragmented nature.

Application of empirical mode decomposition to heart rate variability analysis.

The analysis of heart rate variability, involving changes in the autonomic modulation conditions, demands specific capabilities not provided by either parametric or non-parametric spectral estimation methods. Moreover, these methods produce time-averaged power estimates over the entire length of the record. Recently, empirical mode decomposition and the associated Hilbert spectra have been proposed for non-linear and non-stationary time series. The application of these techniques to real and simulated short-term heart rate variability data under stationary and non-stationary conditions is presented. The results demonstrate the ability of empirical mode decomposition to isolate the two main components of one chirp series and three signals simulated by the integral pulse frequency modulation model, and consistently to isolate at least four main components localised in the autonomic bands of 14 real signals under controlled breathing manoeuvres. In addition, within the short time-frequency range that is recognised for heart rate variability phenomena, the Hilbert amplitude component ratio and the instantaneous frequency representation are assessed for their suitability and accuracy in time-tracking changes in amplitude and frequency in the presence of non-stationary and non-linear conditions. The frequency tracking error is found to be less than 0.22% for two simulated signals and one chirp series.

Effects of fetal respiratory movements on the short-term fractal properties of heart rate variability.

We evaluated the effect of fetal respiratory movements (RM) on the heart rate (HR) fractal dynamics.Abdominal ECG recordings were collected from low-middle-risk pregnant woman at rest. Mean gestational age was 34.8 ± 3.7 weeks. Ultrasound images were simultaneously acquired determining if RM were exhibited by fetuses. 13 pairs of HR series were compared. Each pair included 5 min of data from the same fetus either during the manifestation of RM or when there was no persistent indication of them. Detrended fluctuation analysis was applied to these series for obtaining the scaling exponent α1. HR series were also assessed using the conventional parameters RMSSD and HF power.The main findings of this contribution were the lack of significant changes in the scaling exponent α1 of fetal HR fluctuations as a result of RM. By contrast, HF power and RMSSD did show significant changes associated with the manifestation of fetal RM (p < 0.001 and p < 0.05, respectively). Yet the scaling exponent was the only parameter showing a significant relationship with the particular frequency of fetal RM (r s  = 0.6, p < 0.03). Given the invariability of α1 regarding the manifestation of fetal RM, we consider that the HR short-term fractal properties are convenient for assessing the cardiovascular prenatal regulation.

Fractal and nonlinear changes in the long-term baseline fluctuations of fetal heart rate.

The interpretation of heart rate patterns obtained by fetal monitoring relies on the definition of a baseline, which is considered as the running average heart rate in the absence of external stimuli during periods of fetal rest. We present a study along gestation of the baseline's fluctuations, in relation to fractal and nonlinear properties, to assess these fluctuations according with time-varying attracting levels introduced by maturing regulatory mechanisms. A low-risk pregnancy was studied weekly from the 17th to 38th week of gestation during long-term recording sessions at night (>6 h). Fetal averaged pulse rate samples and corresponding baseline series were obtained from raw abdominal ECG ambulatory data. The fractal properties of these series were evaluated by applying detrended fluctuation analysis. The baseline series were also explored to evaluate nonlinear properties and time ordering by applying the scaling magnitude and sign analyses. Our main findings are that the baseline shows fractal and even nonlinear anticorrelated fluctuations. This condition was specially the case before mid-gestation, as revealed by α values near to unit, yet becoming significantly more complex after 30 weeks of gestation as indicated by α(mag) values >0.5. The structured (i.e. not random) fluctuations and particular nonlinear changes that we found thus suggest that the baseline provides on itself information concerning the functional integration of cardiac regulatory mechanisms.

The autonomic condition of children with congenital hypothyroidism as indicated by the analysis of heart rate variability.

Early treatment by thyroid hormone in congenital hypothyroidism (CH) prevents for mental retardation, but the autonomic condition could well remain impaired. The aim of this study was to get insights about the cardiovascular regulation of CH children by analysing their heart rate variability (HRV) data. RR-interval data of 34 early treated CH children, 24 to 48 months of age, were collected at rest in a clinical environment. In all these cases, CH resulted from an abnormal development of thyroid gland. Data from 29 age matched-controls were also collected in a scholar environment. Short time series were then analysed in time and frequency domains. Detrended fluctuation analysis (DFA) was also applied to obtain the scaling exponent α(1). We found that the HRV obtained from children with CH is diminished as suggested by statistical differences (p<0.05) in the following parameters: RMSDD, pNN20, total power, LF and HF. However, no significant differences were found in the LF/HF ratio, the central frequencies of the LF and HF bands, as well as in α(1). Given that the significant differences were basically reflected in static parameters rather than in the frequency distribution or scale-invariant behaviour of the studied data, we consider that the analysis points toward the effect of treatment on rising heart rate by the thyroid hormone, and not necessarily toward any residual autonomic dysfunction, as the dominant underlying cause of the diminished HRV exhibited by CH children.

Short-term heart rate dynamics of women during labor.

We studied 10 minutes segments of heartbeat interval fluctuations from 18 young women in labor with normal outcome of pregnancy. Data of each studied case were classified into two distinct groups. One group involving segments where the uterine activity was observable (three or more contractions in ten minutes), and the other group of reference having segments with fewer uterine activity or not presenting contractions at all. For comparison, we also included segments collected during the last trimester of gestation prior to labor from a third group of women. Corresponding RR interval series were analyzed to estimate RR(mean), RMSSD, α(1), α(1(MAG)) and α(1(SIGN)) parameters. No significant differences among groups were identified in RMSSD, α(1) and α(1(MAG)) Nevertheless, α(1(SIGN)) did present significant differences in comparison with the last trimester results (p<0.007), revealing a subtle change in the temporal organization of maternal RR series during labor. Results of these parameters then suggest that during labor, despite preserving a concomitant non-linear influence, the maternal short-term autonomic cardiac regulation behaves with less antagonism.

Short-term heart rate dynamics of pregnant women.

Aiming to detect the stage of gestation where dynamical changes of the RR fluctuations may occur, we assessed short-term fluctuations of low risk pregnant women. Ninety six, 10min ECG recordings were collected along gestation (7 to 39 weeks). Corresponding RR fluctuations series were analysed to obtain the RMSSD, α(1), α(1(mag)) and α(1(sign)) parameters. Four groups covering first, second and last trimesters of gestation were conformed. No significant changes in α(1), which was close to unit, and α(1(sign)) among gestational groups were identified. But, in accordance with previous findings, we did find a significant reduction of RMSSD along gestation, and significant short-term changes that indicate a higher degree of nonlinearity after about 26 weeks of gestation (α(1(mag))>0.5)). These results suggest that the short-term heart rate dynamics of low risk pregnant women do not become compromised during gestation, despite the increased haemodynamic demands and other ongoing adaptations. Yet the complexity of the mechanisms involved in the cardiac regulation of pregnant women does seem to increase from mid-pregnancy, possibly owing to new short-term control influences or to modifications regardless the strength of the regulatory interactions.

Applying fractal analysis to short sets of heart rate variability data.

The aim of this study was to explore the interchangeability of fractal scaling exponents derived from short- and long-term recordings of real and synthetic data. We compared the alpha(1) exponents as obtained by detrended fluctuation analysis from RR-interval series (9 am to 6 pm) of 54 adults in normal sinus rhythm, and the alpha(1) estimated from shorted segments of these series involving only 50, 100, 200 and 300 RR intervals. Three series of synthetic data were also analysed. The principal finding of this study is the lack of individual agreement between alpha(1) derived from long and short segments of HRV data as indicated by the existence of bias and low intraclass correlation coefficient (r(i) = 0.158). The extent of variation in the estimation of alpha(1) from real data does not only appear related to segments' length, but also to different dynamics among subjects or lack of uniform scaling behaviour. However, we did find statistical agreement between the means of alpha(1) exponents from long and short segments, even for segments involving just 50 RR intervals. According to results of synthetic series, the 95% confidence interval found for the variation of alpha(1) using segments with 300 samples is [-0.1783 + 0.1828]. Caution should be taken concerning the use of short segments to obtain representative exponents of fractal RR dynamics; a circumstance not fully considered in several studies.

Repeatability of heart rate variability in congenital hypothyroidism as analysed by detrended fluctuation analysis.

The analysis of heart rate fluctuations, or heart rate variability (HRV), may be applied to explore children's neurodevelopment. However, previous studies have reported poor reliability (repeatability) of HRV measures in children at rest and during light exercise. Whether the reliability can be improved by controlling variables such as physical activity, breathing rate and tidal volume, or by selecting non-conventional techniques for analysing the data remains as an open question. We evaluated the short-term repeatability of RR-interval data from medicated children with congenital hypothyroidism (CH). The alpha(1) exponents, obtained by detrended fluctuation analysis (DFA), from the data of 21 children collected at two different sessions were compared. Elapsed days between sessions were 59 +/- 33, and data were obtained during 10 min, trying to restrict the children's activity while being seated. We found statistical agreement between the means of alpha(1) exponents for each session (p = 0.94) and no bias with a low-coefficient variation (9.1%); an intraclass correlation coefficient ri = 0.48 ([0.14 0.72], 95% confidence interval) was also estimated. These findings, which were compared with results obtained by conventional time and frequency techniques, indicate the existence of agreement between the alpha(1) exponents obtained at each session, thereby providing support concerning the repeatability of HRV data as analysed by DFA in children with congenital hypothyroidism. Of particular interest was also the agreement found by using the central frequency of the high-frequency band and the parameter pNN20, both showing better or similar ri than alpha(1) (0.77 [0.57 0.89] and 0.51 [0.17 0.74], respectively), yet considerably better repeatability than other conventional time and frequency parameters.

Prenatal RR fluctuations dynamics: detecting fetal short-range fractal correlations.

OBJECTIVE: Several studies have suggested that the analysis of heart rate variability (HRV) during gestation provides indications of the development or maturation of fetal cardiovascular regulatory mechanisms. In this study, we evaluate the existence of short-range fractal-like correlations in fetal RR fluctuations data from the second half of human gestation. METHODS: Fifty-six short-term abdominal ECG recordings were obtained from low-middle-risk pregnant women. Gestational age varied from estimated 21 weeks to term. For comparison, RR-interval data of 51 healthy adults were also analysed. RESULTS: Principal findings along the gestational period explored were the existence of fractal RR dynamics in prenatal fetal data as revealed by the short-range scaling exponent alpha(1). No significant differences of alpha(1) (p = 0.4770) were found between fetal (median 1.2879) and adult data (median 1.3214), either between the fetal cases before or after 24 weeks (p = 0.6116) despite observing more variation at early stages. However, fetal RR data did involve lower magnitude in comparison with adults as we found significant differences in pNN20 and SDNN values. CONCLUSION: The fetal short-range fractal behaviour of RR data could then be linked to the functional development of the parasympathetic activity, which appears to become manifested before 21 weeks of gestation.

Interpretation of heart rate variability via detrended fluctuation analysis and alphabeta filter.

Detrended fluctuation analysis (DFA), suitable for the analysis of nonstationary time series, has confirmed the existence of persistent long-range correlations in healthy heart rate variability data. In this paper, we present the incorporation of the alphabeta filter to DFA to determine patterns in the power-law behavior that can be found in these correlations. Well-known simulated scenarios and real data involving normal and pathological circumstances were used to evaluate this process. The results presented here suggest the existence of evolving patterns, not always following a uniform power-law behavior, that cannot be described by scaling exponents estimated using a linear procedure over two predefined ranges. Instead, the power law is observed to have a continuous variation with segment length. We also show that the study of these patterns, avoiding initial assumptions about the nature of the data, may confer advantages to DFA by revealing more clearly abnormal physiological conditions detected in congestive heart failure patients related to the existence of dominant characteristic scales.