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Low-frequency Components (low-frequency + component)
Selected AbstractsSympathetic control of short-term heart rate variability and its pharmacological modulationFUNDAMENTAL & CLINICAL PHARMACOLOGY, Issue 4 2007Jean-Luc Elghozi Abstract The static relationship between heart rate (HR) and the activity of either vagal or sympathetic nerves is roughly linear within the physiological range of HR variations. The dynamic control of HR by autonomic nerves is characterized by a fixed time delay between the onset of changes in nerve activity and the onset of changes in HR. This delay is much longer for sympathetically than for vagally mediated changes in HR. In addition, the kinetics of the HR responses shows the properties of a low-pass filter with short (vagal) and long (sympathetic) time constants. These differences might be secondary to differences in nervous conduction times, width of synaptic cleft, kinetics of receptor activation and post-receptor events. Because of the accentuated low-pass filter characteristics of the HR response to sympathetic modulation, sympathetic influences are almost restricted to the very-low-frequency component of HR variability, but the chronotropic effects of vagal stimulation usually predominate over those of sympathetic stimulation in this frequency band. Oscillations in cardiac sympathetic nerve activity are not involved in respiratory sinus arrhythmia (high-frequency component) and make a minor contribution to HR oscillations of approximately 10-s period (low-frequency component of approximately 0.1 Hz), at least in the supine position. In the latter case, HR oscillations are derived mainly from a baroreflex, vagally mediated response to blood pressure Mayer waves. Beta-blockers and centrally acting sympathoinhibitory drugs share the ability to improve the baroreflex control of HR, possibly through vagal facilitation, which might be beneficial in several cardiovascular diseases. [source] Increased heart rate variability correlation between mother and child immediately pre-operationACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 5 2009Y.-C. P. ARAI Background: Maternal distress would correlate with the children's mental status, thereby influencing the activity of the autonomic nervous system (ANS) of the children and mothers. We hypothesized that pre-anesthetic maternal ANS activity, when approaching close to their children's operation time, would correlate with children's ANS activity, and that the values of heart rate variability (HRV) would correlate. Methods: We calculated maternal and children's HRVs and analyzed the relationship between the two. A total of 24 pairs of mother and child were analyzed. Maternal and children's HRVs were recorded from the night before the child's surgery to the arrival to the operation room. Results: The ratios of low-frequency components (LF) to high-frequency components (HF) (LF/HF ratio) of children's and maternal HRVs obtained during the immediate pre-operative period (06:00,08:00 hours) showed a significantly, positive correlation, but no correlation was found for the LF/HF ratios obtained during the pre-operative night. Conclusion: The LF/HF ratios of HRV immediately before surgery in children and mothers showed a significant positive correlation. [source] Traditional versus unobserved components methods to forecast quarterly national account aggregatesJOURNAL OF FORECASTING, Issue 2 2007Gustavo A. Marrero Abstract We aim to assess the ability of two alternative forecasting procedures to predict quarterly national account (QNA) aggregates. The application of Box,Jenkins techniques to observed data constitutes the basis of traditional ARIMA and transfer function methods (BJ methods). The alternative procedure exploits the information of unobserved high- and low-frequency components of time series (UC methods). An informal examination of empirical evidence suggests that the relationships between QNA aggregates and coincident indicators are often clearly different for diverse frequencies. Under these circumstances, a Monte Carlo experiment shows that UC methods significantly improve the forecasting accuracy of BJ procedures if coincident indicators play an important role in such predictions. Otherwise (i.e., under univariate procedures), BJ methods tend to be more accurate than the UC alternative, although the differences are small. We illustrate these findings with several applications from the Spanish economy with regard to industrial production, private consumption, business investment and exports.,,Copyright © 2007 John Wiley & Sons, Ltd. [source] Spatial frequency content of the Cardiff and related acuity testsOPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 1 2006W. N. Charman Abstract In the Cardiff acuity test, simple pictures on an otherwise neutral grey card are defined by borders consisting of a relatively broad white band flanked by black bands each half the width of the white band. Higher levels of acuity correspond to the ability to detect figures defined by narrower borders, the figure size remaining constant. It is sometimes implied that the acuity limit corresponding to each card can be equated with different levels of grating resolution, the total width of the border corresponding to the overall grating period. It is shown that although the spatial frequency spectra of the Cardiff figures, like those of other vanishing optotypes, lack very low-frequency components, they have a complex two-dimensional form. The figures have wide spatial bandwidth and no well-defined discrete frequency components. As a result, the relationship between measured Cardiff and grating acuity will vary somewhat, depending upon the particular optical, neural or other deficits of the individual being tested. [source] Fluctuations in isometric muscle force can be described by one linear projection of low-frequency components of motor unit discharge ratesTHE JOURNAL OF PHYSIOLOGY, Issue 24 2009Francesco Negro The aim of the study was to investigate the relation between linear transformations of motor unit discharge rates and muscle force. Intramuscular (wire electrodes) and high-density surface EMG (13 × 5 electrode grid) were recorded from the abductor digiti minimi muscle of eight healthy men during 60 s contractions at 5%, 7.5% and 10% of the maximal force. Spike trains of a total of 222 motor units were identified from the EMG recordings with decomposition algorithms. Principal component analysis of the smoothed motor unit discharge rates indicated that one component (first common component, FCC) described 44.2 ± 7.5% of the total variability of the smoothed discharge rates when computed over the entire contraction interval and 64.3 ± 10.2% of the variability when computed over 5 s intervals. When the FCC was computed from four or more motor units per contraction, it correlated with the force produced by the muscle (62.7 ± 10.1%) by a greater degree (P < 0.001) than the smoothed discharge rates of individual motor units (41.4 ± 7.8%). The correlation between FCC and the force signal increased up to 71.8 ± 13.1% when the duration and the shape of the smoothing window for discharge rates were similar to the average motor unit twitch force. Moreover, the coefficients of variation (CoV) for the force and for the FCC signal were correlated in all subjects (R2 range = 0.14,0.56; P < 0.05) whereas the CoV for force was correlated to the interspike interval variability in only one subject (R2= 0.12; P < 0.05). Similar results were further obtained from measures on the tibialis anterior muscle of an additional eight subjects during contractions at forces up to 20% of the maximal force (e.g. FCC explained 59.8 ± 11.0% of variability of the smoothed discharge rates). In conclusion, one signal captures most of the underlying variability of the low-frequency components of motor unit discharge rates and explains large part of the fluctuations in the motor output during isometric contractions. [source] | ||||||||||