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High-frequency Components (high-frequency + component)

Distribution by Scientific Domains

Medical Sciences	50%

Selected Abstracts

Sympathetic control of short-term heart rate variability and its pharmacological modulation

FUNDAMENTAL & CLINICAL PHARMACOLOGY, Issue 4 2007
Jean-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]

Intra-seasonal variability of wintertime temperature over East Asia

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2004
Dao-Yi Gong
Abstract There has been a profound warming over East Asia during the winter months (November through to March) over the past few decades. The goal of this study is to address the question of whether the daily temperature has become more variable in conjunction with this warming by using observed temperature data obtained from 155 Chinese and Korean stations. Prior to the analysis, the annual cycle is removed to obtain daily temperature anomalies for each winter for each station. Results show that the intra-seasonal variance generally decreases, implying that the daily temperatures are becoming less variable. Considering all stations as a whole, the rate of change is ,0.49°C2 per decade (equivalent to ,3.59% per decade). The changes are more robust in the northeastern portion of China. In contrast, there are no dominant trends for the skewness coefficients, except for clear negatively skewed trends in northeastern China. These results are consistent with an increase in the number of extremely cold events. Over the region, the frequency of low-temperature extremes (as low as below minus two standard deviations) increases at a rate of change of 0.26 days per decade, significant at the 95% confidence level. Both the Siberian high and Arctic oscillation (AO) exert a notable influence on the temperature variance. Intra-seasonal variance of the Siberian high and AO are significantly correlated with the temperature variance, whereas the seasonal mean state of the AO affects the temperature variance by modulating the high-frequency components of the Siberian high. The intra-seasonal variance of the Siberian high tends to decline at a rate of change of ,10.7% per decade, significant at the 99% level; meanwhile, the mean wintertime AOs have strengthened in the last few decades. These two climate features together make a considerable contribution to the changes in intra-seasonal temperature variance in East Asia. Copyright © 2004 Royal Meteorological Society [source]

Increased heart rate variability correlation between mother and child immediately pre-operation

ACTA ANAESTHESIOLOGICA SCANDINAVICA, Issue 5 2009
Y.-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]

Investigation of the physical mechanisms in rolling bearings during the passage of electric current

LUBRICATION SCIENCE, Issue 4 2005
A. Jagenbrein
Abstract With the increasing use of frequency converters as control units for electric motors, bearing failures caused by the passage of electric current through the bearings are more common. This paper discusses the most relevant parasitic current loops in frequency converter-driven motors that can lead to damage of bearing raceways or deterioration of lubricants. Electrical stray capacitances, which are inevitably present in any electric motor, in combination with the high-frequency components of the drive voltages, have been identified as the source of electric discharge machining (EDM) currents. Based on an EDM equivalent electric circuit, the passage of electric current through the rolling contact of a bearing is simulated. A set-up is introduced to initiate EDM processes in bearings under well-controlled electrical and mechanical conditions. The effect of EDM currents on bearing elements in the presence of different lubricants is studied. Initial results are presented. [source]