Vera Gal

Reflection of heart rate regulation on linear and nonlinear heart rate variability measures

The aim of this study was to examine the dependence of heart rate variability (HRV) measures on RR interval length and to find out relationships between linear and nonlinear measures. The spectral powers in very low (VLF), low (LF) and high (HF) frequency regions, short-term scaling exponent alpha(1) and sample entropy (SampEn) were determined. All spectral powers increased with increasing RR interval length until they reached a plateau. Neighbouring spectral powers were strongly correlated. The largest fraction of the spectrum consisted of VLF (from about 40 to 95%) and the smallest of HF, although HF was most sensitive to changes in RR interval length. SampEn is also increased with increasing RR interval, reaching a plateau. The dependence of alpha(1) on RR showed a deflection point at 0.5 s. Nonlinear measures can be expressed by spectral powers: alpha(1) by a linear function of ln(LF/HF) and SampEn by a quadratic function of ln HF. We concluded that during the day an increase of HR up to 120 beats min(-1) was reflected in a reduction in HF and LF and to a smaller extent in VLF and by decreased complexity and increased correlation in RR interval series. In sleep, HRV measures are at a plateau. We suggest that below intrinsic value, HR is regulated mainly by changes of parasympathetic activity, reflected in linear and nonlinear HRV measures.

Nonlinear and chaos characteristics of heart period time series: Healthy aging and postural change

In this study we investigated nonlinear and linear characteristics of heart period variability with aging in supine and standing posture. Sixty healthy subjects (8-61 years) divided in three age groups participated in the study. Heart period variability was assessed by measurement of short-term scaling exponent, sample entropy, largest Lyapunov exponent and spectral low-frequency and high-frequency power. In standing, there was significant increase in short-term scaling exponent and largest Lyapunov exponent in all subjects, and significant decrease in sample entropy in children (<15 years) and young adults (15-39 years). Increasing age is associated with reduction in sample entropy in supine posture. Mutual effect of aging and postural change was reflected on heart rate and sample entropy. Correlation between low-frequency-to-high-frequency power ratio and short-term scaling exponent was found in supine posture. In standing both low-frequency and high-frequency powers are correlated with short-term scaling exponent and sample entropy. These results show that posture, standing compared to supine, has significant effect on nonlinear properties of heart period variability in healthy subjects while the influence of healthy aging is less pronounced. The findings indicate that intrinsic properties of heart period dynamics, reflected on nonlinear measures, are altered only by robust changes of autonomic modulation of heart rate.

Fast disassembly of microtubules induced by Mg2+ or Ca2+.

The extent and rate of disassembly of microtubules induced by the addition of high concentrations of magnesium and calcium have been measured. At 25 degrees C, the rate constant for microtubule disassembly increases more than ten-fold on increasing [Mg2+] from 4.0 to 20 mM. The process is even more sensitive to [Ca2+], showing similar enhancement on increasing [Ca2+] from 0.5 to 8 mM. Electron microscopy indicates that the disassembly is an end-dependent process. Complete microtubule disassembly occurs at concentrations in excess of 10 and 2 mM for Mg2+ and Ca2+, respectively; this suggests the importance of binding to weak sites for both ions. The sensitivity to ionic composition explains the wide variations in the published values for k-, under varying conditions. The results indicate the potential range of microtubule disassembly rates which may be encountered under different conditions in vitro and in vivo. The highest values of k- (ca. 3000 s-1) would imply microtubule shortening rates in excess of 100 um per minute.

Dependence of heart rate variability on heart period in disease and aging

The aim of this study was to compare the dependence of heart rate variability (HRV) on heart period (RR interval length) under different physiological and pathological states in order to detect changes in HR modulation. The dependence of HRV on the RR interval length in healthy elderly subjects, congestive heart failure (CHF) patients and one patient with a transplanted heart (T) was compared with healthy young subjects. Spectral powers, sample entropy (SampEn) and short-term fractal scaling exponent (alpha1) were determined from 24 h free-running recordings. For the same HR, HRV measures were different in different groups. In healthy subjects HRV measures depended on RR interval length and all spectral powers were highly correlated, although reduced in elderly subjects. SampEn at high HR was the most sensitive quantity to changes induced by aging. In disease, CHF and T, an achievable HR range was decreased, all spectral powers were reduced, but correlated, and the dependence of HRV measures on RR was lost. There was an evident difference in the dependence of nonlinear on linear measures between young subjects and all the other studied groups. In disease the reduction in autonomic control was associated with the decrease in short-range correlation and regularity in RR series. We have concluded that the analysis of HRV measures as functions of RR interval length can reveal important aspects of HR control that might be lost in averaging.

Linear and non‐linear heart rate variability measures in chronic and acute phase of anorexia nervosa

Previous results on heart rate variability (HRV) analysis in anorexia nervosa (AN) include some apparently conflicting data. In order to find out the reason for different results and to improve understanding of autonomic control in AN we compare HRV in acute and chronic AN. Spectral powers, fractal scaling exponent and sample entropy were computed from 24 h RR series derived from Holter ECG recordings in 17 anorexic patients, nine chronic and eight healthy women. We found that all linear and non‐linear HRV measures change in different direction in acute and chronic AN. Acute AN is characterized by decreased HR and increased HRV. In chronic AN, HR is increased, HRV reduced and the difference between awake and sleeping values is high. HRV measures are associated with body mass index only in chronic AN. As HRV measures are significantly different between acute and chronic AN, we propose that HRV analysis might provide additional data in clinical practice.

Complexity of heartbeat interval series in young healthy trained and untrained men

The origin of heart rate variability (HRV) is largely in parasympathetic activity. The direct influence of sympathetic activity and other control mechanisms, especially at an increased HR, is not well understood. The objectives of the study were to investigate the influence of increasing HR on the properties of heartbeat interval (RR) series in young healthy subjects. ECG was recorded in 9 trained and 11 untrained young men during supine rest, standing, incremental running exercise and relaxation. During exercise, a breath-to-breath gas exchange was monitored. The RR time series analysis included the spectral analysis, detrended fluctuations analysis method and sample entropy (SampEn) calculation. During exercise, spectral powers were reduced dramatically in both groups. The dependence of short-term scaling exponent (alpha(1)) on the RR included a characteristic maximum, while SampEn for the same value of the RR had a minimum. The value of HR corresponding to the maximum of alpha(1) and minimum of SampEn (IHR) corresponded to the intrinsic HR obtained by an autonomic blockade. In trained subjects, the curves alpha(1) versus RR and SampEn versus RR were moved toward larger RR, compared with control. For HR values higher than IHR, alpha(1) decreased and SampEn increased. These results reveal that the complexity of the heart rhythm above intrinsic HR decreases with an increase in HR. We suggest that at the highest HR intrinsic heart control is reflected in the heart rhythm. We point out the possibility of developing a new non-invasive method for the determination of intrinsic HR from the curve alpha(1) versus RR.

Correlation properties of heartbeat dynamics

In this study, we investigate correlation properties of fluctuations in heart interbeat (RR) time series in a broad range of physiological and pathological conditions. Using detrended fluctuation analysis (DFA) method we determined short-term (α1) and long-term (α2) scaling exponent. In addition, we calculated standard deviation of RR intervals (SDRR) as the simplest variability measure. We found that the difference between α1 and α2 is related to RR interval length. At the shortest RR intervals, which correspond to extreme physiological and pathological conditions, we found the highest reduction of variability and the biggest difference between scaling exponents. In this case, DFA reveals a white noise over short scales (α1 about 0.5) and strongly correlated noise over large scales (α2 about 1.5). With an increase in RR interval, accompanied by increased variability (increase in parasympathetic control), the difference between α1 and α2 decreases. The difference between scaling exponents disappeared in a state of efficient autonomic control. We suggest that the complexity in heart rhythm is achieved through coupling between intrinsically controlled heart rhythm and autonomic control, and that the model of stochastic resonance mechanism could be applied to this system.

Correlation Properties and Regularity of Heart Period Time Series: Influence of Posture and Heart Disease

Abstract: Nonlinear measures of heart period variability (HPV) were determined in supine rest and standing posture in children and young adults with heart disease and compared to the control. The aim was to study influence of posture and heart disease on heart period dynamics. It was found that standing increases short‐term scaling exponent in all subjects and decreases sample entropy in healthy and less severely ill patients. Severely ill patients compared to control have higher short‐term scaling exponent in supine. Changes in nonlinear measures of HPV upon standing are more pronounced in healthy than diseased subjects due to different vagal activity.

Effect of γ-irradiation on Microtubule Assembly in Vitro

Microtubular protein was exposed to gamma-radiation from 500 to 1000 Gy, Within that dose range its polymerization ability was decreased by 20-60 per cent when samples were irradiated in assembled state, and by 40-75 per cent when irradiated in unassembled state. Microtubules assembled from irradiated subunits were shorter and of more uniform lengths than control microtubules. For the dose of 1000 Gy the mean length and its standard deviation were reduced to about one-half of the values of the control.

Two types of cardiovascular autonomic regulatory responses to the war

In war, many people are not directly hurt, but they feel fear of various intensities for a long time. The response of the cardiovascular autonomic system to these conditions, produced by the bombardment of Yugoslav cities by NATO for 78 days in spring 1999, was investigated. A group of 12 active medical professionals was examined 1 and 5 weeks after the beginning of bombardment and 10 days after the end of bombardment by spectral analysis of heart rate variability. Two types of cardiovascular autonomic responses emerged. One type included a fast reaction and then adaptation; the increased activity of symphaticus compared to parasymphaticus in the beginning of war was followed by a balance of these activities after 1 month and a small increase immediately after the end of bombardment. The other type did not show an increased activity of symphaticus compared to parasymphaticus during the war, but at the end of bombardment the prevalence of vagal tone was observed.