The influence of exercise intensity on the power spectrum of heart rate variability

Summary The power spectral analysis of R-R interval variability (RRV) has been estimated by means of an autoregressive method in seven sedentary males at rest, during steady-state cycle exercise at 21 percent maximal oxygen uptake. (% V _{O 2max}), SEM 2%, 49% V_{O 2max}, SEM 2% and 70% V_{O 2max}, SEM 2% and during recovery. The RRV, i.e. the absolute power of the spectrum, decreased 10, 100 and 500 times in the three exercise intensities, returning to resting value during recovery. In the RRV power spectrum three components have been identified: (1) high frequency peak (HF), central frequency about 0.24 Hz at rest and recovery, and 0.28 Hz, SEM 0.02, 0.37 Hz, SEM 0.03 and 0.48 Hz, SEM 0.06 during the three exercise intensities, respectively; (2) low frequency peak (LF), central frequency about 0.1 Hz independent of the metabolic state; (3) very low frequency component (VLF), <0.05 Hz, no peak observed. The HF peak power, as a percentage of the total power (HF%), averaged 16%, SEM 5% at rest and did not change during exercise, whereas during recovery it decreased to 5%–10%. The LF% and VLF% were about 50% and 35% at rest and during low exercise intensity, respectively. At higher intensities, LF% decreased to 16% and VLF% increased to 70%. During recovery a return to resting values occurred. The HF component may reflect the increased respiratory rate and the LF peak changes the resetting of the baroreceptor reflex with exercise. The hypothesis is made that VLF fluctuations in heart rate might be partially mediated by the sympathetic system.     

Effect of magnitopuncture on sympathetic and parasympathetic nerve activities in healthy drivers--assessment by power spectrum analysis of heart rate variability

The object of this study was to assess the effects of magnitopuncture applied to Dazhui (DU14) point and Neiguan (PC6) points on sympathetic and parasympathetic nerve activities by power spectrum analysis of heart rate variability in healthy drivers during simulated driving. Using power spectrum analysis, the low frequency (LF) and high frequency (HF) components of heart rate variability can be calculated reflecting the sympathetic and parasympathetic activity. The 40 healthy male subjects were randomly divided into two groups: A (study group) and B (control group). All subjects were required to be well rested before the experiment. The subjects of both groups were required to perform a simulated driving task for 3 h. During the driving, magnitopunctures were applied to the DU14 and PC6 points for A while the subject performed the task for 2.5 h, and for B magnitopunctures were applied to non-acupuncture points which were 1.5 cm away from the two acupuncture points respectively over the same time. Subjective response to a questionnaire was obtained after the simulated task in the two groups. At the end of the driving task the LF component in normalized units (NU) had decreased significantly (P<0.05) indicating a reduced sympathetic nerve activity and the HF component (NU) increased significantly (P<0.05) indicating a increased parasympathetic nerve activity for A compared with pre-stimulation while for B no significant differences were observed. There were significant group differences in LF (NU), HF (NU) and LF:HF at the end of the driving task (P<0.05). It was concluded that a modulating effect of magnitopuncture on sympathetic and parasympathetic nerve activities in healthy subjects was associated with the acupuncture points. The findings represent physiological evidence that magnitopuncture may reduce mental fatigue in healthy drivers. Electronic Publication     

Alpha-adrenoceptor gene variants and autonomic nervous system function in a young healthy Japanese population

α_1A-adrenergic receptor (α_1A-AR) regulates the cardiac and peripheral vascular system through sympathetic activation, and α_2A-AR and α_2C-AR subtypes are essential for presynaptic feedback regulation of catecholamine release from the central and peripheral sympathetic nerve. Genetic variations in each human α-AR subtype gene have been identified and have been implicated in hypertension and cardiovascular disease. It is not yet clear whether these genetic variations actually have an effect on sympatho-vagal modulation. The aim of the present study was to evaluate the relation between the five representative genetic polymorphisms of α-AR subtypes (Arg347Cys of α_1A-AR; C-1291G, Asn251Lys, and DraI RFLP of α_2A-AR; and Del322–325 of α_2C-AR) and autonomic nervous system (ANS) function in young and healthy Japanese males. One hundred forty-nine subjects were genotyped for each α-AR polymorphism, and underwent evaluation of ANS function by power spectral analysis of heart rate variability (HRV) during supine rest and in a standing position. In a supine position, the α_1A-AR 347Cys allele was significantly associated with lower HRV sympathetic index (normalized low frequency power [LF(%)] and LF:HF ratio) and higher HRV parasympathetic index [HF(%)]. Meanwhile, subjects with the α_2C-AR Del322–325 allele had markedly higher LF(%) and LF:HF ratio and lower HF(%) than noncarriers. Thus, the α_1A-AR and α_2C-AR genetic variations influence sympatho-vagal balance even in young and healthy normotensive states, which could be postulated to constitute an intermediate phenotype for future pathological episodes of various ANS dysfunction-related diseases.     

Effect of low-dose endurance training on heart rate variability at rest and during an incremental maximal exercise test

We evaluated the effects of low-dose endurance training on autonomic HR control. We assessed the heart rate variability (HRV) of 11 untrained male subjects (36.8 +/- 7.2 years) at rest and during an incremental maximal aerobic exercise test prior to a 7-week preparatory period and prior to and following a 14-week endurance training period, including a low to high intensity exercise session twice a week. Total (0.04-1.2 Hz), low (0.04-0.15 Hz) and high (0.15-1.2 Hz) frequency power of HRV were computed by short-time Fourier transform. The preparatory period induced no change in aerobic power or HRV. The endurance training period increased peak aerobic power by 12% (P < 0.001), decreased the HR (P < 0.01) and increased all HRV indices (P < 0.05-0.01) at absolute submaximal exercise intensities, but not at rest. In conclusion, low-dose endurance training enhanced vagal control during exercise, but did not alter resting vagal HR control.     

The impact of unemployment on heart rate variability: The evidence from the Czech Republic

We examined the relationship between unemployment and heart rate variability (HRV) in a region of high unemployment in the Czech Republic. The study involved 21 involuntarily unemployed and 21 employed men and women aged 30-49 years, matched on number of potentially confounding factors, including age, gender, type of job, health related behavior and body mass index. HRV was assessed in response to a modified orthostatic test. Compared with the employed group, unemployed participants had decreased high frequency HRV (p=0.018), lower root mean square of successive differences (p=0.050), and lower total spectral variability (p=0.022). These findings suggest that unemployment is a potential chronic stressor that may lead to suppression of vagal activity. This may be one mechanism linking unemployment with cardiovascular disease risk.     

Heart rate variability after prolonged spaceflights

Astronauts returning from spaceflight often experience post-flight orthostatic intolerance. This study was designed to determine whether cosmonauts with post-flight syncope could be distinguished from those with no post-flight syncope. The autonomic function was determined in a group of ten subjects, with no previous history of syncope, during a stand test before and after a long-term spaceflight (90 to 198 days). Heart rate (HR) and systolic blood pressure (SBP) were measured beat-by-beat, pre- and post-flight and the spontaneous baroreflex sensitivity and HR variability were studied. Individuals were categorized according to their ability to remain standing for 5 min the day after landing. Three of the ten cosmonauts failed to finish the standing test performed the day after landing (non-finishers). The spontaneous baroreflex slope was reduced in both groups after the spaceflight. The non-finisher group had a lower SBP (P<0.05) at rest in pre-flight tests than the group that completed the test (finisher group). The non-finisher group also had higher indicators of parasympathetic activity when supine, both pre- and post-flight, but this difference disappeared with standing. At the end of the stand test, SBP and HR were lower in non-finisher cosmonauts than the finishers, while HR did not increase compared to early measurements in the stand test of the finisher group. These results suggest an impairment in autonomic control of HR, which might contribute to the fainting response. Electronic Publication     

Mitigating the effect of non-stationarity in spectral analysis—An application to neonate heart rate analysis

In order to mitigate the effect of non-stationarity in frequency domain analysis of data, we propose a modification to the power spectral estimation, a widely used technique to characterize physiological signals. Spectral analysis requires partitioning data into smaller epochs determined by the desired frequency resolution. The modified approach proposed here involves dividing the data within each epoch by the standard deviation of the data for that epoch. We applied this modified approach to cardiac beat-to-beat interval data recorded from a newborn infant undergoing hypothermia treatment for birth asphyxia. The critically ill infant had episodes of tachyarrhythmia, distributed sporadically throughout the study, which affected the stationarity of the heart rate. Over the period of continuous heart rate recording, the infant's clinical course deteriorated progressively culminating in death. Coinciding with this clinical deterioration, the heart rate signal showed striking changes in both low-frequency and high-frequency power indicating significant impairment of the autonomic nervous system. The standard spectral approach failed to capture these phenomena because of the non-stationarity of the signal. Conversely, the modified approach proposed here captured the deteriorating physiology of the infant clearly.     

Orthostatic influence on heart rate and blood pressure variability in trained persons with tetraplegia

To examine the orthostatic influence on heart rate and blood pressure variability in persons with tetraplegia playing wheelchair basketball, ten trained persons with tetraplegia, ten untrained persons with tetraplegia, and ten able-bodied participated in this study. Spectrum analysis of the ECG R-R interval and blood-pressure on a beat-by-beat basis during head-up tilt 60 degrees sitting were performed. The ratio of the high frequency to total frequency (HF/TF) in the R-R interval decreased from supine (0.5 +/- 0.2) to sitting (0.3 +/- 0.2), and the low frequency (LF) power in systolic blood pressure increased from 4.7 +/- 9.1 to 15.0 +/- 13.1 mmHg(2) only in the untrained persons with tetraplegia (P < 0.01). The decrease in the HF/TF ratio in the untrained persons with tetraplegia indicates attenuated parasympathetic activity to the orthostatic challenge and the similar increase in LF power indicate that parasympathetic activity was reduced and sympathetic activity increased only in these persons. These results suggest that training enhances cardiovascular stability in tetraplegic subjects.     

Linear and nonlinear analysis of heart rate variability in healthy subjects and after acute myocardial infarction in patients

The objectives of this study were to evaluate and compare the use of linear and nonlinear methods for analysis of heart rate variability (HRV) in healthy subjects and in patients after acute myocardial infarction (AMI). Heart rate (HR) was recorded for 15 min in the supine position in 10 patients with AMI taking β-blockers (aged 57 ± 9 years) and in 11 healthy subjects (aged 53 ± 4 years). HRV was analyzed in the time domain (RMSSD and RMSM), the frequency domain using low- and high-frequency bands in normalized units (nu; LFnu and HFnu) and the LF/HF ratio and approximate entropy (ApEn) were determined. There was a correlation (P < 0.05) of RMSSD, RMSM, LFnu, HFnu, and the LF/HF ratio index with the ApEn of the AMI group on the 2nd (r = 0.87, 0.65, 0.72, 0.72, and 0.64) and 7th day (r = 0.88, 0.70, 0.69, 0.69, and 0.87) and of the healthy group (r = 0.63, 0.71, 0.63, 0.63, and 0.74), respectively. The median HRV indexes of the AMI group on the 2nd and 7th day differed from the healthy group (P < 0.05): RMSSD = 10.37, 19.95, 24.81; RMSM = 23.47, 31.96, 43.79; LFnu = 0.79, 0.79, 0.62; HFnu = 0.20, 0.20, 0.37; LF/HF ratio = 3.87, 3.94, 1.65; ApEn = 1.01, 1.24, 1.31, respectively. There was agreement between the methods, suggesting that these have the same power to evaluate autonomic modulation of HR in both AMI patients and healthy subjects. AMI contributed to a reduction in cardiac signal irregularity, higher sympathetic modulation and lower vagal modulation.     

Body position affects the power spectrum of heart rate variability during dynamic exercise

Summary The power spectrum analysis of R-R interval variability (RRV) has been estimated by means of an autoregressive method in six men in supine (S) and sitting (C) postures at rest and during steady-state cycle exercise at about 14010, 28%, 45%, 67% of the maximal oxygen consumption (% VO_2max). The total power of RRV decreased exponentially as a function of exercise intensity in a similar way in both postures. Three components were recognized in the power spectra: firstly, a high frequency peak (HF), an expression of respiratory arrhythmia, the central frequency (f _central) of which increased in both S and C from a resting value of about 0.26 Hz to 0.42 Hz at 67% VO_2max; secondly, a low frequency peak (LF) related to arterial pressure control, the f _central of which remained constant at 0.1 Hz in C, whereas in S above 28% VO_2max decreased to 0.07 Hz; and thirdly, a very low frequency component (VLF; less than 0.05 Hz, no f _central). The power of the three components (as a percentage of the total power) depended on the body posture and the metabolic demand. HF% at rest was 30.3 (SEM 6.6) % in S and 5.0 (SEM 0.8) % in C. During exercise HF% decreased by about 30% in S and increased to 19.7 (SEM 5.5) % at 28% VO_2max in C. LF% was lower in S than in C at rest [31.6 (SEM 5.7) % vs 44.9 (SEM 6.4) %; P<0.05], remaining constant up to 28% VO_2max. At the highest intenstities it increased to 54.0 (SEM 15.6) % in S whereas in C it decreased to 8.5 (SEM 1.6) %. VLF represented the remaining power and the change was in the opposite direction to LF. The changes in power spectrum distribution of RRV during exercise depended on the intensity and the body posture. In particular, the LF peak showed opposite trends in S and C tasks, thus suggesting a different readjustment of arterial pressure control mechanisms in relation to the blood distribution and peripheral resistances.     

Autonomic nervous control of heart rate during blood-flow restricted exercise in man

Summary Power spectra of instantaneous heart rate (f _c) allows the estimation of the contribution of sympathetic and parasympathetic control of f _c during steady-state conditions. The present study was designed to examine autonomic control of f _c as influenced by normal dynamic leg exercise and by ischemic leg exercise. Eight subjects performed supine cycle ergometry at 30% of their control peak work rate, with and without blood-flow restriction. Blood-flow restriction was induced by exposing the exercising legs to a supra-atmospheric pressure of 6.7 kPa (leg positive pressure; LPP). The exercise responses of arterial pressure and f _c increased (P<0.05) by LPP exposure. The exaggerated pressor response may be attributed to a chemoreflex drive originating in the ischemic muscles. Exposure to LPP during exercise also produced a significant decrease in parasympathetically mediated high frequency (HF; 0.15-1.00 Hz) fluctuation of f _c, as indicated by a decrease (P<0.05) in percent HF power compared to the control exercise level. During LPP exercise, the sympathetically mediated very low frequency (VLF; 0–0.05 Hz) fluctuation of f _c increased, as indicated by an increase (P<0.05) in percent VLF power above control exercise levels. Both LPP and control exercise conditions decreased (P<0.05) power in all frequency ranges of interest compared to their respective resting conditions. The results suggest that the increase in f _c associated with normal dynamic exercise was mediated predominantly by parasympathetic withdrawal, whereas the exaggerated f _c response during ischemic exercise resulted from a combination of cardiac sympathetic drive and parasympathetic withdrawal. The increase in sympathetic activity is attributable to a muscle chemoreflex drive, which also may have attenuated parasympathetic activity by reciprocal inhibition. Alternatively, augmented central command mediated parasympathetic withdrawal during ischemic exercise.     

心肌缺血中心率变异性的符号动力学分析

目的基于心率变异性（HRV）研究,分析心肌缺血发生过程中心率的动态变化特性以探讨自主神经活动的变化和影响。方法从长时ST段一T波终点（Long．term ST-T）数据库中,选取13个记录中共193个心肌缺血时段及其前后各5min的对照时段。选用符号动力学方法分析不同时段的RR间期序列,利用重复测量的方差分析对缺血前和缺血中以及缺血后结果进行检验;以线性拟合的方式,分析心肌缺血时自主神经响应模式随缺血持续时间的分布。结果较之心肌缺血前后,心肌缺血过程中0v％的显著增加和2UV％的显著减小反映了交感神经活性的增加和迷走神经活性的减弱,但这种自主神经平衡向交感神经更占优移动的趋势随着缺血持续时间的延长而减小,提示可能存在的对心脏的保护作用。结论符号动力学分析为研究心肌缺血中自主神经的调制提供了一个敏感的工具。     

微机心率变异功率谱测量系统

本文报道在在IBM—PC/XT微型计算机上开发的心率变异(HRV)功率谱测量系统技术,用于无创伤性定量测量心脏植物性神经的活动。系统软件采用菜单方式设计,用8088汇编语言和FORTRAN语言编写。系统在程控下,通过对连续心电信号(ECG)的R—R间期采样计数,并利用傅利叶变换的数字技术,计算出心率变异的时域和频域参数。     

Heart rate and autonomic response to stress after experimental induction of worry versus relaxation in healthy,high-worry,and generalized anxiety disorder individuals

Generalized anxiety disorder (GAD) is the most commonly occurring anxiety disorder and has been related to cardiovascular morbidity such as cardiac ischemia, sudden cardiac death, and myocardial infarction. Both GAD and its cardinal symptom – worry – have been shown to promote muted physiological reactivity in response to laboratory and ecological stressors. Importantly, no study to date has examined the concurrent and relative contributions of trait and state worry within healthy controls, (non-clinical) high trait-worry controls, and GAD participants. The present study examined heart rate (HR), respiratory sinus arrhythmia (RSA), and salivary alpha-amylase (sAA) responses to laboratory stress during and following the experimental induction of worry versus relaxation in healthy controls (n = 42), high trait worriers (n = 33) and participants with GAD (n = 76). All groups exhibited increased HR and decreased RSA in response to the stressor, with no differences by condition. Baseline sAA significantly moderated HR and RSA reactivity, such that higher sAA predicted greater increases in HR and decreases in RSA. There was a significant group by baseline sAA interaction such that in GAD, higher baseline sAA predicted decreased change in sAA during stress, whereas higher baseline sAA predicted greater sAA change in healthy controls. High-worry controls fell non-significantly between these groups. The present study provides additional evidence for the effect of worry on diminished HR stress response and points to possible suppression of adrenergic sympathetic stress responses in GAD.     

Correlation between heart rate variability and pulmonary function adjusted by confounding factors in healthy adults

The autonomic nervous system maintains homeostasis, which is the state of balance in the body. That balance can be determined simply and noninvasively by evaluating heart rate variability (HRV). However, independently of autonomic control of the heart, HRV can be influenced by other factors, such as respiratory parameters. Little is known about the relationship between HRV and spirometric indices. In this study, our objective was to determine whether HRV correlates with spirometric indices in adults without cardiopulmonary disease, considering the main confounders (e.g., smoking and physical inactivity). In a sample of 119 asymptomatic adults (age 20-80 years), we evaluated forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV₁). We evaluated resting HRV indices within a 5-min window in the middle of a 10-min recording period, thereafter analyzing time and frequency domains. To evaluate daily physical activity, we instructed participants to use a triaxial accelerometer for 7 days. Physical inactivity was defined as <150 min/week of moderate to intense physical activity. We found that FVC and FEV₁, respectively, correlated significantly with the following aspects of the RR interval: standard deviation of the RR intervals (r =0.31 and 0.35), low-frequency component (r =0.38 and 0.40), and Poincaré plot SD2 (r =0.34 and 0.36). Multivariate regression analysis, adjusted for age, sex, smoking, physical inactivity, and cardiovascular risk, identified the SD2 and dyslipidemia as independent predictors of FVC and FEV₁ (R ²=0.125 and 0.180, respectively, for both). We conclude that pulmonary function is influenced by autonomic control of cardiovascular function, independently of the main confounders.     

Effect of relaxation training on cardiac parasympathetic tone

To examine the hypothesis that the relaxation response is associated with an increase in cardiac parasympathetic tone, the frequency components of heart rate variability during relaxation training were investigated in 16 college students. Electrocardiograms and pneumograms were recorded during a 5-min baseline period followed by three successive 5-min sessions of the autogenic training (relaxation) or by the same periods of quiet rest (control), while subjects breathed synchronously with a visual pacemaker (0.25 Hz). Although neither the magnitude nor the frequeney of respiration showed a significant difference between relaxation and control, the amplitude of the high-frequency component of heart rate variability increased only during relaxation (p= .008). There was no significant difference in the ratio of the low-frequency (0.04–0.15 Hz) to the high-frequency amplitudes. The increased high-frequency amplitude without changes in the respiratory parameters indicates enhanced cardiac parasympathetic tone. Thus, our results support the initial hypothesis of this study. Enhanced cardiac parasympathetic tone may explain an important mechanism underlying the beneficial effect of the relaxation response.     

Stability of heart rate variability indices reflecting parasympathetic activity

Heart rate variability (HRV) is a measure of autonomic influences on heart rate that has frequently been used as a transsituationally consistent biomarker for cardiovascular health and emotional or cognitive functions. The psychometric properties of HRV however remain unclear. In the present study, we examined the reliability and temporal stability of parasympathetic HRV measures and estimated the portion of variance explained by transsituationally consistent trait variance and by effects of the situation and person-situation interaction with structural equation modeling. The results show good reliability of indices reflecting central parasympathetic control over heart rate and that about 40% of the variance of a single HRV measurement can be explained by effects of the situation and person-situation interaction. An aggregation across at least two measurements may be recommended when using HRV as a transsituationally consistent biomarker or trait.     

Baroreflex sensitivity assessment and heart rate variability: relation to maneuver and technique

In the present study, we examined two baroreflex sensitivity (BRS) issues that remain uncertain: the differences among diverse BRS assessment techniques and the association between BRS and vagal outflow. Accordingly, the electrocardiogram and non-invasive arterial pressure were recorded in 27 healthy subjects, during supine with and without controlled breathing, standing, exercise, and recovery conditions. Vagal outflow was estimated by heart rate variability indexes, whereas BRS was computed by alpha-coefficient, transfer function, complex demodulation in low- and high-frequency bands, and by sequence technique. Our results indicated that only supine maneuvers showed significantly greater BRS values over the high frequency than in the low-frequency band. For maneuvers at the same frequency region, supine conditions presented a larger number of significant differences among techniques. The plots between BRS and vagal measures depicted a funnel-shaped relationship with significant log–log correlations (r=0.880–0.958). Very short latencies between systolic pressure and RR interval series in high-frequency band and strong log–log correlations between frequency bands were found. Higher variability among different baroreflex measurements was associated with higher level of vagal outflow. Methodological assumptions for each technique seem affected by non-baroreflex variation sources, and a modified responsiveness of vagal motoneurons due to distinct stimulation levels for each maneuver was suggested. Thus, highest vagal outflows corresponded to greatest BRS values, with maximum respiratory effect for the high-frequency band values. In conclusion, BRS values and differences across the tested techniques were strongly related to the vagal outflow induced by the maneuvers.     

Autonomic interactions and chronotropic control of the heart: Heart period versus heart rate

Autonomic control of the heart varies more linearly with heart period than with rate. Relative linearity confers a greater independence of basal autonomic activation and heart period changes. Thus, heart period appears to be more appropriate for characterizing cardiac phenomena such as autonomic interactions that involve significant baseline shifts. Simulated and published empirical data were used to demonstrate the importance of the chronotropic metric for characterizing autonomic interactions. Simulations revealed a significant autonomic interaction when heart rate, but not heart period, was the chronotropic metric. Published heart rate data also show a substantial autonomic interaction, whereas heart period data do not. These findings suggest that the choice of chronotropic metric can overstate the extent of autonomic interactions on cardiac chronotropic function.     

Modulation of heart rate variability by estrogen in young women undergoing induction of ovulation

Estrogens are involved in the modulation of the cardiovascular system, yet their effects in young women remains largely unknown. Women who undergo ovulation induction treatments attain extremely high estrogen concentrations during a very short time period. The aim of the present study was to evaluate the effects of an acute increase in estrogens on the autonomic nervous system modulation of heart rate variability (HRV). A total of 27 women undergoing ovulation induction and 14 normally menstruating women were prospectively studied. HRV was assessed during nadir and peak estrogen using time domain and power spectral density analyses. A significant increase in high-frequency spectral power (243 ± 77 vs. 188 ± 73 ms²/Hz, P < 0.01) with a significant decrease in the ratio of low to high-frequency power was observed during estrogen peak in women undergoing induction of ovulation. The acute increase in estrogen in women undergoing ovulation induction was associated with vagal activation and altered sympathovagal balance.