ECG artifacts and heart period variability: Don't miss a beat!

The impact of artifacts on estimates of heart period variability were evaluated by modeling the effects of missed R-waves and spurious R-wave detections in actual and simulated heart period series. Results revealed that even a single artifact, occurring within a 128-s interbeat interval series, can impart substantial spurious variance into all commonly analyzed frequency bands, including that associated with respiratory sinus arrhythmia. In fact, the spurious variance introduced by a single artifact may be greater than that associated with true basal heart period variability and can far exceed typical effect sizes in psychophysiological studies. The effects of artifacts are not related to a specific analytical method and are apparent in both frequency and time domain analyses. Results emphasize the importance of artifact detection and resolution for studies of heart period variability.     

Heart period sensitivity to forced oscillations in ventilatory pressure

Heart rate variability analysis is a dynamic method to estimate the autonomic control over the cardiac cycle. Although dysfunction in this control system may appear spontaneously, other deficits may require provocation of the system. In this article we describe a non-invasive method to perturb the autonomic influences on the cardiac cycle. We recorded the ECG and respiratory pressure of ten healthy volunteers while introducing a random forced oscillation pressure wave onto spontaneous respiration. The heart period time series was determined and the power spectra for the 0.05–0.15, 0.15–0.3 and 0.05–0.4 Hz bands were calculated. The random input did not alter mean heart rate. However, the segments with the forced oscillation input demonstrated, on average, a tenfold increase in spectral power averaged across all subjects, with a maximum observed effect of 100-fold increase in power. This increase in power correlated with the respiratory frequency. This study demonstrates that random noise ventilation, such as used in respiratory forced oscillation impedance estimates, significantly alters the autonomic input to cardiac cycle variability in wake subjects.     

Heritability of cardiac vagal control in 24‐h heart rate variability recordings: Influence of ceiling effects at low heart rates

This study estimated the heritability of 24‐h heart rate variability (HRV) measures, while considering ceiling effects on HRV at low heart rates during the night. HRV was indexed by the standard deviation of all valid interbeat intervals (SDNN), the root mean square of differences between valid, successive interbeat intervals (RMSSD), and peak‐valley respiratory sinus arrhythmia (pvRSA). Sleep and waking levels of cardiac vagal control were assessed in 1,003 twins and 285 of their non‐twin siblings. Comparable heritability estimates were found for SDNN (46%–53%), RMSSD (49%–54%), and pvRSA (48%–57%) during the day and night. A nighttime ceiling effect was revealed in 10.7% of participants by a quadratic relationship between mean pvRSA and the interbeat interval. Excluding these participants did not change the heritability estimates. The genetic factors influencing ambulatory pvRSA, RMSSD, and SDNN largely overlap. These results suggest that gene‐finding studies may pool the different cardiac vagal indices and that exclusion of participants with low heart rates is not required.     

Time domain, geometrical and frequency domain analysis of cardiac vagal outflow: effects of various respiratory patterns.

The purpose of this study was to compare the applicability of four different measures of heart rate variability (HRV) in the assessment of cardiac vagal outflow, with special reference to the effect of breathing pattern. The anticholinergic effects of an intravenous glycopyrrolate infusion (5 microg x kg(-1) x h(-1) for 2 h) during spontaneous and controlled (15 min(-1)) breathing rate were investigated in eight volunteers, and the effects of different fixed breathing rates (6-15-24 min(-1)) and hyperventilation in 12 subjects. Cardiac vagal activity was assessed by ECG recordings in which the following measures of HRV were computed: the high-frequency (HF) spectral component, the instantaneous RR interval (RRI) variability (SD1) analysed from the Poincaré plots, the percentage of differences between successive RRIs greater than 50 ms (pNN50), and the square root of the mean squared differences of successive RRIs (RMSSD). On average, glycopyrrolate reduced the HF spectral component by 99.8%, SD1 by 91.3%, pNN50 by 100% and RMSSD by 97.0%. The change of breathing pattern from controlled to spontaneous decreased significantly the HF component and pNN50, but did not affect SD1 or RMSSD. Rapid breathing rate (24 min(-1)) decreased the HF component, but had no effects on the other measures. A controlled breathing rate is needed for a reliable assessment of cardiac vagal outflow by the spectral analysis technique. The quantitative geometrical analysis of short-term RRI variability from the Poincaré plots and the time domain measure RMSSD were not significantly affected by changes in the breathing rate, suggesting that these indices are more suitable for the measurement of cardiac vagal outflow during the 'free-running' ambulatory conditions.     

呼吸性窦性心律不齐定量分析研究进展

呼吸性窦性心律不齐(RSA)是一种心率随呼吸周期性波动的生理现象。近年来在临床上常被用作心迷走神经张力的非侵入式检测指征。但它的定量分析受呼吸参数、体位、体动等因素影响,使得至今还没有建立起一个通用的定量分析标准和校准方法。本文主要介绍了五种常见的定量分析方法:相邻R-R间期差值均方根(RMSSD)法、峰谷(pvRSA)法、逐次呼吸余弦拟合(cosinor fitting)法、谱分析(spectral analysis)法以及时频联合分析(JTFA)法;同时本文介绍了定量分析中常见的呼吸参数校准方法:引导呼吸法、协方差分析统计学控制法、相对呼吸参数残余量法以及潮气量修正法。最后,对如何解决现阶段RSA定量估计中存在的问题做了展望。     

Baroreceptor reflex sensitivity in human neonates: the effect of postmenstrual age

We performed a cross-sectional study in human infants to determine if indices of R–R interval variability, systolic blood pressure (SBP) variability, and baroreceptor reflex sensitivity change with postmenstrual age (PMA: gestational age + postnatal age). The electrocardiogram, arterial SBP and respiration were recorded in clinically stable infants (PMA, 28–42 weeks) in the quiet sleep state in the first days after birth. (Cross-)spectral analyses of R–R interval series and SBP series were performed to calculate the power of low-frequency (LF, indicating baroreceptor reflex activity, 0.04–0.15 Hz) and high-frequency (HF, indicating parasympathetic activity, individualized between the p-10 and p-90 values of respiratory frequency) fluctuations, and transfer function phase and gain. The mean R–R interval, and LF and HF spectral powers of R–R interval series increased with PMA. The mean SBP increased with PMA, but not the LF and HF spectral powers of SBP series. In the LF range, cross-spectral analysis showed high coherence values (> 0.5) with a consistent negative phase shift between R–R interval and SBP, indicating a ∼3 s lag in R–R interval changes in relation to SBP. Baroreceptor reflex sensitivity, calculated from LF transfer gain, increased significantly with PMA, from 5 (preterm) to 15 ms mmHg⁻¹ (term). Baroreceptor reflex sensitivity correlated significantly with the (LF and) HF spectral powers of R–R interval series, but not with the LF and HF spectral powers of SBP series. The principal conclusions are that baroreceptor reflex sensitivity and spectral power in R–R interval series increase in parallel with PMA, suggesting a progressive vagal maturation with PMA.     

The ontogeny of heart period patterning in the rat

Heart period, heart period variability, and respiration frequency were observed and compared longitudinally in rats from birth to 24 days of age. In addition, V, a quantification of respiratory sinus arrhythmia derived via spectral analysis of the heart period pattern, was observed. It was hypothesized that these heart period parameters would be differentially sensitive to the neural control of the heart and would, therefore, reflect different developmental patterns. Heart period and heart period variability indicated developmental patterns consistent with the literature. The ontogeny of V suggests that it may represent the developmental pattern of tonic vagal influences on the heart.     

Influence of respiratory activity on the cardiac response pattern to mental effort

A group of 32 healthy adult volunteers completed three blocks of a reaction time task that varied in the degree of controlled processing load. A rest period preceded each of the task blocks. The task blocks were presented in the order of either increasing or decreasing cognitive load. For each of the six periods, mean values and spectral measures of heart rate and respiration variability were calculated. The spectral measures were obtained for three different frequency bands. Differences between the cardiac measures of the task and preceding rest periods were compared with respect to differences in task load and the order of task presentation. All comparisons were carried out while adjusting for respiratory variability in the corresponding frequency band. The frequency band in which task load-related changes in heart rate variability became manifest appeared to be dependent on the individual's breathing pattern.     

Effect of constant and intermittent vagal stimulation on the heart rate and heart rate variability in rabbits

To examine whether the high-frequency (HF) component of heart rate variability (HRV) reflects fluctuation or tonic level of vagal outflow, we investigated the effects of vagal efferent nerve stimulation (VS) on the heart rate and HRV in anesthetized open-chest rabbit under artificial respiration at a rate of 52 breaths/min (0.86 Hz). A power spectral analysis was performed at baseline and during VS (stimuli at 2 ms, 1-10 V and 5-25 Hz). VS was applied using two different patterns. The first was constant VS; continuous stimulation at graded frequency or voltage to simulate changes in the level of vagal "tone." The second pattern was intermittent VS; stimulation at 0.5 Hz of on-off cycle to simulate fluctuations in vagal efferent activity. The power spectrum at baseline showed a single narrow component at 0.86 Hz, identical to respiration rate. Both the constant and intermittent VS prolonged RR interval. The amplitude of the component at 0.86 Hz remained unaffected by either the constant or intermittent VS, whereas the latter evoked a distinct narrow component at 0.5 Hz, reflecting the on-off cycle of intermittent VS. Our results suggest that the HF component of the power spectrum of HRV measures the magnitude of fluctuations of vagal input associated with respiratory modulation.     

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.     

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.     

Genetic influences on heart rate variability at rest and during stress

We tested whether the heritability of heart rate variability (HRV) under stress is different from rest and its dependency on ethnicity or gender. HRV indexed by root mean square of successive differences (RMSSD) and high-frequency (HF) power was measured at rest and during 3 stressors in 427 European and 308 African American twins. No ethnic or gender differences were found for any measures. There was a nonsignificant increase in heritability of RMSSD (from 0.48 to 0.58) and HF (from 0.50 to 0.58) under stress. Up to 81% and 60% of the heritabilities of RMSSD and HF under stress could be attributed to genes influencing rest levels. The heritabilities due to genes expressed under stress were 0.11 for RMSSD and 0.23 for HF. The findings suggest that, independent of ethnicity and gender, HRV regulation at rest and under stress is largely influenced by the same genes with a small but significant contribution of stress-specific genetic effects.     

New method for the assessment of neonatal respiratory sinus arrhythmia

Respiratory sinus arrhythmia (RSA) is sometimes hard to identify in neonates, in particular when the respiratory rate approaches half the mean heart rate. It is shown that the analysis of the cardiac event series may reveal RSA in cases where traditional heart rate variability analysis methods fail. A signal analysis approach to event series processing is reviewed. The validation of the spectral analysis of event series is illustrated with the help of simulation results. In accordance with this approach, the event series spectrum of a neonate shows a RSA component at 0·9 Hz, which was in agreement with the frequency of the simultaneously measured respiration. The possible implications for research into the causes of sudden infant death syndrome (SIDS) are discussed.     

自主呼吸状态下光电容积脉搏波法对 健康人群心率变异性的分析

目的 分析自主呼吸状态下光电容积脉搏波法对健康人群心率变异性。方法 选取2016年10月~2018年9月我校本科生健康志愿者64名,所有志愿者均行心电图（ECG）及光电容积脉搏波法（PPG）检测,比较3 min的ECG与PPG检测HRV时的RR间期标准差（SDNN）、RR间期差值均方根值（RMSSD）、低频功率与标准化高频功率的比值（LF/HF）,并分析ECG与PPG检测的心率变异性参数的相关性与一致性。结果 64例志愿者3 min的ECG与PPG心率变异性参数比较,差异无统计学意义（P＞0.05）。经Pearson相关性分析发现,PPG检测SDNN、RMSSD、LF/HF与ECG各指标呈正相关（SDNN:r=0.999,P＜0.001;RMSSD:r=0.998,P＜0.001;LF/HF:r=0.998,P＜0.001）;经可靠性分析发现,PPG用于检测健康人群心率变异性与ECG一致性较高（ICC=0.915,95% CI:0.878~0.943,P＜0.001）。结论 健康人群自主呼吸状态下光电容积脉搏波法可用于心率变异性的检测,临床上有较好的应用价值。     

Changes in Heart Period, Heart-Period Variability, and a Spectral Analysis Estimate of Respiratory Sinus Arrhythmia in Response to Pharmacological Manipulations of the Baroreceptor Reflex in Cats

The purpose of the present study was to assess changes in heart period, heart-period variance (HPV), and respiratory sinus arrhythmia (RSA) during manipulations of the baroreceptor reflex in anesthetized cats. Hypertension, induced via phenylephrine infusion, reflexly increases parasym-pathetic activity and decreases sympathetic activity. Hypotension, produced by infusion of nitroprusside, leads to increased sympathetic activity and an inhibition of vagal influences on the heart. Specific autonomic contributions were assessed following administration of practolol, a β-adrenergic blocker, or atropine methyl nitrate. Spectral analysis was used to quantify the component of HPV associated with respiration, i.e., RSA, which is proposed to be sensitive to vagal influences on the heart. The respiratory component of HPV is described by a statistic, V?, which is the sum of the spectral densities of the heart period spectrum across the band of frequencies associated with normal respiration. Hypertension produced an increase in V? which was blocked by atropine infusion, but unaltered by beta-adrenergic blockade. Hypotension decreased V? to near-zero values which persisted after atropine and practolol infusion. These data suggest that V? is determined primarily by vagal factors. Correlations between V? and a previously used criterion measure of vagal tone further support this hypothesis. In contrast, heart period was sensitive to vagal and sympathetic factors. Heart period increases during hypertension were abolished with atropine and heart period decreases due to hypotension were eliminated by beta-adrenergic blockade. These responses are consistent with the notion that heart period is under the control of both sympathetic and vagal factors. Since in an anesthetized preparation RSA is the major source of heart-period variability, HPV responded similarly to V?. The results suggest that the spectral estimate of RSA, V?, is particularly sensitive to blood pressure induced reflexive changes in vagal efferent influences on the heart. It also appears that V? is less influenced by sympathetic factors than heart period, a commonly used estimate of vagal activity.     

The influence of monocular viewing on heart period variability.

Monocular viewing seems to differentially activate the cerebral hemispheres. Viewing with only one eye is accompanied by a relatively greater activation of the contralateral hemisphere. In this study heart periods were measured during binocular, left and right monocular viewing in 29 right-handed males. Power spectral analysis of heart period variability showed that the amount of heart rate fluctuations in the 0.07-0.13 Hz frequency range (the mid-frequency peak of the spectrum) depends on the viewing eye. Only viewing with the left eye produced a significant increase of the mid-frequency peak. It is known that sympathetic activity contributes strongly to the mid-frequency component of the heart rate. Therefore, it is concluded that the effects of left monocular viewing result in an increased sympathetic influence on the sinoatrial node.     

Spectral analysis of heart rate variability and respiration during sleep in cocaine-exposed neonates.

This study's objective was to examine the autonomic control of heart rate and respiration during the neonatal period in human infants with prenatal exposure to cocaine. Four-hour daytime recordings of the electrocardiogram (ECG) were obtained from 15 cocaine-exposed and 13 non-exposed full-term neonates at 2 weeks of age during quiet sleep (QS) and active sleep (AS). For each 1-min epoch of sleep, the power spectrum of the R-R intervals was computed from the ECG to obtain the total power (0-2 Hz), and spectral power in the high-frequency (HFP, 0.3-2 Hz), mid-frequency (MFP, 0.1-0.2 Hz), and low-frequency (LFP, 0.03-0.1 Hz) bands. Respiration was also monitored and processed using similar spectral analysis procedures. Cocaine-exposed neonates showed enhanced heart rate variability reflected by an increase in spectral power across all frequency bands. Spectral power in LFP and MFP was higher in cocaine-exposed neonates during both sleep states, but only in HFP during QS. There were no respiratory patterning differences between the groups to account for these findings. The index of sympathovagal balance (LFP + MFP)/HFP, showed no differences between the groups. We conclude that infants exposed to cocaine in utero show differences in the modulation of heart rate reflecting an increase in both vagal and sympathetic influences.     

Regional cerebral blood flow correlates with heart period and high-frequency heart period variability during working-memory tasks: Implications for the cortical and subcortical regulation of cardiac autonomic activity

The aim of the present study was to characterize the functional relationships between behaviorally evoked regional brain activation and cardiac autonomic activity in humans. Concurrent estimates of regional cerebral blood flow (rCBF; obtained by positron emission tomography), heart period, and high-frequency heart period variability (HF-HPV; an indicator of cardiac parasympathetic activity) were examined in 93 adults (aged 50-70 years) who performed a series of increasingly difficult working-memory tasks. Increased task difficulty resulted in decreased heart period (indicating cardioacceleration) and decreased HF-HPV (indicating decreased cardiac parasympathetic activity). Task-induced decreases in heart period and HF-HPV were associated with concurrent increases and decreases in rCBF to cortical and subcortical brain regions that are speculated to regulate cardiac autonomic activity during behavioral processes: the medial-prefrontal, insular, and anterior cingulate cortices, the amygdala-hippocampal complex, and the cerebellum. These findings replicate and extend a small number of functional neuroimaging studies that suggest an important role for both cortical and subcortical brain systems in human cardiac autonomic regulation.     

Spectral Parameters of Phrenic Nerve Activity in Mature Rats during Electrical Stimulation of Retrotrapezoid Nucleus

In mature rats, electrical stimulation of the retrotrapezoid nucleus increased the amplitude and frequency of the high-frequency peak in the firing spectrum of the phrenic nerve, while the amplitude of medium-frequency peak and the amplitude ratio of medium- to high-frequency peaks decreased. These changes in spectral parameters were associated with accelerated increase in central inspiratory activity, decreased amplitude of phrenic nerve firing, and increased frequency of respiratory rhythm. It is hypothesized that being a relay structure of central chemosensitive mechanism, the retrotrapezoid nucleus regulates parameters of medium- and high-frequency spectral peaks of efferent electrical activity in the respiratory center together with the dorsal and ventral respiratory groups.     

Quantifying electrocardiogram RT-RR variability interactions

A dynamic linear parametric model is designed to quantify the dependence of ventricular repolarisation duration variability on heart period changes and other immeasurable factors. The model analyses the beat-to-beat series of the RR duration and of the interval between R- and T-wave apexes (RT period). Directly from these two signals, a parametric identification procedure and spectral decomposition techniques allow RT variability to be divided into RR-related and RR-unrelated parts and allow the RT-RR transfer function to be calculated. RT variability is driven by RR changes at low frequency (LF, around 0.1 Hz) and high frequency (HF, at the respiratory rate), whereas, at very low frequencies, the RR-unrelated contribution to the total RT variability is remarkable. During tilt at LF the RR-related RT percentage power increases (p<0.02), the RR-unrelated RT percentage power remains unchanged, the gain of the RT-RR relationship largely increases (p<0.001), and the phase is not significantly modified. Both the RR-related and the RR-unrelated RT percentage powers at LF are not affected by controlled respiration, and an increase in the RT-RR gain at HF is observed (p<0.02). The proposed analysis may help to describe the regulation of the ventricular repolarisation process and to extract indexes quantifying the coupling between heart period and ventricular repolarisation interval changes.