Low‐frequency heart rate variability is related to the breath‐to‐breath variability in the respiratory pattern

Changes in heart rate variability (HRV) at “respiratory” frequencies (0.15–0.5 Hz) may result from changes in respiration rather than autonomic control. We now investigate if the differences in HRV power in the low‐frequency (LF) band (0.05–0.15 Hz, HRV_LF) can also be predicted by respiration variability, quantified by the fraction of tidal volume power in the LF (V_LF,n). Three experimental protocols were considered: paced breathing, mental effort tasks, and a repeated attentional task. Significant intra‐ and interindividual correlations were found between changes in HRV_LF and V_LF,n despite all subjects having a respiratory frequency above the LF band. Respiratory parameters (respiratory period, tidal volume, and V_LF,n) could predict up to 79% of HRV_LF differences in some cases. This suggests that respiratory variability is another mechanism of HRV_LF generation, which should be always monitored, assessed, and considered in the interpretation of HRV changes.     

Effects of high altitude acclimatization on heart rate variability in resting humans

The sympatho-vagal nerve interaction at the heart was studied by means of power spectrum analysis of heart rate variability in seven Caucasians (aged 27–35 years) in resting supine and sitting positions before and during 35 days of a sojourn at 5050 m above sea level (asl) and in six Sherpas (aged 22–30 years) at high altitude only. A high frequency peak (HF)-central frequency between 0.20 and 0.33 Hz, a low frequency peak (LF)-central frequency between 0.08 and 0.14 Hz, and a very low frequency component ( < 0.05 Hz), no peak observed, were found in the power spectrum in both positions and independent of altitude. The peak powers, as a percentage of the total power, were affected by both body position and altitude. At sea level the change from a supine to a sitting position yielded a decrease in percentage HF from 25 (SEM 1.9)% to 6.2 (SEM 1.5) % (P < 0.05) and a significant increase in the ratio between LF and HF powers (LF : HF) from 1.7 (SEM 0.4) to 6.9 (SEM 1.6). At altitude compared to sea level in the supine position, percentage HF decreased from 25% to 10.9 (SEM 1.0)% (P < 0.05) and the LF:HF ratio increased from 1.7 to 4.8 (SEM 0.7) (P < 0.05). No changes occurred at altitude in the sitting position either in the peak powers or in the LF:HF ratio, but the central frequency of HF peak increased significantly from 0.25 (SEM 0.02) Hz to 0.32 (SEM 0.01) Hz. In the Sherpas comparable results to the Caucasians were found in both body positions. The high LF:HF ratios observed at altitude in both body positions and groups would suggest that hypoxia caused a shift of sympatho-vagal nerve interaction at rest toward a dominance of the sympathetic system, which was found at sea level only in the sitting position. An acclimatization period of 10 days higher than 2850 m asl and 1 month at 5050 m asl did not modify the interactions of the autonomic systems.     

Resting state connectivity of the medial prefrontal cortex covaries with individual differences in high‐frequency heart rate variability

Resting high‐frequency heart rate variability (HF‐HRV) relates to cardiac vagal control and predicts individual differences in health and longevity, but its functional neural correlates are not well defined. The medial prefrontal cortex (mPFC) encompasses visceral control regions that are components of intrinsic networks of the brain, particularly the default mode network (DMN) and the salience network (SN). Might individual differences in resting HF‐HRV covary with resting state neural activity in the DMN and SN, particularly within the mPFC? This question was addressed using fMRI data from an eyes‐open, 5‐min rest period during which echoplanar brain imaging yielded BOLD time series. Independent component analysis yielded functional connectivity estimates defining the DMN and SN. HF‐HRV was measured in a rest period outside of the scanner. Midlife (52% female) adults were assessed in two studies (Study 1, N = 107; Study 2, N = 112). Neither overall DMN nor SN connectivity strength was related to HF‐HRV. However, HF‐HRV related to connectivity of one region within mPFC shared by the DMN and SN, namely, the perigenual anterior cingulate cortex, an area with connectivity to other regions involved in autonomic control. In sum, HF‐HRV does not seem directly related to global resting state activity of intrinsic brain networks, but rather to more localized connectivity. A mPFC region was of particular interest as connectivity related to HF‐HRV was shared by the DMN and SN. These findings may indicate a functional basis for the coordination of autonomic cardiac control with engagement and disengagement from the environment.     

Moderate alcohol intake is related to increased heart rate variability in young adults: Implications for health and well‐being

Epidemiological literature indicates that the relationship between alcohol consumption and health outcomes reflects a J‐shaped curve such that moderate alcohol consumption confers a protective effect in comparison to abstinence, while heavy consumption is associated with poorer health. While heart rate variability (HRV) may underpin the relationship between drinking and poor health in heavy drinkers, it is unclear whether HRV is increased in moderate, habitual drinkers relative to nonhabitual drinkers. HRV and drinking habits were assessed in 47 volunteers. Results supported hypotheses suggesting that moderate, habitual drinking increases HRV. Although not supported by a significant interaction between drinking group and sex, planned follow‐up analysis also revealed that these findings may be specific to males. Regardless, results highlight HRV as a candidate mechanism for the findings reported in the epidemiological literature.     

Individual differences in resting heart rate variability moderate thought suppression success

Individual differences in heart rate variability (HRV) at rest are thought to represent an individual's capacity for self‐regulation, but it remains unclear whether HRV predicts control over unwanted thoughts. The current study used a thought suppression paradigm in which participants recorded occurrences of a personally relevant intrusive thought over three monitoring periods. Among those instructed to suppress, higher levels of HRV were associated with greater declines in intrusions across the monitoring periods; no such relationship was found among those assigned to a control condition. Resting HRV also interacted with spontaneous thought suppression effort to predict intrusive thought frequency. In both cases, these HRV‐related differences in thought suppression success predicted the generalized distress symptoms common to depression and anxiety. These findings enhance understanding of the relationships between HRV and cognitive control and highlight how individual differences in self‐regulatory capacity impact thought suppression success and emotion regulation.     

Resting heart rate variability is associated with inhibition of conditioned fear

Startle blink as well as skin conductance responses (SCR) are widely used indices of learning processes associated with fear conditioning and extinction. During safety learning, the amygdala is under top‐down inhibitory control by the prefrontal cortex (PFC). The capacity of the PFC to exert inhibitory control over subcortical brain structures may be indexed by resting state vagally mediated heart rate variability (HRV). The present study investigated the association of resting HRV with startle blink and SCR during conditioned fear inhibition and extinction. Participants first learned to discriminate a threat cue (A) signaling an aversive unconditioned stimulus from a safety signal (B), which were each presented together with a third stimulus X (AX+/BX?). Then, both the threat and safety signal were presented together (AB) to test whether the presence of the learned safety signal inhibits the fear response to the danger signal. Finally, AX was presented without reinforcement (AX?) to investigate fear extinction. Higher HRV was associated with pronounced fear inhibition and fear extinction. Resting HRV levels were associated with fear extinction as indexed by startle blink potentiation but not SCR, which presumably reflect more cognitive aspects of learning. Resting HRV may reflect the capacity of the prefrontal cortex to inhibit subcortical fear responses in the presence of safety or when former threat cues are presented in the absence of threat.     

Resting heart rate variability and the startle reflex to briefly presented affective pictures

We have previously shown that persons with low HRV showed potentiated startle responses to neutral stimuli. In the present study we replicated our prior findings and extended them to examine the effects of HRV on the startle magnitude to pictures that were presented outside of conscious awareness. A total of 85 male and female students were stratified via median split on their resting HRV. They were presented pictures for 6 s or for 30 ms. Results indicated that the high HRV group showed the context appropriate startle magnitude increase to unpleasant foreground. The low HRV group showed startle magnitude increase from pleasant to neutral pictures but no difference between the neutral and unpleasant pictures. This pattern of results was similar for the 30 ms and the 6 s conditions. These results suggest that having high HRV may allow persons to more efficiently process emotional stimuli and to better recognize threat and safety signals.     

Focusing neurovisceral integration: Cognition,heart rate variability,and cerebral blood flow

The neurovisceral integration hypothesis suggests in part that cerebral control of autonomic function conveys comparable control of executive function and, hence, correlation among vagally determined high frequency heart rate variability (HF‐HRV), executive function, and regional cerebral blood flow (CBF). In 440 middle‐aged men and women, resting HF‐HRV was related to regional CBF derived from a resting arterial spin‐labeled MRI scan and to seven neuropsychological tests of executive function. Despite some intercorrelations, regression modeling failed to support integrated central control of HF‐HRV and executive function. Integration between autonomic and cognitive control appears more circumscribed than the general integration suggested by the neurovisceral integration hypothesis.     

Unconditioned and conditioned effects of intravenous insulin and glucose on heart rate variability in healthy men

We examined whether an injection of intravenous insulin and intravenous glucose would affect frequency-domain measures of heart rate variability (HRV), i.e., the high-frequency (HF-) band and the ratio of the low frequency (LF-) to the HF-band in healthy humans. Using a classical conditioning protocol, we also assessed whether the measures of HRV are subject to classical conditioning. Thirty healthy men were divided into three groups, given a conditioned stimulus (CS) and an intravenous injection of either insulin (0.05 IU/kg) in Group 1, glucose (15%, 0.5 g/kg) in Group 2, or placebo (physiological saline [0.9%]) in Group 3 during the 4-day acquisition phase. All subjects were given an olfactory CS (rosewood-peppermint smell) and placebo injection on day 5 (test). Due to their high inter-individual variability, HF and LF/HF-ratio were analysed by intragroup comparisons, using a pre-injection baseline interval (min -15 to -5), and three functional post-injection intervals: a) the interval to the maximum insulin level, i. e. insulin peak (min 0-5) in Groups 1 and 2, b) the interval to the maximum of insulin-induced hypoglycaemia (min 20-25) in Group 1, and c) the end of the session (min 70-75). On days 1 to 4, we found significant increases of the HF-band from baseline to interval min 0-5 in Group 1, and an even more pronounced increase in the glucose-treated Group 2. At the test (Day 5), both experimental groups responded with an HF-increase in the interval of the former insulin peak, and also at the other measurement intervals, reflecting some general increase of vagal activity remaining as a conditioned response. On days 1 to 4, the HF-band was positively correlated with the change of peripheral insulin levels in Group 1, reaching statistical significance on days 3 and 4. This pattern only emerged in tendency on Day 4 in Group 2. In conclusion, insulin triggers an increase in parasympathetic tone at maximum hyperinsulinaemia, and our data support the notion that this response pattern can become classically conditioned.     

Changes in resting heart rate variability across the menstrual cycle

Heart rate variability (HRV) is a noninvasive indicator of autonomic control. This study examines HRV changes across a normal menstrual cycle and proposes a novel piecewise function controlling for the effects of breathing on HRV spectral parameters. A resting ECG was collected from 13 women at five points in their menstrual cycle. Both heart rate and breathing rate increased across the cycle (p < .01) while time‐domain variability decreased (p = .04). Use of the piecewise function for breathing rate in HRV spectral analysis was confirmed by a substantial increase in model goodness‐of‐fit. HRV spectral parameters, controlled for breathing with the piecewise function, confirm that the decrease in variability is likely due to a parasympathetic withdrawal, since high frequency HRV decreases (p = .02).     

Higher resting heart rate variability predicts skill in expressing some emotions

Vagally mediated heart rate variability (vmHRV) is a measure of cardiac vagal tone, and is widely viewed as a physiological index of the capacity to regulate emotions. However, studies have not directly tested whether vmHRV is associated with the ability to facially express emotions. In extending prior work, the current report tested links between resting vmHRV and the objectively assessed ability to facially express emotions, hypothesizing that higher vmHRV would predict greater expressive skill. Eighty healthy women completed self‐reported measures, before attending a laboratory session in which vmHRV and the ability to express six emotions in the face were assessed. A repeated measures analysis of variance revealed a marginal main effect for vmHRV on skill overall; individuals with higher resting vmHRV were only better able to deliberately facially express anger and interest. Findings suggest that differences in resting vmHRV are associated with the objectively assessed ability to facially express some, but not all, emotions, with potential implications for health and well‐being.     

Cortical thickness,resting state heart rate,and heart rate variability in female adolescents

Resting state heart rate variability (HRV) is a psychophysiological marker that has gained increasing research interest, in particular in developmental neuroscience. HRV has been shown to be associated with mental and physical health, beyond simple measures of heart rate (HR) and shows inter‐ and intraindividual variance across aging. Recently, three studies reported on a positive correlation between resting state HRV and cortical thickness in selected regions of interest (ROIs) in adult samples. Structural thickness, HRV, and HR change during the sensitive period of adolescence. Previously, no study has addressed the structural concomitants of resting HR and HRV in adolescents. Cortical thickness (3‐T MRI), HR, and HRV were recorded in 20 healthy, female adolescents (mean age: 15.92 years; SD = 1.06; range: 14–17). In line with existing research in adults, cortical thickness in a number of ROIs was associated with resting state HRV but not HR. The comparison of regression analyses using the Bayes factor revealed evidence for a correlation between HRV and cortical thickness of the bilateral rostral anterior cingulate cortex. However, unlike in adults, greater cortical thickness was associated with reduced HRV in female adolescents. Analyses on HR showed no superior model fit. Results suggest that greater HRV might be beneficial for cortical development during adolescence (cortical thinning). On the other hand, cortical development might determine changes in autonomic nervous system function in adolescents. Future studies are needed to replicate these findings in larger samples including boys and to test these hypotheses in longitudinal designs.     

Spectral analysis of heart rate variability using the integral pulse frequency modulation model

Spectral analysis of heart rate variability (HRV) is a widely accepted approach for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. As a rule, the non-parametric methods for HRV spectral analysis are tested using the integral pulse frequency modulation (IPFM) model. However, published results with simulated HRV signals show differences requiring further development of the existing methods. With the aim of improving estimation accuracy, an entirely IPFM-based method for HRV analysis is investigated. According to this method, the spectra are computed by finding the least squares solution of two matrix equations that are derived using the IPFM model and involve irregular samples of a signal representing the HRV. The method is validated with various synthesised signals (in all tests, the relative errors of the power estimates at the modulating frequencies are within 3%, and the relative power of the spurious terms is less than 0.8% only) and is furthermore applied to the spectral analysis of RR interval series obtained from diabetic children. The results, with simulated and real HRV signals, show that the developed method yields very accurate estimations of the spectral region below half the mean heart rate. Moreover, it allows the detection and assessment of certain genuine modulating components beyond the traditional frequency limit of the HRV spectra.     

Breathing frequency bias in fractal analysis of heart rate variability

Detrended Fluctuation Analysis (DFA) is an algorithm widely used to determine fractal long-range correlations in physiological signals. Its application to heart rate variability (HRV) has proven useful in distinguishing healthy subjects from patients with cardiovascular disease. In this study we examined the effect of respiratory sinus arrhythmia (RSA) on the performance of DFA applied to HRV. Predictions based on a mathematical model were compared with those obtained from a sample of 14 normal subjects at three breathing frequencies: 0.1 Hz, 0.2 Hz and 0.25 Hz. Results revealed that: (1) the periodical properties of RSA produce a change of the correlation exponent in HRV at a scale corresponding to the respiratory period, (2) the short-term DFA exponent is significantly reduced when breathing frequency rises from 0.1 Hz to 0.2 Hz. These findings raise important methodological questions regarding the application of fractal measures to short-term HRV.     

Reproducibility of heart rate variability and baroreflex sensitivity measurements in children

Despite their extensive use, the reproducibility of cardiac autonomic measurements in children is not well-known. We investigated the reproducibility of short-term continuous measurements of heart rate (HR), heart rate variability (HRV, time and frequency domain), and spontaneous baroreflex sensitivity (BRS, frequency domain) in the supine and standing position in 57 children (11.2 ± 0.7 years, 52.6% boys). Reproducibility between two sessions within a two-week interval was evaluated by intraclass correlation coefficients (ICCs), standard error of measurement, coefficients of variation (CVs), limits of agreement, and Bland-Altman plots. HR and HRV were moderately-to-highly (ICC = .63-.79; CV = 5.7%-9.7%) and BRS moderately (ICC = .49-.63; CV = 11.4%-14.0%) reproducible. While the BRS measurements were slightly less reproducible than the HR and HRV measurements, all can be reliably applied in research, thus implicating sufficient capacity to detect real differences between children. Still, clinical studies focusing on individual changes in cardiac autonomic functioning need to address the considerable random variations that may occur between test-retest measurements.     

Assessment of heart rate variability derived from finger-tip photoplethysmography as compared to electrocardiography

Heart rate variability (HRV) is traditionally derived from RR interval time series of electrocardiography (ECG). Photoplethysmography (PPG) also reflects the cardiac rhythm since the mechanical activity of the heart is coupled to its electrical activity. Thus, theoretically, PPG can be used for determining the interval between successive heartbeats and heart rate variability. However, the PPG wave lags behind the ECG signal by the time required for transmission of pulse wave. In this study, finger-tip PPG and standard lead II ECG were recorded for five minutes from 10 healthy subjects at rest. The results showed a high correlation (median = 0.97) between the ECG-derived RR intervals and PPG-derived peak-to-peak (PP) intervals. PP variability was accurate (0.1 ms) as compared to RR variability. The time domain, frequency domain and Poincaré plot HRV parameters computed using RR interval method and PP interval method showed no significant differences (p < 0.05). The error analysis also showed insignificant differences between the HRV indices obtained by the two methods. Bland-Altman analysis showed high degree of agreement between the two methods for all the parameters of HRV. Thus, HRV can also be reliably estimated from the PPG based PP interval method.     

Respiratory control of heart rate

Summary Cross spectrum techniques have been used to characterise the relationships between breathing rate, sinus rhythm and peripheral blood flow. The breathing rate modulates both sinus rhythm and peripheral blood flow between certain limits, with a characteristic frequency amplitude pattern and entrains the Traube-Hering-Mayer waves within set limits. Beyond the upper limit of 0.4 Hz (24 breath min^–1) no modulation occurs. The lower limit is estimated in our subjects able to breathe at rates of 0.033 Hz (2 breath min^–1) the lower limit of modulation is approximately 0.05 Hz (3 breath min^–1) in most individuals. These observations have serious implications for heart rate measurements in sinus rhythm below 90 beat min^–1.     

Relationships between resting heart rate,heart rate variability and sleep characteristics among female collegiate cross‐country athletes

Even though sleep has been shown to be influenced by athletes’ training status, the association with resting heart rate and heart rate variability remains unclear. The purpose of this study was to compare the changes in and relationships between resting heart rate, heart rate variability and sleep characteristics across a female collegiate cross‐country season. Ten NCAA Division I collegiate female cross‐country athletes (mean ± SD; age, 19 ± 1 year; height, 167.6 ± 7.6 cm; body mass, 57.7 ± 10.2 kg; VO_2max, 53.3 ± 5.9 ml kg^?1 min^?1) participated in this study. Resting heart rate, heart rate variability and the percentage of time in slow wave sleep were captured using a wrist‐worn multisensor sleep device throughout the 2016 competitive cross‐country season (12 weeks). Linear mixed‐effects models and magnitude‐based inferences were used to assess differences between each week. Pearson product moment correlations were used to investigate relationships between variables. Resting heart rate at the end of the season, specifically during weeks 10–12 (mean ± SE; week 10, 48 ± 2; week 11, 48 ± 3; week 12, 48 ± 3), showed a practically meaningful increase compared to the beginning of the season, weeks 2–4 (week 2, 44 ± 2; week 3, 45 ± 2; week 4, 44 ± 2). Higher resting heart rate (r = 0.55) and lower heart rate variability (r = ?0.62) were largely associated with an increase in percentage of time spent in slow wave sleep. These data suggest that when physiological state was impaired, meaning the physiological restorative demand was higher, the percentage of time in slow wave sleep was increased to ensure recovery. Thus, it is important to implement sleep hygiene strategies to promote adequate slow wave sleep when the body needs physiological restoration.