The role of the autonomic nervous system in secretory
immunoglobulin A (sIgA) responses to laboratory challenge
was explored in a study in which sIgA and cardiovascular
activity were recorded at rest and during mental arithmetic
and paced breathing. These tasks were selected to preferentially
engage the sympathetic and parasympathetic nervous systems,
respectively. Mental arithmetic elicited a mixed pattern
of increased alpha- and beta-adrenergic activity and a
reduction in parasympathetic activity; diastolic blood
pressure, total peripheral resistance, and systolic blood
pressure increased, preejection period shortened, and heart
rate variability decreased. In contrast, paced breathing
primarily elicited an increase in parasympathetic activity;
heart rate variability increased. Mental arithmetic also
provoked an increase in sIgA concentration but no change
in saliva volume, whereas paced breathing affected neither
sIgA concentration nor saliva volume. These data suggest
that sIgA responses to laboratory challenges are mediated
by sympathetic rather than parasympathetic processes.

Explored the effect of repeating a laboratory stressor and a real-life stress exposure on the correlation of the reactivity to both types of stressors. 49 male university students’ HRs were monitored during exposure to the same stressful lab task 3 times, with 1-wk intervals. Ss’ HRs were also monitored during 2 examinations. Despite the repeated exposures, the relationship between the average response to the repeated lab task and to the exams remained small and insignificant. The moderate association between reactivity to lab tasks and to real-life stress is not primarily a problem of reliability, but rather of the difference in the nature of the 2 stress situations. The difference in time scale may frustrate a real-life to lab comparison, and different physiological mechanisms may be operating during short and prolonged exposure to stress. (PsycINFO Database Record (c) 2016 APA, all rights reserved)

Impedance cardiography has been used increasingly to measure human physiological responses to emotional and mentally engaging stimuli. The validity of large-scale ensemble averaging of ambulatory impedance cardiograms was evaluated for preejection period (PEP), interbeat interval, and dZ/dt(min) amplitude. We tested whether the average of “classical” 60-sec ensemble averages across periods with fixed activity, posture, physical load, social situation, and location could be accurately estimated from a single large-scale ensemble average spanning these entire periods. Impedance and electrocardiograms were recorded for about 24-h from 21 subjects. Recordings were scored by seven raters, using both methods for each subject. Good agreement (average intraclass correlation coefficient was .91) between both ensemble averaging methods was found for all three cardiac function measures. The results indicate that for unambiguous ambulatory impedance cardiograms, large-scale ensemble averaging is valid, which makes measuring prolonged changes in cardiac sympathetic activity by measuring ambulatory PEP feasible even in large epidemiological samples.

To understand the underlying genetic and environmental sources of individual variation in basal cortisol levels, we collected salivary cortisol at awakening and at six fixed time points during the day in adult twins and their singleton siblings. Reported time of awakening was verified with heart rate and body movement recordings. Cortisol data were available for 199 MZ twins, 272 DZ twins and 229 singleton siblings from 309 twin families. No differences in cortisol means and variances were found between twins and singleton siblings. Additionally, the correlations for DZ twins and siblings were not significantly different, indicating generalizability of twin study results to the general population. Genetic model fitting showed heritability for cortisol levels during the awakening period (34% for cortisol level at awakening and 32% for cortisol level at 30min after awakening) but not for cortisol levels later during the day. The current study shows that, while cortisol levels in the awakening period are influenced by genetic factors, cortisol levels throughout most of the day are not heritable, indicating that future gene finding studies for basal cortisol should focus on the first hour post-awakening.

This study assessed the heritability of ambulatory heart period, respiratory sinus arrhythmia (RSA), and respiration rate and tested the hypothesis that the well-established correlation between these variables is determined by common genetic factors. In 780 healthy twins and siblings, 24-h ambulatory recordings of ECG and thorax impedance were made. Genetic analyses showed considerable heritability for heart period (37%-48%), RSA (40%-55%), and respiration rate (27%-81%) at all daily periods. Significant genetic correlations were found throughout. Common genes explained large portions of the covariance between heart period and RSA and between respiration rate and RSA. During the afternoon and night, the covariance between respiration rate and RSA was completely determined by common genes. This overlap in genes can be exploited to increase the power of linkage studies to detect genetic variation influencing cardiovascular disease risk.

Background— Reduced heart rate variability (HRV) is a prognostic factor for cardiac disease and cardiac mortality. Understanding the sources of individual differences in HRV may increase its diagnostic use and provide new angles for preventive therapy. To date, the contribution of genetic and environmental factors to the variance in HRV has not been investigated during prolonged periods of ambulatory monitoring in a naturalistic setting.

Methods and Results— In 772 healthy twins and singleton siblings, ambulatory ECG was recorded during 24 hours. Two time domain measures of HRV were used: the standard deviations of all normal-to-normal intervals across 5-minute segments (SDNN index) and the root mean square of successive differences between adjacent normal RR intervals (RMSSD). Multivariate genetic analyses across 4 periods of day (morning, afternoon, evening, night) yielded significant estimates for genetic contribution to the mean ambulatory SDNN index (ranging from 35% to 47%) and the mean ambulatory RMSSD (ranging from 40% to 48%).

Conclusions— Ambulatory HRV measures are highly heritable traits that can be used to support genetic association and linkage studies in their search for genetic variation influencing cardiovascular disease risk.

The extent to which various measures of ambulatory respiratory sinus arrhythmia (RSA) capture the same information across conditions in different subjects remains unclear. In this study the root mean square of successive differences (RMSSD), peak valley RSA (pvRSA), and high frequency power (HF power) were assessed during ambulatory recording in 84 subjects, of which 64 were retested after about 3 years. We used covariance structure modeling to test the equality of the correlations among three RSA measures over two test days and three conditions (daytime sitting or walking and nighttime sleep) and in groups with low, medium, and high mean heart rate (HR), or low, medium, and high mean respiration rate (RR). Results showed that ambulatory RMSSD, pvRSA, and HF power are highly correlated and that their correlation is stable across time, ambulatory conditions, and a wide range of resting HR and RR values. RMSSD appears to be the most cost-efficient measure of RSA.

Introduction: Overactivity of the sympathetic nervous system (SNS) plays a pivotal role in the development of cardiovascular disease. This involvement suggests that the genetic susceptibility to adverse cardiovascular events may derive in part from individual differences in SNS activity. Methods and Results: To establish a genetic contribution to SNS activity, we measured sympathetic effects on cardiac contractility in 755 healthy adult twins and their singleton siblings. The preejection period (PEP) and the ratio of PEP to the left ventricular ejection time (PEP/LVET ratio) were derived from ambulatory recordings of the ECG and thorax impedance. During this type of prolonged recordings in a real life setting, the extent of cardiac sympathetic activity will vary with the demands of daily activities. Therefore, the genetic architecture of both indices was examined separately across three daytime periods (morning, afternoon, evening), and during nighttime sleep. Results showed significant genetic contribution to PEP (48–62%) over all daily periods. Heritability estimates for PEP/LVET ratio ranged between 35% and 58%. Cardiac sympathetic activity during the waking and sleep periods was largely influenced by genetic factors that were common to the entire 24-hour period. During sleep, additional genetic influences emerged that accounted for 8% of the variance in PEP. Conclusion: Impedance-derived measures of sympathetic effects on cardiac contractility show substantial heritability across all periods of the day and during sleep.

Recently, devices have become available that allow non-invasive measurement of stroke volume and cardiac output through ambulatory thorax impedance recording. If such recordings have adequate temporal stability, they offer great potential to further our understanding of how repeated or chronic cardiovascular activation in response to naturalistic events may contribute to cardiovascular disease. In this study, 24h ambulatory impedance-derived systolic time intervals, stroke volume and cardiac output were measured in 65 healthy subjects across an average time span of 3 years and 4 months. Stability was computed separately for sleep and daytime recordings. To avoid confounding by differences in posture and physical activity across measurement days, temporal stability was computed using sitting activities only. During the day intraclass correlations were moderate for stroke volume (.29–.46) and cardiac output (.33–.46) and good for systolic time intervals (.55–.81). When test–retest comparison was limited to two comparable days (two work days or two leisure days), correlations for both SV (.42–.46) and CO (.43–.50) improved. Conclusion: Moderate long-term temporal stability is found for individual differences in ambulatory stroke volume and cardiac output measured by impedance cardiography.

Objective
To examine whether ceiling effects at long inter beat intervals (IBIs)cause an underestimation of cardiac vagal control in regular exercisers by time and frequency-domain measures of respiratory sinus arrhythmia (RSA).
Methods
24-hour ECG and respiration recordings were performed in 26 regularly exercising subjects, actively engaged in aerobic training for the past year, and enrolled in supervised training in the six weeks pre-study, and in 26 age- and sex-matched non-exercisers. Sleep and waking levels of cardiac vagal control were estimated by RSA obtained through the peak–valley method, by the standard deviation of the IBIs, the root mean square of successive IBIs, and the high frequency IBI spectral power.
Results
In 11 of the exercisers the IBI–RSA relationship was characterized by a quadratic relationship. This reflected a ceiling effect at very long IBI values attained by regular exercisers, particularly during the nighttime recording. Irrespective of this ceiling effect, RSA as well as other heart rate variability (HRV) measures was still significantly larger in the exercisers with a quadratic IBI–RSA relationship than in non-exercisers or exercisers with a linear IBI–RSA relationship.
Conclusions
We conclude that a subgroup of regular exercisers is characterized by a low heart rate paired to high levels of cardiac vagal control. In these exercisers, vagal control is underestimated from HRV measures in ambulatory recordings. Inspection of the IBI–RSA relationship should be routinely added when HRV measures are used to index cardiac vagal control.