Day-by-day variability of spontaneous baroreflex sensitivity measurements: implications for their reliability in clinical and research applications

Reproducibility of autonomic cardiovascular function and hemodynamics at rest and during recovery from exercise

Smartwatches and home-based blood pressure (BP) devices have permitted easy use of heart rate variability (HRV) and BP to identify the recovery status of users after acute exercise training. The reproducibility of HRV and BP after exercise in healthy young participants is not well known. Eighteen participants (age 27 ± 6 years, female n = 8) performed test and retest aerobic exercises (cycling, 30 min, 60% of peak workload, W) and a control session in randomized order. RMSSD, high and low-frequency power of RR intervals, and BP were measured at rest and 30-60 min after interventions. The relative reproducibility was assessed by the intraclass correlation coefficient (ICC) and 95% confidence interval (95% CI). The absolute reproducibility was evaluated using the coefficient of variation (CV%). HRV indices revealed moderate-to-excellent reproducibility at rest (ICC 0.81-0.86; 95% CI 0.53-0.95) but not after exercise (ICC -0.06 to 0.60; 95% CI -1.85 to 0.85). Systolic BP had a good-to-excellent reproducibility before (ICC 0.93; 95% CI 0.81-0.98, CV% 4.2) and after exercise (ICC 0.93; 95% CI 0.81-0.97, CV% 4.2). The reproducibility of HRV indices is poor after exercise in young participants. However, the reproducibility of BP is excellent at rest and after aerobic exercise.

Synchronization of the Processes of Autonomic Control of Blood Circulation in Humans Is Different in the Awake State and in Sleep Stages

The influence of higher nervous activity on the processes of autonomic control of the cardiovascular system and baroreflex regulation is of considerable interest, both for understanding the fundamental laws of the functioning of the human body and for developing methods for diagnostics and treatment of pathologies. The complexity of the analyzed systems limits the possibilities of research in this area and requires the development of new tools. Earlier we propose a method for studying the collective dynamics of the processes of autonomic control of blood circulation in the awake state and in different stages of sleep. The method is based on estimating a quantitative measure representing the total percentage of phase synchronization between the low-frequency oscillations in heart rate and blood pressure. Analysis of electrocardiogram and invasive blood pressure signals in apnea patients in the awake state and in different sleep stages showed a high sensitivity of the proposed measure. It is shown that in slow-wave sleep the degree of synchronization of the studied rhythms is higher than in the awake state and lower than in sleep with rapid eye movement. The results reflect the modulation of the processes of autonomic control of blood circulation by higher nervous activity and can be used for the quantitative assessment of this modulation.

Sugar-sweetened soft drink consumption acutely decreases spontaneous baroreflex sensitivity and heart rate variability

In healthy humans, fructose-sweetened water consumption increases blood pressure variability (BPV) and decreases spontaneous cardiovagal baroreflex sensitivity (cBRS) and heart rate variability (HRV). However, whether consuming commercially available soft drinks containing high levels of fructose elicits similar responses is unknown. We hypothesized that high-fructose corn syrup (HFCS)-sweetened soft drink consumption increases BPV and decreases cBRS and HRV to a greater extent compared with artificially sweetened (diet) and sucrose-sweetened (sucrose) soft drinks and water. Twelve subjects completed four randomized, double-blinded trials in which they drank 500 mL of water or commercially available soft drinks matched for taste and caffeine content. We continuously measured beat-to-beat blood pressure (photoplethysmography) and R-R interval (ECG) before and 30 min after drink consumption during supine rest for 5 min during spontaneous and paced breathing. BPV was evaluated using standard deviation (SD), average real variability (ARV), and successive variation (SV) methods for systolic and diastolic blood pressure. cBRS was assessed using the sequence method. HRV was evaluated using the root mean square of successive differences (RMSSD) in R-R interval. There were no differences between conditions in the magnitude of change from baseline in SD, ARV, and SV (P ≥ 0.07). There were greater reductions in cBRS during spontaneous breathing in the HFCS (-3 ± 5 ms/mmHg) and sucrose (-3 ± 5 ms/mmHg) trials compared with the water trial (+1 ± 5 ms/mmHg, P < 0.03). During paced breathing, HFCS evoked greater reductions in RMSSD compared with water (-26 ± 34 vs. +2 ± 26 ms, P < 0.01). These findings suggest that sugar-sweetened soft drink consumption alters cBRS and HRV but not BPV.

Myths and methodologies: Reliability of non-invasive estimates of cardiac autonomic modulation during whole-body passive heating

Observed individual variability in cardiac baroreflex sensitivity (cBRS) and heart rate variability (HRV) is extensive, especially during exposure to stressors such as heat. A large part of the observed variation may be related to the reliability (consistency) of the measurement. We therefore examined the test-retest reliability of cBRS and HRV measurements on three separate occasions in 14 young men (age: 24 (SD 5) years), at rest and during whole-body heating (water-perfused suit) to raise and clamp oesophageal temperature 0.6°C, 1.2°C and 1.8°C above baseline. Beat-to-beat measurements of RR interval and systolic blood pressure (BP) were obtained for deriving HRV (from RR), and cBRS calculated via (i) the spontaneous method, α coefficients and transfer function analysis at each level of heat strain, and (ii) during forced oscillations via squat-stand manoeuvres (0.1 Hz) before and after heating. Absolute values and changes in all cBRS estimates were variable but generally consistent with reductions in parasympathetic activity. cBRS estimates demonstrated poor absolute reliability (coefficient of variation ≥25%), but relative reliability (intraclass correlation coefficient; ICC) of some frequency estimates was acceptable (ICC ≥0.70) during low-heat strain (ICC: 0.56-0.74). After heating, forced oscillations in BP demonstrated more favourable responses than spontaneous oscillations (better reliability, lower minimum detectable change). Absolute reliability of HRV estimates were poor, but relative reliability estimates were often acceptable (≥0.70). Our findings illustrate how measurement consistency of cardiac autonomic modulation estimates are altered during heat stress, and we demonstrate the possible implications on research design and data interpretation.

Different estimation methods of spontaneous baroreflex sensitivity have different predictive value in heart failure patients

OBJECTIVE

Several methods have been developed so far to estimate cardiac baroreflex sensitivity (BRS) through the analysis of spontaneous fluctuations of systolic arterial pressure (SAP) and R-R interval. Their relative performance in predicting cardiac mortality in heart failure patients is not known. The aim of this study was to compare the prognostic value of a set of representative indexes of spontaneous BRS in these patients.

METHODS

We studied 228 stable, moderate-to-severe heart failure patients with reduced ejection fraction, in sinus rhythm, who underwent an 8-min supine recording of ECG, arterial blood pressure and respiration during paced breathing (15 breaths/min). BRS was estimated according to the following methods: sequence (BRSSeq); nonparametric transfer function in the low-frequency band (BRSTF_NP-LF); parametric spectral computed in the low-frequency and high-frequency bands (BRSPS-LF and BRSPS-HF); parametric transfer function computed in the low-frequency and high-frequency bands (BRSTF_P-LF and BRSTF_P-HF); model-based closed loop (BRSCL); and bivariate phase-rectified signal averaging (BRSPRSA).

RESULTS

During a median follow-up of 36 months, 45 patients experienced a cardiac event. Only BRSTF_NP-LF, BRSPS-LF, BRSTF_P-LF and BRSPRSA were significantly associated with the outcome (P < 0.01), and statistical significance remained (P ≤ 0.03) after adjusting for clinical covariates. BRSTF_NP-LF and BRSPRSA also significantly improved the risk classification.

CONCLUSION

This study shows that different spontaneous BRS indexes have different predictive value in patients with heart failure. It also shows that the prognostic information of BRS estimates is linked to SAP and RR oscillations in the low-frequency band.

Reliability of autonomic and vascular components of baroreflex sensitivity in adolescents

Improvements in the autonomic and vascular systems are implicated in cardiovascular disease risk reduction. Baroreflex sensitivity (BRS) is composed of vascular and autonomic components. This study aimed to investigate between- and within-day reliability of BRS and its autonomic and vascular determinants in adolescents. Thirteen male adolescents (14·1 ± 0·5 y) participated in this study. For between-day reliability, participants completed four experimental visits separated by a minimum of 48-h. For within-day reliability, participants repeated BRS assessments three times in the morning with one hour between the measures. BRS was evaluated using the cross-spectral gain (LFgain) between blood pressure and heart rate interval. BRS was further divided into: 1) vascular component using arterial compliance (AC); and 2) autonomic component measured as LFgain divided by AC (LFgain/AC). LFgain, AC and LFgain/AC presented between-day coefficient of variation (CV) of 20%, 17%, and 20%, respectively. Similarly, variables associated with blood pressure control, such as cardiac output, mean arterial pressure, heart rate and total peripheral resistance, presented CVs ranging from 6% to 15%. Within-day reliability was poorer compared to between-day for LFgain (25%), AC (25%), and LFgain/AC (31%), as well as all hemodynamic variables (CVs from 11% to 22%, except heart rate with presented CV of 6%). This study indicates suitable between- and within-reliability of BRS and its autonomic and vascular determinants, as well as hemodynamic variables associated with BRS, in adolescents.

Fitness, Fatness, Physical Activity, and Autonomic Function in Midlife

PURPOSE

Although low cardiorespiratory fitness (CRF), physical inactivity, and obesity are associated with impaired autonomic function, they are also extensively interrelated. The present study aimed to assess the extent to which they contribute to autonomic function independently of each other.

METHODS

At the age of 46 yr, 1383 men and 1761 women without cardiorespiratory diseases and diabetes underwent assessments of vagally mediated heart rate (HR) variability (root mean square of successive differences in R-R interval (rMMSD)), peak HR during a submaximal step test (CRF), and 60-s HR recovery (HRR). Moderate-to-vigorous physical activity (MVPA; ≥3.5 METs, 2 wk) was measured by wrist-worn accelerometer and body fat percentage (Fat%) by bioimpedance.

RESULTS

In men, CRF and Fat% were significantly associated with higher rMSSD (standardized β = 0.31 and -0.16) and HRR (β = 0.19 and -0.18), whereas higher MVPA was linked with higher HRR (β = 0.13) when including CRF, MVPA, and Fat% in the initial regression. After adjustments for other lifestyle and cardiometabolic factors, CRF remained significantly associated with rMMSD (β = 0.24) and HRR (β = 0.14), as did MVPA with HRR (β = 0.11). In women, CRF was associated with rMSSD (β = 0.23) and HRR (β = 0.15), and MVPA (β = 0.17) and Fat% (β = -0.07) with HRR, when CRF, MVPA, and Fat% were adjusted for each other. After further adjustments, CRF remained a significant determinant of rMSSD (β = 0.20) and HRR (β = 0.13), as did MVPA with HRR (β = 0.15). The final models explained 23% and 21% of variation in rMSSD and HRR in men, and 10% and 12% in women, respectively.

CONCLUSIONS

CRF was a more important determinant of cardiac autonomic function than MVPA and body fat. Furthermore, MVPA but not body fat was independently associated with cardiac autonomic function in both men and women.

Baroreflex control of renal sympathetic nerve activity in early heart failure assessed by the sequence method

KEY POINTS

The integrity of the baroreflex control of sympathetic activity in heart failure (HF) remains under debate. We proposed the use of the sequence method to assess the baroreflex control of renal sympathetic nerve activity (RSNA). The sequence method assesses the spontaneous arterial pressure (AP) fluctuations and their related changes in heart rate (or other efferent responses), providing the sensitivity and the effectiveness of the baroreflex. Effectiveness refers to the fraction of spontaneous AP changes that elicits baroreflex-mediated variations in the efferent response. Using three different approaches, we showed that the baroreflex sensitivity between AP and RSNA is not altered in early HF rats. However, the sequence method provided evidence that the effectiveness of baroreflex in changing RSNA in response to AP changes is markedly decreased in HF. The results help us better understand the baroreflex control of the sympathetic nerve activity.

ABSTRACT

In heart failure (HF), the reflex control of the heart rate is known to be markedly impaired; however, the baroreceptor control of the sympathetic drive remains under debate. Applying the sequence method to a series of arterial pressure (AP) and renal sympathetic nerve activity (RSNA), we demonstrated a clear dysfunction in the baroreflex control of sympathetic activity in rats with early HF. We analysed the baroreflex control of the sympathetic drive using three different approaches: AP vs. RSNA curve, cross-spectral analysis and sequence method between AP and RSNA. The sequence method also provides the baroreflex effectiveness index (BEI), which represents the percentage of AP ramps that actually produce a reflex response. The methods were applied to control rats and rats with HF induced by myocardial infarction. None of the methods employed to assess the sympathetic baroreflex gain were able to detect any differences between the control and the HF group. However, rats with HF exhibited a lower BEI compared to the controls. Moreover, an optimum delay of 1 beat was observed, i.e. 1 beat is required for the RSNA to respond after AP changing, which corroborates with the findings related to the timing between these two variables. For delay 1, the BEI of the controls was 0.45 ± 0.03, whereas the BEI of rats with HF was 0.29 ± 0.09 (P < 0.05). These data demonstrate that while the gain of the baroreflex is not affected in early HF, its effectiveness is markedly decreased. The analysis of the spontaneous changes in AP and RSNA using the sequence method provides novel insights into arterial baroreceptor reflex function.

Variations of perioperative baroreflex sensitivity in hypertensive and normotensive patients

BACKGROUND

Impaired baroreflex sensitivity (BRS) is a marker of autonomous dysfunction, which may play an important role in the long-term development of hypertension. Perioperative patients with hypertension are rapidly increasing in all populations worldwide.

OBJECTIVE

To estimate the value of BRS for hypertension in a surgery cohort.

METHODS

An observational and cross-sectional study was performed, involving 96 patients who underwent video-assisted thoracoscopic pulmonary lobectomy or segmentectomy. Invasive blood pressure and heart rate before and after giving nitroglycerin were measured in 48 normotensive patients and 48 hypertensive patients (25 and 23 undergoing regular treatment hypertensive [HR] and irregular treatment hypertensive [HI], respectively) while entering the operation room, 30 minutes after skin incision, and 10 minutes after skin suture.

RESULTS

BRS_preoperative of hypertensive group (Group H) was lower than normotensive group (Group N) (3.49 ± 1.55 vs. 5.75 ± 4.15 ms/mmHg; P < 0.05). BRS_{intraoperative} of Group H was lower than Group N (1.70 ± 1.18 vs. 2.84 ± 1.11 ms/mmHg; P < 0.05). BRS_preoperative of patients in irregular-treated group (Group HI) was lower than regular-treated group (Group HR) (2.92 ± 1.36 vs. 4.31 ± 1.87 ms/mmHg; P < 0.05). BRS_{intraoperative} of Group HI was lower than Group HR (1.45 ± 0.90 vs. 2.08 ± 1.82 ms/mmHg; P < 0.05). BRS_{intraoperative} and BRS_{postoperative} were lower than BRS_preoperative in each group (P < 0.01).

CONCLUSION

The perioperative autonomic nerve function was significantly impaired in hypertensive patients. This effect was less pronounced for those who were on regular antihypertensive treatment. Attention should be paid to maintain the stability of cardiovascular function, to ensure patients can go through perioperative period safely.

The arterial baroreflex effectiveness index in risk stratification of chronic heart failure patients who are candidates for cardiac resynchronization therapy

INTRODUCTION

Baroreflex function is an independent marker of prognosis in heart failure (HF). However, little is known about its relation to response to cardiac resynchronization therapy (CRT). The aim of this study is to assess arterial baroreflex function in HF patients who are candidates for CRT.

METHODS

The study population consisted of 25 patients with indication for CRT, aged 65±10 years, NYHA functional class ≥III in 52%, QRS width 159±15 ms, left ventricular ejection fraction (LVEF) 29±5%, left ventricular end-systolic volume (LVESV) 150±48 ml, B-type natriuretic peptide (BNP) 357±270 pg/ml, and peak oxygen consumption (peak VO2) 18.4±5.0 ml/kg/min. An orthostatic tilt test was performed to assess the baroreflex effectiveness index (BEI) by the sequence method. This group was compared with 15 age-matched healthy individuals.

RESULTS

HF patients showed a significantly depressed BEI during tilt (31±12% vs. 49±18%, p=0.001). A lower BEI was associated with higher BNP (p=0.038), lower peak VO2 (p=0.048), and higher LVESV (p=0.031). By applying a cut-off value of 25% for BEI, two clusters of patients were identified: lower risk cluster (BEI >25%) QRS 153 ms, LVESV 129 ml, BNP 146 pg/ml, peak VO2 19.0 ml/kg/min; and higher risk cluster (IEB ≤25%) QRS 167 ms, LVESV 189 ml, BNP 590 pg/ml, peak VO2 16.2 ml/kg/min.

CONCLUSIONS

Candidates for CRT show depressed arterial baroreflex function. Lower BEI was observed in high-risk HF patients. Baroreflex function correlated closely with other clinical HF parameters. Therefore, BEI may improve risk stratification in HF patients undergoing CRT.

Assessment of baroreflex sensitivity from spontaneous oscillations of blood pressure and heart rate: proven clinical value?

The baroreceptor-heart rate reflex (baroreflex sensitivity, BRS) is a key mechanism contributing to the neural regulation of the cardiovascular system. Several methods have been proposed so far to assess BRS by analyzing the spontaneous beat-to-beat fluctuations of arterial blood pressure and heart rate. These methods are inherently simple, non-invasive and low-cost. This study is an attempt to address the question of whether spontaneous baroreflex methods have proven to be of value in the clinical setting. In the first part of this article, we critically review most representative clinical studies using spontaneous BRS techniques either for risk stratification or treatment evaluation, these being major issues in the clinical management of the patients. In the second part, we address two important aspects of spontaneous BRS measurements: measurability and reliability. Estimation of BRS in the studies selected for the review was performed according to the sequence, transfer function, alpha-index and phase-rectified signal averaging method. Arterial blood pressure was recorded non-invasively during supine, short-term (<30 min) laboratory recordings. The conclusion from this review is that spontaneous BRS techniques have been shown to be of great value in clinical practice but further work is needed to confirm the validity of previous findings and to widen the field of clinical applications. Measurability and reliability can be a major issue in the measurement of spontaneous BRS, particularly in some patient populations like post-myocardial infarction and heart failure patents. Main causes of poor measurability are: non-sinus rhythm, a high rate of ectopic beats and the need for recorded time series of RR interval and arterial blood pressure to satisfy the constraints of the different BRS estimation algorithms. As for reliability, within-subject variability is rather high in the measurements of spontaneous BRS and, therefore, should be carefully taken into account when BRS measurements are used to detect treatment effects in individual patients.

Eight-year parallel change in baroreflex sensitivity and memory function in a sample of healthy older adults

OBJECTIVES

To examine whether changes in the cardiac autonomic nervous system (ANS) over time, as expressed by baroreflex sensitivity (BRS), were associated with long-term changes in cognitive performance in elderly individuals without dementia.

DESIGN

Community-based 8-year longitudinal study.

SETTING

Clinical settings.

PARTICIPANTS

Individuals aged 66.9 ± 0.9 (N = 425).

MEASUREMENTS

At baseline and follow-up, subjects underwent a clinical interview, autonomic and vascular measurements, and a neuropsychological evaluation including attentional, executive, and memory tests using standardized Z-scores. BRS was defined as being normal, moderate, or severe alteration at each evaluation. On the basis of the longitudinal changes subjects were stratified as being stable, moderate or improved.

RESULTS

Mean attentional, executive, and memory change Z-scores were -0.41 ± 0.9, -0.15 ± 0.7, and -0.14 ± 0.8, respectively. BRS of 56% of the subjects remained unchanged, of 20% decreased, and of 24% improved. After regression analysis, the worsened BRS group was 1.88 times as likely to have greater memory change as the group with stable BRS (P = .02). No significant association was found between longitudinal change in BRS and attentional and executive changes.

CONCLUSION

In healthy elderly individuals, BRS decrease was associated with a weak but significant decrease in memory function. The long-term effect of BRS on memory changes may suggest a role of the ANS in cognitive decline.

A randomized controlled trial of acupuncture in stable ischemic heart disease patients

BACKGROUND

Heart rate variability (HRV) is reduced in stable ischemic heart disease (SIHD) patients and is associated with sudden cardiac death (SCD). We evaluated the impact of traditional acupuncture (TA) on cardiac autonomic function measured by HRV in SIHD patients.

METHODS

We conducted a randomized controlled study of TA, sham acupuncture (SA), and waiting control (WC) in 151 SIHD subjects. The TA group received needle insertion at acupuncture sites, the SA group received a sham at non-acupuncture sites, while the WC group received nothing. The TA and SA groups received 3 treatments/week for 12 weeks. 24-Hour, mental arithmetic stress, and cold pressor (COP) HRV was collected at entry and exit, along with BP, lipids, insulin resistance, hs-CRP, salivary cortisol, peripheral endothelial function by tonometry (PAT), and psychosocial variables.

RESULTS

Mean age was 63 ± 10; 50% had prior myocardial infarction. Comparison of WC and SA groups demonstrated differences consistent with the unblinded WC status; therefore by design, the control groups were not merged. Exit mental stress HRV was higher in TA vs. SA for markers of parasympathetic tone (p ≤ 0.025), including a 17% higher vagal activity (p=0.008). There were no differences in exit 24-hour or COP HRV, BP, lipids, insulin resistance, hs-CRP, salivary cortisol, PAT, or psychosocial variables.

CONCLUSIONS

TA results in intermediate effects on autonomic function in SIHD patients. TA effect on HRV may be clinically relevant and should be explored further. These data document feasibility and provide sample size estimation for a clinical trial of TA in SIHD patients for the prevention of SCD.

CONDENSED ABSTRACT

We conducted a randomized, single-blind trial of traditional acupuncture (TA) vs. sham acupuncture (SA) vs waiting control (WC) in stable ischemic heart disease (SIHD) patients to evaluate cardiac autonomic function measured by heart rate variability (HRV). Exit mental stress HRV was higher in the TA compared to SA group for time and frequency domain markers of parasympathetic tone (all p ≤ 0.025), including a 17% higher vagal activity (p=0.008). These data document feasibility and provide sample size estimation for an outcome-based clinical trial of TA in SIHD patients for the prevention of sudden cardiac death.

Interval and continuous exercise elicit equivalent postexercise hypotension in prehypertensive men, despite differences in regulation

Equicaloric bouts of interval (IE: 5 × 2:2 min at 85% and 40% maximal oxygen uptake) and steady state (SS: 21 min at 60% maximal oxygen uptake) exercise were performed by 13 older prehypertensive males on separate days, at equivalent times of day, to assess the influence of exercise mode on postexercise hypotension (PEH). Exercise conditions were compared with a control session. Cardiovascular measures were collected for 30 min prior to, and 60 min following exercise. PEH, as measured by mean postexercise systolic blood pressure (SBP) decrease (IE: -4 ± 6 mm Hg; SS: -3 ± 4 mm Hg; control: 4 ± 4 mm Hg), area under the SBP curve (IE: -240 ± 353 mm Hg·min; SS: -192 ± 244 mm Hg·min), and minimum SBP achieved (IE: -15 ± 7 mm Hg; SS: -13 ± 7 mm Hg), was equivalent after both conditions. Stroke volume was significantly reduced (IE: -14.6 ± 16.0 mL; SS: -10.1 ± 14.2 mL, control -1.7 ± 2.2 mL) and heart rate was significantly elevated (IE: 13 ± 8 beats·min⁻¹; SS: 7.9 ± 8 beats·min⁻¹; control: -2 ± 3 beats·min⁻¹) postexercise after both conditions. Cardiac output and total peripheral resistance were nonsignificantly decreased and increased postexercise, respectively. Baroreflex sensitivity (BRS) was reduced following IE (p < 0.05) and heart rate variability (HRV) parameters were reduced after both conditions, with IE eliciting larger and longer reductions in some indices. The results from the current study indicate that older prehypertensive adults experience similar PEH following equicaloric bouts of IE and SS exercise despite larger alterations in HRV and BRS elicited by IE.

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.