Measurement of parasympathetic activity from 24-hour ambulatory electrocardiograms and its reproducibility and sensitivity in normal subjects, patients with symptomatic myocardial ischemia, and patients with diabetes mellitus

Describing heart rate variability in patients with chronic atrial fibrillation during hospitalization for COVID-19

INTRODUCTION

Myriad cardiovascular manifestations have been reported with COVID-19. We previously reported that failure of PR interval shortening with increasing heart rate (HR) in patients with COVID-19 is associated with adverse outcomes. Here, we report on heart rate variability (HRV) and clinical outcomes in patients with chronic atrial fibrillation (cAF) hospitalized for COVID-19.

METHODS

A retrospective review of admitted COVID-19 patients with cAF between 1 March 2020 to 30 June 2020 was performed. HRV in cAF was compared during pre-COVID-19 and COVID-19 admissions; we selected pre-COVID-19 ECGs with HRs that were within 10 beats per minute of the COVID-19 ECGs. Mean HR and each RR interval were recorded. Time-domain measurements of HR variability were then calculated (SDSD, RMSSD, pNN50). Clinical outcomes during COVID-19 were correlated to indices of HRV.

RESULTS

A total of 184 ECGs (95 pre-COVID-19, 89 COVID-19) from 38 cAF in-patients were included. Mean age 78.6 ± 11.4 years, male 44.7%. The mean number of ECGs analyzed per patient pre-COVID-19 was 2.50 and during COVID-19 was 2.34. Comparing pre-COVID-19 versus COVID-19 ECGs showed: mean HR (95.9 ± 24.3 vs. 101.6 ± 22.8 BPM; P = .10), SDSD (109.0 ± 50.6 vs. 90.3 ± 37.2 ms; P < .01), RMSSD (184.1 ± 80.4 vs. 147.3 ± 59.8 ms; P < .01), pNN50 (73.8 ± 16.3 vs. 65.6 ± 16.6%; P < .01). Patients who had a smaller pNN50 during a COVID-19 admission had increased mortality (50.0% vs. 14.3%; log-rank test P = .02).

CONCLUSION

In patients with cAF, the HRV was reduced during COVID-19 compared with prior illnesses at similar average heart rates. Patients with the most depressed HRV as measured by pNN50 had an associated increase in mortality compared with patients whose HRV was preserved.

Mental Health Outcome Measures in Environmental Design Research: A Critical Review

AIM

During the last several decades, researchers have produced abundant evidence of the environmental impacts on stress, attention, and physical activity. More recently, scholars have turned their focus to the influence environments have on mental wellness. Therefore, a critical review of this more recent research is both timely and crucial for setting the future research agenda.

METHODS

In this article, we examined 65 papers published between 2008 and 2019 that examined the environmental correlates of a wide variety of mental health outcomes. We coded each study by type of environment, research design, mental health measurement scale used, and p-value.

RESULTS

We categorized the research studies into six groups based on mental health outcomes: emotions, moods, vitality, executive function, stress, and general well-being. Our review revealed several trends among the studies, including a heavy focus on nature and outdoor environments with little attention to workplace or residential environments; a lack of consensus on how to operationalize the environment; a heavy reliance on self-reported ratings using a wide variety of scales, many focused on the same outcome; and a disproportionate focus on short-term health effects.

CONCLUSIONS

There is a need for greater consensus on research constructs and health outcome measurements, focused on a wider variety of environmental settings and scales, in order to better inform evidence-based environmental design practice.

The prognostic value of the frontal QRS-T angle is comparable to cardiovascular autonomic neuropathy regarding long-term mortality in people with diabetes. A population based study

Cardiovascular autonomic neuropathy (CAN) is a well known prognostic marker in diabetes. A large angle between the QRS- and the T-wave vector (QRS-T angle) in the electrocardiogram (ECG) has recently been introduced as another marker of poor prognosis.

AIM

To assess and compare the long-term predictive power of the frontal plane QRS-T angle with CAN in people with diabetes.

METHODS

In 1992-93 people with diabetes in the municipality of Horsens, Denmark, were identified by the prescription method andan age and gender stratified sample of 240 individuals with diabetes were randomly selected. The presence of CAN was defined using the heart rate response to Valsalva manoeuvre. The QRS-T angle was read using the method described by Gandhi. In July 2015 vital statistics were obtained fromthe Danish Civil Registration System.

RESULTS

178 individuals accepted to participate in the study, of which 153 (86%) completed the Valsalva manoeuvre and had sinus rhythm. Total observation time was 21.5 (0.18) years, in which 99 (65%) individuals died. An elevated QRS-T angle and the presence of cardiovascular autonomic neuropathy were both found to be significant predictors of death. In Cox regression analyses, adjusting for the effect of gender, age, duration of diabetes, BMI, total-cholesterol, diabetes type, haemoglobin A1c, smoking status, hypertension and previous MI, an independent prognostic value was found for the QRS-T angle as well as the Valsalva ratio.

CONCLUSION

A large QRS-T angle and the presence of cardiovascular autonomic neuropathy are both strong and independent long-term predictors of all-cause mortality in people with diabetes.

Autonomic Predictors of Hospitalization Due to Heart Failure Decompensation in Patients with Left Ventricular Systolic Dysfunction

Introduction

Autonomic nervous system balance can be significantly deteriorated during heart failure exacerbation. However, it is still unknown whether these changes are only the consequence of heart failure decompensation or can also predict development thereof. Objectives were to verify if simple, non-invasive autonomic parameters, such as baroreflex sensitivity and short-term heart rate variability can provide independent of other well-known clinical parameters information on the risk of heart failure decompensation in patients with left ventricular systolic dysfunction.

Methods

In 142 stable patients with left ventricular ejection fraction ≤ 40%, baroreflex sensitivity and short-term heart rate variability, as well as other well-known clinical parameters, were analyzed. During 23 ± 9 months of follow-up 19 patients were hospitalized due to the heart failure decompensation (EVENT).

Results

Pre-specified cut-off values of baroreflex sensitivity (≤2.4 ms/mmHg) and low frequency power index of heart rate variability (≤19 ms²) were significantly associated with the EVENTs (hazard ratio 4.43, 95% confidence interval [CI] 1.35–14.54 and 5.41, 95% CI 1.87–15.65 respectively). EVENTs were also associated with other parameters, such as left ventricular ejection fraction, NYHA class, diuretic use, renal function, brain natriuretic peptide and hemoglobin level, left atrial size, left and right ventricular heart failure signs. After adjusting baroreflex sensitivity and low frequency power index for each of the abovementioned parameters, autonomic parameters were still significant predictors of hospitalization due to the heart failure decompensation.

Conclusion

Simple, noninvasive autonomic indices can be helpful in identifying individuals with increased risk of hospitalization due to the heart failure decompensation among clinically stable patients with left ventricular systolic dysfunction, even when adjusted for other well-known clinical parameters.

Reproducibility of Heart Rate Variability Indices in Children with Cystic Fibrosis

Fundamental to the potential utilisation of heart rate variability (HRV) indices as a prognostic tool is the reproducibility of these measures. The purpose of the present study was therefore to investigate the reproducibility of 24-hour derived HRV indices in a clinical paediatric population. Eighteen children (10 boys; 12.4 ± 2.8 years) with mild to moderate Cystic Fibrosis (CF; FVC: 83 ± 12% predicted; FEV1: 80 ± 9% predicted) and eighteen age- and sex-matched controls (10 boys; 12.5 ± 2.7 years) wore a combined ECG and accelerometer for two consecutive days. Standard time and frequency domain indices of HRV were subsequently derived. Reproducibility was assessed by Bland-Altman plots, 95% limits of agreement and intra-class correlation coefficients (ICC). In both groups, there was no systematic difference between days, with the variables demonstrating a symmetrical, homoscedastic distribution around the zero line. The time domain parameters demonstrated a good to excellent reproducibility irrespective of the population considered (ICC: 0.56 to 0.86). In contrast, whilst the frequency domain parameters similarly showed excellent reproducibility in the healthy children (ICC: 0.70 to 0.96), the majority of the frequency domain parameters illustrated a poor to moderate reproducibility in those with CF (ICC: 0.22 to 0.43). The exceptions to this trend were the normalised LF and HF components which were associated with a good to excellent reproducibility. These findings thereby support the utilisation of time and relative frequency domain HRV indices as a prognostic tool in children with CF. Furthermore, the present results highlight the excellent reproducibility of HRV in healthy children, indicating that this may be a useful tool to assess intervention effectiveness in this population.

Mindfulness meditation, well-being, and heart rate variability: a preliminary investigation into the impact of intensive Vipassana meditation

Mindfulness meditation has beneficial effects on brain and body, yet the impact of Vipassana, a type of mindfulness meditation, on heart rate variability (HRV) - a psychophysiological marker of mental and physical health - is unknown. We hypothesised increases in measures of well-being and HRV, and decreases in ill-being after training in Vipassana compared to before (time effects), during the meditation task compared to resting baseline (task effects), and a time by task interaction with more pronounced differences between tasks after Vipassana training. HRV (5-minute resting baseline vs. 5-minute meditation) was collected from 36 participants before and after they completed a 10-day intensive Vipassana retreat. Changes in three frequency-domain measures of HRV were analysed using 2 (Time; pre- vs. post-Vipassana)× 2 (Task; resting baseline vs. meditation) within subjects ANOVA. These measures were: normalised high-frequency power (HF n.u.), a widely used biomarker of parasympathetic activity; log-transformed high frequency power (ln HF), a measure of RSA and required to interpret normalised HF; and Traube-Hering-Mayer waves (THM), a component of the low frequency spectrum linked to baroreflex outflow. As expected, participants showed significantly increased well-being, and decreased ill-being. ln HF increased overall during meditation compared to resting baseline, while there was a time∗task interaction for THM. Further testing revealed that pre-Vipassana only ln HF increased during meditation (vs. resting baseline), consistent with a change in respiration. Post-Vipassana, the meditation task increased HF n.u. and decreased THM compared to resting baseline, suggesting post-Vipassana task-related changes are characterised by a decrease in absolute LF power, not parasympathetic-mediated increases in HF power. Such baroreflex changes are classically associated with attentional load, and our results are interpreted in light of the concept of 'flow' - a state of positive and full immersion in an activity. These results are also consistent with changes in normalised HRV reported in other meditation studies.

Heart rate variability reproducibility during exercise

The use of heart rate variability (HRV) parameters during exercise is not supported by appropriate reliability studies. In 80 healthy adults, ECG was recorded during three 6 min bouts of exercise, separated by 6 min of unloaded cycling. Two bouts were at a moderate intensity while the final bout was at a heavy exercise intensity. This protocol was repeated under the same conditions on three occasions, with a controlled start time (pre-determined at the first visit). Standard time and frequency domain indices of HRV were derived. Reliability was assessed by Bland–Altman plots, 95% limits of agreement and intraclass correlation coefficients (ICC). The sample size required to detect a mean difference ≥30% of the between-subject standard deviation was also estimated. There was no systematic change between days. All HRV parameters demonstrated a high degree of reproducibility during baseline (ICC range: 0.58–0.75), moderate (ICC: 0.58–0.85) and heavy intensity exercise (ICC range: 0.40–0.76). The reproducibility was slightly diminished during heavy intensity exercise relative to both unloaded baseline cycling and moderate exercise. This study indicates that HRV parameters can be reliably determined during exercise, and it underlines the importance of standardizing exercise intensity with regard to fitness levels if HRV is to be reliably determined.

Reliability of heart rate variability in patients with type 2 diabetes mellitus

AIMS

Heart rate variability may be used to assess diabetic cardiac autonomic neuropathy. The aim of the present study was to determine the reliability of standard short-term clinical measurements of heart rate variability in patients with Type 2 diabetes.

METHODS

In 24 patients with Type 2 diabetes (11 male, age 61 ± 9 years), parameters of heart rate variability in the time domain (standard deviation of RR intervals, coefficient of variation of RR intervals and root mean square of successive RR interval differences) and frequency domain (very low frequency, low frequency, high frequency and total spectral power) were derived from a 5-min electrocardiograph recorded during two laboratory visits separated by 16 ± 8 days. Absolute and relative reliability were assessed by 95% limits of random variation and the intraclass correlation coefficient, respectively. Categorical agreement of classifications of heart rate variability and sample size estimates for clinical trials were calculated.

RESULTS

Despite no significant difference in mean heart rate variability between tests, 95% limits of random variation indicated that repeated measurements were between 58% higher/37% lower (most reliable parameter; coefficient of variation of RR intervals) and 443% higher/82% lower (least reliable parameter; very low frequency power) than the first measure. The intraclass correlation coefficient ranged from 0.58 to 0.90 and sample size requirements from 20 to 93 patients per group. Agreement of categories of heart rate variability ranged from 79 to 96%.

CONCLUSIONS

Short-term clinical measurements of heart rate variability in patients with Type 2 diabetes are characterized by poor absolute reliability, but substantial to good relative reliability, suggesting greater clinical utility in diagnosis than in sequential follow-up.

Heart rate variability analysis using a ballistocardiogram during Valsalva manoeuvre and post exercise

We introduced a novel non-constrained technique for estimating heart rate variability (HRV) using a ballistocardiogram (BCG). To assess whether the BCG signal can be used to analyse the cardiac autonomic modulation, HRV parameters derived from the BCG signal (ballistocardiographic HRV, B-HRV) were statistically compared with the HRV parameters from the ECG signal during rest and under two different experimental conditions that induce cardiac autonomic rhythm changes: the Valsalva manoeuvre and static exercise. Time domain, frequency domain and nonlinear analyses were individually performed on 15 healthy subjects to assess whether the BCG can be used to analyse the cardiac autonomic modulation under each condition. For all subjects, the proposed method had averages of relative errors of 5.01 ± 4.72, 5.64 ± 4.83 and 5.98 ± 5.80% for resting, Valsalva and post-exercise sessions, respectively, and the correlation coefficients between the reference (ECG) and proposed (BCG) methods are 0.97, 0.98 and 0.98, for resting, Valsalva and post-exercise sessions, respectively. During cardiac autonomic changes, the B-HRV parameters changed in a pattern that is very similar to the variations in the HRV parameters based on Student's t-test results. In addition, some of the B-HRV parameters changed according to cardiac autonomic rhythms controlled by sympathetic and parasympathetic activities during the experiments. These findings indicate that BCG can provide an accurate and reliable means to evaluate autonomic system activation by HRV in its unconstrained way.

Evaluation of unconstrained monitoring technology used in the smart bed for u-health environment

OBJECTIVE

Since ubiquitous technology is an emerging paradigm in healthcare, we wanted to evaluate its feasibility in the healthcare area. As a first step, we evaluated the feasibility of the smart bed. In this study, a series of experiments were conducted to evaluate the smart bed.

MATERIALS AND METHODS

Smart bed is a continuous ballistocardiogram monitoring device developed by Seoul National University. Eleven healthy subjects participated in the study. Each subject slept in the smart bed for two nights. To measure meaningful sleep periods, noisy sections were removed from the original signal. The subject's heart activity was obtained by automatic peak detection, using the AF2 peak detection algorithm. In this study, the feasibility of the smart bed was evaluated in terms of the meaningful sleep periods and the accuracy of automatic peak detection.

RESULTS

On an average, 92.8% of sleep duration was meaningful. The accuracy of peak detection algorithm was also evaluated. The algorithm proved to have an accuracy of 95%; thus, we observe that 95% of the peak detection results were correct in comparison with the manual results.

CONCLUSIONS

Based on the experimental results, we conclude that the smart bed can be a useful device for long-term monitoring in ubiquitous healthcare environments.

Cardiovascular autonomic function in Cushing's syndrome

Cardiac autonomic dysfunction is associated with increased cardiovascular mortality. No data on sympathovagal balance are available in patients with Cushing's syndrome, in whom cardiovascular risk is high. We studied 10 patients with newly diagnosed Cushing's syndrome (1 male/9 females; age mean+/-SD, 47+/-10 yr) and 10 control subjects matched for age, sex, body mass index, and cardiovascular risk factors. In both groups there were 7 patients with arterial hypertension, 3 with diabetes mellitus, and 2 with obesity. Cardiac autonomic function was evaluated by analysis of short time heart rate variability (HRV) measures in frequency domain over 24-h, daytime, and nighttime. The 24-h ambulatory blood pressure monitoring and echocardiography were also performed. In comparison with controls, patients with Cushing's syndrome had lower 24-h (1.3+/-0.6 vs 3.7+/-1.5, mean+/-SD, p<0.01), daytime (2.0+/-1.4 vs 4.5+/-1.6, p<0.01), and night-time (1.0+/-0.4 vs 3.5+/-2.3, p<0.01) low-frequency/ high frequency (LF/HF) power ratio. In the presence of similar LF power, the difference was due to elevation in HF power in Cushing's syndrome compared to controls: 24-h, 12.7+/-6.7 vs 5.8+/-2.8, p<0.01; daytime, 10.2+/-7.3 vs 4.5+/-2.1, p<0.05; nighttime, 14.2+/-7.0 vs 7.8+/-4.7, p<0.05. Eight Cushing patients vs 4 controls had a non-dipping blood pressure profile. At echocardiography, Cushing patients had a greater left ventricular mass index and/or relative wall thickness, and impaired diastolic function, compared with controls. Compared to controls, patients with Cushing's syndrome showed a sympathovagal imbalance, characterized by a relatively increased parasympathetic activity. Whether this autonomic alteration is meant to counterbalance cortisol-induced effects on blood pressure and cardiac structure/function or has a different pathophysiological significance is still unknown.

Altered cardiovascular vagal responses in nonelderly female patients with subclinical hyperthyroidism and no apparent cardiovascular disease

OBJECTIVE

Subclinical hyperthyroidism (SH) has been associated with exercise intolerance, changes in cardiac morphology, atrial arrhythmias and sympathovagal imbalance. The aim of this study was to evaluate the vagal reserve and modulation by a sympathetic stimulus in nonelderly patients with SH without cardiovascular problems.

DESIGN

We carried out a cross-sectional study, comparing data of the heart rate variability (HRV) of SH patients and healthy controls at rest and after vagal and sympathetic stimulation.

PATIENTS

We studied 16 female patients with at least 6 months of SH and 16 healthy female controls with the same median age (40 vs. 34.5 years).

MEASUREMENTS

We used the tilt test, with electrocardiographic record at rest, during the respiratory sinus arrhythmia (RSA) manoeuvre and after tilting, in order to analyse HRV in the frequency domain (%high frequency (HF) and low/high frequency ratio (LF/HF) using Biopotentials Captation System software.

RESULTS

The median TSH level was 0.03 mU/l in patients and 1.37 mUI/l in controls. The median free T4 was 1.37 ng/dl in patients and 1.20 ng/dl in controls. Patients demonstrated a significantly smaller difference between %HF during the RSA and %HF at rest than controls (median -7.5 vs. 36.6, P < 0.001). There was a lower difference between LF/HF ratio after tilting and LF/HF ratio at rest in patients than in controls (1.5 vs. 5.3, P = 0.005).

CONCLUSION

Subclinical hyperthyroidism affects cardiovascular autonomic balance in otherwise apparently healthy nonelderly females by blunting vagal responses.

The assessment of autonomic function in chronic atrial fibrillation: description of a non-invasive technique based on circadian rhythm of atrioventricular nodal functional refractory periods

AIMS

Heart rate variability (HRV) parameters can be used to assess autonomic function and to predict outcome, but this has been done exclusively in patients with sinus rhythm. Atrial fibrillation (AF) is the commonest sustained arrhythmia and is particularly prevalent in heart failure. We have developed a simple index to assess autonomic function in patients with chronic AF.

METHODS AND RESULTS

Forty patients with chronic AF (>1 month) and symptoms of heart failure underwent ambulatory 24 h electrocardiography recording as well as evaluation of symptoms, exercise capacity (6 min walk distance), ventricular function (echocardiography and radionuclide ventriculography), and neuroendocrine activation. A number of standard HRV parameters shown to have prognostic significance in sinus rhythm were also determined. A modified in-house HRV statistical programme was used to filter labelled QRS intervals and to compute the 5th percentile RR interval in each hour. This parameter has been shown to approximate the functional refractory period (FRP) of the atrioventricular node (AVN). A cosine curve was fitted to hourly 5th percentile RR intervals for each patient and from this was estimated the diurnal change in hourly 5th percentile RR interval (approximating DeltaFRP of the AVN) and, by inference, diurnal variation in sympathovagal input to the AVN. Digoxin was the sole agent permitted for control of ventricular rate. DeltaFRP of the AVN varied and revealed a significant correlation, on multivariate analysis, with mean RR interval (P<0.001), SDARR (SD of 5-min average RR intervals during 24 h, P<0.001), and NYHA class of heart failure (classes III and IV heart failure vs. classes I and II, P=0.02). SDARR has previously been shown independently to predict mortality in patients with chronic AF and heart failure.

CONCLUSION

This analysis describes a novel non-invasive method for assessing autonomic function in chronic AF. Whether DeltaFRP in chronic AF patients can independently predict adverse prognosis or sudden death requires further study.

Test-Retest Reliability of Heart Rate Variability and Respiration Rate at Rest and during Light Physical Activity in Normal Subjects

BACKGROUND

A variable that remains stable over repeated measurements (in stable conditions) is ideal for tracking modifications of the clinical state. The aim of the present study is to examine test-retest reliability of time-domain heart rate variability and respiration rate measurements using a portable device on normal subjects during rest and light physical activity.

METHODS

Twenty-six normal subjects [18 females and 8 males aged 28 +/- 6 years and 34 +/- 12 years (mean +/- SD), respectively] underwent two measurements for time-domain heart rate variability (SDNN and RMSSD) and respiration rate, with 7 days in between. Measurements took place under three conditions: lying down in a laboratory, cycling in a laboratory and sleeping in an ambulatory surrounding. Reliability was assessed statistically by calculating intra-class correlation coefficients (ICC).

RESULTS

Reliability was found to be good to excellent for both time-domain heart rate variability (SDNN: ICC values between 0.74 and 0.85, RMSSD: ICC values between 0.75 and 0.98) and for respiration rate (ICC values between 0.77 and 0.96).

CONCLUSIONS

Both time-domain heart rate variability and respiration rate can be reliably assessed. However, we advise reliability research in a clinical setting before using the device for tracking modifications in a clinical state.

Vagal modulation and aging

Cardiovascular disease accounts for approximately 30% of all deaths worldwide, and will only worsen as the world's population ages. It is well-established that age, per se, is a major risk factor and contributor to all cardiovascular morbidities and mortalities. However, environmental factors, including a lack of exercise, appear to play a critical role in the onset and progression of cardiovascular disease. This paper reviews the literature on cardiac variability and aging and addresses risk factors associated with aging that can be modified and possibly attenuate the decline of heart rate variability with aging, including exercise training to increase vagal modulation. Thus, results of the studies described in this review support a potential benefit of increasing or maintaining fitness in order to slow the decline of parasympathetic control of HR with normal aging.

Heart rate variability and circadian variations in type 1 diabetes mellitus

Diabetic autonomic neuropathy (DAN) commonly complicates diabetes and is associated with increased mortality rates over 5 yr. This fact denotes the significance of DAN prevention, mainly with effective glycemic control. However, total prevention of autonomic neuropathy in diabetic patients is not achievable. Thus, the timely detection of DAN and the use of effective means to improve autonomic nervous system function or slow down its progression become of utmost significance. Heart rate variability (HRV) is a technique that measures the beat-to-beat variability in RR intervals, which reflects changes in autonomic activity and their impact on cardiovascular function. Circadian variation in time and frequency domains of heart variability has been shown to correlate with circadian rhythm of ambulatory ischemia and suggests that relative changes in vagal and sympathetic tone at different times during the day may have a direct relationship to the severity of clinical events. Forty-seven (21 boys and 26 girls) type I insulin-dependent diabetics and 46 control subjects (19 boys and 27 girls) were included in the study. Our investigation demonstrated that overall HRV is markedly depressed in diabetes mellitus (DM). All time domain parameters except standard deviation of all 5-min mean RR intervals and all frequency domain indices maintain significant circadian variation. These changes in overall HRV and HRV circadian rhythms reflect significant reductions in cardiac parasympathetic activity and, possibly, increased sympathetic tone.

Cardiac vagal outflow after aerobic training by analysis of high-frequency oscillation of the R–R interval

This study was designed to assess the effect of aerobic training on the dynamics between the R-R interval length and the high-frequency (HF) oscillation of the R-R interval. Seventeen healthy males (26+/-2 years) participated in an 8-week aerobic training intervention. The mean HF spectral power (0.15-0.4 Hz) of the R-R interval and the mean R-R interval length were analyzed from 24-h recordings. HF power was also analyzed in 5-min sequences and plotted as a function of the corresponding mean R-R interval length. The relationship between the R-R interval length and the HF power was analyzed by a quadratic regression model. The relationship was defined as saturated if the distinct deflection point of the model occurred before the maximum R-R interval. Otherwise, the relationship was defined as linear. Additionally, the mean HF power was calculated from the linear portion of the R-R interval versus the HF power regression curve (HF index). Before the training intervention, seven subjects had a saturated HF power. After the intervention, five new cases of saturated HF power were observed. The mean HF power of the 24-h recording did not change in the group with a saturated HF power before training (7.4+/-0.8 vs. 7.6+/-0.8 ms(2)), but the HF index increased (6.7+/-0.7 vs. 7.1+/-0.7 ms(2), P<0.05). We conclude that enhanced vagal activity due to aerobic training increases the prevalence of the saturation of the HF oscillation of the R-R interval variability in healthy subjects. HF power calculated from unsaturated area detects more accurately subtle changes in the vagally mediated beat-to-beat variability of the R-R interval after aerobic training than the mean 24-h HF power.

Prognostic value of ventricular arrhythmias and heart rate variability in patients with unstable angina

OBJECTIVES

To assess the prognostic value of ventricular arrhythmias (VA) and heart rate variability (HRV) in patients with unstable angina.

DESIGN

Multicentre prospective study.

SETTING

17 cardiological centres in Italy.

PATIENTS

543 consecutive patients with unstable angina and preserved left ventricular function (ejection fraction >or=40%) enrolled in the SPAI (Stratificazione Prognostica dell'Angina Instabile) study.

METHODS

Patients underwent 24 h ECG Holter monitoring within 24 h of hospital admission. Tested variables were frequent ventricular extrasystoles (>or=10/h), complex (that is, frequent or repetitive) VA, and bottom quartile values of time-domain and frequency-domain HRV variables. Primary end points were in-hospital and six-month total and cardiac deaths.

RESULTS

Eight patients died in hospital (1.5%) and 32 (5.9%, 29 cardiac) during follow up. Both complex VA and frequent extrasystoles were strongly predictive of death in hospital and at follow up, even after adjustment for clinical (age, sex, cardiac risk factors and history of myocardial infarction) and laboratory (troponin I, C reactive protein and transient myocardial ischaemia on Holter monitoring) variables. At univariate analysis bottom quartile values of three HRV variables (standard deviation of RR intervals index, low-frequency amplitude and low to high frequency ratio) were associated with in-hospital death, and bottom quartile values of most HRV variables predicted six-month fatal events. At multivariate Cox survival analysis reduced low-frequency amplitude was consistently found to be independently associated with fatal end points.

CONCLUSION

In patients with unstable angina with preserved myocardial function, both VA and HRV are independent predictors of in-hospital and medium-term mortality, suggesting that these factors should be taken into account in the risk stratification of these patients.

The reliability of short-term measurements of heart rate variability

Short-term assessment of heart rate variability (HRV) is a non-invasive technique to examine ANS function. Within the literature, HRV is commonly referred to as a reliable measurement technique. The aim of this review was to assess the accuracy of this description based upon a comprehensive review of the available data concerning reliability of short-term HRV measures. Reviewing only studies using appropriate statistical analyses, it was determined that reliability coefficients for HRV measures were highly varied. Coefficients of variation ranged from <1% to >100%. Similar variation was found in studies using the intraclass correlation coefficient values, and limits of agreement. Reliability coefficients reported displayed some distinct patterns. Firstly, where measurements were made during interventions such as tilt or pharmacological stimulation, reliability was poorer than when HRV was measured at rest. Secondly, clinical populations displayed poorer reliability than healthy subjects. There was little effect of test-retest duration on reliability and although no single HRV measurement appeared less reliable than another, there was evidence that optimal data collection conditions for specific frequency domain measures exist. Describing HRV as a reliable measurement technique appears to be a gross oversimplification, as results of reliability studies are heterogeneous, and dependent on a number of factors. Further studies are required, particularly in clinical populations to assess HRV reliability. Authors should refer to coefficients from similar populations measured under similar conditions when making future sample size calculations.

Reproducibility of heart rate variability, blood pressure variability and baroreceptor sensitivity during rest and head-up tilt

OBJECTIVE

Previous studies have indicated moderate-to-poor reproducibility of heart rate variability (HRV) but the reproducibility of blood pressure variability (BPV) and spectral measures of baroreceptor sensitivity (BRS) are not well established.

METHODS

We measured normal-to-normal heart beat (RR) interval and finger blood pressure (Finapres) in 14 healthy individuals on three different days. The protocol was 1 h of supine rest and 1 h of 60-degree head-up tilt. Time-series of consecutive 300-s segments as well as 1024-s segments of RR intervals and systolic, diastolic and mean blood pressures were extracted for the assessment of day-to-day and short-term reproducibility. Power spectrum analysis (Fourier) and transfer function analysis was performed. Reproducibility was assessed using the coefficient of variation (CV). The reproducibility of the mean RR interval, mean systolic, diastolic and mean blood pressure was good (CV<10 %). However, there was only moderate-to-poor reproducibility of the spectral parameters of HRV (CV range 18-36%) and BPV (16-44%) and moderate reproducibility of BRS (14-20%).

CONCLUSION

Spectral estimates of BRS had only moderate reproducibility although it was better than the spectral estimates of HRV and BPV.

Heart rate variability fraction--a new reportable measure of 24-hour R-R interval variation

BACKGROUND

The scatterplot of R-R intervals has several unique features. Its numerical evaluation may produce a new useful index of global heart rate variability (HRV) from Holter recordings.

METHODS

Two-hundred and ten middle-aged healthy subjects were enrolled in this study. The study was repeated the next day in 165 subjects. Each subject had a 24-hour ECG recording taken. Preprocessed data were transferred into a personal computer and the standard HRV time-domain indices: standard deviation of total normal R-R intervals (SDNN), standard deviation of averaged means of normal R-R intervals over 5-minute periods (SDANN), triangular index (TI), and pNN50 were determined. The scatterplot area (0.2-1.8 second) was divided into 256 boxes, each of 0.1-second interval, and the number of paired R-R intervals was counted. The heart rate variability fraction (HRVF) was calculated as the two highest counts divided by the number of total beats differing from the consecutive beat by <50 ms. The HRVF was obtained by subtracting this fraction from 1, and converting the result to a percentage.

RESULTS

The normal value of the HRVF was 52.7 +/- 8.6%. The 2-98% range calculated from the normal probability plot was 35.1-70.3%. The HRVF varied significantly with gender (female 48.7 +/- 8.4% vs male 53.6 +/- 8.6%, P = 0.002). The HRVF correlated with RRI (r = 0.525) and showed a similar or better relationship with SDNN (0.851), SDANN (0.653), and TI (0.845) than did the standard HRV measures with each other. Bland-Altman plot showed a good day-by-day reproducibility of the HRVF, with the intraclass correlation coefficient of 0.839 and a low relative standard error difference (1.8%).

CONCLUSION

We introduced a new index of HRV, which is easy for computation, robust, reproducible, easy to understand, and may overcome the limitations that belong to the standard HRV measures. This index, named HRV fraction, by combining magnitude, distribution, and heart-rate influences, might become a clinically useful index of global HRV.

On the genesis of myocardial ischemia

About three quarters of myocardial ischemic events are triggered by the autonomic nervous system. The pathognomonic constellation is a combination of an almost complete withdrawal of tonic vagal activity with increased sympathetic activity. The reduction of tonic vagal activity, which is characteristic for ischemic heart disease, and the acute withdrawal of vagal drive preceding the onset of ischemia are not dependent on coronary artery disease. In this paper, the pathophysiological steps that lead from sympathetic-parasympathetic imbalance to myocardial ischemia shall be discussed. A considerable increase of aerobic glycolysis within the myocardium as a result of the autonomic imbalance is of special importance in this process.

Effects of aerobic training on heart rate dynamics in sedentary subjects

This study was designed to assess the effects of moderate- and high-volume aerobic training on the time domain and on spectral and fractal heart rate (HR) variability indexes. Sedentary subjects were randomized into groups with moderate-volume training (n = 20), high-volume training (n = 20), and controls (n = 15). The training period was 8 wk, including 6 sessions/wk at an intensity of 70-80% of the maximum HR, lasting for 30 min/session in the moderate-volume group and 60 min/session in the high-volume group. Time domain, frequency domain, and short-term fractal scaling measures of HR variability were analyzed over a 24-h period. Mean HR decreased from 70 +/- 7 to 64 +/- 8 beats/min and from 67 +/- 5 to 60 +/- 6 beats/min (P < 0.001 for both) for the moderate- and high-volume training groups, respectively. The normalized high-frequency spectral component increased in both groups (P < 0.05). The normalized low-frequency component decreased significantly (P < 0.05), resulting in a marked decrease in low frequency-to-high frequency ratio in both groups. In addition, short-term scaling exponent decreased in both groups (P < 0.001). There were no significant differences in the changes of HR variability indexes between groups. Aerobic training in sedentary subjects results in altered autonomic regulation of HR toward vagal dominance. A moderate training volume is a sufficient intervention to induce these beneficial effects.

Cardiac function after orthotopic liver transplantation and the effects of immunosuppression: a prospective randomized trial comparing cyclosporin (Neoral) and tacrolimus

There are several case reports in the literature that describe cardiac complications in the first few weeks after orthotopic liver transplantation (OLT) in patients receiving tacrolimus as their primary immunosuppressive therapy. In this study, we investigated the cardiac function of patients on tacrolimus (T) compared with those on cyclosporin (C) (Neoral; Novartis, Basel, Switzerland) immunosuppression, after OLT, in a prospective randomized trial. We randomized 40 adult patients with cirrhosis to either T or C with azathioprine and prednisolone immunosuppression and followed up on them for 3 months after OLT. All had detailed clinical, biochemical, electrocardiographic and echocardiographic assessments at regular intervals. Abnormalities in cardiac function were common after OLT and significant deterioration in left ventricular diastolic function was demonstrable up to 3 months in both patient groups. Cardiac function was similar in the T and C arms and no significant electrocardiographic differences were observed, although reduced heart rate variability (HRV) and higher mean serum brain natriuretic peptide (BNP) levels were identified in the T group. The percentage increase in posterior wall thickness was higher in the T group. Cardiac dysfunction as shown by worsening echocardiographic measures of left ventricular diastolic function and by clinical cardiac events is common in the first 3 months after OLT in patients with cirrhosis. HRV and BNP values in the T group were worse than in the C group, but this was not translated to an increase in cardiac clinical events in this study.

Functional assessment of heart rate variability: physiological basis and practical applications

The autonomic nervous system dynamically controls the response of the body to a range of external and internal stimuli, providing physiological stability in the individual. With the progress of information technology, it is now possible to explore the functioning of this system reliably and non-invasively using comprehensive and functional analysis of heart rate variability. This method is already an established tool in cardiology research, and is increasingly being used for a range of clinical applications. This review describes the theoretical basis and practical applications for this emerging technique.

Reproducibility of a non-invasive real-time measure of cardiac parasympathetic activity

The NeuroScope is a new device which produces a continuous real-time index of cardiac parasympathetic activity (the CIPA) and accurately measures RR intervals. The reproducibility of the CIPA has not yet been assessed. This study was designed to assess the reproducibility of a 5 minute recording of the CIPA using the NeuroScope and compare it with that of conventional heart rate variability (HRV) measures. 50 subjects (31 male, 19 female) aged 21-77 years were recruited. 11 of these were apparently healthy and 39 were patients with a variety of cardiological problems. Two 5 minute recordings of CIPA and RR intervals for each subject were made with a 2-3 minute break between recordings. The limits of agreement for the CIPA were such that the second estimate was between 72 and 140% of the first estimate, 95% of the time. The limits of agreement for other indices of HRV, namely SDNN, rMSSD, In low-frequency HRV and In high-frequency HRV, were 61-157%, 68-153%, 77-131% and 76-133%. The short-term reproducibility of a 5 minute recording of CIPA is moderate and better than simple time-domain but not frequency-domain measures of HRV.

Correlation and stability of heart rate and ventricular ectopy variability in patients with congestive heart failure

The stability of indexes of heart rate variability and their possible association with spontaneous variability of ventricular ectopy was examined in 13 patients with advanced congestive heart failure over 14 consecutive days of 24-hour ambulatory electrocardiographic recording. It was found that time and frequency domain measures of heart rate variability are stable over time and are inversely correlated with spontaneous variability of ventricular ectopy.

Phase I study of cinchonine, a multidrug resistance reversing agent, combined with the CHVP regimen in relapsed and refractory lymphoproliferative syndromes

Overexpression of P-glycoprotein (P-gp) in cancer cells reduces intracellular accumulation of various anticancer drugs including anthracyclines and vinca alkaloids. This multidrug resistance (MDR) phenotype can be reversed in vitro by a number of non-cytotoxic drugs. We have identified the quinine's isomer cinchonine as a potent MDR reversing agent, both in vitro and in animal models. Here, we report an open phase I dose escalation trial in patients with refractory or relapsed malignant lymphoid diseases. Cinchonine dihydrochloride was administered by continuous i.v. infusion for 48 h and escalated over five dose levels ranging from 15 to 35 mg/kg/d. Cinchonine infusion started 24 h before i.v. doxorubicin (25 mg/m2), vinblastine (6 mg/m2), cyclophosphamide (600 mg/m2) and methylprednisolone (1 mg/kg/d) (CHVP regimen) and lasted for 24 h after chemotherapy infusion. Thirty-four patients received 87 cycles of CHVP/cinchonine. The MTD of cinchonine administered by continuous i.v. infusion was 30 mg/kg/d. Prolonged cardiac repolarization was the main dose-limiting toxicity. No ventricular arrhythmia including 'torsade de pointes' was observed. An MDR reversing activity was identified in the serum from every patient and correlated with cinchonine serum level. When infused at 30 mg/kg/d, cinchonine demonstrated a limited influence on doxorubicin pharmacokinetic. We conclude that i.v. infusion of cinchonine might be started 12 h before MDR-related chemotherapy infusion and requires continuous cardiac monitoring but no reduction of cytotoxic drug doses.

Effects of short-term psychological stress on the time and frequency domains of heart-rate variability

The purpose of this study was to test the hypothesis that short-term psychological stress produces significant changes in sympathovagal activity. A simple, noninvasive method was used measuring the timing and frequency of heart-rate variability (HRV). 30 normal healthy subjects were assigned into two age- and sex matched groups. In the experimental condition a 5-min. psychological stress test, predominantly based on the Stroop Word Color Conflict Test, was employed in a competitive setting and included a financial inducement to produce psychological strain. Analysis showed that during psychological stress a significant reduction in the timing and frequency of heart rate variability was observed. The standard deviation of interbeat intervals decreased. A significant increase in heart rate was also observed. Within the frequency domain, a significant reduction in the high frequency component of HRV and a significant increase in the low frequency component were observed. There was also a significant increase in the low frequency to high frequency ratio. Self-evaluation of physical tension and emotional state measured by visual analog scales also showed significant increases following psychological stress. No significant differences were observed on any variables within the control group. The results indicate a shift towards sympathetic predominance as a result of parasympathetic withdrawal and demonstrates that this psychological stress test is effective in provoking a characteristic defence-arousal reaction. This simple, cost-effective method of analysing heart rate variability is suitable for detection of short-term changes in sympathovagal balance.

Reproducibility of abnormal heart rate variability indices: the case of hypertensive sleep apnoea syndrome

Before heart rate variability can be used as an investigational tool in the clinical setting, its reproducibility must be known. We studied heart rate variability four times during 44 weeks in 15 hypertensive patients with sleep apnoea syndrome. Time and frequency domain analytical approaches were used during the spontaneous and controlled breathing tests, orthostatic manoeuvre and the cold pressor test. Alterations in resting heart rate were taken into account using the coefficient of component variance. In general, the response of heart rate variability was abnormal and variability was reduced in the hypertensive patients with sleep apnoea syndrome compared with reference data. Time domain measures of heart rate variability demonstrated generally better reproducibility over four recordings than frequency domain measures in these hypertensive patients with sleep apnoea syndrome. On the other hand, the reproducibility of frequency domain measures depended on the specific conditions: during orthostatic manoeuvre and cold pressor test the best reproducibility was found using normalized units. In the reference data set, there were no significant differences between the two heart rate variability recordings during any of the autonomic nervous function tests. In this follow-up study we found that simple procedures such as the controlled breathing test show good reproducibility. More complex tests such as the orthostatic manoeuvre require special attention in order to obtain acceptable reproducibility of heart rate variability measurements. Quantitatively minor changes in the variability indices when the overall variability is reduced exert major effects on the results. Therefore we suggest that reproducibility data obtained in healthy volunteers are not valid for the interpretation of data in patients with damaged cardiovascular autonomic control.

Prospective study of heart rate variability and mortality in chronic heart failure: results of the United Kingdom heart failure evaluation and assessment of risk trial (UK-heart)

BACKGROUND

Patients with chronic heart failure (CHF) have a continuing high mortality. Autonomic dysfunction may play an important role in the pathophysiology of cardiac death in CHF. UK-HEART examined the value of heart rate variability (HRV) measures as independent predictors of death in CHF.

METHODS AND RESULTS

In a prospective study powered for mortality, we recruited 433 outpatients 62+/-9.6 years old with CHF (NYHA functional class I to III; mean ejection fraction, 0.41+/-0.17). Time-domain HRV indices and conventional prognostic indicators were related to death by multivariate analysis. During 482+/-161 days of follow-up, cardiothoracic ratio, SDNN, left ventricular end-systolic diameter, and serum sodium were significant predictors of all-cause mortality. The risk ratio for a 41.2-ms decrease in SDNN was 1.62 (95% CI, 1.16 to 2.44). The annual mortality rate for the study population in SDNN subgroups was 5.5% for >100 ms, 12.7% for 50 to 100 ms, and 51.4% for <50 ms. SDNN, creatinine, and serum sodium were related to progressive heart failure death. Cardiothoracic ratio, left ventricular end-diastolic diameter, the presence of nonsustained ventricular tachycardia, and serum potassium were related to sudden cardiac death. A reduction in SDNN was the most powerful predictor of the risk of death due to progressive heart failure.

CONCLUSIONS

CHF is associated with autonomic dysfunction, which can be quantified by measuring HRV. A reduction in SDNN identifies patients at high risk of death and is a better predictor of death due to progressive heart failure than other conventional clinical measurements. High-risk subgroups identified by this measurement are candidates for additional therapy after prescription of an ACE inhibitor.

Reproducibility of tilt table testing in patients with vasovagal syncope and its relation to variations in autonomic nervous system activity

To assess the variability of head-up tilt table testing, 35 patients with vasovagal syncope, shown by a positive tilt table test, underwent a second test 1 week later. Also, on the day before each test, spectral and time-domain indexes of heart rate variability were derived from Holter recordings to examine the stability of autonomous nervous system activity in these patients. Fifteen healthy volunteers served as a control group and also underwent two tilt table tests with prior Holter monitoring. Twenty-one (60%) of the 35 patients had a second positive test. None of the patients in the control group experienced syncope during either of the tests. The heart rate variability measures in the control group varied slightly from 1 day to the other, in contrast to the syncopal patients, where only low frequency spectral power and the mean of all 5-minute standard deviations of RR intervals showed comparable behavior. The indexes which reflect parasympathetic activity exhibited significant fluctuations in the syncopal patients. These fluctuations were due entirely to the patients who did not reproduce the outcome of the tilt table test, where high parasympathetic tone was associated with the positive test and normal parasympathetic tone with the negative test. In contrast, the patients with two positive tests had high parasympathetic tone during both test periods, with low individual variability. In conclusion, patients with vasovagal syncope show variations in vagal autonomic tone and appear to be more prone to syncope when their parasympathetic tone is elevated.

Short- and long-term reproducibility of heart rate variability in patients with long-standing type I diabetes mellitus

Heart rate variability (HRV) has been used to assess cardiac autonomic function noninvasively, understand the pathophysiologic mechanisms of heart disease, evaluate therapy, and assess long-term prognosis. We examined both the short- and long-term reproducibility of the time and frequency domain HRV parameters in 23 type I diabetics over a 12-month interval. Entry criteria included juvenile onset diabetes before age 35 years, >24-year duration of diabetes, diabetes difficult to control, and albuminuria. Standardized noninvasive autonomic testing and 24-hour ambulatory electrocardiographic recordings were obtained. Fifteen men and 8 women (mean age 36.7 years) were enrolled. Fifty-three percent of the men and 75% of the women were smokers, and women had higher cholesterol than men. All HRV parameters were markedly decreased when compared with normal persons. Using Pearson correlation, the time domain indicators of parasympathetic activity demonstrated very strong correlations at 3 and 6 months compared with baseline, with good correlations at 1 year. The average SD of all 5-minute RR intervals maintained a very strong correlation for the entire year (r >0.94). In the frequency domain, the measures of parasympathetic and sympathetic activity maintained a solid correlation for the entire study period. Reproducibility of HRV was also examined using repeated-measures analysis of variance. The time and frequency domain parameters demonstrated very little variation over the study period of 12 months. Thus, our investigation demonstrated that HRV in long-term diabetics using 24-hour ambulatory recordings is abnormal and reproducible over a 12-month interval; very little variation in all HRV parameters, especially in parameters of parasympathetic activity, occurred during the study period.

Effects of digoxin on time domain measures of heart rate variability in patients with stable chronic cardiac failure: withdrawal and comparison group studies

The effect on heart rate variability of adding digoxin to a diuretic and ACE inhibitor was studied in patients with chronic stable cardiac failure. Digoxin was found to increase heart rate variability, especially those measures of heart rate variability thought to represent parasympathetic activity. The withdrawal of digoxin led to a decrease in heart rate variability to pre-treatment levels. Whilst digoxin in standard doses does not alter prognosis in chronic cardiac failure, it does have potentially beneficial neurohumoral effects. If the increase in heart rate variability, which represents beneficial neurohumoral modulation, can be divorced from the potentially detrimental effects, perhaps by using smaller doses, then there may be a role for digoxin in the treatment of chronic cardiac failure.

Preservation of autonomic function following successful reperfusion with streptokinase within 12 hours of the onset of acute myocardial infarction

Successful reperfusion following thrombolysis results in increased heart rate variability in the first 24 hours after administration. Preservation of autonomic function may contribute to improved prognosis when coronary artery patency is restored with intravenous thrombolysis.

Reproducibility of time-domain indexes of heart rate variability in patients with vasovagal syncope

The aim of this study was to examine whether the indexes of heart rate variability (HRV) are stable from day-to-day in patients with vasovagal syncope and whether the stability of the HRV indexes is linked with that of the clinical results of the tilt test. Nineteen patients with a history of syncopal episodes and a positive tilt test underwent a second test 1 week later. Of these, 11 (group P-P) also had a positive second test, whereas 8 (group P-N) had a negative second test. Fifteen healthy volunteers served as a control group. Five time domain indexes were derived: the mean of all coupling intervals between normal beats (mean NN), the SD about the mean of all coupling intervals between normal beats (SDNN), the mean of all 5-minute standard deviations of NNs (SD), the proportion of adjacent normal RR intervals differing by >50 ms (pNN50), the root-mean-square of the difference between successive RRs (rMSSD) and the standard deviations of 5-minute mean NN intervals (SDANN). The control group showed good reproducibility of all HRV indexes (slope 0.86 to 0.97). The syncopal patients taken as a whole had significantly less reproducibility than the controls in the pNN50 parameter. This difference was due entirely to the patients in the P-N group, who had a remarkable lack of reproducibility in both the pNN50 and rMSSD measures (slope pNN50, 0.52; rMSSD, 0.78), whereas the P-P group had a reproducibility of all HRV indexes, which was no different from that in controls (slope 0.83 to 1.04). In patients with vasovagal syncope, certain HRV measures that express parasympathetic tone did not show the high reproducibility found in normal subjects. Syncopal patients who lack reproducibility in these HRV parameters also show a lack of reproducibility in the clinical result of tilt testing.