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Heart rate (HR) variability (HRV) during ambulatory recordings may be affected by individual differences in daily physical activity (PA). However, the influence of various levels of PA on different measures of HRV is not exactly known. We examined the association between simultaneously measured HRV and objective PA data obtained with an accelerometer during waking hours among 45 healthy adults. Bouts of PA were identified from minute-by-minute accelerometer data as metabolic equivalent (METs) values and calculated as mean METs for 30 min. HRV was analyzed concurrently. Within-individual correlation analyses and sign tests were performed to study the relationships between various HRV indexes and PA. The mean PA time was 15:44 [+ or -] 1:01 h, and the mean MET was 1.91 [+ or -] 0.14. HR and sample entropy, but not the other measures of HRV, had a significant relationship with PA, as shown by both correlation analyses (r = 0.64, P = 0.021, and r = -0.55, P = 0.022, respectively) and sign tests (P < 0.0001 for both). Beat-to-beat R-R interval fluctuation expressed as SD 1 also demonstrated a significant relation to PA according to the sign test (P = 0.037) and a trend of association according to the correlation analysis (r = -0.40, P = 0.129). The complexity measure of HRV, in addition to average HR and the short-term index of HRV (SD 1), is significantly influenced by the level of PA during ambulatory conditions. Long-term HRV indexes remained relatively stable at various activity levels, making them the most robust indexes for the assessment of cardiac autonomic function during free-running ambulatory conditions. accelerometer; cardiovascular autonomic regulation doi:10.1152/ajpheart.00856.2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ahj.2009.07.023 Byline: Jari A. Laukkanen (a)(b), Timo H. Makikallio (b)(c), Jussi Kauhanen (a), Sudhir Kurl (a) Abstract: Adrenoceptors mediate contraction of vascular smooth muscle and induce coronary vasoconstriction in humans. A deletion variant of the human [alpha].sub.2B-adrenoreseptor of glutamic acid residues has been associated with impaired receptor desensitization. This receptor variant could, therefore, be involved in cardiovascular diseases associated with enhanced vasoconstriction. Our aim was to study whether an insertion/deletion (I/D) polymorphism in the [alpha].sub.2B-adrenoceptor gene is associated with the risk for sudden cardiac death. Author Affiliation: (a) Research Institute of Public Health, School of Public Health and Clinical Nutrition, University of Kuopio, Kuopio, Finland (b) Department of Internal Medicine, Lapland Central Hospital, Rovaniemi, Finland (c) Division of Cardiology, Department of Internal Medicine, University of Oulu, Oulu, Finland Article History: Received 22 April 2009; Accepted 10 July 2009 Article Note: (footnote) The US National Heart, Lung, and Blood Institute grant HL44199.

The determinants of heart rate (HR) recovery after exercise are not well known, although attenuated HR recovery is associated with an increased risk of cardiovascular mortality. Because acetylcholine receptor subtype M2 (CHRM2) plays a key role in the cardiac chronotropic response, we tested the hypothesis that, in healthy individuals, the CHRM2 gene polymorphisms might be associated with HR recovery 1 rain after the termination of a maximal exercise test, both before and after endurance training. The study population consisted of sedentary men and women (n = 95, 42 [+ or -] 5 yr) assigned to a training (n = 80) or control group (n = 15). The study subjects underwent a 2-wk laboratory-controlled endurance training program, which included five 40-min sessions/wk at 70-80% of maximal HR. HR recovery differed between the intron 5 rs324640 genotypes at baseline (C/C, -33 [+ or -] 10; C/T, -33 [+ or -] 7; and T/T, -40 [+ or -] 11 beats/min, P = 0.008). Endurance training further strengthened the association: the less common C/C homozygotes showed 6 and 12 beats/rain lower HR recovery than the C/T heterozygotes or the T/T homozygotes (P = 0.001), respectively. A similar association was found between A/T transversion at the 3'-untranslated region of the CHRM2 gene and HR recovery at baseline (P = 0.025) and after endurance training (P = 0.005). These data suggest that DNA sequence variation at the CHRM2 locus is a potential modifier of HR recovery in the sedentary state and after short-term endurance training in healthy individuals. genotype; exercise training; cardiovascular autonomic function

The amplitude of low-frequency (LF) oscillations of heart rate (HR) usually reflects the magnitude of sympathetic activity, but during some conditions, e.g., physical exercise, high sympathetic activity results in a paradoxical decrease of LF oscillations of HR. We tested the hypothesis that this phenomenon may result from a feedback inhibition of sympathetic outflow caused by circulating norepinephrine (NE). A physiological dose of NE (100 ng x [kg.sup.-1] x [min.sup.-1]) was infused into eight healthy subjects, and infusion was continued alter [alpha]-adrenergic blockade [with phentolamine (Phe)]. Muscle sympathetic nervous activity (MSNA) from the peroneal nerve, LF (0.04-0.15 Hz) and high frequency (HF; 0.15-0.40 Hz) spectral components of HR variability, and systolic blood pressure variability were analyzed at baseline, during NE infusion, and during NE infusion after Phe administration. The NE infusion increased the mean blood pressure and decreased the average HR (P < 0.01 for both). MSNA (10 [+ or -] 2 vs. 2 [+ or -] 1 bursts/min, P < 0.01), LF oscillations of HR (43 [+ or -] 13 vs. 35 [+ or -] 13 normalized units, P < 0.05), and systolic blood pressure (3.1 [+ or -] 2.3 vs. 2.0 [+ or -] 1.1 mm[Hg.sup.2], P < 0.05) decreased significantly during the NE infusion. During the NE infusion after PHE, average HR and mean blood pressure returned to baseline levels. However, MSNA (4 [+ or -] 2 bursts/ min), LF power of HR (33 [+ or -] 9 normalized units), and systolic blood pressure variability (1.7 [+ or -] 1.1 mm[Hg.sup.2]) remained significantly (P < 0.05 for all) below baseline values. Baroreflex gain did not change significantly during the interventions. Elevated levels of circulating NE cause a feedback inhibition on sympathetic outflow in healthy subjects. These inhibitory effects do not seem to be mediated by pressor effects on the baroreflex loop but perhaps by a presynaptic autoregulatory feedback mechanism or some other mechanism that is not prevented by a nonselective [alpha]-adrenergic blockade. heart rate dynamics; catecholamines; blood pressure oscillation

Saturation of high-frequency oscillations of R-R intervals in healthy subjects and patients after acute myocardial infarction during ambulatory conditions. Am J Physiol Heart Circ Physiol 287: H1921-H1927, 2004. First published July 8, 2004; doi: 10.1152/ajpheart.00433.2004.--This study was designed to assess the relationship between R-R interval length and heart rate (HR) variability in healthy subjects and patients after an acute myocardial infarction (AMI). Twenty-four-hour ambulatory ECG recordings were obtained for 76 healthy subjects and 82 post-AMI patients. The high-frequency (HF, 0.15-0.4 Hz) spectral power of R-R intervals was analyzed in 5-min sequences over 24 h and plotted as a function of the corresponding mean R-R interval length. Quadratic regression model was used to study the relationship between R-R interval length and HF power. If a distinct deflection point (R-[R.sub.o]) occurred in the quadratic regression (r > 0.50) model before maximum R-R interval, indicating the plateau of HF power, the relationship between R-R interval and HF power was defined as saturated. Otherwise, the relationship was defined as linear (r > 0.50) or low correlated (r > 0.50). The relationship was saturated in 35, linear in 38, and low correlated in 3 healthy subjects. In post-AMI patients, the relationship was saturated in 9 subjects, linear in 44 subjects, and low correlated in 29 patients. The HF power analyzed from the 24-h period did not differ between the saturated and linear groups, but when analyzed from the linear portion only, HF spectral power was smaller in the linear than the saturated group both among healthy subjects (P < 0.05) and post-AMI patients (P < 0.05). Saturation of the HF oscillations of R-R intervals is a common phenomenon in healthy subjects and also present in post-AMI patients during ambulatory conditions. This saturation effect may bias the quantification of cardiac vagal function when HR variability is analyzed from Holter recordings. heart rate variability; cardiovascular regulation; vagal activity

Individual responses to aerobic training vary from almost none to a 40% increase in aerobic fitness in sedentary subjects. The reasons for these differences in the training response are not well known. We hypothesized that baseline cardiovascular autonomic function may influence the training response. The study population included sedentary male subjects (n = 39, 35 [+ or -] 9 yr). The training period was 8 wk, including 6 sessions/wk at an intensity of 70-80% of the maximum heart rate for 30-60 min/session. Cardiovascular autonomic function was assessed by measuring the power spectral indexes of heart rate variability from 24-h R-R interval recordings before the training period. Mean peak [O.sub.2] uptake increased by 11 [+ or -] 5% during the training period (range 2-19%). The training response correlated with age (r = -0.39, P = 0.007) and with the values of the high-frequency (HF) spectral component of R-R intervals (HF power) analyzed over the 24-h recording (r = 0.46, P = 0.002) or separately during the daytime hours (r = 0.35, P = 0.028) and most strongly during the nighttime hours (r = 0.52, P = 0.001). After adjustment for age, HF power was still associated with the training response (e.g., P = 0.001 analyzed during nighttime hours). These data show that cardiovascular autonomic function is an important determinant of the response to aerobic training among sedentary men. High vagal activity at baseline is associated with the improvement in aerobic power caused by aerobic exercise training in healthy sedentary subjects. cardiovascular autonomic function; vagal activity; aerobic training response

Cardiac research has established that the heart rate is affected by accentuated sympathovagal interaction, causing deterministic chaotic behaviour. Experiments to measure the heart beat's fixed R-R interval are described. The ongoing measurement of the heart beat's R-R interval is more appropriate in dynamics studies than commonly used methods of establishing heart rate variability.

The effects of age and physical fitness on the modulation of cardiac rate by the activity of the vagus nerve are investigated by monitoring the variability of the R-R segment in the Poincare plots derived from male volunteers at rest and during exercise. Results indicate that the age factor is evident in the vagal activity at rest while the effects of physical conditioning are seen during exercise.

The dynamics of beat-to-beat heart rate (HR) were analyzed using the Poincare plot (R-R- interval) method. HR was assessed with or without atropine exposure followed by exercise in healthy subjects. Atropine produced a progressive reduction in beat-to-beat R-R interval variability (SD1) which was sustained at the zero level during exercise. It exerted similar effects on continuous long-term R-R interval variability (SD2), but produced the reverse results in the SD1/SD2 ratio.

Determinants and inter-subject variations of fractal and complexity measures of R-R interval variability were studied in a random population of 200 healthy middle-aged women (age 51 [+ or -] 6 yr) and 189 men (age 50 [+ or -] 6 yr) during controlled conditions in the supine and sitting positions. The short-term fractal exponent ([[Alpha].sub.1]) was lower in women than men in both the supine (1.18 [+ or -] 0.20 vs. 1.12 [+ or -] 0.17, P [is less than] 0.01) and sitting position (P [is less than] 0.001). Approximate entropy (ApEn), a measure of complexity, was higher in women in the sitting position (1.16 [+ or -] 0.17 vs. 1.07 [+ or -] 0.19, P [is less than] 0.001), but no gender-related differences were observed in ApEn in the supine position. Fractal and complexity measures were not related to any other demographic, laboratory, or lifestyle factors. Intersubject variations in a fractal measure, [[Alpha].sub.1] (e.g., 1.15 [+ or -] 0.20 in the supine position, z value 1.24, not significant), and in a complexity measure, ApEn (e.g., 1.14 [+ or -] 0.18 in the supine position, z value 1.44, not significant), were generally smaller and more normally distributed than the variations in the traditional measures of heart rate variability (e.g., standard deviation of R-R intervals 49 [+ or -] 21 ms in the supine position, z value 2.53, P [is less than] 0.001). These results in a large random population sample show that healthy subjects express relatively little interindividual variation in the fractal and complexity measures of heart rate behavior and, unlike the traditional measures of heart rate variability, they are not related to lifestyle, metabolic, or demographic variables. However, subtle gender-related differences are also present in fractal and complexity measures of heart rate behavior. gender; heart rate variability; nonlinear methods; risk factors

Tulppo, Mikko P., Richard L. Hughson, Timo H. Makikallio, K. E. Juhani Airaksinen, Tapio Seppanen, and Heikki V. Huikuri. Effects of exercise and passive head-up tilt on fractal and complexity properties of heart rate dynamics. Am J Physiol Heart Circ Physiol 280: H1081-H1087, 2001.--tk;1Passive head-up tilt and exercise result in specific changes in the spectral characteristics of heart rate (HR) variability as a result of reduced vagal and enhanced sympathetic outflow. Recently analytic methods based on nonlinear system theory have been developed to characterize the nonlinear features in HR dynamics. This study was designed to assess the changes in the fractal and complexity measures of HR behavior during the passive head-up tilt and during exercise. Fractal exponent ([[Alpha].sub.1]) and approximate entropy (ApEn), measures of short-term correlation properties and overall complexity of HR, respectively, along with spectral components of HR variability were analyzed during a passive head-up tilt test (n = 10) and a low-intensity steady-state exercise (n = 20) in healthy subjects. We observed that [[Alpha].sub.1] increased during the tilt test (from 0.85 [+ or -] 0.22 to 1.48 [+ or -] 0.20; P [is less than] 0.001) and during the exercise (from 1.00 [+ or -] 0.22 to 1.37 [+ or -] 0. 14; P [is less than] 0.001). ApEn also increased during the exercise (from 1.04 [+ or -] 0.11 to 1. 11 [+ or -] 0.08; P [is less than] 0.05), but it did not change during the tilt test. The normalized high-frequency spectral component decreased and the low-frequency component increased similarly during both the exercise and the tilt test (P [is less than] 0.001 for all). Exercise and passive tilt result in an increase of short-term fractal correlation properties of HR dynamics, which is related to changes in the balance between the low- and high-frequency oscillations in controlled situations. Overall complexity of HR dynamics increases during exercise but not during passive tilt. variability; approximate entropy; detrended fluctuation analysis

BACKGROUND. Previous cross-sectional studies have suggested that cardiac autonomic regulation, measured as heart rate (HR) variability, is altered upon ageing, and that altered HR variability may predict mortality. However, there are no longitudinal follow-up reports of the changes of HR dynamics in elderly subjects. AIM & METHOD. This study was designed to examine the longitudinal changes in the spectral, fractal, and complexity measures of HR variability in a random sample of 41 elderly subjects (mean age 69±4 years), who underwent repeated 24-hour Holter recordings at the baseline and after 16 years' follow-up. Several cardiovascular risk factors were determined at the baseline. RESULTS. A time-domain index, standard deviation of N-N intervals (SDNN) (142±34 msec versus 133±50 msec, not significant (NS)), and the high frequency spectral component of HR variability (219±222 msec 2 versus 268±287 msec 2 , NS) did not change during the follow-up. The low frequency power (LF) of HR variability decreased from 678±654 msec 2 to 436±651 msec 2 ( P <0.01). Among the fractal HR variability indexes, the power-law slope (ß) (-1.31±0.20 versus -1.47±0.21, P <0.001) and the short-term fractal exponent a1 (1.16±0.19 versus 1.06±0.18, P <0.01) decreased significantly. Approximate entropy remained unchanged. CONCLUSIONS. The magnitude of total HR variability and the respiratory vagal modulation of HR do not change over time in the elderly. However, the spectral and fractal characteristics of HR behavior still undergo alterations upon ageing. [ABSTRACT FROM AUTHOR]

BACKGROUND. Experimental studies have suggested that R-R interval dynamics during ventricular fibrillation (VF) have organized features, but whether dynamic behavior of non-sustained VF differs from sustained VF is unknown. AIM. The purpose of this study was to investigate whether the dynamics of R-R intervals during non-sustained VF differs from the dynamics during sustained VF. METHODS. A group of 67 patients undergoing routine implantable cardioverter defibrillator (ICD) testing was studied. Forty-three VF events containing mean of 38 local cardiac activation intervals before the termination by ICD shock were analyzed. From intracardiac electrogram recordings, the ratio between the short and long term variability (SD1/SD2) and fractal scaling exponent (α) were analyzed. After the initial analyses, data sets were randomized and reanalyzed. Local activation dynamics were then also compared in seven patients with both sustained and spontaneously terminating VF episodes. RESULTS. Randomization caused a change in the VF dynamics from organized toward less organized dynamics (α) from 1.08 ± 0.57 to 0.81 ± 0.45, P < 0.05 and SD1/SD2 from 0.80 ± 0.23 to 1.04 ± 0.20, P < 0.01). Spontaneously terminating VF showed more organized dynamics than sustained VF terminated by shock (P < 0.05). CONCLUSIONS. Local cardiac activation dynamics during initial phase of human VF shows organized dynamics. Spontaneously terminating VF episodes have more structured dynamics than sustained VF. Thus, the dynamic behavior of local cardiac activation intervals may be related to the maintenance of ventricular tachyarrhythmias. [ABSTRACT FROM AUTHOR]

Studies the heart rate dynamics before spontaneous onset of ventricular fibrillation (VF) in patients with healed myocardial infarcts. Clinical, angiographic and electrophysiologic data; Heart rate interval variability Possible pathophysiologic mechanisms of abnormal short-term dynamics and the propensity for VF.

Assesses the utility of selected dynamic analysis methods of heart rate variability in predicting mortality in a population of patients with acute myocardial infarction and depressed left ventricular function. Ambulatory electrocardiographic recordings and analysis of heart rate variability; Univariable predictors of mortality and their relative risks.

Studies whether fractal and complexity measures of heart rate (HR) behavior are altered in patients with stable angina pectoris. Altered correlation properties in PR interval dynamics of patients with stable angina; Better performance of short-term fractal scaling exponent in detecting abnormalities in HR behavior.

Explores benefits of dynamic analysis of heart beat variability towards the prediction of ventricular tachycardia (VT) post myocardial infarction. Clinical profile of postinfarction patients; Low frequency spectral power in study groups; Short-term scaling exponent in VT groups.