Background— Altered fractal heart rate (HR) dynamics occur during various disease states, but the physiological background of abnormal fractal HR behavior is not well known. We tested the hypothesis that the fractal organization of human HR dynamics is determined by the balance between sympathetic and vagal outflow. Methods and Results— A short-term fractal scaling exponent (α ₁ ) of HR dynamics, analyzed by the detrended fluctuation analysis (DFA) method, and the high-frequency (HF) and low-frequency (LF) spectral components of R-R intervals (0.15 to 0.4 Hz; n=13), along with muscle sympathetic nervous activity (MSNA) from the peroneus nerve (n=11), were assessed at rest and during cold face and cold hand immersion in healthy subjects. During cold face immersion, HF power increased (from 6.9±1.3 to 7.6±1.2 ln ms ² , P <0.01), as did MSNA (from 32±17 to 44±14 bursts/100 heartbeats, P <0.001), and LF/HF ratio decreased ( P <0.01). Cold hand immersion resulted in a similar increase in MSNA (from 34±17 to 52±19 bursts/100 heartbeats, P <0.001) but a decrease in HF spectral power (from 7.0±1.3 to 6.5±1.1 ln ms ² , P <0.05) and an increase in the LF/HF ratio ( P <0.05). The fractal scaling index α ₁ decreased in all subjects (from 0.85±0.27 to 0.67±0.30, P <0.0001) during cold face immersion but increased during cold hand immersion (from 0.77±0.22 to 0.97±0.20, P <0.01). Conclusions— The fractal organization of human HR dynamics is determined by a delicate interplay between sympathetic and vagal outflow, with the breakdown of fractal HR behavior toward more random dynamics occurring during coactivation of sympathetic and vagal outflow.

To investigate the combination of heart rate turbulence (HRT) and deceleration capacity (DC) as risk predictors in post-infarction patients with left ventricular ejection fraction (LVEF) > 30%. We enrolled 2343 consecutive survivors of acute myocardial infarction (MI) (30% (cumulative 5-year mortality rates of 37.9% and 7.8%, respectively). Among patients with LVEF > 30%, SAF identified another high-risk group of 117 patients with 37 deaths (cumulative 5-year mortality rates of 38.6% and 6.1%, respectively). Merging both high-risk groups (i.e. LVEF ≤ 30% and/or SAF) doubled the sensitivity of mortality prediction compared with LVEF ≤ 30% alone (21.1% vs. 42.1%, P < 0.001) while preserving 5-year mortality rate (38.2%). In post-MI patients with LVEF>30%, SAF identifies a high-risk group equivalent in size and mortality risk to patients with LVEF ≤ 30%.

Purpose:To test the utility of HR variability (HRV) in daily exercise prescription in moderately active (approximately two exercises per week) men and women.Methods:A total of 21 men and 32 women were divided into standard training (ST: males = 7 and females = 7), HRV-guided training (HRV-I:

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 ± 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 O₂ uptake increased by 11 ± 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.