Fractal frequency scaling of heart period variability is used as a concise index of overall cardiac control. However, no prior study has assessed within-individual reproducibility of fractal indices of heart period, or reported how the estimated indices respond to autonomic blockade. Therefore, we examined fractal properties of the heart period from ten young, healthy individuals during three separate experimental sessions under control (saline) conditions and twice under combined autonomic blockade (atenolol and atropine sulfate) conditions. Under each condition, R-R intervals were recorded with the subject in the supine and the 40 deg upright tilt positions during 20 min of controlled breathing in each position. We calculated the fractal scaling exponent using detrended fluctuation analysis and estimated confidence intervals of the scaling exponents for each R-R interval time series within each individual. In the control condition, upright tilt significantly increased the scaling exponents (from 0.73 ± 0.11 (±s.d., session 1), 0.72 ± 0.10 (session 2) and 0.75 ± 0.13 (session 3) to 0.82 ± 0.12, 0.82 ± 0.11 and 0.84 ± 0.10; Student's paired t-test, t= 2.79, P= 0.02; t= 2.80, P= 0.02; and t= 2.07, P= 0.07). However, neither the absolute scaling exponents nor their change in response to upright tilt were reproducible (Lin's concordance coefficient less than 0.9, P > 0.1 for all comparisons). Following autonomic blockade, the scaling exponents were significantly increased (supine: 1.08 ± 0.13 and 1.08 ± 0.14; tilt: 1.07 ± 0.21 and 1.08 ± 0.14) for both experimental sessions (two-way repeated-measures ANOVA; F17,1= 40.89, P < 0.001 and F17,1= 42.72, P < 0.001) regardless of position. However, within individuals, the scaling exponents failed to distinguish between control and blockade for half of the subjects in at least one experimental session. Thus, fractal scaling exponents are not reproducible within individuals and do not reliably reflect the autonomic mechanisms responsible for heart period variability. In fact, data from combined blockade suggest that physiological effects of autonomic outflow may mask intrinsic fractal behaviour of the sinoatrial node.
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机译：心周期变异性的分形频率定标用作总体心脏控制的简洁指标。但是，以前没有研究评估心律分形指数在个体内的可再现性，也没有报道估计的指数对自主神经阻滞的反应。因此，我们在对照（盐水）条件下进行的三个独立实验中，在组合的自主神经阻滞剂（阿替洛尔和硫酸阿托品）条件下进行了两次独立的实验，研究了十名年轻健康个体的心脏周期的分形特性。在每种情况下，R-R间隔记录为受试者在仰卧和40度直立倾斜位置中在每个位置的20分钟受控呼吸过程中。我们使用去趋势波动分析计算了分形标度指数，并估算了每个个体中每个R-R间隔时间序列的标度指数的置信区间。在控制条件下，直立倾斜显着增加了缩放指数（从0.73±0.11（±sd，第1阶段），0.72±0.10（第2阶段）和0.75±0.13（第3阶段）增加到0.82±0.12、0.82±0.11和0.84 ±0.10;学生配对t检验，t = 2.79，P = 0.02; t = 2.80，P = 0.02; t = 2.07，P = 0.07）。但是，绝对比例指数和它们对直立倾斜的响应变化均不可再现（Lin的一致性系数小于0.9，所有比较的P> 0.1）。自主神经阻滞后，两个实验阶段（双向重复测量方差分析； F17,1 = 40.89，双向：重复测量方差分析； F17,1 = 40.89）显着增加了缩放指数（仰卧：1.08±0.13和1.08±0.14；倾斜度：1.07±0.21和1.08±0.14）。 P <0.001，而F17,1 = 42.72，P <0.001），无论位置如何。然而，在个体中，缩放指数未能在至少一个实验阶段中区分一半对象的控制和封锁。因此，分形标度指数在个体内是不可再现的，并且不能可靠地反映出导致心周期变异的自主机制。实际上，来自联合封锁的数据表明，自主流出的生理效应可能掩盖了窦房结的固有分形行为。
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