Introduction of Application of Gini Coefficient to Heart Rate Variability Spectrum for Mental Stress Evaluation

Abstract Background: The Gini coefficient is a statistical tool generally used by economists to quantify income inequality. However, it can be applied to any kind of data with unequal distribution, including heart rate variability (HRV). Objectives: To assess the application of the Gini coefficient to measure inequality in power spectral density of RR intervals, and to use this application as a psychophysiological indicator of mental stress. Methods: Thirteen healthy subjects (19 ± 1.5 years) participated in this study, and their RR intervals were obtained by electrocardiogram during rest (five minutes) and during mental stress (arithmetic challenge; five minutes). These RR intervals were used to obtain the estimates of power spectral densities (PSD). The limits for the PSD bands were defined from 0.15 to 0.40 Hz for high frequency band (HF), from 0.04 to 0.15 Hz for low frequency band (LF), from 0.04 to 0.085 Hz for first low frequency sub-band (LF1) and from 0.085 to 0.15 Hz for second low frequency sub-band (LF2). The spectral Gini coefficient (SpG) was proposed to measure the inequality in the power distribution of the RR intervals in each of above-mentioned HRV bands. SpG from each band was compared with its respective traditional index of HRV during the conditions of rest and mental stress. All the differences were considered statistically significant for p < 0.05. Results: There was a significant decrease in HF power (p = 0.046), as well as significant increases in heart rate (p = 0.004), LF power (p = 0.033), LF2 power (p = 0.019) and LF/HF (p = 0.002) during mental stress. There was also a significant increase in SpG(LF) (p = 0.009) and SpG(LF2) (p = 0.033) during mental stress. Coefficient of variation showed SpG has more homogeneity compared to the traditional index of HRV during mental stress. Conclusions: This pilot study suggested that spectral inequality of Heart Rate Variability analyzed using the Gini coefficient seems to be an independent and homogeneous psychophysiological indicator of mental stress. Also, HR, LF/HF, SpG(LF) of HRV are possibly important, reliable and valid indicators of mental stress.

摘要 研究背景：基尼系数（Gini coefficient）是经济学家普遍用于量化收入不平等的统计工具，但其亦可应用于任何存在分布不均的数据集，包括心率变异性（heart rate variability, HRV）。研究目的：评估基尼系数在测量RR间期功率谱密度不平等中的应用价值，并将该应用作为心理应激的心理生理指标。研究方法：本研究纳入13名健康受试者（年龄19±1.5岁），在静息状态（5分钟）与心理应激状态（算术挑战任务，5分钟）下通过心电图采集其RR间期，并基于此计算功率谱密度（power spectral density, PSD）估计值。功率谱频段的划分标准为：高频段（high frequency band, HF）0.15~0.40 Hz，低频段（low frequency band, LF）0.04~0.15 Hz，第一低次子频段（first low frequency sub-band, LF1）0.04~0.085 Hz，第二低次子频段（second low frequency sub-band, LF2）0.085~0.15 Hz。本研究提出频谱基尼系数（spectral Gini coefficient, SpG）以量化上述各HRV频段内RR间期功率分布的不平等程度。将各频段的SpG与对应传统HRV指标分别在静息与心理应激条件下进行比较，所有统计学差异均以p<0.05作为显著性判定标准。研究结果：心理应激状态下，高频段功率（p=0.046）显著降低，而心率（p=0.004）、低频段功率（p=0.033）、第二低次子频段功率（p=0.019）及LF/HF比值（p=0.002）均显著升高；同时，低频段频谱基尼系数SpG(LF)（p=0.009）与第二低次子频段频谱基尼系数SpG(LF2)（p=0.033）亦显著升高。变异系数分析显示，相较于传统HRV指标，心理应激状态下的SpG具有更高的同质性。研究结论：本初步试点研究表明，基于基尼系数分析的心率变异性频谱不平等性可作为一种独立且一致性良好的心理应激心理生理指标。此外，心率、LF/HF比值及HRV的SpG(LF)或许是重要、可靠且有效的心理应激评估指标。