ABC | Volume 113, Nº4, October 2019

Original Article Sánchez-Hechavarría et al. Inequality in HRV spectrum for evaluation of mental stress Arq Bras Cardiol. 2019; 113(4):725-733 Table 1 – Traditional and Spectral Gini Indices of Heart Rate Variability during rest and mental stress Variables Rest Mental Stress Effect Size Gates' delta p value X [*] SD [¥] CV (%) X [*] SD [¥] CV (%) HRV Index HR (bpm) 80.32 [75.5] 10.52 [15.6] 13.09 96.41 [91.4] 11.78 [21.1] 12.22 1.52 Large 0.004 RMSSD (ms) 47.36 [44.10] 22.95 [26.45] 48.45 33.52 [32.20] 17.98 [27.30] 53.63 0.60 Medium 0.009 EDR (Hz) 0.24 [0.25] 0.05 [0.06] 20.82 0.21 [0.22] 0.04 [0.08] 23.73 -0,6 Small 0.064 Traditional Indices [Bandwidth] LF (ms2/Hz) |0.04-0.15 Hz| 844.78 [689.27] 627.95 [789.86] 74.33 1373.44 [1123.02] 1003.01 [1560.15] 73.02 0.84 Medium 0.033 HF (ms2/Hz) |0.15-0.40 Hz| 1281.96 [986.91] 1429.36 [848.70] 111.49 758.91 [517.83] 691.12 [1001.75] 91.06 -0.36 Small 0.046 LF1 (ms2/Hz) |0.04-0.085 Hz] 291.79 [283.39] 200.64 [180.87] 68.76 267.57 [235.48] 174.23 [224.52] 65.11 -0.12 Small 0.650 LF2 (ms2/Hz) |0.085-0.15 Hz| 533.69 [435.59] 421.36 [580.17] 78.95 1086.58 [726.52] 861.88 [1308.89] 79.32 1.31 Large 0.019 LF/HF (ratio) 1.00 [0.69] 0.88 [0.79] 88.2 2.31 [1.93] 0.93 [1.60] 40.34 1.48 Large 0.002 Gin Spectral Indices [Bandwidth] SpG(LF) |0.04-0.15 Hz| 0.29 [0.29] 0.06 [0.08] 20.40 0.40 [0.39] 0.10 [0.16] 25.62 1.66 Large 0.009 SpG(HF) |0.15-0.40 Hz| 0.50 [0.50] 0.08 [0.15] 17.35 0.45 [0.47] 0.09 [0.14] 20.00 -0.54 Small 0.133 SpG(LF1) |0.04-0.085 Hz| 0.24 [0.21] 0.06 [0.07] 25.70 0.23 [0.22] 0.08 [0.12] 36.86 -0.19 Small 0.382 SpG(LF2) |0.085-0.15 Hz| 0.28 [0.27] 0.07 [0.12] 26.22 0.35 [0.38] 0.10 [0.16] 29.71 0.85 Medium 0.033 p < 0.05. Mean (X), SD: standard deviation; CV: coefficient of variation; HRV: heart rate variability; HR: heart rate; RMSSD: Root Mean Square of the Successive Differences; EDR: ECG-derived; Respiration Rate; LF: low frequency; HF: high frequency; SpG: spectral; Gini coefficient.Median [*] and Interquartile Range [¥]. To the best of our knowledge, this is the first study to apply the Gini coefficient to power spectrums of HRV signal/RR intervals to measure inequality in distribution of power. Conceptually, a Gini coefficient of zero means that the power is distributed equally for all frequencies within a spectral bandwidth. In contrast, a Gini coefficient of 1 suggests that there is a single frequency with the most power within a specific spectral bandwidth, and all other frequencies in the bandwidth have no power. In other words, increase in the Gini coefficient value suggests that there are few frequencies with the most power within that frequency band compared to before. The results showed that there was a significant increase in SpG(LF) during mental stress compared to rest, meaning that during mental stress, not only there was an increase in total power in LF, but also the total power distribution became more unequal and certain frequencies gained the most power. It is noteworthy that the LF2 sub-band (0.085-0.15 Hz) showed increased inequality, as changes in SpG(LF) and Table 2 – Correlations between Traditional and Spectral Gini Indices of HRV during mental stress and rest HRV Indices SpG(LF) SpG(HF) SpG(LF1) SpG(LF2) Rest Mental Stress Total Rest Mental Stress Total Rest Mental Stress Total Rest Mental Stress Total HR 0.313 -0.413 0.587‡* 0.306* -0.140 -0.110 0.566†* -0.112 ,151 -0.025 0.463 0.409 RMSSD -0.084 0.432 -0.029 -0.153 -0.446 -0.173 0.192* 0.053 ,122 -0.216 0.267 -0.084 EDR -0.177 -0.47 -0.466† -,031 -0.293 -0.055 -0.330* -0.037 -0.179 0.010 -0.404 -0.320 LF 0.264* -0.005 0.296* - - - -0.335* 0.016 -0.177* 0.593 †* 0.180 0.333* HF - - - -0.192* 0.078 -0.026* - - - - - - LF/HF 0.220* -0.039 0.397 †* 0.253* -0.038 -0.002* 0.104* -0.207 -0.008* 0.379* -0.048 0.387* LF1 0.258* -0224 0.017* - - - -0.319* -0.054 -0.217* - - - LF2 0.231* 0.041 0.335* - - - - - -0.158* 0.582 †* 0.17 ,335 SpG(LF) - - - - - - 0.390* 0.153 0.177* 0.829 ‡ 0.682 † 0.698 ‡ Note: † p < 0.05; ‡ p < 0.01; *Spearman’s correlation, for the HRV indices that did not accept normal distribution in Shapiro-Wilk test. HRV: heart rate variability; HR: heart rate; RMSSD: Root Mean Square of the Successive Differences; EDR: ECG-derived; Respiration Rate; LF: low frequency; HF: high frequency; SpG: spectral; Gini coefficient. 728