ABC | Volume 113, Nº4, October 2019

Original Article Introduction of Application of Gini Coefficient to Heart Rate Variability Spectrum for Mental Stress Evaluation Miguel Enrique Sánchez-Hechavarría, 1 Shreya Ghiya, 2 Ramon Carrazana-Escalona, 3 Sergio Cortina-Reyna, 3 Adán Andreu-Heredia, 3 Carlos Acosta-Batista, 4 Nicolás Armando Saá-Muñoz 5 Departamento de Ciencias Básicas y Morfología - Facultad de Medicina - Universidad Católica de la Santísima Concepción, 1 Concepción – Chile Department of Kinesiology, San Francisco State University, 2 San Francisco – USA Departamento de Ciencias Basicas Biomédicas - Facultad de Medicina 1 - Universidad de Ciencias Médicas de Santiago de Cuba, 3 Santiago de Cuba – Cuba Hospital Universitario Calixto García - Universidad de Ciencias Médicas de La Habana, 4 La Habana – Cuba Centro de Simulación - Departamento de Ciencias Clínicas y Preclínicas - Facultad de Medicina - Universidad Católica de la Santísima Concepción, 5 Concepción – Chile Mailing Address: Miguel Enrique Sánchez-Hechavarría • Alonso de Ribera, 2850. Postal Code 4090541. Concepción – Chile Email: misanchez@ucsc.cl , miguel.sanchez881119@gmail.com Manuscript received June 09, 2018, revised manuscript March 05, 2019, accepted May 15, 2019 DOI: 10.5935/abc.20190185 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. (Arq Bras Cardiol. 2019; 113(4):725-733) Keywords: Gini Coefficient; Heart, Rate; Stress, Psychological/physiopathology; Action, Spectrum; Parasympathetic Nervous System; Simpathetic Nervous System. Introduction The Gini coefficient is a statistical tool typically used in economics to measure income inequality. However, it can be applied to any data with an unequal distribution, including Heart Rate Variability (HRV). HRV is the spectrum of time interval between successive heartbeats (RR interval) over a specific period. This study proposes a novel application of the Gini coefficient to measure the inequality of the power spectral density of RR intervals. Physical or mental imbalance caused by noxious stimuli can induce stress to normal homeostasis. If the stress on the system becomes chronic, the sympathetic nervous system stays activated, which can cause physical, psychological, and behavioral abnormalities. 1 Sympathetic nervous system sensitivity to mental stress increases over time and it can increase the risk of future cardiovascular diseases. 2 HRV measurement has been adopted as a non-invasive and relatively easy method for objective assessment of the severity of stress. 3 It is a physiological phenomenon of variation in the time interval between heartbeats (RR interval) and is commonly used as a measure of autonomic nervous system activity. 4-7 Power spectral density (PSD) describes the transformation of periodic oscillations of the heart-rate signals 725