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 3 – Values of Factor Loadings of Traditional and Spectral Gini Indices of Heart Rate Variability during rest and mental stress from Principal Component Analysis Variable Dimension 1 Dimension 2 HR -0.5240 1.3612 RMSSD -0.7615 0.1257 EDR -0.2707 -1.1683 LF 1.4742 0.2518 HF 1.2896 -0.6571 LF1 1.4674 -0.2996 LF2 1.3519 0.3851 LF/HF (ratio) -0.2641 1.2657 SpG(LF) 0.3048 1.4026 SpG(HF) 0.3397 0.0928 SpG(LF1) -0.2806 0.2438 SpG(LF2) 0.7623 1.0909 Explained variance by 0.3125 0.2578 Total explained variance: 0.5703 ROC curve and other efficacy values for Traditional and Spectral Gini Indices of Heart Rate Variability are described in Table 4. The cutoff points of the different indicators can be observed in the differentiation of the psychophysiological states obtained from the Youden Index of the ROC curve. Out of the 12 variables studied here, only HR (cutoff point: 83.350 bpm; p = 0.001), LF/HF (cutoff point: 1.02; p = 0.001) and SpG(LF) (cutoff point: 0.356; p = 0.011) show high values of sensitivity, specificity, Youden Index and area under the curve (p < 0.05). 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 Table 4 – Efficacy of Traditional and Spectral Gini Indices of Heart Rate Variability in the discrimination of rest and mental stress Variables Cutoff point Sensitivity Specificity Youden Index Area Under Curve p value HR 83.350 bpm 1.00 0.769 0.769 0.870 0.001 RMSSD 37.70 ms 0.385 0.307 -0.308 0.325 0.130 EDR 0.2299 Hz 0.385 0.307 -0.308 0.308 0.096 LF 1120.44 ms 2 /Hz 0.538 0.769 0.308 0.651 0.191 HF 623.83 ms 2 /Hz 0.385 0.230 -0.385 0.343 0.174 LF1 239.99 ms 2 /Hz 0.462 0.384 -0.154 0.450 0.663 LF2 581.42 ms 2 /Hz 0.769 0.692 0.462 0.698 0.086 LF/HF (ratio) 1.02 1.00 0.769 0.769 0.870 0.001 SpG(LF) 0.356 0.692 0.923 0.615 0.793 0.011 SpG(HF) 0.505 0.231 0.53846 -0.231 0.373 0.270 SpG(LF1) 0.203 0.538 0.0769 -0.385 0.420 0.489 SpG(LF2) 0.274 0.692 0.615 0.308 0.722 0.054 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. is a reflex of the parasympathetic activity, and that the LF and LF/HF components are a reflex of both sympathetic and parasympathetic activity. 4 Breathing rate can influence HRV variables noticeably. 14,30 Bernardi et al. 14 have further reported that regardless of the amount of stress involved in the mental task, low breathing rate usually contributes to increase in LF power of HRV. Although there was a decrease in breathing rate during stress compared to rest in the present study, the EDR was 0.21 ± 0.04 Hz or 12.6 ± 0.24 br/min, which is not within the LF components in the RR power spectrum. In other words, in the present study, respiration rate was not responsible for increased LF power during mental stress. There are few studies examining the contributing factors to LF power of HRV in depth. In their recent study, Roach et al. 31 reported that 75% of the contribution to LF power comes from fluctuations called ripples, and these ripples are probably due to arterial baroreceptor functions. Reyes del Paso et al. 32 have showed a strong association between baroreflex activity and mental stress. Vaschillo et al. 33 have investigated the subdivision of LF in two separate components in young binge drinkers and suggested that these two divisions functionally indicate two distinct physiological parameters. LF1 represents vascular tone baroreflex and LF2 represents heart rate baroreflex activity. As noted earlier, data analysis from the current study showed increased LF power and decreased HF power during mental stress, along with increased SpG(LF) and SpG(LF2). It is possible that, under stress, a healthy cardiovascular system generates more LF oscillations, especially with power mostly around 0.1Hz frequencies, to regain homeostasis. This possibility is supported by Bates et al., 34 who evaluated real‑time changes in RR interval spectrum in response to placebo and alcohol. Bates et al. 34 suggested that under alcohol or other adverse conditions, one of the main adaptations includes maintaining low frequency oscillations even at the expense of high frequency oscillations. This can also explain the lack of changes in SpG(HF) under mental stress. That study also suggested that low frequency oscillations are useful to generate resonance for better adaptation, and 0.1 Hz is one of several resonance frequencies. The current study supports significant increase in LF subdivision during mental stress, and future studies are recommended to investigate the association of 0.1Hz frequency to arterial baroreflex activity for better understanding of the mechanism of physiological adaptations during mental stress. 730