Postgraduate A.S. Ushakov
Postgraduate N.E. Kleshchenkova
Associate Professor, Dr.Biol. A.V. Nenasheva
Postgraduate S.I. Astakhov
Postgraduate K.E. Ryabina
South Ural State University, Chelyabinsk

Keywords: heart rate variability, orthostatic test, spectral analysis, stress index.

Introduction. The study of the variability of circulatory indices, as markers of activity of the levels of regulation, is one of the central issues of the modern world developmental physiology [1, 2, 3]. Nowadays, cardiovascular diseases and risk factors are developing at a young age (children and adolescents) with ever increasing frequency. In this case, heart rate variability (HRV) records help detect cardiovascular autonomic disturbances at early stages [8].

However, to apply the normative clinical data obtained it is necessary to consider that the assessment of the functional state of the cardiovascular system in this age group was carried out mainly among adolescents who already had some disturbances, or among children doing sports professionally. Consequently, there is a need to obtain the normative data in virtually healthy children, too, with due regard to their gender, age and environment. In terms of modern educational space, an adolescent's body is repeatedly affected by the external and internal factors [3, 4]. Thus, a long-term educational process, an increased number of physical education lessons, preparation for examination tests, psychological stress cause stable distress, which, first and foremost, reflects upon the cardiovascular system and levels of its regulation [9]. The analysis of the temporal and frequency characteristics of the variability of circulatory indices helps evaluate heart rate regulation. Health deviations detected when acquiring the research data may indicate cardiac pathologies [10]. HRV is treated as a standard, reliable and non-invasive method of the overall quantitative and qualitative assessment of neurovegetative functions, made in a continuous, integrated manner [8].

Objective of the research was to examine heart rate variability at rest and in active orthostatic posture in adolescent girls not doing sports.

Methods and structure of the study. The study involved 11^th graders (girls (n=20) aged 17-18 years) of the comprehensive school not doing sports. Schoolgirls with concurrent diseases, for which physical loads are contraindicated, and/or schoolgirls, who had had acute respiratory diseases not later than seven days before the examination, which could interfere with the evaluation of HRV and neurocognitive assessment, did not participate in the study. HRV was recorded by means of ECG in the prone position and in the active orthostatic posture for 500 heart beats, against the background of spontaneous breathing, using the computer-based system “Kentavr II PC” by “Microlux”, Chelyabinsk. The statistical data processing was made using the “Biospectrum 2.0” software (developed by A.N. Ragozin, D.Yu. Kononov). We measured the total spectrum power (TSP (mm Hg²), absolute (mm Hg²) and relative (%) power of oscillations within the ranges [1, 2, 4] (very low frequency - VLF; low frequency - LF; high frequency - HF), spectral mode and median (Mo and Me, Hz).

The study was conducted on an individual basis, in a school medical room. When interpreting the results of the spectral analysis, we used the general notion of the regulatory nature of the slow wave oscillations [1, 4, 7]. The parameters of the spectral analysis of heart rate variability, applied in this study, are presented in Table 1.

Table 1. Cardiorhythmogram indices used to assess heart rate variability

The obtained results were statistically significant at p<0.05.

Results and discussion. The findings of our research are presented in Table 2.

Table 2. Mathematical analysis of heart rate in adolescent girls (active orthostatic test against the background of spontaneous breathing)

* results are statistically significant at p<0.05

As seen from Table 2, the duration of RR intervals in the standing position is reduced by 20%. The total spectrum power is reduced by 47%. The contribution of respiratory regulation in the orthostatic test is reduced by 86% in absolute values and by 64% in relative ones. Similar results were obtained by other authors [5, 6]. The contribution of the suprasegmental level of regulation (VLF/Ptot) and the contribution of the sympathetic division of the autonomous nervous system (ANS) (LF/Ptot) increased by 41% and 38%, respectively. Stress index (SI) went up by 28%.

The reduction of the duration of RR intervals in the orthostatic test indicates an increase in the sympathetic effects on heart rate regulation. Similar results were obtained by other authors [7, 8]. In addition, as exemplified by the orthostatic test, there is an increase in the contribution of VLF. The findings testify to the activation of the higher autonomic centers. Similar data were obtained by other authors. For instance, according to N.A. Mikhaylov [2], the reduction of the total spectrum power and the increase of the contribution of the sympathetic ANS to the active orthostatic test are a physiological response.

Conclusion. We received an adequate response of hemodynamic indices to the active orthostatic test, characterized by the reduction of the duration of RR intervals, which testifies to heightened sympathetic effects on heart rate regulation, decrease in the total spectrum power (TP) and impact of the regulation of respiration (HF), increased contribution of the suprasegmental level of regulation (VLF), increased contribution of the sympathetic division of ANS (LF) and SI; however, it should be noted that the differences in the HRV ranges in schoolgirls may be due to the menstrual cycle phases, as the difference in the ovarian hormone balance may affect the changes in the autonomic functions. Our study suggests that heart rate variability may serve as a sensitive indicator of the academic load intensity.

References

1. Baranov A.A. Fundamental'nye i prikladnye problemy pediatrii na sovremennom etape (Fundamental and applied issues of modern pediatrics) / A.A. Baranov, L.A. Shcheplyagina // Ros. pediatr. zhurnal. – 2005. – № 3. – P. 4–7.
2. Kazin E.M. Vliyanie psikhofiziologicheskogo potentsiala na adaptatsiyu k uchebnoy deyatel'nosti (Psycho-physiological capacities and adaptation to learning activities) / E.M. Kazin // Fiziologiya cheloveka. – 2002. –­ V. 28. –­ № 3. – P. 23­-29.
3. Mikhaylov N.A. Funktsional'naya asimmetriya i variabel'nost' serdechnogo ritma u shkol'nikov (Functional asymmetry and heart rate variability in schoolchildren) / N.A. Mihaylov, D.A. Dimitriev // Sovremennye problemy nauki i obrazovaniya. – 2011. – № 5. P. 70-­76.
4. Mikhaylova L.A. Pokazateli tsentral'noy gemodinamiki i osobennosti regulyatsii serdechnogo ritma u starsheklassnikov (Central hemodynamics indices and heart rate regulation features in senior pupils) / L.A. Mikhaylova // // Vestnik YuUrGU. Seriya «Obrazovanie, zdravookhranenie, fizicheskaya kul'tura» (SUSU Bulletin. "Education, health care, physical culture" series). – 2013.­ – V. 13.­ – № 4. – P. 59-­63 .
5. Petrova V.K. Reaktsii nasosnoy funktsii serdtsa detey i podrostkov na funktsional'nye nagruzki (Cardiac pump function responses in children and adolescents to functional load) / V.K. Petrova // Filologiya i kul'tura . – 2011. – № 25. – P. 82-­84.
6. Sabiriyanova E.S. Osobennosti dinamiki chastotno-vremennykh kharakteristik pokazateley tsentral'nogo krovoobrashcheniya u devochek i devushek v aktivnom ortostaticheskom polozhenii (Dynamics of time-frequency characteristics of central circulation indices in girls and young women in active orthostatic posture) / E.S. Sabiriyanova, A.R. Sabiriyanov, V.V. Epishev // Vestnik YuUrGU. Seriya «Obrazovanie, zdravookhranenie, fizicheskaya kul'tura» (SUSU Bulletin. "Education, health care, physical culture" series). – 2013. – V. 13. –­ № 2. – P. 53-­56.
7. Judith Esi van der Zwan, Wieke de Vente, Anja C. Huizink, Susan M. Bögels, Esther I. de Bruin Physical activity, mindfulness meditation, or heart rate variability biofeedback for stress reduction: a randomized controlled trial // Applied Psychophysiology and Biofeedback. – 2015. – V. 40, № 4. – P. 257-268.
8. Tejinder Kaur Brar, K.D. Singh, Avnish Kumar, Effect of different phases of menstrual cycle on heart rate variability (hrv) // Journal of Clinical and Diagnostic Research. – 2015. – V. 10. – Р. 3-5.
9. Vivek Kumar Sharma, Senthil Kumar Subramanian, Vinayathan Arunachalam, Rajathi Rajendran Heart rate variability in adolescents – normative data stratified by sex and physical activity // Journal of Clinical and Diagnostic Research. – 2015. – V. 10. – P. 8-13.