ИСТИНА

Adaptation or maladaptation of the body can be assessed by activity of its regulatory systems. One of the manifestations of such activity is heart rate variability (HRV) as a correlate of autonomous regulation of the cardiovascular system. Aim: to study seasonal fluctuations of HRV indicators in primary school students (7-12 years, 1st–4th grades). We were interested in a retrospective analysis of this phenomenon (comparison of current results with similar data from 2004-2007) and the correlation with body mass index (BMI). We analyzed the dynamics of semi-annual changes of HRV indicators (the "delta" of indicators, in % of the initial level), as well as blood pressure (BP) and its variability (BPV). Testing was performed twice a year: in late March–early April, which corresponds to the beginning of the school year, and in late September–early October, i.e. at the end of the school year. A spiroarteriocardiorhythmograph was used. The volume of samples in 2016-2019 was 214 people, 701 repeated measurements, in 2004-2007 317 people, 810 repeated measurements. Of all the HRV and BPV parameters, seasonal fluctuations were revealed only for systolic BP and the LF/HF ratio in the HRV spectrum. In 2016-2019, an increase in systolic BP over the school year (fall – spring) and a decrease in LF/HF (from 2nd to 5th grade) were revealed in the examined primary school students. However, in 2004-2007, seasonal changes in children of the same age were opposite: a decrease in systolic BP during the school year and an increase in LF/HF (from 1st to 5th grade). When the children were divided into subgroups by BMI values, testing at the end of the fourth grade showed that seasonal fluctuations in HRV and BPV parameters were most pronounced in the group with BMI values ranging from the 15th to the 84th percentile. However, there were no significant differences between children from groups with different BMI (underweight in the range below the 15th percentile, overweight in the range of 85-95%, obesity in the range above the 95th percentile). The obtained data attest to the presence of seasonal fluctuations in HRV and BPV parameters in children aged 7-11 years. These fluctuations do not depend on BMI value, but their pattern inverted from 2004-2007 to 2016-2019. We hypothesized that these changes are related to evolution of the educational environment, its intensification, and introduction of computer technologies, but this assumption needs additional verification.