Heart rate variability as an indicator of stress in women during wartime

. The impact of post-traumatic stress related to the war was analyzed in 91 women who were in Kyiv or the Kyiv region during active combat in February and March 2022. Some of the examined women had been under occupation for an extended period. In contrast, others, from May 2022 to May 2023, experienced the strong influence of war factors such as missile and artillery shelling, staying in bomb shelters, or other shelters. Due to the military actions, the examined individuals were subjected to significant stress factors, which inevitably affected their psychoemotional state. To compare, data from 79 women examined before the Russian-Ukrainian war were used as a control group. The groups of women examined before and during the war did not differ in anthropometric indicators. To understand the age-related characteristics of civilian women, they were distributed into age groups of 30-44, 45-59, and 60-74 years. ECG registration was carried out using the ECG-recorder DiaCard (Solvaig, Ukraine). The analysis of heart rate variability (HRV) was performed by application DiaCard v. 1.0.0.73. ECG measurements were taken from 10:00 a.m. to 12:00 p.m., in the supine and standing positions (5 min). The study of HRV in women during wartime and the comparison of obtained results with the pre-war period revealed that war-related stress induces changes in HRV characteristics of various types of stress. In the examined women during wartime, there is a decrease in overall HRV, primarily due to HF oscillations, indicating a reduction in parasympathetic activity of the autonomic nervous system (ANS). Such changes are typical for mild stress. Changes in HRV under conditions of military stress depend on the age of individuals and are more pronounced in young and elderly women. No significant shifts in heart rate variability were observed in individuals of average age.

In most studies, heart rate variability (HRV) indices changed in response to stress induced by various methods.The most common factor associated with variations in heart rate variability was low parasympathetic activity, characterized by a decrease in HF and an increase in LF.Reduced heart rate variability and suppressed parasympathetic activation increase vulnerability to future stress.Changes in HRV in individuals with clinical anxiety disorders have been reported [1].In one study, patients with generalized anxiety disorder exhibited lower HF components of HRV during both baseline calm and anxious states, consistent with reduced modulation of the heart and vagus nerve compared to the control group, where anxiety was not induced [2].Yeragani et al. (1993) reported lower LF and VLF components among patients with panic disorder compared to the control group [3].In contrast, Rechlin et al. (1994) found a higher VLF component, in line with increased sympathetic modulation of HRV in patients with panic disorder compared to a control group matched for age and gender [4].In the study by Orsila R. et al. (2008), a modest inverse relationship between perceived emotional stress and the normalized HF component of HRV was demonstrated [5].This suggests a dampening of cardiacvagal modulation of HRV among both men and women who experienced higher levels of stress.This association was found to be independent of age, gender, anxiety, and cardiorespiratory fitness.The authors concluded that the cardiac-vagal spectral component of HRV may be sensitive to recent experiences of persistent emotional stress, regardless of an individual's current state.
Taking into consideration the observations of stress-associated variations in HRV and existing neurobiological evidence, HRV can be utilized as an objective assessment of stress.HRV can be considered a tool that reflects cardiac activity and the overall autonomic state of health, rather than specific diseases or pathological conditions.Since the concept of stress encompasses both biological and psychological factors, the assessment of stress should integrate objective and physiological evaluations, as well as selfreports, using HRV in clinical practice.The study aimed to investigate the feasibility of using HRV analysis for stress assessment in women during wartime.

Materials and methods
The study included 170 practically healthy women aged from 30 to 74 years, who were examined at the Department of Clinical Physiology and Pathology of Internal Organs of the State Institution «D.F.Chebotarev Institute of Gerontology of the National Academy of Medical Sciences of Ukraine».
The impact of war-related post-traumatic stress on the organism was analysed in 91 women who were in Kyiv or the Kyiv region during active combat in February and March 2022.A portion of those examined had spent an extended period under occupation.Others, from May 2022 to May 2023, experienced significant war-related factors such as missile and artillery bombardments, staying in bomb shelters, or other forms of shelter.Due to the military actions, the examined individuals were subjected to strong stressors, which undoubtedly affected their psychoemotional state.
The data for comparison were obtained from 79 women examined before the Russo-Ukrainian war (control group).The groups of women examined before and during the war did not differ in terms of anthropometric indicators.To explore the age characteristics of civilian women, they were categorized into age groups of 30-44, 45-59, and 60-74 years (Tab.1).All subjects were instructed to avoid alcohol or caffeinated drinks after 10:00 pm.(22:00) the night before the examination.In addition, they refrained from smoking 1 hour before the measurement.ECG measurements were taken from 10:00 a.m. to 12:00 p.m., in the supine and standing positions (5 min).During the ECG recording, the subject was instructed to breathe according to his normal rate.ECG registration was carried out using the ECG-recorder DiaCard (Solvaig, Ukraine).The analysis of HRV was performed by application DiaCard v. 1.0.0.73.(Tab.2).

Results and discussion
In assessing the cardiovascular system's condition, Heart Rate Variability (HRV) indices are widely used, characterizing the functional activity of the sympathetic and parasympathetic divisions of the autonomic nervous system and their influence on the heart and blood vessels [6, 7, 8, 9-11, 12, 13].Additionally, HRV can serve as an indicator of stress [14, 15-17, 18, 19, 20, 21, 22, 23].During mental stress, vagal activity decreases, and during physical stress, baroreflex activity also decreases.Intense stress leads to an increase in sympathetic activity, manifested by an augmentation of slow waves in the heart rate.Tab. 3 presents HRV data for women aged 30 to 74 before and during the war.As evident from the obtained data, there is a significant reduction in overall heart rate variability (TP) in women in the supine position.Additionally, indices of parasympathetic nervous system activity (RMSSD, pNN50, HF) are significantly lower in those examined during the war (p<0.05).The power of the LF component of HRV did not change significantly (Table 3).In the upright position, there are no significant changes.Therefore, the observed changes in HRV (reduced RMSSD, pNN50, HF) can be interpreted as a state of increased physiological stress.
As indicated by the obtained data (Tab.3), before the war, orthostatic influence resulted in a decrease in overall HRV (RMSSD, SDNN, TP) and parasympathetic activity indices (pNN50, HF).Before the war, there was also a significant alteration in the sympathovagal balance in response to orthostatic influence, favouring sympathetic activity.During the war, these changes are practically absent, except for a minor decrease in pNN50, indicating disturbances in baroreflex control of hemodynamics.
It is known that HRV indices change with ageing [20,21].Therefore, for correct comparison of mean values of indices before and during the war, all examined individuals were divided into three age groups: under 45 years, 45-59 years, and 60 years and older.
Tab. 4 presents HRV indices in women aged 30-44 before and during the war.Total HRV (TP) shows a tendency to decrease in the supine position.Simultaneously, indices of parasympathetic activity (pNN50, HF) significantly decrease (p<0.05).The power of the low-frequency component of heart rate variability (LF) and LF/HF do not undergo significant changes.In the upright position during the war, there are no significant shifts in HRV.The observed changes in HRV during the wartime period (reduced pNN50, HF) can be interpreted as a state of "mild" stress.Note: * -a statistically significant difference before and during the war: p< 0.05 Tab. 5 presents data for women in the age group of 45-59 before and during the war.As evident from the obtained data, total Heart Rate Variability (SDNN, TP) and its components (RMSSD, pNN50, LF, HF) do not undergo significant changes (p>0.05) in both supine and upright positions.The shifts of these indices also remain unchanged, indicating greater resilience to stress in women of this age group.The reduction in HRV occurs due to high-frequency, parasympathetic activity -statistical and spectral indicators of this activity significantly decrease RMSSD, pNN50, and HF.The power of the lowfrequency component of HRV (LF) remains unchanged.The ratio of low-to high-frequency oscillations, LF/HF, in the supine position tends to increase (p < 0.1).This ratio characterizes the sympathovagal balance, and an increase indicates a shift towards sympathetic dominance.In the upright position, there are no significant shifts in HRV and its components.Under orthostatic stress during the war, there is a significantly smaller change in total HRV (SDNN), as well as vagal (pNN50) and baroreflex (LF) activity.This indicates a disturbance in baroreflex regulation.
Thus, the observed reduction in parasympathetic activity indices in older women during the war (RMSSD, pNN50, HF) can be interpreted as their being in a state of moderate stress.

Conclusions.
1.The study of HRV in civilian women during wartime and the comparison of the results with the prewar period revealed that the stress associated with war induces HRV changes characteristic of different types of stress.2. In the examined women during wartime, a reduction in overall HRV is observed, primarily due to HF oscillations, indicating a decrease in the parasympathetic activity of the Autonomic Nervous System (ANS).Such changes are typical for mild stress.3. Changes in HRV under conditions of military stress depend on individuals' age and are more pronounced in young and elderly women.In middle age, HRV shifts are not detected on average.

Table 2 HRV scores in the time domain and frequency domain
Note: the term "NN" is used in place of RR-interval ECG to emphasize the fact that the processed beats are "normal" beats Statistical processing of the obtained data was carried out with the help of Excel 2007 and Statistica 7 (StatSoft, USA) programs.Standard statistical procedures, including variation and regression analyses, were used.

Table 5 Mean values of HRV indices in women aged 45-59 before and during the war (M ± m)
-a statistically significant difference before and during the war: p< 0.05 Tab.6 presents data for women in the age group of 60-74 before and during the war.As evident from the obtained data, total HRV (SDNN) undergoes a significant change (p < 0.05) in the supine position, showing a reduction. Note:

Table 6 Mean values of HRV indices in women aged 60-74 before and during the war (M ± m)
Note: -a statistically significant difference before and during the war: p< 0.05