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Brain & Heart Automatic cardiac autonomic modulation assessment

Table 10. Pearson’s correlation coefficients between PNSi, Average SNSi values were slightly above normal during
SNSi, and BSTRi and the HRV spectral components regular daily activity and, as expected, significantly lower
during baseline daily activity, physical effort, and during NREM (Table 5), with no significant age-related
psychophysiological stress variations observed (Table 6). In contrast, the BSTRi was
PNSi SNSi BSTRi significantly higher in Age3 compared to both Age1 and
Baseline Age2. This unexpected result may indicate that “healthy
elderly” individuals exhibit a more complex adaptation
VLFpow FFT ms 2 0.470** −0.497** −0.533**
of CAM, potentially influenced by a moderate increase in
LFpow FFT ms 2 0.492** −0.530** −0.626** physiological stress. However, further research involving
HFpow FFT ms 2 0.549** −0.484** −0.545** a larger population is needed to confirm this preliminary
TOTpow FFT ms 2 0.557** −0.575** −0.668** observation.
LF/HF ratio FFT −0.132 0.018 −0.033 Although the evaluation of PNSi and SNSi was not
Physical effort affected by the duration of the time segments used for
VLFpow FFT ms 2 0.510** −0.465** −0.468** calculation, BSTRi values during high stress were higher
LFpow FFT ms 2 0.681** −0.638** −0.638** (though not significantly) when calculated from 2-min
HFpow FFT ms 2 0.579** −0.659** −0.660** intervals (Table 8), which may be more suitable for assessing
TOTpow FFT ms 2 0.599** −0.591** −0.602** the complex brain–heart interaction during critical events.
However, we believe that the assessment of Kubios’ BSTRi
LF/HF ratio FFT 0.031 0.036 0.051 “normality range” requires further investigation. Notably,
Psychophysiological an interesting finding from this study, which has not been
VLFpow FFT ms 2 0.596** −0.644** −0.624** reported previously, is that average BSTRi values around 45
LFpow FFT ms 2 0.787** −0.756** −0.727** and even exceeding 80 conventional units were observed at
HFpow FFT ms 2 0.636** −0.627** −0.611** peak stress during police tactical training. This suggests a
TOTpow FFT ms 2 0.805** −0.779** −0.750** variable combination of physical and psychophysiological
LF/HF ratio FFT −0.147 0.190 0.209 stress, with individual peaks exceeding 60 in five cases and
above 80 in one case (Figure 5). If squared (since Kubios’
Note: **P < 0.01. stress index is the square root of Baevsky’s values), the
Abbreviation: FFT: Fast Fourier transform.
observed values of 45–80 would correspond to 2025–6400
stressful stimuli. 19,20 In our study, VLFpower was higher Baevsky conventional units, which are extremely high
during daily activity than during NREM sleep across all and theoretically inconsistent with a healthy status and
43,44
age groups, confirming its association with homeostatic adequate functional capabilities. This was not the case
mechanisms such as thermoregulation, baroreflex activity, for our police officers; however, in a few instances, values
and the renin–angiotensin system. However, although exceeding 60 during highly stressful tactical scenarios
20
significant, the correlations found between VLFpower and were linked to poor tactical performance. Moreover,
PNSi (positive correlation) and with SNS and BSTR indexes similar values were consistently achieved and tolerated by
(negative correlation) were only moderate (Table 10). a 75-year-old volunteer during intensive effort without any
subjective clinical symptoms or functional or ECG changes.
Notably, a recent study reported a time-varying increase Furthermore, as suggested by space medicine, individual
in VLFpower that followed the same trend as heart rate physiological responses in critical situations may not align
and the Kubios Baevsky stress index, suggesting that with average statistical normality, as adaptive reactions
51
VLFpower may serve as a marker for stress-related increases can vary based on individual psychophysiological and
in intrinsic cardiac sympathetic activity. 21,52 However, the functional capabilities. Interestingly, previous reports
43
VLF spectral component is generally viewed as indicative of indicated that a similar BSTRi value in a highly fit police
slower physiological mechanisms, sympathovagal balance, officer, without significant physical effort, was linked to a
and a reduction in vagal tone during mental stress. 20,21,53 loss of situational control, resulting in operational failure
The discrepancy between that study and our findings may and the sudden onset of paroxysmal arrhythmia, attributed
be attributed to the VLF filtering effect of the detrending to a marked acute CAM imbalance. Under such intense
51
with the smoothness priors function used in our study. and uncontrolled psychophysiological stress, which can
Despite the notable changes in linear parameters lead to immobilization (freezing) and an inability to
associated with the well-known decline of HRV due to respond to external threats, HRV decreases significantly.
aging, average PNSi values remained within the suggested An unexpected increase in the delayed HF component has
12
normal range during both daily activity and NREM sleep. been noted in ultra-short-term HRV analysis, aligning
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Volume 2 Issue 4 (2024) 12 doi: 10.36922/bh.3503