We aimed to study the complexity and time asymmetry of short-term heart rate variability (HRV) as an index of complex neurocardiac control in response to stress using symbolic dynamics and time irreversibility methods. ECG was recorded at rest and during and after two stressors (Stroop, arithmetic test) in 70 healthy students. Symbolic dynamics parameters (NUPI, NCI, 0V%, 1V%, 2LV%, 2UV%), and time irreversibility indices (P%, G%, E) were evaluated. Additionally, HRV magnitude was quantified by linear parameters: spectral powers in low (LF) and high frequency (HF) bands. Our results showed a reduction of HRV complexity in stress (lower NUPI with both stressors, lower NCI with Stroop). Pattern classification analysis revealed significantly higher 0V% and lower 2LV% with both stressors, indicating a shift in sympathovagal balance, and significantly higher 1V% and lower 2UV% with Stroop. An unexpected result was found in time irreversibility: significantly lower G% and E with both stressors, P% index significantly declined only with arithmetic test. Linear HRV analysis confirmed vagal withdrawal (lower HF) with both stressors; LF significantly increased with Stroop and decreased with arithmetic test. Correlation analysis revealed no significant associations between symbolic dynamics and time irreversibility. Concluding, symbolic dynamics and time irreversibility could provide independent information related to alterations of neurocardiac control integrity in stress-related disease.

Autism spectrum disorder (ASD) is a serious neurodevelopmental disorder, associated with autonomic dysregulation. However, the pathomechanism leading to autonomic abnormalities is still unclear. The aim of this study was to assess autonomic nervous system (ANS) activity during baseline in homogenous group of autistic children using electrodermal activity (EDA), as an index of sympathetic activity and short-term heart rate variability (HRV) reflecting predominantly cardiac vagal control. Fifteen ASD boys and 15 healthy age-matched boys at the age of 7-15 years were examined. The continuous EDA and ECG were recorded during resting phase in a supine position. Evaluated parameters: EDA amplitude (µS), RR interval, spectral power, peak frequency and power spectral density in low (LF-HRV: 0.04-0.15 Hz) and high-frequency (HF-HRV: 0.15-0.4 Hz) bands of HRV spectral analysis. In ASD group we found significantly shortened RR intervals (729±20 ms vs. 843±30 ms, p=0.005), lower mean EDA (0.66±0.13 µS vs. 1.66±0.42 µS, p=0.033), reduced spectral activity and power spectral density in HF-HRV compared to controls (2.93±0.12 ms2 vs. 3.38±0.10 ms2, p=0.01; 4.12±0.10 ms2/Hz vs. 4.56±0.11 ms2/Hz, p=0.008, respectively). We suggest that impairment in resting autonomic regulation associated with ASD could represent an important pathomechanism leading to potential cardiovascular complications in ASD.

Respiratory sinus arrhythmia (RSA) is accepted as a peripheral marker of cardiac-linked parasympathetic regulation. According to polyvagal theory, the RSA is also considered as the index of emotion regulation. The neurovisceral integration model posits that parasympathetic modulation of the heart marked by RSA is related to complex nervous regulation associated with emotional and cognitive processing. From this perspective, high resting RSA amplitude associated with a greater withdrawal during stressors and subsequent recovery could represent a flexible and adaptive physiological response system to a challenge. Conversely, low resting RSA accompanied by an inadequate reactivity to stress might reflect maladaptive regulatory mechanisms. The RSA reactivity is different with various types of stressors: while the RSA decreases to cognitive tasks indicating a vagal withdrawal, the RSA magnitude increases to emotional challenge indicating an effective cognitive processing of emotional stimuli. The RSA reactivity to stress could have important implications for several mental disorders, e.g. depressive or anxiety disorder. It seems that the study of the RSA, as a non-invasive index of ‘brain-heart’ communication, could provide important information on the pathway linked to mental and physical health.

The reactions of human organism to changes of internal or external environment termed as stress response have been at the center of interest during recent decades. Several theories were designed to describe the regulatory mechanisms which maintain the stability of vital physiological functions under conditions of threat or other environmental challenges. However, most of the models of stress reactivity were focused on specific aspects of the regulatory outcomes – physiological (e.g. neuroendocrine), psychological or behavioral regulation. Recently, a novel complex theory based on evolutionary and developmental biology has been introduced. The Adaptive Calibration Model of stress response employs a broad range of the findings from previous theories of stress and analyzes the responsivity to stress with respect to interindividual differences as a consequence of conditional adaptation – the ability to modify developmental trajectory to match the conditions of the social and physical environment. This review summarizes the contributions of the most important models in the field of stress response and emphasizes the importance of complex analysis of the psycho-physiological mechanisms. Moreover, it outlines the implications for nonpharmacological treatment of stress-related disorders with the application of biofeedback training as a promising tool based on voluntary modification of neurophysiological functions.