Determining autonomic sympathetic tone and reactivity using Baevsky's stress index

The extrinsic autonomic nervous system is critical in controlling most organ functions and is involved in the pathophysiology of many chronic diseases. However, its assessment plays a minor role in the clinical practice of diagnosis and treatment outside cardiology. Since sympathetic dysfunction is related to diseases such as diabetes, chronic stress, and urinary and gastrointestinal motor dysfunction, an autonomic assessment is warranted. Here, we evaluate the Baevsky stress index (SI) to assess sympathetic tone and reactivity based on heart rate variability. We start with discussing Baevsky's original stress index. We propose an optimized calculation of SI and assess the SI of 73 self-declared healthy subjects in the age groups 16-35 yr, 35-50 yr, and 50+ yr at supine baseline and in response to postural change from supine to standing. Normality assessment and kernel density analysis identified two subgroups: one we deemed to have normal autonomic functioning, and an outlier group with significantly higher baseline sympathetic index (SI) and sympathetic reactivity to standing. Using a Gaussian mixture model, we determined normal SI values and values for autonomic stress and autonomic dysfunction. This study provides a needed start to evaluate sympathetic dysfunction using heart rate variability.NEW & NOTEWORTHY To help diagnose autonomic dysfunction in chronic diseases, Baevsky's stress index is evaluated for the assessment of sympathetic tone and reactivity. Ranges of control values are given for different age groups, and significant differences are shown with patients with chronic gastrointestinal motility disorders.