Changes in Autonomic Balance, Cardiac Parasympathetic Modulation, and Cardiac Baroreflex Gain in Older Adults Under Different Orthostatic Stress Conditions

<b>Background</b>: As we age, our autonomic function declines, resulting in altered autonomic balance during postural transitions. These changes can affect the dynamic interplay between sympathetic and parasympathetic modulation, compromising short-term compensatory responses to active standing. <b>Objectives</b>: This study aimed to compare heart rate variability (HRV) at baseline, cardiac parasympathetic modulation (CPM), and cardiac baroreflex gain (CRG) between younger adults (YA) and older adults (OA) following active standing orthostatic stress. A secondary objective was to analyze the incidence of orthostatic intolerance (OI) symptoms. <b>Methods:</b> Participants (<i>n</i> = 76) completed sit-to-stand and lie-to-stand maneuvers with continuous beat-to-beat blood pressure and heart rate (HR, electrocardiogram). HRV at baseline was analyzed in both time and frequency domains. CPM was measured by the HR 30:15 ratio on standing. CBG was determined as the ratio of HR and SBP changes (ΔHR/ΔSBP) at specific phase time points (30 s, 60 s, 180 s, and 420 s). <b>Results</b>: At baseline, OA showed reduced Standard Deviation of RR intervals (SDRR), Root Mean Square of Successive Differences (RMSSD), low-frequency (LF), and high-frequency (HF) power, and elevated LF/HF ratio (all <i>p</i> < 0.05), indicating a shift toward sympathetic dominance. During active standing orthostatic stress, OA demonstrated a lower HR30:15 ratio and CBG in later phases (phases 2-4) (all <i>p</i> < 0.05). Also, OA reported more symptoms (14%) of OI than YA (0%) (<i>p</i> = 0.041). <b>Conclusions</b>: These findings indicate that older adults have impaired autonomic function characterized by reduced HRV, CPM and CBG responses. These impairments lead to diminished autonomic regulation under active-standing orthostatic stress and a higher incidence of OI symptoms.