Normal Heart Rate with Tilt, Yet Autonomic Dysfunction in Persons with Down Syndrome

Persons with Down syndrome (DS) exhibit altered autonomic function at rest and in response to adrenergic stimuli. It is unknown whether a subset of persons with DS that have similar HR responses to a task would have similar responses in HR variability (HRV). Purpose: This study aimed to compare cardiac autonomic function during upright tilt using HRV analysis in persons with and without DS when persons with and without DS were matched for the change in HR. Methods: Persons with (25 T 2 yr; 30.4 T 1.9 kgImj2 , n = 15) and without DS (27 T 2 yr; 24.7 T 1.1 kgImj2 , n = 15) were matched on their HR response to a 5-min tilt at 80-, whereas a subset of persons with DS (28 T 3 yr; 33.5 T 2.0 kgImj2 , n = 11) were not matched for the change in HR. HRV was assessed in both the frequency (natural log transformation (Ln) of low frequency (LF), high frequency (HF), LF/HF ratio, and total power (TP)) and time domains (root mean square of successive differences [RMSSD]). Results: Changes in HR were similar in DS-matched and control but lower in DS-not matched. Tilt effects were observed for LnHF, LNTP, and RMSSD in all groups (P G 0.05). Both groups of persons with DS exhibited reduction in LnLF, with no change in the control group (P G 0.05). The increase in LF/HF was greater in the group without DS when compared with that in DS-not matched (8.71 T 2.38 vs 2.34 T 1.39, P G 0.05) but not when compared with that in DS-matched (3.59 T 1.10, P = 0.075). Conclusions: Despite similar HR response to passive upright tilt in the DS-matched, we still observed reduced sympathetic dominance in response to upright tilt in persons with DS