Reduced Sympathetic Modulation on the SA-node in Response to Upright Tilt is Related to Blunted Maximal Heart Rate in Individuals with Down Syndrome

Abstract

0895 Heart rate (HR) responses to upright tilt and maximal treadmill exercise are blunted in individuals with Down Syndrome (DS). It is thought these blunted responses are caused by sympathoinhibition. Cardiac autonomic regulation is often measured by heart rate variability (HRV). PURPOSE: We investigated the HRV response to upright tilt and the relationship between HRV and maximal HR in 13 patients with DS and 12 controls. METHODS: ECG tracings were collected continuously during a 10min supine rest followed by 10min of upright tilt at 80 degrees. ECG data were stored and later analyzed for HRV. HRV analysis included calculations of high frequency (HF) (parasympathetic indicator) and low frequency (LF) power (sympathetic indicator), LF/HF ratio, and root mean square of successive differences (RMSSD) (time domain parasympathetic). Maximal HR was obtained from a graded exercise treadmill test. RESULTS: The groups were of similar age (DS = 28.6, control = 28.3 yrs) and weight (DS = 78, control = 77 kg). The HRV results are presented below. None of the HRV variables were different between groups at rest. The controls increased LF power, LF/HF ratio, and normalized LF power in response to tilt significantly more than patients with DS (p < .05). Maximal HR was lower in subjects with DS (163 vs. 188 bpm, p < 0.05) and was positively correlated with LF (nu) (r = 0.76, p < 0.001). CONCLUSIONS: Patients with DS exhibited decreased levels of sympathetic modulation in response to tilt consistent with altered autonomic regulation, and previously reported reductions in sympathetic excitation. These data suggest individuals with DS have a reduced ability to respond to physical stressors, consistent with autonomic dysfunction. Our data suggests reduced sympathetic tone may explain low maximal HR in subjects with DS.Table: No Caption Available.