Factors Contributing to Heart Rate Variability in Individuals With Mental Retardation With or Without Down Syndrome

Abstract

0894 Individuals with Down Syndrome (DS) exhibit an impaired ability to respond to physical stressors associated with a dysfunction of the autonomic nervous system (ANS). Heart rate variability (HRV) is a valid and non-invasive assessment tool of the ANS in both healthy and diseased population. Age, body composition and fitness level are associated with HRV in healthy individuals, but these associations are unknown in individuals with mental retardation (MR) with or without DS. PURPOSE: To investigate the relationship between age, body mass index (BMI), maximum oxygen consumption (VO2peak) and resting HRV in individuals with MR with or without DS. METHODS: ECG tracings were collected continuously during a 5min supine rest in individuals with MR with DS (N = 58), without DS (N = 15) and controls (N = 30). ECG data were analyzed for HRV in both time (SD of R-R interval, Dev > 50%, RMS of successive differences) and frequency domain [total power (TP), low frequency (LF), high frequency (HF)] using an autoregressive model. RESULTS: Individuals with MR were significantly younger (20 yrs) than both DS (24 yrs) and control (26 yrs) groups. Individuals with DS had a higher BMI (31.5 kg/m2) vs. the other groups (MR: 23.9 kg/m2, Control: 25.5 kg/m2). Individuals with DS had a lower VO2peak (24 ml/kg/min) compared to individuals with MR (34.3 ml/kg/min), and controls (41.25 ml/kg/min). Total HRV power was different between groups in both time and frequency domains, and only total power could be significantly predicted. Stepwise regression revealed that VO2peak and group membership were the strongest predictors of both TP (R 2 = 0.14) and SD (R2 = 0.16). CONCLUSION: VO2peak is a predictor of total power of HRV across all groups, but group membership further improves the prediction. These data suggest aerobic fitness is related to parasympathetic influence on the SA node, but this influence varies between groups. These variables explain only a small fraction of the shared variance, thus other non-measured factors contribute more to HRV.