Decreased autonomic nervous system activity as assessed by heart rate variability in patients with chronic tetraplegia

Objectives: To assess the alterations of autonomic nervous functions and sympathovagal balance of the subjects with spinal cord injuries (SCIs) in different levels by heart rate variability analysis. Design: Prospective, observational study. Setting: All participants were recruited from the outpatient clinic from National Taiwan University Hospital, which is a tertiary referral center. Participants: Thirty-one patients with traumatic chronic complete SCI (more than 6 months): 14 with paraplegia (Group A), and 17 with tetraplegia (Group B). Main Outcome Measures: Heart rate variability assessed by 24-hour Holter monitoring. Results: Two patients in Group A and 1 in Group B were excluded from final data analysis because of poor recording data. Two time domain variables, the standard deviation (SD) of all normal RR intervals (SDNN) and the mean of the SDs of all normal RR intervals for all 5-minute intervals (SDNNi), over 24 hours were decreased in Group B. All time domain variables, SDNN, SDNNi, root mean square of the successive normal RR interval difference (rMSSD), and the percentage of RR intervals differing >50msec from the preceding one (pNN50), were decreased during the nighttime recordings (all p<.05) in Group B. The very-low-frequency, low-frequency (LF), and high-frequency (HF) components of the power spectrum of the RR intervals were also decreased in Group B (p < .05), irrespective of the daytime and nighttime recordings. The LF-to-HF ratio did not differ significantly in these two groups, indicating the maintained sympathovagal balance in the chronic SCI patients. Conclusion: These findings suggested that the autonomic nervous system activity was depressed in the patients with chronic tetraplegia, but the autonomic nervous system still maintained homeostasis.