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 sy mpathovagal balance of the subjects with spinal cord injuries (SCIs) in dif ferent levels by heart rate variability analysis. Design: Prospective, observational study. Setting: All participants were recruited from the outpatient clinic from Na tional 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 (Gro up B). Main Outcome Measures: Heart rate variability assessed by 24-hour Holter mo nitoring. 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, th e 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), ove r 24 hours were decreased in Group B. All time domain variables, SDNN, SDNN i, root mean square of the successive normal RR interval difference (rMSSD) , and the percentage of RR intervals differing >50msec from the preceding o ne (pNN50), were decreased during the nighttime recordings (all p<.05) in G roup B. The very-low-frequency, low-frequency (LF), and high-frequency (HF) components of the power spectrum of the RR intervals were also decreased i n Group B (p < .05), irrespective of the daytime and nighttime recordings. The LF-to-HF ratio did not differ significantly in these two groups, indica ting the maintained sympathovagal balance in the chronic SCI patients. Conclusion: These findings suggested that the autonomic nervous system acti vity was depressed in the patients with chronic tetraplegia, but the autono mic nervous system still maintained homeostasis.