The QT Variability Index and Cardiac Autonomic Modulation: Perspectives from Apparently Healthy Men with Spinal Cord Injury

OBJECTIVE: To describe the effect of spinal cord injury (SCI) on the QT variability index (QTVI). METHODS: Digital electrocardiograms from 113 age-matched men (40 with tetraplegia, 26 with high paraplegia, 17 low paraplegia and 31 controls) were analyzed. RR interval, heart rate (HR) variability [total power (TP(RR)), low frequency (LF(RR)) and high frequency (HF(RR))], QT interval duration, Bazett HR-corrected QT (QTc), QT interval variance (QTVN) and QTVI were compared. RESULTS: Significant group main effects were present for RR, QTc, TP(RR) and LF(RR), but not QT duration, QTVN or HF(RR). Post hoc comparisons revealed the following: (1) longer RR in controls versus subjects with high paraplegia and low paraplegia, and in subjects with tetraplegia versus high paraplegia and low paraplegia; (2) QTc was longer in subjects with low paraplegia versus controls and shorter in subjects with tetraplegia versus high paraplegia, and (3) TP(RR) and LF(RR) were different in controls and subjects with high paraplegia compared to those with low paraplegia. QTVI was significantly elevated in all SCI groups compared to controls. Significant negative correlations between QTVI and HF(RR) were observed in all SCI groups, and TP(RR) and LF(RR) in subjects with tetraplegia and high paraplegia only. Age was negatively correlated in controls. CONCLUSIONS: QTVI is negatively affected in otherwise healthy SCI men compared to age-matched controls. This observation appears to reflect the attenuation of vagal modulation, sympathetic impairment above the sixth thoracic vertebra and/or a heightened degree of cardiovascular disease risk.