Diurnal variation of heart rate variability in individuals with spinal cord injury

Background: Heart rate variability (HRV) may provide objective information
about cardiogenic autonomic balance in individuals with spinal cord injury (SCI). The
aim of this study was to characterize the diurnal variation of HRV in individuals with SCI
at lesion level T6 and above and lesion level below T6.

Methods: This was a retrospective analysis of a prior cross-sectional study. Individuals
with chronic SCI underwent 24 h recording of the time between consecutive R waves
(RR interval) to derive parameters of HRV as follows: standard deviation of all normalto-
normal R–R intervals (SDNN) and square root of the mean of the squared differences
between successive R–R intervals (RMSSD) (time domain); and high frequency power
(HF), low-frequency power (LF), very low frequency power (VLF), ultra-low frequency
power (ULF) and total power (TP) (frequency domain). Changes in the magnitude
of HRV outcomes over the 24 h period were investigated using a novel multi-component
cosinor model constrained to the form of a three-harmonic Fourier series.

Results: Participants were grouped as lesion level T6 and above (n = 22) or below T6
(n = 36). Most of them were male (n = 40, 69 %) and the median age (interquartile
range) was 50.5 (28) years. Both groups exhibited similar diurnal patterns in most
HRV metrics. The lowest values occurred in the late afternoon (4–6 pm) and gradually
increased, peaking around midnight to early morning (1–6 am). Exceptions included
RMSSD, which peaked before midnight, and ULF, which showed a double peak pattern
that peaked from 11 am to 1 pm and 4–6 am in participants with lesion level at T6
and above. The HRV values in participants with lesion level T6 and above were generally
lower than participants with lesion level below T6, except for peak values of RMSSD,
HF and LF.

Conclusion: This study demonstrated substantial diurnal variation of HRV in participants
with SCI in both groups of participants. In clinical and research settings, diurnal
variations in HRV must be taken into consideration.