Heart rate variability in spinal cord injury: Asymptomatic orthostatic hypotension is a confounding variable

Abstract

Study designobservational study. ObjectivesHeart rate variability (HRV) studies have shown that the low-frequency (LF) domain is decreased in postural challenge of spinal cord injury (SCI). However, there are inconsistencies in the literature regarding HRV with postural challenge. We believed that asymptomatic orthostatic hypotension (OH) may contribute to the discrepancies in the literature. We also evaluated the time-domain parameters of HRV to evaluate for changes in SCI postural challenge. We looked at the association of Frequency and Time-domain HRV parameters to blood pressure (BP) and postural challenge in SCI based on groups with and without OH. SettingSCI clinic. MethodsPatients from SCI clinic were enrolled and 5-minute electrocardiogram (ECG) recordings were performed before and after a postural challenge from supine to sitting position with blood pressure (BP) recordings at each time point. Between and within group analyses were performed based on tetraplegia, paraplegia, and the presence of asymptomatic OH. ResultsThere were 6 controls and 18 SCI subjects, 4 with OH based on change in diastolic BP. After postural challenge, mean heart rate was lower in tetraplegia compared to controls in all groupings. When OH was removed from the groups, total power was consistently reduced in tetraplegia compared to paraplegia across all positions. There were both time- and frequency-domain differences observed in SCI with OH compared to SCI without. ConclusionsAutonomic variability is increased in SCI with OH and can lead to misinterpretation of findings if asymptomatic OH is included in samples. Our data suggest that there are frequency- and time-domain differences in HRV variables in those with OH compared to those without. Future HRV studies should account for asymptomatic OH and include analyses of the time-domain variables in postural challenge of SCI.