Heart Rate Variability and Physical Activity Level after Spinal Cord Injury

Jon N. Myers, PhD; K Gladysheva, BA; Heather E. Brown, MSPT; Inder Perkash, MD; B. Jenny Kiratli, PhD

Objectives: Beat-to-beat variations in heart rate (heart rate variability, or HRV) have recently been recognized as markers of autonomic function, cardiovascular health, outcomes associated with cardiovascular disease, and a sedentary lifestyle. Among individuals with spinal cord injury (SCI), recent studies have demonstrated that patterns of altered HRV are useful in characterizing the physiology associated with injury level, but little is known about the influence of habitual activity on HRV in this population. Our objective was to evaluate HRV responses relative to both level of injury and physical activity patterns following SCI.

Methods: Twenty-one subjects (11 tetraplegic, 10 paraplegic) with complete SCI agreed to participate. The subject sample included patients admitted to the inpatient unit who had experienced at least 3 months of extended bedrest (n=5, "restricted activity") and an outpatient sample who were not restricted in their activities (n=16, "unrestricted activity"). Subjective determination of activity level was recorded for subjects with unrestricted activity. HRV measures were obtained from a portable Schiller MT-100 Holter recorder worn for a period of 24 hours. HRV was expressed as the standard deviation of normal-to-normal beats (SDNN), which excludes ectopic beats, and the mean of standard deviation of normal R to R intervals calculated over five minute intervals (SDNN-index) during both day and night. PNN50 represents the percent of R to R intervals differing 50 ms from the preceding one.

Results: There were no significant differences in day or nighttime HRV parameters in comparisons of subjects with tetraplegia versus paraplegia although there was a trend towards higher values in paraplegia. When categorized by habitual activity level (for the unrestricted activity group only), active subjects demonstrated a trend toward higher values than sedentary in both tetraplegia and paraplegia. However, in comparisons between those with severely restricted versus unrestricted activity, significant reduction was found in HRV parameters with restricted activity regardless of injury level; this was more apparent at night.

Table: Effect of substantial reduction in activity on HRV parameters (Mean (SD))

p between 0.6 - 1.0, **p 0.001, in comparisons between restricted vs. unrestricted activity

Conclusions: In this pilot analysis, HRV was primarily unrelated to level of injury but appeared to reflect the activity status of the individual. Moreover, HRV was significantly reduced among subjects with extremely limited activity levels. The latter finding is similar to that observed in ambulatory subjects suggesting that abnormal HRV may be a modifiable risk factor in persons with SCI warranting intervention. In a recently funded study, we are expanding these findings with a comprehensive study of the association between autonomic function, reflected by HRV, daily physical activity patterns, and cardiovascular health in SCI.

Acknowledgment: This study was funded by VA Rehabilitation Research and Development Service, project #B2549-R.