Altered Circadian Rhythmicity in Blood Pressure with Spinal Cord Injury is Not Explained by Differences in Physical Activity

Introduction Individuals with spinal cord injury (SCI) demonstrate attenuation or absence of nocturnal reductions in blood pressure (BP) and heart rate (HR) (Rosado-Rivera et al., 2011; J Spinal Cord Med, 34, 395). Diminished rhythmicity is associated with increased incidence of cardiovascular disease and mortality (Verdecchia et al., 1993; Circ, 88; 986). Differences in the levels of physical activity (PA), due to impaired motor functioning in SCI, may contribute to these findings. Therefore, the purpose of this study is to examine the impact physical activity on the circadian rhythms in BP and HR in paraplegics and tetraplegics.

Methods Ten healthy able-bodied controls, 9 paraplegic (spinal cord lesion between T4-T12) and 6 tetraplegic (spinal cord lesion between C4-C7) SCI individuals underwent simultaneous 24-h ambulatory BP, HR and activity monitoring. Blood pressure and HR were measured every 15 min and a diary of events was kept throughout this period which included time of sleep and time of waking. Ambulatory accelerometry was used to measure changes in PA across 15 min periods preceding each BP/HR-measurement. Data were then averaged hourly and re-ordered to the time of waking for each individual. Systolic (SBP) and diastolic (DBP) and HR were analyzed with within-subject generalized estimating equations, and then covariate controlled for changes in PA. Data are presented as mean±SD.

Results Significant interactions between time of day and group factors were evident for SBP, DBP, HR and PA. Generally, SBP was 14±8 mmHg and HR 15±4 beats·min^-1 lower in tetraplegics compared to paraplegics. Significant interactions remained for SBP, DBP and HR after controlling for physical activity (P<0.05). SBP was significantly lower in tetraplegics compared with paraplegics 5, 6, 13, 14 and 21 hrs post-waking (range 13-20 mmHg) and controls 5, 6, 9 and 13 hrs post-waking (range 16-35 mmHg). DBP was significantly lower in tetraplegics than paraplegics 6, 9, 11, 22 and 23 hrs post-waking (range 8-14 mmHg) and controls 1, 22 and 24 hrs post-waking (range 10-12 mmHg). Importantly, Paraplegics and controls were not different following covariate control for activity.

Conclusions Differences in the level of physical activity between SCI individuals compared with healthy controls do not explain the altered circadian rhythmicity of BP and HR. The elevated cardiovascular incidence is likely due to inherent changes in the autonomic nervous system as a direct result of the injury.