Influence of sympathetic vascular regulation on heart-rate scaling structure : spinal cord lesion as a model of progressively impaired autonomic control

Estimation of self-similarity is a promising tool for quantifying alterations in cardiovascular dynamics. To evaluate the as yet unexplored influence of sympathetic vascular regulation on the scaling exponent, namely on the parameter characterizing self-similarity, we studied patients with a spinal cord injury as a model of progressively impaired vascular control. We considered 24 able-bodied subjects (AB) and 23 paraplegics with increasing lesion levels: between T-12 and L-4 (n=7); T-5 and T-11 (n=9); and C-6 and T-4 (n=7). We recorded the heart rate in three conditions characterized by increasing sympathetic activation: supine (SUP), sitting (SIT) and exercise (EXE). We calculated the scaling exponent by detrended fluctuation analysis (H-DFA). Sympathetic activation had different effects on H, depending on the lesion level. H-DFA, tended to decrease in AB from SUP (0.85+0.02; mean-+SEM) and SIT (0.84+0.02) to EXE (0.79+0.02). It remained constant in the T-12-L-4 group (0.92+0.04, 0.94+0.05 and 0.94+0.04, respectively), while it increased significantly in the T-5-T-11 group (0.88+0.07, 0.94+0.05, 1.00+0.08) and increased even more in the C-6-T-4 group (0.83+0.07, 0.91+0.05, 1.06+0.06). Results suggest that heart-rate self-similarity depends on vascular sympathetic control, because it is altered by spinal-cord lesions, even when cardiac neural control is intact.