Influence of autonomic impairment on blood-pressure and heart-rate scaling structures

Self similarity is a promising tool for quantifying alterations in cardiovascular dynamics, although the effect of the autonomic control on the scaling structure of cardiovascular signals is still unknown. To address this issue, we studied spinal-cord injured subjects as a model of progressively impaired vascular control. We considered 24 able-bodied subjects (AB) and 23 paraplegics with lesion at different levels: between T12 and L4 (N=7); T5 and T11 (N=9); and C6 and T4 (N=7). We recorded blood pressure and heart rate in three conditions characterized by increasing sympathetic activation: supine (SUP); sitting (SIT); and exercise (EXE). We calculated the scaling exponent of mean arterial pressure, HMAP, and of R-R interval, HRRI, by detrended fluctuation analysis. The sympathetic activation had different effects on the scaling exponent, depending on the lesion level. HRRI did not change significantly from SUP to SIT and to EXE in the AB and T12-L4 group, while it increased in the T5-T11 and C6-T4 groups. Also for HMAP sympathetic activation produced changes which depend on the level of the spinal lesion. In particular, our results suggest that heart-rate self similarity depends on the vascular sympathetic control, because it is altered by the spinal-cord lesion even when the cardiac neural control is intact.