Positive and negative feedback mechanisms in the neural regulation of cardiovascular function in healthy and spinal cord-injured humans

Background-We tested the hypothesis that in humans, hypertension/tachycardia and hypotension/bradycardia nonbaroreflex sequences that occur within spontaneous arterial pressure (AP) and R-R interval fluctuations are an expression of positive feedback mechanisms neurally regulating the cardiovascular system. Methods and Results-We studied 15 spinal cord-injured (SCI) subjects (8 tetraplegics and 7 paraplegics) and 8 healthy subjects. The occurrence of nonbaroreflex (NBseq) and baroreflex (Bseq) sequences, ie, hypertension-bradycardia and hypotension-tachycardia sequences, was assessed during rest and head-up tilt (HUT). The ratio between Bseq and NBseq (B/NB ratio) was also calculated. In resting conditions, the occurrence of NBseq was significantly lower (P < 0.05) in tetraplegics (7.9 +/- 1.5) than in paraplegics (16.2 +/- 3.2) and normal subjects (19.0 +/- 3.5), whereas the occurrence of Bseq was not significantly different between the 3 groups (38.6 +/- 11.9 versus 45.4 +/- 6.0 versus 47.0 +/- 11.9). In tetraplegics, the B/NB ratio showed a marked, significant decrease (from 8.4 +/- 4.2 to 1.9 +/- 0,8, P < 0.05) in response to HUT, whereas in normal subjects, it showed a significant increase (from 3.5 +/- 0.7 to 9.4 +/- 2.7, P<0.05). In paraplegics, the B/NB ratio did not change significantly in response to HUT (from 4.5 +/- 1.6 to 4.8 +/- 1.1). Conclusions-Our data suggest that nonbaroreflex sequences occur in humans and might represent the expression of an integrated, neurally mediated, feed-forward type of short-term cardiovascular regulation that is able to interact dynamically with feedback mechanisms of baroreflex origin.