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(Circulation. 2001;103:1250.)
© 2001 American Heart Association, Inc.

Clinical Investigation and Reports

Positive and Negative Feedback Mechanisms in the Neural Regulation of Cardiovascular Function in Healthy and Spinal Cord–Injured Humans

J. M. Legramante, MD; G. Raimondi, MD; M. Massaro, MD; F. Iellamo, MD

From Dipartimento di Medicina Interna, Universitá di Roma "Tor Vergata," Rome, Italy.

Correspondence to Jacopo M. Legramante, MD, Dipartimento di Medicina Interna, Universitá di Roma "Tor Vergata," Via O. Raimondo, snc, 00173 Rome, Italy. E-mail Iellamo{at}med.uniroma2.it

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.

Key Words: reflex • baroreceptors • nervous system, autonomic

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