Investigating feed-forward neural regulation of circulation from analysis of spontaneous arterial pressure and heart rate fluctuations

Background-Analysis of spontaneous fluctuations in systolic arterial pressu re (SAP) and pulse interval (PI) reveals the occurrence of sequences of con secutive beats characterized by SAP and PI changing in the same (+PI/+SAP a nd -PI/-SAP) or opposite (-PI/+SAP and +PI/-SAP) direction. Although the fo rmer reflects baroreflex regulatory mechanisms, the physiological meaning o f -PI/+SAP and +PI/-SAP is unclear. We tested the hypothesis that -PI/+SAP and +PI/-SAP "nonbaroreflex" sequences represent a phenomenon modulated by the autonomic nervous system reflecting a feed-forward mechanism of cardiov ascular regulation. Methods and Results-We studied anesthetized rabbits before and after (1) co mplete autonomic blockade (guanethidine+propranolol+atropine, n=13; CAB), ( 2) sympathetic blockade (guanethidine+propranolol, n=15; SE), (3) parasympa thetic blockade (atropine, n=16), (4) sinoaortic denervation (n=10; SAD), a nd (5) controlled respiration (n=10; CR). Nonbaroreflex sequences were defi ned as greater than or equal to 3 beats in which SAP and PI of the followin g beat changed in the opposite direction. CAB reduced the number of nonbaro reflex sequences (19.1+/-12.3 versus 88.7+/-36.6, P<0.05), as did SE (25.3/-11.7 versus 84.6+/-23.9, P<0.001) and atropine (11.2+/-6.8 versus 94.1+/- 32.4, P<0.05). SE concomitantly increased baroreflex sensitivity (1.18+/-0. 11 versus 0.47+/-0.09 ms/mm Hg, P<0.01). SAD and CR did not significantly a ffect their occurrence. Conclusions-These results suggest that nonbaroreflex sequences represent th e expression of an integrated, neurally mediated, feed-forward type of shor t-term cardiovascular regulation able to interact dynamically with the feed back mechanisms of baroreflex origin in the control of heart period.