Comparison of aortic and carotid baroreflex stimulus-response characteristics in humans

In order to characterize the stimulus-response relationships of the arteria l, aortic, and carotid baroreflexes in mediating cardiac chronotropic funct ion, we measured heart rate (HR) responses elicited by acute changes in mea n arterial pressure (MAP) and carotid sinus pressure (CSP) in 11 healthy in dividuals. Arterial (aortic + carotid) baroreflex control of HR was quantif ied using ramped changes in MAP induced by bolus injection of phenylephrine (PE) and sodium nitroprusside (SN). To assess aortic-cardiac responses, ne ck pressure (NP) and suction (NS) were applied during PE and SN administrat ion, respectively, to counter alterations in CSP thereby isolating the aort ic baroreflex. Graded levels of NP and NS were delivered to the carotid sin us using a customized neck collar device to assess the carotid-cardiac baro reflex, independent of drug infusion. The operating characteristics of each reflex were determined from the logistic function of the elicited HR respo nse to the induced change in MAP. The arterial pressures at which the thres hold was located on the stimulus-response curves determined for the arteria l, aortic and carotid baroreflexes were not significantly different (72 +/- 4, 67 +/- 3, and 72 +/- 4 mm Hg, respectively, P > 0.05). Similarly, the M AP at which the saturation of the reflex responses were elicited did not di ffer among the baroreflex arcs examined (98 +/- 3, 99 +/- 2, and 102 +/- 3 mm Hg. respectively). These data suggest that the baroreceptor populations studied operate over the same range of arterial pressures. This finding ind icates each baroreflex functions as both an important anti-hypotensive and anti-hypertensive mechanism. In addition, this investigation describes a mo del of aortic baroreflex function in normal healthy humans, which may prove useful in identifying the origin of baroreflex dysfunction in disease- and training-induced conditions. (C) 2001 Elsevier Science B.V. All rights res erved.