title: Enhanced open-loop but not closed-loop cardiac baroreflex sensitivity during orthostatic stress in humans.
abstract: The neural interaction between the cardiopulmonary and arterial baroreflex may be critical for the regulation of blood pressure during orthostatic stress. However, studies have reported conflicting results: some indicate increases and others decreases in cardiac baroreflex sensitivity (i.e., gain) with cardiopulmonary unloading. Thus the effect of orthostatic stress-induced central hypovolemia on regulation of heart rate via the arterial baroreflex remains unclear. We sought to comprehensively assess baroreflex function during orthostatic stress by identifying and comparing open- and closed-loop dynamic cardiac baroreflex gains at supine rest and during 60° head-up tilt (HUT) in 10 healthy men. Closed-loop dynamic "spontaneous" cardiac baroreflex sensitivities were calculated by the sequence technique and transfer function and compared with two open-loop carotid-cardiac baroreflex measures using the neck chamber system: 1) a binary white-noise method and 2) a rapid-pulse neck pressure-neck suction technique. The gain from the sequence technique was decreased from -1.19 ± 0.14 beats·min(-1)·mmHg(-1) at rest to -0.78 ± 0.10 beats·min(-1)·mmHg(-1) during HUT (P = 0.005). Similarly, closed-loop low-frequency baroreflex transfer function gain was reduced during HUT (P = 0.033). In contrast, open-loop low-frequency transfer function gain between estimated carotid sinus pressure and heart rate during white-noise stimulation was augmented during HUT (P = 0.01). This result was consistent with the maximal gain of the carotid-cardiac baroreflex stimulus-response curve (from 0.47 ± 0.15 beats·min(-1)·mmHg(-1) at rest to 0.60 ± 0.20 beats·min(-1)·mmHg(-1) at HUT, P = 0.037). These findings suggest that open-loop cardiac baroreflex gain was enhanced during HUT. Moreover, under closed-loop conditions, spontaneous baroreflex analyses without external stimulation may not represent open-loop cardiac baroreflex characteristics during orthostatic stress.
authors: Department of Biomedical Engineering, Toyo University, Kawagoe-Shi, Saitama Japan.
authors: Toshinari
authors: T
authors: Akimoto
authors: Jun
authors: J
authors: Sugawara
authors: Daisuke
authors: D
authors: Ichikawa
authors: Nobuyuki
authors: N
authors: Terada
authors: Paul J
authors: PJ
authors: Fadel
authors: Shigehiko
authors: S
authors: Ogoh
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D017704:Baroreflex; D001794:Blood Pressure; D003935:Diagnostic Techniques, Cardiovascular; D004244:Dizziness / Q000503:physiopathology*; D004562:Electrocardiography; D006339:Heart Rate; D006801:Humans; D008297:Male; D008365:Manometry; D008955:Models, Cardiovascular; D011187:Posture; D011237:Predictive Value of Tests; D018667:Tilt-Table Test; D013997:Time Factors; D055815:Young Adult