Enhanced open-loop but not closed-loop cardiac baroreflex sensitivity during orthostatic stress in humans

Akimoto T, Sugawara J, Ichikawa D, Terada N, Fadel PJ, Ogoh S. Enhanced open-loop but not closed-loop cardiac baroreflex sensitivity during orthostatic stress in humans. Am J Physiol Regul Integr Comp Physiol 301: R1591-R1598, 2011. First published September 7, 2011; doi:10.1152/ajpregu.00347.2011.-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.