Upright posture increases the strength of baroreflex coupling between heart period and blood pressure in human subjects

Background: a positive peak at negative time shifts of the cross-correlation function (rCCF) between heart period and systolic blood pressure is consistent with baroreflex control (Auton Neurosci 2011;162:66-71). In human subjects, rCCF is higher in deep non-rapid-eye-movement (NREM) sleep than in wakefulness, indicating tighter cardiac baroreflex control (Am J Physiol 2008;294:R1686-92). Aim: we investigated whether rCCF also varies during wakefulness as a function of posture. Methods: 8 adult healthy male subjects were studied during overnight polysomnography and during daytime while awake and lying supine, sitting, standing actively, and with 65° passive head-up tilt (HUT). Data before (supine rest) and during HUT were also obtained in a replication group of 58 adult healthy males. In each subject, rCCF was computed on beat-to-beat values of heart period (electrocardiography) and systolic blood pressure (Portapres model 2, FMS) over 5-min windows. Data are expressed as mean ± SEM and analysed with ANOVA and t-tests (significance at P < 0.05). Results: rCCF increased significantly from lying supine (-0.04 ± 0.04) to sitting (0.45 ± 0.06) and active standing (0.63 ± 0.04), when it was higher than in deep NREM sleep (0.41 ± 0.03). rCCF during HUT (0.49 ± 0.07) was significantly higher than while lying supine, but did not differ significantly compared to either active standing or sitting. HUT increased rCCF in 91% of the subjects (53/58) in the replication group, the difference being significant at group level (0.39 ± 0.02 vs. 0.08 ± 0.02). Conclusions: rCCF varies during wakefulness as a function of posture, indicating that upright posture increases the strength of the baroreflex coupling between heart period and blood pressure.