title: Physiological Mechanisms Mediating the Coupling between Heart Period and Arterial Pressure in Response to Postural Changes in Humans.
abstract: The upright posture strengthens the coupling between heart period (HP) and systolic arterial pressure (SAP) consistently with a greater contribution of the arterial baroreflex to cardiac control, while paradoxically decreasing cardiac baroreflex sensitivity (cBRS). To investigate the physiological mechanisms that mediate the coupling between HP and SAP in response to different postures, we analyzed the cross-correlation functions between low-frequency HP and SAP fluctuations and estimated cBRS with the sequence technique in healthy male subjects during passive head-up tilt test (HUTT,n= 58), during supine wakefulness, supine slow-wave sleep (SWS), and in the seated and active standing positions (n= 8), and during progressive loss of 1 L blood (n= 8) to decrease central venous pressure in the supine position. HUTT, SWS, the seated, and the standing positions, but not blood loss, entailed significant increases in the positive correlation between HP and the previous SAP values, which is the expected result of arterial baroreflex control, compared with baseline recordings in the supine position during wakefulness. These increases were mirrored by increases in the low-frequency variability of SAP in each condition but SWS. cBRS decreased significantly during HUTT, in the seated and standing positions, and after blood loss compared with baseline during wakefulness. These decreases were mirrored by decreases in the RMSSD index, which reflects cardiac vagal modulation. These results support the view that the cBRS decrease associated with the upright posture is a byproduct of decreased cardiac vagal modulation, triggered by the arterial baroreflex in response to central hypovolemia. Conversely, the greater baroreflex contribution to cardiac control associated with upright posture may be explained, at least in part, by enhanced fluctuations of SAP, which elicit a more effective entrainment of HP fluctuations by the arterial baroreflex. These SAP fluctuations may result from enhanced fluctuations of vascular resistance specific to the upright posture, and not be driven by the accompanying central hypovolemia.
authors.affiliation: Department of Biomedical and Neuromotor Sciences, University of Bologna Bologna, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Silvani
authors.affiliation: Department of Biomedical and Neuromotor Sciences, University of BolognaBologna, Italy; IRCCS Bologna Institute of Neurological SciencesBologna, Italy.
authors.forename: Giovanna
authors.initials: G
authors.lastname: Calandra-Buonaura
authors.affiliation: Center for Research and Education in Special Environments, Department of Exercise and Nutrition Sciences, University at Buffalo Buffalo, NY, USA.
authors.forename: Blair D
authors.initials: BD
authors.lastname: Johnson
authors.affiliation: Department of Anesthesiology, Mayo Clinic Rochester, MN, USA.
authors.forename: Noud
authors.initials: N
authors.lastname: van Helmond
authors.affiliation: IRCCS Bologna Institute of Neurological Sciences Bologna, Italy.
authors.forename: Giorgio
authors.initials: G
authors.lastname: Barletta
authors.affiliation: IRCCS Bologna Institute of Neurological Sciences Bologna, Italy.
authors.forename: Anna G
authors.initials: AG
authors.lastname: Cecere
authors.affiliation: Department of Anesthesiology, Mayo Clinic Rochester, MN, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Joyner
authors.affiliation: Department of Biomedical and Neuromotor Sciences, University of BolognaBologna, Italy; IRCCS Bologna Institute of Neurological SciencesBologna, Italy.
authors.forename: Pietro
authors.initials: P
authors.lastname: Cortelli
keywords: arterial blood pressure; baroreflex; central venous pressure; heart; hemorrhage; seated position; standing position; tilt table test