title: Cardiovascular re-adjustments and baroreflex response during clinical reambulation procedure at the end of 35-day bed rest in humans.
abstract: During the reambulation procedure after 35-day head-down tilt bed rest (HDTBR) for 9 men, we recorded for the first time heart rate (HR; with electrocardiogram) and arterial pressure profiles (fingertip plethysmography) for 5 min in HDTBR and horizontal (SUP) positions, followed by 12 min in standing position, during which 4 subjects fainted (intolerant, INT) and were laid horizontal again (Recovery). We computed: mean arterial pressure (P̄; pressure profiles integral mean), stroke volume (SV; obtained with Modelflow method), and cardiac output (Q̇; SV × HR). All cardiovascular data remained stable in HDTBR and SUP for both groups (EXP). Taking the upright posture, EXP showed a decrease in SV and an increase in HR, becoming significantly different from SUP within 1 min. Further evolution of these parameters kept Q̇ stable in both groups until the second minute of standing. Afterward, in INT, P̄ precipitated without further HR increases: SV stopped being corrected and Q̇ reached 2.9 ± 0.4 L·min(-1) at the last 15 s of standing. Sudden drop in P̄ allowed identification of a low-pressure threshold in INT (70.7 ± 12.9 mm Hg), after which syncope occurred within 80 s. During Recovery, baroreflex curves showed a flat phase (P̄ increase, HR stable), followed by a steep phase (P̄ increased, HR decreased, starting when P̄ was 84.5 ± 12.5 mm Hg and Q̇ was 9.6 ± 1.5 L·min(-1)). INT, in contrast with tolerant subjects, did not sustain standing because HR was unable to correct for the P̄ drop. These results indicate a major role for impaired arterial baroreflexes in the onset of orthostatic intolerance.
authors.affiliation: Département de Neuroscience Fondamentales, Centre Médical Universitaire, Université de Genève, 1 Rue Michel Servet, CH-1211 Genève 4, Switzerland. alessandra.adami@unige.ch
authors.forename: Alessandra
authors.initials: A
authors.lastname: Adami
authors.forename: Paolo
authors.initials: P
authors.lastname: Pizzinelli
authors.forename: Aurélien
authors.initials: A
authors.lastname: Bringard
authors.forename: Carlo
authors.initials: C
authors.lastname: Capelli
authors.forename: Mara
authors.initials: M
authors.lastname: Malacarne
authors.forename: Daniela
authors.initials: D
authors.lastname: Lucini
authors.forename: Boštjan
authors.initials: B
authors.lastname: Simunič
authors.forename: Rado
authors.initials: R
authors.lastname: Pišot
authors.forename: Guido
authors.initials: G
authors.lastname: Ferretti
mesh_terms: D017704:Baroreflex; D001510:Bed Rest; D002302:Cardiac Output; D002319:Cardiovascular System; D006339:Heart Rate; D006801:Humans