title: Mechanisms of increase in cardiac output during acute weightlessness in humans.
abstract: Based on previous water immersion results, we tested the hypothesis that the acute 0-G-induced increase in cardiac output (CO) is primarily caused by redistribution of blood from the vasculature above the legs to the cardiopulmonary circulation. In seated subjects (n = 8), 20 s of 0 G induced by parabolic flight increased CO by 1.7 ± 0.4 l/min (P < 0.001). This increase was diminished to 0.8 ± 0.4 l/min (P = 0.028), when venous return from the legs was prevented by bilateral venous thigh-cuff inflation (CI) of 60 mmHg. Because the increase in stroke volume during 0 G was unaffected by CI, the lesser increase in CO during 0 G + CI was entirely caused by a lower heart rate (HR). Thus blood from vascular beds above the legs in seated subjects can alone account for some 50% of the increase in CO during acute 0 G. The remaining increase in CO is caused by a higher HR, of which the origin of blood is unresolved. In supine subjects, CO increased from 7.1 ± 0.7 to 7.9 ± 0.8 l/min (P = 0.037) when entering 0 G, which was solely caused by an increase in HR, because stroke volume was unaffected. In conclusion, blood originating from vascular beds above the legs can alone account for one-half of the increase in CO during acute 0 G in seated humans. A Bainbridge-like reflex could be the mechanism for the HR-induced increase in CO during 0 G in particular in supine subjects.
authors.affiliation: Department of Biomedical Sciences, Faculty of Health Sciences, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen, Denmark. lonnie@sund.ku.dk
authors.forename: Lonnie G
authors.initials: LG
authors.lastname: Petersen
authors.forename: Morten
authors.initials: M
authors.lastname: Damgaard
authors.forename: Johan C G
authors.initials: JC
authors.lastname: Petersen
authors.forename: Peter
authors.initials: P
authors.lastname: Norsk
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology*; D005260:Female; D006112:Gravitation; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D007866:Leg / Q000098:blood supply; D008297:Male; D008875:Middle Aged; D011187:Posture / Q000502:physiology; D012039:Regional Blood Flow / Q000502:physiology; D013152:Splanchnic Circulation / Q000502:physiology; D013318:Stroke Volume / Q000502:physiology; D016683:Supine Position / Q000502:physiology; D014655:Vascular Resistance / Q000502:physiology; D014893:Weightlessness / Q000009:adverse effects*; D018474:Weightlessness Simulation