Prolonged head down bed rest-induced inactivity impairs tonic autonomic regulation while sparing oscillatory cardiovascular rhythms in healthy humans.
BACKGROUND
Physical inactivity represents a major risk for cardiovascular disorders, such as hypertension, myocardial infarction or sudden death; however, underlying mechanisms are not clearly elucidated. Clinical and epidemiological investigations suggest, beyond molecular changes, the possibility of an induced impairment in autonomic cardiovascular regulation. However, this hypothesis has not been tested directly.


METHODS
Accordingly, we planned a study with noninvasive, minimally intrusive, techniques on healthy volunteers. Participants were maintained for 90 days strictly in bed, 24 h a day, in head-down (-6 degrees ) position (HDBR). Physical activity was thus virtually abolished for the entire period of HDBR. We examined efferent muscle sympathetic nerve activity, as a measure of vascular sympathetic control, baroreceptor reflex sensitivity, heart rate variability (assessing cardiovagal regulation), RR and systolic arterial pressure and low-frequency and high-frequency normalized components (as a window on central oscillatory regulation). Measures were obtained at rest and during simple maneuvers (moderate handgrip, lower body negative pressure and active standing) to assess potential changes in autonomic cardiovascular responsiveness to standard stimuli and the related oscillatory profiles.


RESULTS
HDBR transiently reduced muscle sympathetic nerve activity, RR, heart rate variability and baroreceptor reflex sensitivity late during HDBR or early during the recovery phase. Conversely, oscillatory profiles of RR and systolic arterial pressure variability were maintained throughout. Responsiveness to test stimuli was also largely maintained.


CONCLUSIONS
Prolonged inactivity as induced by HDBR in healthy volunteers reduces both cardiovagal and vascular sympathetic regulation, while largely maintaining peripheral responsiveness to standardized stimuli and sparing the functional structure of central oscillatory cardiovascular regulation.
Ferretti
Guido
G
Département de Neurosciences Fondamentales, Centre Médical Universitaire, Genève, Switzerland.
Iellamo
Ferdinando
F
Pizzinelli
Paolo
P
Kenfack
Marcel A
MA
Lador
Frédéric
F
Lucini
Daniela
D
Porta
Alberto
A
Narkiewicz
Krzysztof
K
Pagani
Massimo
M
D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D001341:Autonomic Nervous System / Q000502:physiology*; D017704:Baroreflex / Q000502:physiology; D001510:Bed Rest; D001794:Blood Pressure; D002320:Cardiovascular Physiological Phenomena; D018475:Head-Down Tilt / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D011187:Posture / Q000502:physiology*; D013599:Systole; D013997:Time Factors; D014630:Vagus Nerve / Q000502:physiology; D014666:Vasomotor System / Q000502:physiology