Effect of human head flexion on the control of peripheral blood flow in microgravity and in 1 g.
This study evaluated, in six healthy subjects, whether head flexion, which stimulates the vestibular system and the tonic neck receptors, interferes with cardiovascular regulation. Arterial parameters were measured continuously using a pulsed Doppler ultrasound probe during parabolic flights with subjects either in the supine craned-head position (control) or in the supine anterior neck flexion bent-neck position. Exposure to 0 g induced a fluid shift towards the head (stroke volume +8%, P<0.05). Compared to the control situation the mean (SD) blood flow in the femoral artery decreased [ -10 (9)% vs +1 (10)%; P<0.05], and the ratio cerebral artery:femoral artery blood flow ( : ) increased [+8 (14)% vs -4 (7)%; P<0.05], in the bent-neck position. Thus, neck flexion without otolith loading (subject in 0 g) favoured cerebral perfusion during the exposure to 0 g. The return to 1 g, even in the supine position, induced a fluid shift towards the lower limbs. From 0 to 1 g, reduced less [ +6 (8)% vs -1 (8)%; P<0.05], and the : decreased more [-11 (9)% vs 0 (10)%; P<0.05], in the bent-neck position than in the control position. Thus the redistribution of peripheral blood flow in response to the fluid shift towards the legs was less efficient in the bent-neck position. In 0 g environment the passive flexion of the neck (neck receptor stimulation only) increased resistance in the femoral artery [ R(fa) +20 (21)%; P<0.05] and reduced the [-15(10)%; P<0.07] which increased the redistribution of flow towards the brain [; +12 (7)%; P<0.07]. This response was of lower amplitude when both otoliths and neck muscle were stimulated (neck flexion in 1 g) [ R(fa)+9 (7)%, P<0.05; -9 (12), NS; : 0 (12), NS]. We suggest that otolith and neck muscle stimulation (by neck flexion) trigger opposite vascular effects in response to a fluid shift towards the legs.
Unite de Médecine et Physiologie Spatiales, Département de Médecine Nucléaire et Ultrasons, Centre Hospitalier Universitaire Trousseau, 37044 Tours, France.
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S
Herault
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N
Tobal
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H
Normand
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Roumy
P
P
Denise
P
P
Arbeille
D000222:Adaptation, Physiological; D000328:Adult; D001132:Arm / Q000098:blood supply; D001783:Blood Flow Velocity; D001794:Blood Pressure; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005263:Femoral Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D006112:Gravitation; D006257:Head / Q000502:physiology*; D006339:Heart Rate; D006801:Humans; D007866:Leg / Q000098:blood supply; D008297:Male; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D009470:Muscle Spindles / Q000502:physiology; D009333:Neck / Q000502:physiology; D010037:Otolithic Membrane / Q000502:physiology; D011187:Posture / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013318:Stroke Volume; D016683:Supine Position; D014463:Ultrasonography; D014655:Vascular Resistance; D014893:Weightlessness; D018474:Weightlessness Simulation / Q000379:methods