No effect of venoconstrictive thigh cuffs on orthostatic hypotension induced by head-down bed rest

Orthostatic intolerance (OI) is the most serious symptom of cardiovascular deconditioning induced by head-down bed rest or weightlessness. Wearing ven oconstrictive thigh cuffs is an empirical countermeasure used by Russian co smonauts to limit the shift of fluid from the lower part of the body to the cardio-cephalic region. Our aim was to determine whether or not thigh cuff s help to prevent orthostatic hypotension induced by head-down bed rest. We studied the effect of thigh cuffs on eight healthy men. The cuffs were wor n during the day for 7 days of head-down bed rest. We measured: orthostatic tolerance (stand tests and lower body negative pressure tests), plasma vol ume (Evans blue dilution), autonomic influences (plasma noradrenaline) and baroreflex sensitivity (spontaneous baroreflex slope). Thigh cuffs limited the loss of plasma volume (thigh cuffs: -201 +/- 37 mL vs. control: -345 +/ - 42 mL, P < 0.05), the degree of tachycardia and reduction in the spontane ous baroreflex sensitivity induced by head-down bed rest. However, the impa ct of thigh cuffs was not sufficient to prevent OI (thigh cuffs: 7.0 min of standing time vs. control: 7.1 min). Decrease in absolute plasma volume an d in baroreflex sensitivity are known to be important factors in the aetiol ogy of OI induced by head-down bed rest. However, dealing with these factor s, using thigh cuffs for example, is not sufficient to prevent OI. Other fa ctors such as venous compliance, microcirculatory changes, peripheral arter ial vasoconstriction and vestibular afferents must also be considered.