Enhancing circulation to lower limbs during head-down tilt by warming upper body and thighs

BACKGROUND: Long-duration spaceflight results in deconditioning of the cardiovascular system, loss of fluid volume, bone demineralization, and atrophy of skeletal muscles, particularly affecting the lower limbs. We hypothesized that it is possible to improve blood circulation to the lower extremities in simulated microgravity by forcing the blood to deliver heat to the feet through heating parts of the upper body and thighs. METHODS: In Study 1, seven men and four women were assessed in an environmental chamber with head-down tilt (HDT) at 14 degrees, wearing a newly developed shortened multi-compartment liquid cooling/warming garment (SLCWG) with local tubing networks covering parts of the head, torso, thigh, arms, and hands, with fingers, lower leg, and feet exposed. Study 2 was the same as Study 1 with a new cohort of four men and two women, and the assessment of toe blood perfusion on all subjects. Heat was applied as follows: Stage 1--SLCWG inlet water temperature 33 degrees C to stabilize comfort; Stage 2--inlet water temperature 8-10 degrees C (in combination with HDT) to reach a criterion of 25 degrees C finger temperature (Tfing); and Stage 3--inlet water temperature 45 degrees C to restore Tfing to 33 degrees C. RESULTS: Improvement of foot circulation by delivering more heat to the upper body and thighs was noted; increases in toe temperature (Ttoe) suggest enhanced perfusion. From Stage 2 to 3, there were significant increases in Ttoe (p < 0.05), a significant decrease in diastolic BP (DBP) (p < 0.05), and a significant change across stages in subjective perception of foot comfort (p < 0.001) and foot heat (p < 0.06). Further, toe blood perfusion increased significantly from Stage 2 to 3 (p < 0.05). CONCLUSIONS: Moderate partial heating of the upper body/thighs improved blood circulation in the feet in simulated microgravity by delivering heat to the lower extremities through restriction of heat exchange with the environment in the heated body parts. This technique could serve as a supplemental countermeasure for increasing blood circulation to the lower extremities.