Skin surface cooling improves orthostatic tolerance following prolonged head-down bed rest

Prolonged exposure to microgravity, as well as its ground-based analog, head-down bed rest (HDBR), reduces orthostatic tolerance in humans. While skin surface cooling improves orthostatic tolerance, it remains unknown whether this could be an effective countermeasure to preserve orthostatic tolerance following HDBR. We therefore tested the hypothesis that skin surface cooling improves orthostatic tolerance after prolonged HDBR. Eight subjects (six men and two women) participated in the investigation. Orthostatic tolerance was determined using a progressive lower-body negative pressure (LBNP) tolerance test before HDBR during normothermic conditions and on day 16 or day 18 of 6° HDBR during normothermic and skin surface cooling conditions (randomized order post-HDBR). The thermal conditions were achieved by perfusing water (normothermia ∼34°C and skin surface cooling ∼12-15°C) through a tube-lined suit worn by each subject. Tolerance tests were performed after ∼30 min of the respective thermal stimulus. A cumulative stress index (CSI; mmHg LBNP·min) was determined for each LBNP protocol by summing the product of the applied negative pressure and the duration of LBNP at each stage. HDBR reduced normothermic orthostatic tolerance as indexed by a reduction in the CSI from 1,037 ± 96 mmHg·min to 574 ± 63 mmHg·min (P < 0.05). After HDBR, skin surface cooling increased orthostatic tolerance (797 ± 77 mmHg ·min) compared with normothermia (P < 0.05). While the reduction in orthostatic tolerance following prolonged HDBR was not completely reversed by acute skin surface cooling, the identified improvements may serve as an important and effective countermeasure for individuals exposed to microgravity, as well as immobilized and bed-stricken individuals.