Efficacy of fluid loading as a countermeasure to the hemodynamic and hormonal changes of 28-h head-down bed rest

After exposure to microgravity, or head-down bed rest (HDBR), fluid loading is often used with the intent of increasing plasma volume and maintaining mean arterial pressure during orthostatic stress. Nine men (aged 18-32 years) underwent three randomized trials with lower body negative pressure (LBNP) before and after: (1) 4-h of sitting with fluid loading (1 g sodium chloride/125 mL of water starting 2.5-h before LBNP), (2) 28-h of 6-degree HDBR without fluid loading, and (3) 28-h of 6-degree HDBR with fluid loading. LBNP was progressive from 0 to -40 mmHg. After 28-h HDBR, fluid loading did not protect against the loss of plasma volume (-280 ± 64 mL without fluid loading, -207 ± 86 with fluid loading, P = 0.472) nor did it protect against a drop of mean arterial pressure (P = 0.017) during LBNP (Post-28 h HDBR response from 0 to -40 mmHg LBNP: 88 ± 4 to 85 ± 4 mmHg without fluid loading and 93 ± 4 to 88 ± 5 mmHg with fluid loading, P = 0.557 between trials). However, fluid loading did protect against the loss of stroke volume index and central venous pressure observed after 28-h HDBR. Fluid loading also attenuated the increase of angiotensin II seen after 28-h HDBR and throughout the LBNP protocol (Post-28 h HDBR response from 0 to -40 mmHg LBNP: 16.6 ± 3.4 to 23.7 ± 5.0 pg/mL without fluid loading and 6.1 ± 0.8 to 12.2 ± 2.3 pg/mL with fluid loading, P < 0.001 between trials). Our results indicate that fluid loading did not protect against plasma volume loss due to HDBR or change blood pressure responses to LBNP. However, changes in central venous pressure, stroke volume and fluid regulatory hormones could potentially influence longer duration studies and those with more severe orthostatic stress.