CARDIOVASCULAR-RESPONSES TO STANDING - EFFECT OF HYDRATION

Many astronauts experience intolerance to orthostatic stress after spa ce flight, despite the ingestion of salt tablets and water equivalent to 0.9% saline just before their return to Earth. Previous research in dicates that the ingestion of 1.07% saline solution increased plasma v olume more than did 0.9% saline. Therefore, the authors hypothesized t hat the 1.07% saline would be more effective in reducing orthostatic s tress during standing. In this study, six men (22-47 years) performed a 5-minute ''stand test'' (5 minutes supine followed by 5 minutes stan ding) under four hydration conditions: 1) hypohydrated (HYPO, 20 mg in travenous [IV] Lasix), 2) euhydrated (EU), 3) rehydrated with 1 L 0.9% saline 2 hours after Lasix, or 4) rehydrated with 1 L 1.07% saline. S tand tests were done 4 to 5 hours after rehydration. Plasma volume was reduced 10% after Lasix, and was restored by both rehydration solutio ns. When subjects stood, their diastolic pressure, mean pressure, hear t rate (HR), and peripheral resistance increased (P <.05), and their s troke volume (SV), cardiac output (CO), and thoracic fluid (TF, by imp edance cardiography) decreased (P <.05). Systolic arterial pressure (S BP) increased when subjects stood after saline, but decreased if subje cts were HYPO or EU (P <.05 for 1.07% versus HYPO and EU). Heart rate (HR), another indicator of orthostatic stress, did not differ among hy dration states. During the last minute of the stand test, TF was great er if subjects had fluid countermeasures. Stroke volume, CO, and TF we re significantly less during minute 5 of standing than during minute 3 . Whether they would continue to fall in a longer stand test is not kn own. The results for SBP indicate that 1.07% saline may have advantage s over 0.9% saline as a countermeasure to postspace-flight or postbedr est orthostatic intolerance.